ASTM D4174-23

Standard Practice for Cleaning, Flushing, and Purification of Petroleum Fluid Hydraulic Systems

SIGNIFICANCE AND USE
4.1 Proper fluid condition is essential for the satisfactory performance and long life of the equipment. Prerequisites for proper lubrication and component performance are: (1) a well-designed hydraulic system, (2) the use of a suitable fluid, and (3) a maintenance program including proper filtration methods to ensure that the fluid is free of contaminants. These prerequisites are meaningless unless the hydraulic system is initially cleaned to a level that will prevent component damage on initial start up or when debris may be dislodged by any system upset.  
4.2 The cleaning and flushing of both new and used systems are accomplished by essentially the same procedure. In new systems, the emphasis is on the removal of contaminants introduced during the manufacture, storage, field fabrication, and installation. In used systems, the emphasis is on the removal of contaminants that are generated during operations, from failures that occur during operation; or contaminants introduced during overhaul. Both new and used systems may benefit from high velocity flushing to remove materials that can collect in hard to drain pockets or normally non-wetted surfaces.  
4.3 While the flushing and cleaning philosophies stated in this practice are applicable to all primary and servo hydraulic systems, the equipment specified herein does not apply to compact systems that use relatively small volumes of fluid unless they are servo systems where it is economically justified.  
4.4 It should be emphasized that the established procedures to be followed for flushing and cleaning the hydraulic systems should be accomplished through the cooperative efforts and agreement of the equipment manufacturer, the installer, the flushing service vendor, the operator, and the fluid supplier. No phase of these procedures should be undertaken without a thorough understanding of the possible effects of improper system preparation. The installation and cleaning and flushing of the equipment should not b...
SCOPE
1.1 This practice covers aid for the equipment manufacturer, the installer, the oil supplier and the operator in coordinating their efforts towards obtaining and maintaining clean petroleum fluid hydraulic systems. Of necessity, this practice is generalized due to variations in the type of equipment, builder's practices, and operating conditions. Constant vigilance is required throughout all phases of design, fabrication, installation, flushing, testing, and operation of hydraulic systems to minimize and reduce the presence of contaminants and to obtain optimum system reliability.  
1.2 This practice is presented in the following sequence:    
Section  
Scope  
1  
Referenced Documents  
2  
Terminology  
3  
Significance and Use  
4  
Types of Contamination  
5  
General  
5.1  
Water  
5.2  
Soluble Contaminants  
5.3  
Insoluble Contaminants  
5.4  
Lodged Contamination  
5.4.2.1  
Suspended or Loose Contamination  
5.4.2.2  
Contamination Control  
6  
General  
6.1  
Initial Filling  
6.1.1  
In-Service Units  
6.1.2  
Connection of Contamination Control System  
6.1.3  
Piping or Tubing Contamination Control System  
6.1.4  
Contamination Control Procedures  
6.2  
Full Flow Contamination Control  
6.2.1  
Bypass Contamination Control  
6.2.2  
Batch Contamination Control  
6.2.3  
Contamination Control Processes  
6.3  
Gravity  
6.3.1  
Mechanical  
6.3.2  
Centrifuge  
6.3.2.1  
Filters  
6.3.2.2  
Supplementary Methods  
6.3.3  
Limitations of Contamination Control Devices  
6.3.4  
Storage  
7  
General  
7.1  
Inspection  
8  
General  
8.1  
System Components  
8.2  
Valves, Strainers, and Coolers  
8.2.1  
Sumps and Tanks  
8.2.2  
Control Devices  
8.2.3  
Pumps  
8.2.4  
Flushing Program  
9  
General  
9.1  
Preparation of System for Flushing  
9....

General Information

• Status: Published
• Publication Date: 30-Jun-2023
• ICS: 23.100.01 - Fluid power systems in general
• Technical Committee: D02 - Petroleum Products, Liquid Fuels, and Lubricants
• Drafting Committee: D02.N0 - Hydraulic Fluids

Relations

• Refers: ASTM D445-24 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
• Effective Date: 01-Apr-2024
• Refers: ASTM D7546-24 - Standard Test Method for Determination of Moisture in New and In-Service Lubricating Oils and Additives by Relative Humidity Sensor
• Effective Date: 01-Feb-2024
• Refers: ASTM D7647-24 - Standard Test Method for Automatic Particle Counting of Lubricating and Hydraulic Fluids Using Dilution Techniques to Eliminate the Contribution of Water and Interfering Soft Particles by Light Extinction
• Effective Date: 01-Feb-2024
• Refers: ASTM D4175-23a - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 15-Dec-2023
• Refers: ASTM D445-23 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
• Effective Date: 01-Nov-2023
• Refers: ASTM D4175-23e1 - Standard Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
• Effective Date: 01-Jul-2023
• Refers: ASTM F311-08(2020) - Standard Practice for Processing Aerospace Liquid Samples for Particulate Contamination Analysis Using Membrane Filters
• Effective Date: 01-Apr-2020
• Refers: ASTM D664-11a(2017) - Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration
• Effective Date: 01-May-2017
• Refers: ASTM D445-16 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
• Effective Date: 15-Dec-2016
• Refers: ASTM D4006-16 - Standard Test Method for Water in Crude Oil by Distillation
• Effective Date: 01-Jun-2016
• Refers: ASTM D7546-15 - Standard Test Method for Determination of Moisture in New and In-Service Lubricating Oils and Additives by Relative Humidity Sensor
• Effective Date: 01-Apr-2015
• Refers: ASTM D974-14e1 - Standard Test Method for Acid and Base Number by Color-Indicator Titration
• Effective Date: 01-Dec-2014
• Refers: ASTM D445-14e1 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
• Effective Date: 01-Jul-2014
• Refers: ASTM D445-14 - Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
• Effective Date: 01-Jul-2014
• Refers: ASTM D7042-14 - Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer (and the Calculation of Kinematic Viscosity)
• Effective Date: 01-May-2014

Overview

ASTM D4174-23: Standard Practice for Cleaning, Flushing, and Purification of Petroleum Fluid Hydraulic Systems outlines essential procedures and best practices to ensure the cleanliness and reliability of hydraulic systems using petroleum-based fluids. Maintaining clean hydraulic fluid is critical for system performance, longevity, and minimizing equipment downtime. This practice applies to new installations as well as to maintenance and overhaul of existing hydraulic systems, emphasizing coordination among equipment manufacturers, installers, oil suppliers, service vendors, and operators.

Key Topics

• Contamination Types:
  The standard identifies major forms of contamination in hydraulic systems:

  • Water (fresh and sea water), introducing risks of corrosion, oxidation, and instability.
  • Soluble materials such as cleaning agents, incompatible lubricants, and oxidation products that can alter critical fluid properties.
  • Insoluble particles including rust, fibers, sand, and airborne solids, which can cause abrasive wear and clogging.
  • Erroneous fluid additions and deterioration of hydraulic fluids.

• Contamination Control:
  Effective contamination control is an ongoing program starting from system design through fabrication, storage, installation, and during regular operation. Core elements include:

  • Use of filtration during initial system fill (generally 3-10 micron filters).
  • Regular monitoring and maintenance of contamination control systems.
  • Strategic design of fluid take-off and return lines to optimize removal of solids and water.
  • Instrumentation (such as pressure gauges and alarms) to monitor filter health and prompt timely maintenance.

• Cleaning and Flushing Procedures:
  Whether for new systems (removing manufacturing and installation debris) or used systems (removing operational debris and contaminants after a failure or major overhaul), the procedures involve:

  • High-velocity flushing for effective removal of entrained and lodged contaminants.
  • Avoiding flushing through sensitive components; using appropriate bypasses.
  • Preparatory inspection and protection steps to prevent introduction of new contaminants.

• Methods of Contaminant Removal:
  The practice describes several methods suitable for different contamination scenarios:

  • Gravity separation: Effective for dense particles, often used in storage tanks.
  • Mechanical filtration: Removes solid contaminants down to fine particulate sizes.
  • Centrifugation and coalescers: For separating water and solids from fluid.
  • Supplementary techniques: Such as vacuum dehydration and adsorption for specific contaminant types.

Applications

• New System Commissioning:
  Ensures systems are delivered free from fabrication, assembly, and installation contaminants, supporting reliable start-up.
• Maintenance and Overhaul:
  Guides the removal of buildup and newly introduced contaminants following component repair, system failure, or fluid change.
• Continuous Operation:
  Provides strategies for ongoing contamination detection and control, including regular inspection, fluid sampling, and laboratory analysis to maintain optimum fluid condition.
• Industrial Sectors:
  Applicable wherever petroleum-based hydraulic systems are used, including manufacturing, energy, marine applications, aerospace ground equipment, construction, and process industries.

Related Standards

• ASTM D445: Kinematic Viscosity of Transparent and Opaque Liquids
• ASTM D664/D974: Acid Number of Petroleum Products
• ASTM D2709/D4006: Water and Sediment in Fuels and Oils
• ASTM D4175: Terminology Relating to Petroleum Products, Fuels, and Lubricants
• ASTM D7042: Dynamic Viscosity and Density Measurement
• ASTM D7546/D7647: Determination of Moisture and Particle Counting in Hydraulic Fluids
• ANSI B93.2/B93.19: Fluid Power Terminology and Sampling Methods

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ASTM D4174-23 provides a foundational protocol for hydraulic system cleaning and purification, reducing risks associated with contamination, and delivering practical guidance for improved equipment reliability across industries where hydraulic power is essential. Proper application of this standard extends component life, increases system efficiency, and lowers maintenance costs.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D4174 − 23
Standard Practice for
Cleaning, Flushing, and Purification of Petroleum Fluid
Hydraulic Systems
This standard is issued under the fixed designation D4174; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
Section
Filters 6.3.2.2
1.1 This practice covers aid for the equipment manufacturer,
Supplementary Methods 6.3.3
the installer, the oil supplier and the operator in coordinating Limitations of Contamination Control Devices 6.3.4
Storage 7
their efforts towards obtaining and maintaining clean petro-
General 7.1
leum fluid hydraulic systems. Of necessity, this practice is
Inspection 8
generalized due to variations in the type of equipment, build- General 8.1
System Components 8.2
er’s practices, and operating conditions. Constant vigilance is
Valves, Strainers, and Coolers 8.2.1
required throughout all phases of design, fabrication,
Sumps and Tanks 8.2.2
Control Devices 8.2.3
installation, flushing, testing, and operation of hydraulic sys-
Pumps 8.2.4
tems to minimize and reduce the presence of contaminants and
Flushing Program 9
to obtain optimum system reliability.
General 9.1
Preparation of System for Flushing 9.2
1.2 This practice is presented in the following sequence:
Fluid Heating Prior to Flushing 9.3
Selection of Flushing Oils 9.4
Section
System Operation Fluid 9.4.1
Scope 1
Special Flushing Oil 9.4.2
Referenced Documents 2
Flushing Oil Selection Guide 9.4.3
Terminology 3
Flushing Procedure for New Systems 9.5
Significance and Use 4
Flushing Oil Charge 9.5.1
Types of Contamination 5
Cleaning of Filtration Devices 9.5.2
General 5.1
Cleaning of System Components 9.5.3
Water 5.2
System Flushing and Flush Acceptance Criteria 9.5.4
Soluble Contaminants 5.3
Draining of Flushing Oil 9.5.5
Insoluble Contaminants 5.4
Displacement Oil 9.5.6
Lodged Contamination 5.4.2.1
Interim Corrosion Protection 9.5.7
Suspended or Loose Contamination 5.4.2.2
New Fluid Charge 9.5.8
Contamination Control 6
Flushing of Used Systems 9.6
General 6.1
General Guidelines 9.6.1
Initial Filling 6.1.1
Procedure 9.6.2
In-Service Units 6.1.2
System Maintenance 10
Connection of Contamination Control System 6.1.3
Preinstallation 10.2
Piping or Tubing Contamination Control System 6.1.4
In-Service Units 10.3
Contamination Control Procedures 6.2
Decision to Flush In-Service Hydraulic Systems 10.4
Full Flow Contamination Control 6.2.1
Fluid Condition Monitoring 11
Bypass Contamination Control 6.2.2
Fluid Sampling Techniques 11.2
Batch Contamination Control 6.2.3
Visual Inspection 11.3
Contamination Control Processes 6.3
Laboratory Analysis 11.4
Gravity 6.3.1
Fluid Cleanliness Criteria 11.5
Mechanical 6.3.2
General Information 12
Centrifuge 6.3.2.1
Centrifuge Ratings 12.2
Coalescence 12.3
Vacuum Dehydration 12.4
This practice is under the jurisdiction of ASTM Committee D02 on Petroleum
Adsorption 12.5
Products, Liquid Fuels, and Lubricants and is the direct responsibility of D02.N0 on
Hydraulic Fluids.
1.3 The values stated in SI units are to be regarded as the
Current edition approved July 1, 2023. Published July 2023. Originally approved
standard. The values given in parentheses are for information
in 1982. Last previous edition approved in 2017 as D4174 – 17. DOI: 10.1520/
only.
D4174-23.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4174 − 23
1.4 This standard does not purport to address all of the Particulate Contamination Analysis)
safety concerns, if any, associated with its use. It is the
3. Terminology
responsibility of the user of this standard to establish appro-
priate safety, health, and environmental practices and deter-
3.1 Definitions:
mine the applicability of regulatory limitations prior to use.
3.1.1 For definitions of terms used in this practice, refer to
1.5 This international standard was developed in accor-
Terminology D4175.
dance with internationally recognized principles on standard-
3.2 Definitions of Terms Specific to This Standard:
ization established in the Decision on Principles for the
3.2.1 nominal filtration rating, n—an arbitrary micrometre
Development of International Standards, Guides and Recom-
value indicated by a filter manufacturer. Due to lack of
mendations issued by the World Trade Organization Technical
reproducibility this rating is deprecated. (ANSI B93.2)
Barriers to Trade (TBT) Committee.
4. Significance and Use
2. Referenced Documents
4.1 Proper fluid condition is essential for the satisfactory
2.1 ASTM Standards:
performance and long life of the equipment. Prerequisites for
D445 Test Method for Kinematic Viscosity of Transparent
proper lubrication and component performance are: (1) a
and Opaque Liquids (and Calculation of Dynamic Viscos-
well-designed hydraulic system, (2) the use of a suitable fluid,
ity)
and (3) a maintenance program including proper filtration
D664 Test Method for Acid Number of Petroleum Products
methods to ensure that the fluid is free of contaminants. These
by Potentiometric Titration
prerequisites are meaningless unless the hydraulic system is
D974 Test Method for Acid and Base Number by Color-
initially cleaned to a level that will prevent component damage
Indicator Titration
on initial start up or when debris may be dislodged by any
D1744 Test Method for Determination of Water in Liquid
system upset.
Petroleum Products by Karl Fischer Reagent (Withdrawn
3 4.2 The cleaning and flushing of both new and used systems
2016)
are accomplished by essentially the same procedure. In new
D2709 Test Method for Water and Sediment in Middle
systems, the emphasis is on the removal of contaminants
Distillate Fuels by Centrifuge
introduced during the manufacture, storage, field fabrication,
D4006 Test Method for Water in Crude Oil by Distillation
and installation. In used systems, the emphasis is on the
D4175 Terminology Relating to Petroleum Products, Liquid
removal of contaminants that are generated during operations,
Fuels, and Lubricants
from failures that occur during operation; or contaminants
D7042 Test Method for Dynamic Viscosity and Density of
introduced during overhaul. Both new and used systems may
Liquids by Stabinger Viscometer (and the Calculation of
benefit from high velocity flushing to remove materials that can
Kinematic Viscosity)
collect in hard to drain pockets or normally non-wetted
D7546 Test Method for Determination of Moisture in New
surfaces.
and In-Service Lubricating Oils and Additives by Relative
Humidity Sensor 4.3 While the flushing and cleaning philosophies stated in
D7647 Test Method for Automatic Particle Counting of this practice are applicable to all primary and servo hydraulic
Lubricating and Hydraulic Fluids Using Dilution Tech- systems, the equipment specified herein does not apply to
compact systems that use relatively small volumes of fluid
niques to Eliminate the Contribution of Water and Inter-
fering Soft Particles by Light Extinction unless they are servo systems where it is economically justi-
fied.
F311 Practice for Processing Aerospace Liquid Samples for
Particulate Contamination Analysis Using Membrane Fil-
4.4 It should be emphasized that the established procedures
ters
to be followed for flushing and cleaning the hydraulic systems
F312 Test Methods for Microscopical Sizing and Counting
should be accomplished through the cooperative efforts and
Particles from Aerospace Fluids on Membrane Filters
agreement of the equipment manufacturer, the installer, the
F313 Test Method for Insoluble Contamination of Hydraulic
flushing service vendor, the operator, and the fluid supplier. No
Fluids by Gravimetric Analysis (Withdrawn 1988)
phase of these procedures should be undertaken without a
2.2 ANSI Standards:
thorough understanding of the possible effects of improper
B93.2 Glossary of Terms for Fluid Power
system preparation. The installation and cleaning and flushing
B93.19 Method for Extracting Fluid Samples from the Lines
of the equipment should not be entrusted to persons lacking in
of an Operating Hydraulic Fluid Power System (for
experience.
5. Types of Contamination
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
5.1 General—Hydraulic systems can become contaminated
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
from a variety of sources. Generally, there are five categories of
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website.
contamination: (1) water, (2) fluid soluble material, (3) fluid
The last approved version of this historical standard is referenced on
insoluble material, (4) erroneous fluid additions, and (5)
www.astm.org.
hydraulic fluid deterioration. Properly designed systems can
Available from American National Standards Institute (ANSI), 25 W. 43rd St.,
4th Floor, New York, NY 10036. normally control water and insoluble contaminants; however,
D4174 − 23
when it is necessary to remove soluble contaminants, a fluid 5.3.1.6 Initiate additive-water interaction that can cause
change and flush are required. emulsification, possible additive loss, instability, impaired
purification equipment performance, foaming, and air entrain-
5.2 Water—Water is almost always present in hydraulic
ment.
fluids. It may be present in solution or in a free or emulsified
5.3.1.7 Accelerate oxidation.
form. Water can exist in solution at varying concentrations
depending on the nature of the fluid, the temperature, and so
5.3.2 When a soluble contaminant is present, the fluid
forth. For example, hydraulic fluid may hold 50 ppm of water supplier and he equipment manufacturer should be consulted
at 21 °C (70 °F) and 250 ppm at 71 °C (160 °F). The water in
regarding the advisability of continued use of the fluid or
solution has no adverse effect on lubricating properties of the
replacing it with a new charge.
fluid and causes no corrosion; however, when fluid passes
5.4 Insoluble Contaminants:
through a cooler some water may come out of solution and
5.4.1 Insoluble contaminants normally encountered are
become free water in the form of finely dispersed droplets.
metal particles (including rust) of all types and sizes, fibers,
Many contaminants hinder the separation of this free water
airborne solids, sand, and other nonmetallic particles. These
from the fluid by settling and may cause an emulsion. In
contaminants are often the result of improper manufacturing
hydraulic fluids, the emulsion impairs circulation, interfere
techniques, improper shipping and storage practices, and
with lubrication and adversely affect contamination control
careless installation of hydraulic systems. Some of the effects
equipment.
of solid contamination are:
5.2.1 Water contamination can be classified as either fresh
or sea water, as encountered in land or marine systems. Fresh
5.4.1.1 Abrasive wear or sticking of components such as:
water enters the hydraulic system from moist air as control valve poppets, cylinders, piston rods, and seals.
condensation, through improperly located vents, leaks in
5.4.1.2 Faulty control functioning, particularly plugged
coolers, and steam heaters, and because of improper operation.
fluid lines/filter plugging.
Sea water, in marine hydraulic systems, enters through leaks in
5.4.1.3 Reduced fluid stability.
coolers, faulty manhole gaskets, faulty sump tank seals and
5.4.1.4 Sludge formation.
improperly located vents. Sea and brackish water can also
5.4.1.5 Increased foaming tendency.
present a problem when used as a coolant in land-based units.
Water contamination in hydraulic fluids can: 5.4.1.6 Stabilized water-oil emulsions/accelerated oxidation
5.2.1.1 Promote fluid oxidation. by catalytic effect of metal particles.
5.2.1.2 Reduce fluid stability.
5.4.2 Harmful contamination can exist in the hydraulic
5.2.1.3 Promote sludge.
system in two forms:
5.2.1.4 Promote foaming.
5.4.2.1 Lodged Contamination—These contaminants may
5.2.1.5 Form emulsions.
become dislodged by high fluid flows and temperature differ-
5.2.1.6 Promote rusting and corrosion.
entials or by induced vibration during flushing. Contamination
5.2.1.7 Cause additive depletion and drop-out.
can be lodged in unflushed pockets or settled on the bottom of
5.2.1.8 Adversely affect lubricating properties.
tanks. Unless this contaminant is removed, it becomes dis-
5.2.1.9 Promote bacteria growth.
lodged during startup or during system upsets. Experience,
5.2.1.10 Alter fluid viscosity. good judgement, and careful inspection by the installation
5.2.1.11 Adversely affect fine filtration (that is, excessive supervisor must be relied upon to determine when such dirt has
back pressure). been satisfactorily removed.
5.2.1.12 Promote cavitation.
5.4.2.2 Suspended or Loose Contamination:
5.2.2 In the case of severe salt water contamination, it is
5.4.2.3 Contaminants suspended in the fluid can be gener-
necessary to remove the operating fluid and clean and flush the
ated by particles coming loose from pipe, hose, hydraulic
hydraulic systems.
components, tank walls generally caused by high fluid velocity,
5.3 Soluble Contaminants: wear debris, and vibration. Suspended contaminant can be
measured, as described in 11.3 and 11.4. To prevent the level of
5.3.1 Soluble contaminants in hydraulic systems include
suspended contaminant from getting beyond acceptable limits,
cleaning chemicals, solvents, rust preventives, incompatible
lubricants, flushing oils, extraneous oils, oxidation products, hydraulic system filtration can be augmented with a bypass
contamination control system (fluid filter or centrifuge). Pref-
gasket sealants, and assembly lubricants. These contaminants
cannot be removed by conventional fluid contamination control erably a full flow filter or a full flow filter plus bypass
equipment. Normally, a new charge of fluid accompanied with purification is provided. When a full flow filter is used, a
a displacement flush oil is required to correct the problem. bypass purification system may not be required.
Fluid soluble contaminants can:
5.4.2.4 The bypass or full flow system, or both, can be in
5.3.1.1 Change the fluid viscosity.
operation during the flush operation as well as on a continuous
5.3.1.2 Alter the flash point.
basis during hydraulic system operation. High-velocity flushes
5.3.1.3 Change the color.
will require appropriately sized full flow filters. The rated flow
5.3.1.4 Result in sludge deposits. capacity per hour of a bypass system should be 10 % to 20 %
5.3.1.5 Attack elastomeric seals. of the total system fluid volume.
D4174 − 23
6. Contamination Control 6.1.3.5 When severe emulsification with water occurs, raise
the fluid temperature for a limited time (less than 2 h) to a
6.1 General—Contamination control in a hydraulic system
maximum of 82 °C (180 °F) to facilitate breaking of the
is the complete program of monitoring and maintaining a clean
emulsion. Prolonged operation above 82 °C (180 °F) may
fluid. Contamination control must begin with the design,
prematurely oxidize the hydraulic fluid.
manufacture, and installation of the hydraulic system and
6.1.3.6 Overheating of the hydraulic fluid can cause crack-
continue throughout the life of the system. When making
ing that can result in severe viscosity reduction. This can be
inspections or working in or around a unit, care must be taken
prevented by maintaining heater skin temperatures below
to prevent contaminants from entering the system. When work
121 °C (250 °F), and ensuring that fluid hydraulic pumps are
that generates contaminants is being performed in the vicinity
operated during heating. Ensure that the steam heating pressure
of the hydraulic system, the system components must be
is less than 34.5 kPa (5 psig). Experience has shown that a safe
protected even to the extent of suspending operations, and
2 2
watt density for electrical heaters is 0.77 W ⁄cm (5 W ⁄in. ).
requiring system components to be sealed until the contami- 2 2
Higher densities up to 3.1 W ⁄cm (20 W ⁄in. ) have been used
nating activity has ceased. The contamination control system
with adequate circulation to avoid exceeding the allowable
must be capable of removing water and particulate matter
heater skin temperatures indicated above. Temperature controls
consistent with contamination tolerance and system cleanliness
are installed to maintain these maximum levels. At all times,
requirements.
heating elements must be totally immersed, a fluid level control
6.1.1 Initial Filling—When initially filling the hydraulic
will provide adequate protection. Control heating of the oil so
system, fluids are to be filtered through filters rated from 3 μm
as not to go below the minimum in fluid viscosity recom-
to 10 μm (as needed) with a beta ratio >1000 as they are being
mended for the pumps.
transferred into the reservoir. The contamination control sys-
6.1.4 Piping or Tubing Contamination Control System—The
tem is ready for operation prior to the hydraulic system fill and
interconnecting piping or tubing and contamination control
is operating throughout flushing. See Section 9.5 for new
system must be as clean as the initial hydraulic system. The
installation flushing details.
bypass contamination control system, when present, is piped
6.1.2 In-Service Units—The contamination control system
separately with no connection to the hydraulic fluid piping or
is in operation as long as the hydraulic system is in service. Its
tubing. It should be designed so that the contamination control
operation is frequently and regularly monitored to assure that it
system can take suction from and discharge to any of the
is performing adequately and to determine the need for its
following: (Safeguards are provided to prevent fluid being
maintenance.
drained below minimum fluid level in the sump).
6.1.3 Connection of Contamination Control System—The 6.1.4.1 Main hydraulic reservoir.
external fluid take-off from the circulating system to the
6.1.4.2 Storage tank.
contamination control system is from the lowest point of the
6.1.4.3 Auxiliaries, see details in following section.
fluid sump or reservoir, to facilitate removal of solid contami-
6.1.5 Bypass contamination control system suction and
nants and water.
discharge piping or tubing size is sized for turbulent flow at all
6.1.3.1 Piping between the reservoir and the contamination times and based on the capability of the filtration device.
Piping includes: bypass around fluid heater, pressure relief
control system is designed to minimize the potential for the
loss of fluid that results from piping or equipment failure. This valves on inlet and outlet pumps, sample cocks on fluid inlet
and outlet, and check valves and stop valves as required. A
means short runs and the fewest possible joints. Piping is sized
so as to provide sufficient flow velocity to carry water and dirt drain line is installed from the water removal equipment to a
sludge tank or dirty fluid tank.
to the contamination control system.
6.1.6 The suction lines from any reservoir or tank are
6.1.3.2 The fluid return line to the reservoir is located as far
situated at the lowest point to facilitate removal of solid
removed as possible from the take-off for the contamination
contaminants and water. Piping between systems should be
control system. The return line must contain suitable means to
designed to minimize fluid loss by sloping the lines in the
prevent back flow or siphoning and terminate below the fluid
direction of flow for proper drainage.
level. To prevent loss of fluid through back flow, auxiliary
6.1.7 Fluid return lines to reservoirs should be as far as
connections are provided for oil supply from and to station
possible from the take off line and discharge below the normal
storage tanks. Fluid sampling valves with suitable locking
fluid operating level. The lines contain a sight flow glass near
devices are provided before and after the filtration system.
the reservoir.
6.1.3.3 Instrumentation such as a differential pressure gage
and an alarm is provided with the filters to enable the operator
6.2 Contamination Control Procedures—Contamination
to determine if the contamination control system is functioning
control systems normally employ one or more of the following
properly and also to signal or indicate the necessity for
procedures to assure the most efficient removal of water and
changing or cleaning various filter elements. This is particu-
solid contaminants.
larly important in automated systems.
6.2.1 Full-Flow Contamination Control—The most effec-
6.1.3.4 Filters that never increase differential pressure can tive means of maintaining clean fluid is by full flow treatment
be as much as concern as a high differential pressure. This while the fluid is being circulated during flushing and during
could indicate improper installation or damaged filter allowing normal operation of the unit. This method is capable of
bypass. removing solids rather than water since the equipment is a full
D4174 − 23
flow (filter/strainer). When water contamination is present, it 6.3.2.1 Centrifuge—Centrifugation is a means of separating
can normally be controlled by a bypass type system. fluids of different density and removing solid contaminants
from fluids by utilizing centrifugal force developed by rotating
6.2.2 Bypass Contamination Control:
the fluids at a high speed. For hydraulic fluid, the degree of
6.2.2.1 In a bypass mode, a portion of the fluid is continu-
separation is dependent upon the flow rate, viscosity of the
ously withdrawn, the contaminants are removed and the fluid
fluid and the density of the solids and specific gravity of the
then returned to the reservoir. Continuous bypass although less
fluid contaminants. Commercial units are sized to attain a
efficient than full flow, is preferred over batch filtration because
the system can continue in operation even though the hydraulic specific level of separation. Clean fluid and separated water are
continuously discharged automatically by the centrifuge;
system has been shut down. When a centrifuge, dehydrator, or
coalescer is used, both water and solid contaminants are sludge and solid contaminants remain in the centrifuge bowl
and are periodically removed manually, or automatically, as in
removed although the presence of particulate matter will
reduce the water removal effectiveness of the coalescer. Thus, the self-cleaning type centrifuge. These units are commonly
called purifiers or clarifiers. With centrifugal purifiers, the
the bypass system is the primary system considered over batch
purification. solids and water are removed so they are no longer in contact
with the fluid that reduces the self catalyzing effects on
6.2.2.2 The capacity of a bypass system is at least 10 % to
oxidation.
20 % of the total fluid circulation flow rate.
6.3.2.2 Filters—Mechanical filters remove solid contami-
6.2.2.3 The contamination control system should be capable
nants by passing fluid through restrictions that trap the solid
of maintaining particulate matter and free water contamination
particles. Depending upon the choice of filter media, particles
below limits established for the system by the equipment
as small as or less than one micrometre can be removed. As
manufacturer.
contaminants are removed and collected on the filter element,
6.2.3 Batch Contamination Control:
the pressure drop across the filter increases, ultimately requir-
6.2.3.1 Batch methods are generally employed for filtering
ing replacement or cleaning of the elements.
fluid in the storage system. However, when severe fluid
6.3.3 Supplementary Methods—Supplementary methods for
contamination is observed in a storage tank, appropriate
contamination control are coalescers, vacuum dehydration, and
valving for the use of a continuous bypass device (perhaps
adsorption (for descriptions of these methods see 12.3, 12.4,
portable) is recommended. The source of contamination in the
and 12.5). Contamination control by the preceding methods
system is located and corrected.
can, under certain conditions, deplete the hydraulic fluid
6.2.3.2 When the fluid storage tank is likely to encounter
additives.
low temperatures, a prescribed heater will be required. This is
to heat the fluid to obtain a suitable viscosity for the filtration 6.3.4 Limitations of Contamination Control Devices—
Centrifuges are gravity related and effectiveness is dependent
system and for operation of the hydraulic pumps. (Warning—
Proper controls must be provided to avoid overheating of upon particle size and density and gravitational force devel-
oped. Most mechanical-type filters are ineffective for water
noncirculated fluid.).
removal. Coalescers are designed primarily for water removal
6.3 Contamination Control Processes—The basic methods
and are limited by the solid contamination, viscosity, and
for removal of contaminants are gravity, centrifugation, and
surfactants in the fluid. Vacuum dehydrators and air stripping
mechanical filtration.
have low single-pass water removal efficiency. Certain types of
6.3.1 Gravity:
adsorbents can affect the fluid’s chemical composition and
6.3.1.1 Gravity purification can only separate out contami-
should only be used after verification.
nants that are heavier than the fluid. The rate of separation is
dependent on viscosity of the fluid, particle size and specific
7. Storage
gravity of the contaminants, and quiesence of the fluid. Gravity
separation is often accomplished during quiescent fluid storage 7.1 General:
and must be supplemented by one or more of the other 7.1.1 During storage, protect all components from rust,
contamination control methods.
contaminants, and damage as much as possible. Undercover
6.3.1.2 A method for reducing the contaminant load on storage with proper vent breather filers is recommended.
filters is to heat the fluid in a tank and allow the solids and the Monthly inspections are recommended, and corrective steps
water to settle. This settling tank should be equipped with must be taken when found necessary. Take care during inspec-
tions to minimize disturbance of equipment protection.
low-pressure steam heating coils or low-watt density electrical
resistance heaters of designated size and a drain that terminates
7.1.2 The protection of all uncoated components requires
in a waste fluid tank. The settling tank is connected to the that an application of some type of rust preventive be used to
contamination control system suction and discharge lines. This
protect ferrous surfaces from corrosion during the storage and
settling tank is equipped with a drop line to the hydraulic sump, installation phases. Remove the preservative compounds by
or reservoir and the valve on this line is locked closed. The
flushing the system with regular hydraulic fluid or oil solvent,
capacity of the settling tank should be sufficient to hold the although hand cleaning of some components is also used.
entire operating charge. Settled material can be more readily
However, the flush oil and preservative must be compatible to
removed from gravity tanks, when the tanks are provided with preclude foaming, the formation of emulsions, or the break-
sloped, conical, or vee bottoms.
down of hydraulic fluid additives. Once the rust preventive is
6.3.2 Mechanical: removed, the ferrous surfaces are subject to rust unless care is
D4174 − 23
taken to keep all surfaces oil-wetted. To lessen these undesir- spatter, loose paint, etc. Apply a coating of rust-preventive oil,
able effects vapor space inhibited (VSI) oils can be used. The or a vapor space inhibited oil, and seal all openings. Repainting
general requirements for the use of the vapor space inhibited
is not recommended.
oils are:
8.2.3 Control Devices—It is important that control devices
7.1.2.1 Wet all surfaces with vapor space inhibited oils after
be installed by qualified personnel. Extreme care is taken to
cleaning.
prevent accidental contamination to these devices. Prevent
7.1.2.2 Do not drain but add sufficient oil to provide a
accidental contamination of the associated piping during in-
reservoir of the oil in the assembly.
stallation.
7.1.2.3 Seal the component to prevent loss of vapor phase
8.2.4 Pumps—Prior to assembly and installation, inspect
protection and intrusion of contaminants.
pumps for the presence of hard film coatings or contamination.
7.1.3 Most of these VSI oils are fully compatible with
When such coatings or contaminants are present, thoroughly
regular hydraulic fluids and flushing oils, and draining or
clean and coat the pumps with rust preventive or vapor space
removal may not be necessary. However, follow the oil
inhibited oil.
manufacturer’s recommendation. In addition, it has been
shown that these oils will provide some residual protection to
9. Flushing Program
the system and lessen corrosion after the oils have been drained
or displaced.
9.1 General—For a unit that is field assembled, keeping the
7.1.4 These VSI oils are available in a range of viscosities.
hydraulic system piping or tubing clean enough so that flushing
However, when an oil of significantly higher viscosity than the
is not necessary is economically and practically impossible.
flushing oil is used, draining of the assembled system to limit
Thus, it is generally recognized that an oil flush must take place
the amount of the higher viscosity VSI oil to 10 % is
after the piping or tubing has been installed but before the
recommended to prevent significantly increasing the viscosity
system goes into operation. However, the success of this oil
of the flushing oil.
flush depends on: (1) the success of the efforts to keep dirt out
initially, and (2) the proper conduct of the flush (the subject of
8. Inspection
this procedure). A successful flush means that clean pipe and
system components are obtained with minimum time and
8.1 General:
effort. The cleaning and flushing of both new and used systems
8.1.1 All components of the hydraulic system that are
are accomplished by essentially the same procedure. New
fabricated and assembled at a manufacturer’s facility and
systems will often benefit from high velocity flushing. In new
received as unit for installation in the system are defined as
systems, the emphasis is on the removal of contaminants
preassembled components.
introduced during the manufacture, storage, field fabrication
8.1.2 Inspect the preassembled components upon receipt to
determine condition and degree of protection. All seals and and installation. In used systems, the emphasis is on the
removal of contaminants that are generated by failures that
caps intended to exclude moisture and dirt are checked for
integrity and replaced as required. When the initial or subse- occur during operation, or introduced during operation or
quent inspection discloses dirt or rusting, the item is immedi- overhaul. Do not flush through valves, bearings, or other
ately cleaned, represerved, and sealed as required. Due to the critical components. Use inspected clean bypasses. Tight
variety of equipment and materials, details to cover each case clearances in the system and any flow orifice should always be
cannot be specified. For painted (coated) components, the removed to promote turbulent flow conditions, allowing faster
coating is inspected for integrity and renewed as necessary. flushing time.
9.1.1 A successful flush should keep the dirt out is achieved
8.2 Systems Components:
by the cooperation and diligence of many parties. Examples of
8.2.1 Valves, Strainers, and Coolers—Inspect valves, strain-
these efforts are listed as follows:
ers and coolers, when received from the manufacturer, for
contamination. When there is evidence of hard film protective 9.1.1.1 Design system to allow successful cleaning.
coating or contamination, the unit should be dismantled and all
9.1.1.2 Properly clean and preserve the piping or tubing and
the parts thoroughly c
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D4174 − 17 D4174 − 23
Standard Practice for
Cleaning, Flushing, and Purification of Petroleum Fluid
Hydraulic Systems
This standard is issued under the fixed designation D4174; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This practice covers aid for the equipment manufacturer, the installer, the oil supplier and the operator in coordinating their
efforts towards obtaining and maintaining clean petroleum fluid hydraulic systems. Of necessity, this practice is generalized due
to variations in the type of equipment, builder’s practices, and operating conditions. Constant vigilance is required throughout all
phases of design, fabrication, installation, flushing, testing, and operation of hydraulic systems to minimize and reduce the presence
of contaminants and to obtain optimum system reliability.
1.2 This practice is presented in the following sequence:
Section
Scope 1
Referenced Documents 2
Terminology 3
Significance and Use 4
Types of Contamination 5
General 5.1
Water 5.2
Soluble Contaminants 5.3
Insoluble Contaminants 5.4
Lodged Contamination 5.4.2.1
Suspended or Loose Contamination 5.4.2.2
Contamination Control 6
General 6.1
Initial Filling 6.1.1
In-Service Units 6.1.2
Connection of Contamination Control System 6.1.3
Piping or Tubing Contamination Control System 6.1.4
Contamination Control Procedures 6.2
Full Flow Contamination Control 6.2.1
Bypass Contamination Control 6.2.2
Batch Contamination Control 6.2.3
Contamination Control Processes 6.3
Gravity 6.3.1
Mechanical 6.3.2
Centrifuge 6.3.2.1
Filters 6.3.2.2
Supplementary Methods 6.3.3
Limitations of Contamination Control Devices 6.3.4
Storage 7
General 7.1
This practice is under the jurisdiction of ASTM Committee D02 on Petroleum Products, Liquid Fuels, and Lubricants and is the direct responsibility of D02.N0 on
Hydraulic Fluids.
Current edition approved Dec. 1, 2017July 1, 2023. Published January 2018July 2023. Originally approved in 1982. Last previous edition approved in 20152017 as
D4174 – 15.D4174 – 17. DOI: 10.1520/D4174-17.10.1520/D4174-23.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D4174 − 23
Section
Inspection 8
General 8.1
System Components 8.2
Valves, Strainers, and Coolers 8.2.1
Sumps and Tanks 8.2.2
Control Devices 8.2.3
Pumps 8.2.4
Flushing Program 9
General 9.1
Preparation of System for Flushing 9.2
Fluid Heating Prior to Flushing 9.3
Selection of Flushing Oils 9.4
System Operation Fluid 9.4.1
Special Flushing Oil 9.4.2
Flushing Oil Selection Guide 9.4.3
Flushing Procedure for New Systems 9.5
Flushing Oil Charge 9.5.1
Cleaning of Filtration Devices 9.5.2
Cleaning of System Components 9.5.3
System Flushing and Flush Acceptance Criteria 9.5.4
Draining of Flushing Oil 9.5.5
Displacement Oil 9.5.6
Interim Corrosion Protection 9.5.7
New Fluid Charge 9.5.8
Flushing of Used Systems 9.6
General Guidelines 9.6.1
Procedure 9.6.2
System Maintenance 10
Preinstallation 10.2
In-Service Units 10.3
Decision to Flush In-Service Hydraulic Systems 10.4
Fluid Condition Monitoring 11
Fluid Sampling Techniques 11.2
Visual Inspection 11.3
Laboratory Analysis 11.4
Fluid Cleanliness Criteria 11.5
General Information 12
Centrifuge Ratings 12.2
Coalescence 12.3
Vacuum Dehydration 12.4
Adsorption 12.5
1.3 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.
1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.5 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
D445 Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity)
D664 Test Method for Acid Number of Petroleum Products by Potentiometric Titration
D974 Test Method for Acid and Base Number by Color-Indicator Titration
D1774D1744 Test Method for Elastic Properties of Textile FibersDetermination of Water in Liquid Petroleum Products by Karl
Fischer Reagent (Withdrawn 2000)2016)
D2709 Test Method for Water and Sediment in Middle Distillate Fuels by Centrifuge
D4006 Test Method for Water in Crude Oil by Distillation
D4175 Terminology Relating to Petroleum Products, Liquid Fuels, and Lubricants
D7042 Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer (and the Calculation of Kinematic
Viscosity)
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
The last approved version of this historical standard is referenced on www.astm.org.
D4174 − 23
D7546 Test Method for Determination of Moisture in New and In-Service Lubricating Oils and Additives by Relative Humidity
Sensor
D7647 Test Method for Automatic Particle Counting of Lubricating and Hydraulic Fluids Using Dilution Techniques to
Eliminate the Contribution of Water and Interfering Soft Particles by Light Extinction
F311 Practice for Processing Aerospace Liquid Samples for Particulate Contamination Analysis Using Membrane Filters
F312 Test Methods for Microscopical Sizing and Counting Particles from Aerospace Fluids on Membrane Filters
F313 Test Method for Insoluble Contamination of Hydraulic Fluids by Gravimetric Analysis (Withdrawn 1988)
2.2 ANSI Standards:
B93.2 Glossary of Terms for Fluid Power
B93.19 Method for Extracting Fluid Samples from the Lines of an Operating Hydraulic Fluid Power System (for Particulate
Contamination Analysis)
3. Terminology
3.1 Definitions:
3.1.1 For definitions of terms used in this practice, refer to Terminology D4175.
3.2 Definitions:Definitions of Terms Specific to This Standard:
3.2.1 nominal filtration rating—rating, n—an arbitrary micrometre value indicated by a filter manufacturer. Due to lack of
reproducibility this rating is deprecated. (ANSI B93.2)
4. Significance and Use
4.1 Proper fluid condition is essential for the satisfactory performance and long life of the equipment. Prerequisites for proper
lubrication and component performance are: (1) a well-designed hydraulic system, (2) the use of a suitable fluid, and (3) a
maintenance program including proper filtration methods to ensure that the fluid is free of contaminants. These prerequisites are
meaningless unless the hydraulic system is initially cleaned to a level that will prevent component damage on initial start up or
when debris may be dislodged by any system upset.
4.2 The cleaning and flushing of both new and used systems are accomplished by essentially the same procedure. In new systems,
the emphasis is on the removal of contaminants introduced during the manufacture, storage, field fabrication, and installation. In
used systems, the emphasis is on the removal of contaminants that are generated during operations, from failures that occur during
operation; or contaminants introduced during overhaul. Both new and used systems may benefit from high velocity flushing to
remove materials that can collect in hard to drain pockets or normally non-wetted surfaces.
4.3 While the flushing and cleaning philosophies stated in this practice are applicable to all primary and servo hydraulic systems,
the equipment specified herein does not apply to compact systems that use relatively small volumes of fluid unless they are servo
systems where it is economically justified.
4.4 It should be emphasized that the established procedures to be followed for flushing and cleaning the hydraulic systems should
be accomplished through the cooperative efforts and agreement of the equipment manufacturer, the installer, the flushing service
vendor, the operator, and the fluid supplier. No phase of these procedures should be undertaken without a thorough understanding
of the possible effects of improper system preparation. The installation and cleaning and flushing of the equipment should not be
entrusted to persons lacking in experience.
5. Types of Contamination
5.1 General—Hydraulic systems can become contaminated from a variety of sources. Generally, there are five categories of
contamination: (1) water, (2) fluid soluble material, (3) fluid insoluble material, (4) erroneous fluid additions, and (5) hydraulic
fluid deterioration. Properly designed systems can normally control water and insoluble contaminants; however, when it is
necessary to remove soluble contaminants, a fluid change and flush are required.
5.2 Water—Water is almost always present in hydraulic fluids. It may be present in solution or in a free or emulsified form. Water
can exist in solution at varying concentrations depending on the nature of the fluid, the temperature, and so forth. For example,
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036.
D4174 − 23
hydraulic fluid may hold 50 ppm of water at 21 °C (70 °F) and 250 ppm at 71 °C (160 °F). The water in solution has no adverse
effect on lubricating properties of the fluid and causes no corrosion; however, when fluid passes through a cooler some water may
come out of solution and become free water in the form of finely dispersed droplets. Many contaminants hinder the separation of
this free water from the fluid by settling and may cause an emulsion. In hydraulic fluids, the emulsion impairs circulation, interfere
with lubrication and adversely affect contamination control equipment.
5.2.1 Water contamination can be classified as either fresh or sea water, as encountered in land or marine systems. Fresh water
enters the hydraulic system from moist air as condensation, through improperly located vents, leaks in coolers, and steam heaters,
and because of improper operation. Sea water, in marine hydraulic systems, enters through leaks in coolers, faulty manhole gaskets,
faulty sump tank seals and improperly located vents. Sea and brackish water can also present a problem when used as a coolant
in land-based units. Water contamination in hydraulic fluids can:
5.2.1.1 Promote fluid oxidation.
5.2.1.2 Reduce fluid stability.
5.2.1.3 Promote sludge.
5.2.1.4 Promote foaming.
5.2.1.5 Form emulsions.
5.2.1.6 Promote rusting and corrosion.
5.2.1.7 Cause additive depletion and drop-out.
5.2.1.8 Adversely affect lubricating properties.
5.2.1.9 Promote bacteria growth.
5.2.1.10 Alter fluid viscosity.
5.2.1.11 Adversely affect fine filtration (that is, excessive back pressure).
5.2.1.12 Promote cavitation.
5.2.2 In the case of severe salt water contamination, it is necessary to remove the operating fluid and clean and flush the hydraulic
systems.
5.3 Soluble Contaminants:
5.3.1 Soluble contaminants in hydraulic systems include cleaning chemicals, solvents, rust preventives, incompatible lubricants,
flushing oils, extraneous oils, oxidation products, gasket sealants, and assembly lubricants. These contaminants cannot be removed
by conventional fluid contamination control equipment. Normally, a new charge of fluid accompanied with a displacement flush
oil is required to correct the problem. Fluid soluble contaminants can:
5.3.1.1 Change the fluid viscosity.
5.3.1.2 Alter the flash point.
5.3.1.3 Change the color.
5.3.1.4 Result in sludge deposits.
5.3.1.5 Attack elastomeric seals.
5.3.1.6 Initiate additive-water interaction that can cause emulsification, possible additive loss, instability, impaired purification
equipment performance, foaming, and air entrainment.
D4174 − 23
5.3.1.7 Accelerate oxidation.
5.3.2 When a soluble contaminant is present, the fluid supplier and he equipment manufacturer should be consulted regarding the
advisability of continued use of the fluid or replacing it with a new charge.
5.4 Insoluble Contaminants:
5.4.1 Insoluble contaminants normally encountered are metal particles (including rust) of all types and sizes, fibers, airborne
solids, sand, and other nonmetallic particles. These contaminants are often the result of improper manufacturing techniques,
improper shipping and storage practices, and careless installation of hydraulic systems. Some of the effects of solid contamination
are:
5.4.1.1 Abrasive wear or sticking of components such as: control valve poppets, cylinders, piston rods, and seals.
5.4.1.2 Faulty control functioning, particularly plugged fluid lines/filter plugging.
5.4.1.3 Reduced fluid stability.
5.4.1.4 Sludge formation.
5.4.1.5 Increased foaming tendency.
5.4.1.6 Stabilized water-oil emulsions/accelerated oxidation by catalytic effect of metal particles.
5.4.2 Harmful contamination can exist in the hydraulic system in two forms:
5.4.2.1 Lodged Contamination—These contaminants may become dislodged by high fluid flows and temperature differentials or
by induced vibration during flushing. Contamination can be lodged in unflushed pockets or settled on the bottom of tanks. Unless
this contaminant is removed, it becomes dislodged during startup or during system upsets. Experience, good judgement, and
careful inspection by the installation supervisor must be relied upon to determine when such dirt has been satisfactorily removed.
5.4.2.2 Suspended or Loose Contamination:
5.4.2.3 Contaminants suspended in the fluid can be generated by particles coming loose from pipe, hose, hydraulic components,
tank walls generally caused by high fluid velocity, wear debris, and vibration. Suspended contaminant can be measured, as
described in 11.3 and 11.4. To prevent the level of suspended contaminant from getting beyond acceptable limits, hydraulic system
filtration can be augmented with a bypass contamination control system (fluid filter or centrifuge). Preferably a full flow filter or
a full flow filter plus bypass purification is provided. When a full flow filter is used, a bypass purification system may not be
required.
5.4.2.4 The bypass or full flow system, or both, can be in operation during the flush operation as well as on a continuous basis
during hydraulic system operation. High-velocity flushes will require appropriately sized full flow filters. The rated flow capacity
per hour of a bypass system should be 1010 % to 20 % of the total system fluid volume.
6. Contamination Control
6.1 General—Contamination control in a hydraulic system is the complete program of monitoring and maintaining a clean fluid.
Contamination control must begin with the design, manufacture, and installation of the hydraulic system and continue throughout
the life of the system. When making inspections or working in or around a unit, care must be taken to prevent contaminants from
entering the system. When work that generates contaminants is being performed in the vicinity of the hydraulic system, the system
components must be protected even to the extent of suspending operations, and requiring system components to be sealed until
the contaminating activity has ceased. The contamination control system must be capable of removing water and particulate matter
consistent with contamination tolerance and system cleanliness requirements.
6.1.1 Initial Filling—When initially filling the hydraulic system, fluids are to be filtered through filters rated from 3-μm to
D4174 − 23
10-μm3 μm to 10 μm (as needed) with a beta ratio >1000 as they are being transferred into the reservoir. The contamination control
system is ready for operation prior to the hydraulic system fill and is operating throughout flushing. See Section 9.5 for new
installation flushing details.
6.1.2 In-Service Units—The contamination control system is in operation as long as the hydraulic system is in service. Its
operation is frequently and regularly monitored to assure that it is performing adequately and to determine the need for its
maintenance.
6.1.3 Connection of Contamination Control System—The external fluid take-off from the circulating system to the contamination
control system is from the lowest point of the fluid sump or reservoir, to facilitate removal of solid contaminants and water.
6.1.3.1 Piping between the reservoir and the contamination control system is designed to minimize the potential for the loss of
fluid that results from piping or equipment failure. This means short runs and the fewest possible joints. Piping is sized so as to
provide sufficient flow velocity to carry water and dirt to the contamination control system.
6.1.3.2 The fluid return line to the reservoir is located as far removed as possible from the take-off for the contamination control
system. The return line must contain suitable means to prevent back flow or siphoning and terminate below the fluid level. To
prevent loss of fluid through back flow, auxiliary connections are provided for oil supply from and to station storage tanks. Fluid
sampling valves with suitable locking devices are provided before and after the filtration system.
6.1.3.3 Instrumentation such as a differential pressure gage and an alarm is provided with the filters to enable the operator to
determine if the contamination control system is functioning properly and also to signal or indicate the necessity for changing or
cleaning various filter elements. This is particularly important in automated systems.
6.1.3.4 Filters that never increase differential pressure can be as much as concern as a high differential pressure. This could
indicate improper installation or damaged filter allowing bypass.
6.1.3.5 When severe emulsification with water occurs, raise the fluid temperature for a limited time (less than 2 h) to a maximum
of 82 °C (180 °F) to facilitate breaking of the emulsion. Prolonged operation above 82 °C (180 °F) may prematurely oxidize the
hydraulic fluid.
6.1.3.6 Overheating of the hydraulic fluid can cause cracking that can result in severe viscosity reduction. This can be prevented
by maintaining heater skin temperatures below 121 °C (250 °F), and ensuring that fluid hydraulic pumps are operated during
heating. Ensure that the steam heating pressure is less than 34.5 kPa (5 psig). Experience has shown that a safe watt density for
2 2 2 2
electrical heaters is 0.77 W ⁄cm (5 W ⁄in. ). Higher densities up to 3.1 W ⁄cm (20 W ⁄in. ) have been used with adequate
circulation to avoid exceeding the allowable heater skin temperatures indicated above. Temperature controls are installed to
maintain these maximum levels. At all times, heating elements must be totally immersed, a fluid level control will provide adequate
protection. Control heating of the oil so as not to go below the minimum in fluid viscosity recommended for the pumps.
6.1.4 Piping or Tubing Contamination Control System—The interconnecting piping or tubing and contamination control system
must be as clean as the initial hydraulic system. The bypass contamination control system, when present, is piped separately with
no connection to the hydraulic fluid piping or tubing. It should be designed so that the contamination control system can take
suction from and discharge to any of the following: (Safeguards are provided to prevent fluid being drained below minimum fluid
level in the sump).
6.1.4.1 Main hydraulic reservoir.
6.1.4.2 Storage tank.
6.1.4.3 Auxiliaries, see details in following section.
6.1.5 Bypass contamination control system suction and discharge piping or tubing size is sized for turbulent flow at all times and
based on the capability of the filtration device. Piping includes: bypass around fluid heater, pressure relief valves on inlet and outlet
pumps, sample cocks on fluid inlet and outlet, and check valves and stop valves as required. A drain line is installed from the water
removal equipment to a sludge tank or dirty fluid tank.
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6.1.6 The suction lines from any reservoir or tank are situated at the lowest point to facilitate removal of solid contaminants and
water. Piping between systems should be designed to minimize fluid loss by sloping the lines in the direction of flow for proper
drainage.
6.1.7 Fluid return lines to reservoirs should be as far as possible from the take off line and discharge below the normal fluid
operating level. The lines contain a sight flow glass near the reservoir.
6.2 Contamination Control Procedures—Contamination control systems normally employ one or more of the following
procedures to assure the most efficient removal of water and solid contaminants.
6.2.1 Full-Flow Contamination Control—The most effective means of maintaining clean fluid is by full flow treatment while the
fluid is being circulated during flushing and during normal operation of the unit. This method is capable of removing solids rather
than water since the equipment is a full flow (filter/strainer). When water contamination is present, it can normally be controlled
by a bypass type system.
6.2.2 Bypass Contamination Control:
6.2.2.1 In a bypass mode, a portion of the fluid is continuously withdrawn, the contaminants are removed and the fluid then
returned to the reservoir. Continuous bypass although less efficient than full flow, is preferred over batch filtration because the
system can continue in operation even though the hydraulic system has been shut down. When a centrifuge, dehydrator, or
coalescer is used, both water and solid contaminants are removed although the presence of particulate matter will reduce the water
removal effectiveness of the coalescer. Thus, the bypass system is the primary system considered over batch purification.
6.2.2.2 The capacity of a bypass system is at least 10 % to 20 % of the total fluid circulation flow rate.
6.2.2.3 The contamination control system should be capable of maintaining particulate matter and free water contamination below
limits established for the system by the equipment manufacturer.
6.2.3 Batch Contamination Control:
6.2.3.1 Batch methods are generally employed for filtering fluid in the storage system. However, when severe fluid contamination
is observed in a storage tank, appropriate valving for the use of a continuous bypass device (perhaps portable) is recommended.
The source of contamination in the system is located and corrected.
6.2.3.2 When the fluid storage tank is likely to encounter low temperatures, a prescribed heater will be required. This is to heat
the fluid to obtain a suitable viscosity for the filtration system and for operation of the hydraulic pumps. (Warning—Proper
controls must be provided to avoid overheating of noncirculated fluid.).
6.3 Contamination Control Processes—The basic methods for removal of contaminants are gravity, centrifugation, and
mechanical filtration.
6.3.1 Gravity:
6.3.1.1 Gravity purification can only separate out contaminants that are heavier than the fluid. The rate of separation is dependent
on viscosity of the fluid, particle size and specific gravity of the contaminants, and quiesence of the fluid. Gravity separation is
often accomplished during quiescent fluid storage and must be supplemented by one or more of the other contamination control
methods.
6.3.1.2 A method for reducing the contaminant load on filters is to heat the fluid in a tank and allow the solids and the water to
settle. This settling tank should be equipped with low-pressure steam heating coils or low-watt density electrical resistance heaters
of designated size and a drain that terminates in a waste fluid tank. The settling tank is connected to the contamination control
system suction and discharge lines. This settling tank is equipped with a drop line to the hydraulic sump, or reservoir and the valve
on this line is locked closed. The capacity of the settling tank should be sufficient to hold the entire operating charge. Settled
material can be more readily removed from gravity tanks, when the tanks are provided with sloped, conical, or vee bottoms.
6.3.2 Mechanical:
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6.3.2.1 Centrifuge—Centrifugation is a means of separating fluids of different density and removing solid contaminants from fluids
by utilizing centrifugal force developed by rotating the fluids at a high speed. For hydraulic fluid, the degree of separation is
dependent upon the flow rate, viscosity of the fluid and the density of the solids and specific gravity of the fluid contaminants.
Commercial units are sized to attain a specific level of separation. Clean fluid and separated water are continuously discharged
automatically by the centrifuge; sludge and solid contaminants remain in the centrifuge bowl and are periodically removed
manually, or automatically, as in the self-cleaning type centrifuge. These units are commonly called purifiers or clarifiers. With
centrifugal purifiers, the solids and water are removed so they are no longer in contact with the fluid that reduces the self catalyzing
effects on oxidation.
6.3.2.2 Filters—Mechanical filters remove solid contaminants by passing fluid through restrictions that trap the solid particles.
Depending upon the choice of filter media, particles as small as or less than one micrometre can be removed. As contaminants are
removed and collected on the filter element, the pressure drop across the filter increases, ultimately requiring replacement or
cleaning of the elements.
6.3.3 Supplementary Methods—Supplementary methods for contamination control are coalescers, vacuum dehydration, and
adsorption (for descriptions of these methods see 12.3, 12.4, and 12.5). Contamination control by the preceding methods can, under
certain conditions, deplete the hydraulic fluid additives.
6.3.4 Limitations of Contamination Control Devices—Centrifuges are gravity related and effectiveness is dependent upon particle
size and density and gravitational force developed. Most mechanical-type filters are ineffective for water removal. Coalescers are
designed primarily for water removal and are limited by the solid contamination, viscosity, and surfactants in the fluid. Vacuum
dehydrators and air stripping have low single-pass water removal efficiency. Certain types of adsorbents can affect the fluid’s
chemical composition and should only be used after verification.
7. Storage
7.1 General:
7.1.1 During storage, protect all components from rust, contaminants, and damage as much as possible. Undercover storage with
proper vent breather filers is recommended. Monthly inspections are recommended, and corrective steps must be taken when found
necessary. Take care during inspections to minimize disturbance of equipment protection.
7.1.2 The protection of all uncoated components requires that an application of some type of rust preventive be used to protect
ferrous surfaces from corrosion during the storage and installation phases. Remove the preservative compounds by flushing the
system with regular hydraulic fluid or oil solvent, although hand cleaning of some components is also used. However, the flush
oil and preservative must be compatible to preclude foaming, the formation of emulsions, or the breakdown of hydraulic fluid
additives. Once the rust preventive is removed, the ferrous surfaces are subject to rust unless care is taken to keep all surfaces
oil-wetted. To lessen these undesirable effects vapor space inhibited (VSI) oils can be used. The general requirements for the use
of the vapor space inhibited oils are:
7.1.2.1 Wet all surfaces with vapor space inhibited oils after cleaning.
7.1.2.2 Do not drain but add sufficient oil to provide a reservoir of the oil in the assembly.
7.1.2.3 Seal the component to prevent loss of vapor phase protection and intrusion of contaminants.
7.1.3 Most of these VSI oils are fully compatible with regular hydraulic fluids and flushing oils, and draining or removal may not
be necessary. However, follow the oil manufacturer’s recommendation. In addition, it has been shown that these oils will provide
some residual protection to the system and lessen corrosion after the oils have been drained or displaced.
7.1.4 These VSI oils are available in a range of viscosities. However, when an oil of significantly higher viscosity than the flushing
oil is used, draining of the assembled system to limit the amount of the higher viscosity VSI oil to 10 % is recommended to prevent
significantly increasing the viscosity of the flushing oil.
8. Inspection
8.1 General:
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8.1.1 All components of the hydraulic system that are fabricated and assembled at a manufacturer’s facility and received as unit
for installation in the system are defined as preassembled components.
8.1.2 Inspect the preassembled components upon receipt to determine condition and degree of protection. All seals and caps
intended to exclude moisture and dirt are checked for integrity and replaced as required. When the initial or subsequent inspection
discloses dirt or rusting, the item is immediately cleaned, represerved, and sealed as required. Due to the variety of equipment and
materials, details to cover each case cannot be specified. For painted (coated) components, the coating is inspected for integrity
and renewed as necessary.
8.2 Systems Components:
8.2.1 Valves, Strainers, and Coolers—Inspect valves, strainers and coolers, when received from the manufacturer, for
contamination. When there is evidence of hard film protective coating or contamination, the unit should be dismantled and all the
parts thoroughly cleaned with a petroleum solvent or oil soluble cleaner. In all cases, use clean rags (not waste cloths).
8.2.1.1 Internal surfaces that cannot be reached are flushed with a petroleum solvent or oil soluble cleaner. (Warning—
Combustible. Skin irritant on repeated contact. Aspiration hazard.)
8.2.1.2 After internal surfaces have been cleaned, they must be thoroughly dried with moisture-free air. All cleaned surfaces are
coated immediately by spraying them with an oil supplier approved rust-preventive oil, vapor space inhibitor, or suitable lube oil.
After all surfaces have been coated, reassemble the equipment and cap all openings.
8.2.2 Sumps and Tanks—Thoroughly inspect sumps, reservoirs, or tanks. Manually remove rust, mill scale, weld spatter, loose
paint, etc. Apply a coating of rust-preventive oil, or a vapor space inhibited oil, and seal all openings. Repainting is not
recommended.
8.2.3 Control Devices—It is important that control devices be installed by qualified personnel. Extreme care is taken to prevent
accidental contamination to these devices. Prevent accidental contamination of the associated piping during installation.
8.2.4 Pumps—Prior to assembly and installation, inspect pumps for the presence of hard film coatings or contamination. When
such coatings or contaminants are present, thoroughly clean and coat the pumps with rust preventive or vapor space inhibited oil.
9. Flushing Program
9.1 General—For a unit that is field assembled, keeping the hydraulic system piping or tubing clean enough so that flushing is
not necessary is economically and practically impossible. Thus, it is generally recognized that an oil flush must take place after
the piping or tubing has been installed but before the system goes into operation. However, the success of this oil flush depends
on: (1) the success of the efforts to keep dirt out initially, and (2) the proper conduct of the flush (the subject of this procedure).
A successful flush means that clean pipe and system components are obtained with minimum time and effort. The cleaning and
flushing of both new and used systems are accomplished by essentially the same procedure. New systems will
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