Water-tube boilers and auxiliary installations - Part 11: Requirements for limiting devices of the boiler and accessories

This European Standard specifies requirements for limiters (or limiting devices) which are incorporated into safety systems for water-tube boilers as defined in EN 12952-1.
A limiter (or limiting device) can be either:
-   a safety accessory as defined in the Pressure Equipment Directive, Article 1, clause 2.1.3, and needs to include the safety logic and final actuator, or
-   one element of a safety system, for example, a self-monitoring water level sensor used as part of a safety accessory as defined in the Pressure Equipment Directive, Article 1, clause 2.1.3. The overall boiler protection function shall be provided in association with additional safety logic (where appropriate) and a final actuator.
The design requirements and examination of functional capability for the limiters are covered in this European Standard.
For an explanation of the extent of the limiter (or limiting device) see Figure A.1.

Wasserrohrkessel und Anlagenkomponenten - Teil 11: Anforderungen an Begrenzungseinrichtungen an Kessel und Zubehör

Diese Europäische Norm legt die Anforderungen an Begrenzer (oder Begrenzungseinrichtungen) fest, die in Sicherheitssystemen für Wasserrohrkessel nach EN 12952-1 eingebaut sind.
Ein Begrenzer (oder eine Begrenzungseinrichtung) kann entweder
   ein Ausrüstungsteil mit Sicherheitsfunktion sein, wie in der Druckgeräterichtlinie, Artikel 1, 2.1.3, festgelegt, das mit einer Sicherheitskette und einem Stellglied versehen ist, oder
   ein Element eines Sicherheitssystems sein, z. B. ein selbstüberwachender Wasserstandfühler als Teil eines Ausrüstungsteils mit Sicherheitsfunktion nach der Festlegung in der Druckgeräterichtlinie, Artikel 1, 2.1.3. Die Schutzfunktion für den gesamten Kessel wird mit einer zusätzlichen Sicherheitskette (wo angemessen) und
einem Stellglied abgedeckt.
Diese Europäische Norm umfasst die Anforderungen an die Ausführung und die Prüfung der Funktionssicherheit der Begrenzer.
Der Funktionsumfang des Begrenzers (oder der Begrenzungseinrichtung) ist in Bild A.1 verdeutlicht.

Chaudières à tubes d'eau et installations auxiliaires - Partie 11: Exigences pour les dispositifs de limitation de la chaudière et de ses accessoires

Vodocevni kotli in pomožne napeljave - 11. del: Zahteve za omejilne naprave kotla in opremo

General Information

Status
Published
Publication Date
03-Jul-2007
Current Stage
9060 - Closure of 2 Year Review Enquiry - Review Enquiry
Due Date
02-Dec-2021
Completion Date
02-Dec-2021

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2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Vodocevni kotli in pomožne napeljave - 11. del: Zahteve za omejevalne naprave kotla in opremeWasserrohrkessel und Anlagekomponenten - Teil 11: Anforderungen an Begrenzungseinrichtungen an Kessel und ZubehörChaudieres a tubes d'eau et installations auxiliaires - Partie 11: Exigences pour les dispositifs de limitation de la chaudiere et de ses accessoiresWater-tube boilers and auxiliary installations - Part 11: Requirements for limiting devices of the boiler and accessories27.060.30Grelniki vode in prenosniki toploteBoilers and heat exchangersICS:Ta slovenski standard je istoveten z:EN 12952-11:2007SIST EN 12952-11:2007en,fr,de01-september-2007SIST EN 12952-11:2007SLOVENSKI

STANDARD
SIST EN 12952-11:2007

EUROPEAN STANDARDNORME EUROPÉENNEEUROPÄISCHE NORMEN 12952-11July 2007ICS 27.040 English VersionWater-tube boilers and auxiliary installations - Part 11:Requirements for limiting devices of the boiler and accessoriesChaudières à tubes d'eau et installations auxiliaires - Partie11: Exigences pour les dispositifs de limitation de lachaudière et de ses accessoiresWasserrohrkessel und Anlagekomponenten - Teil 11:Anforderungen an Begrenzungseinrichtungen an Kesselund ZubehörThis European Standard was approved by CEN on 26 May 2007.CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this EuropeanStandard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such nationalstandards may be obtained on application to the CEN Management Centre or to any CEN member.This European Standard exists in three official versions (English, French, German). A version in any other language made by translationunder the responsibility of a CEN member into its own language and notified to the CEN Management Centre has the same status as theofficial versions.CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland,France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal,Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.EUROPEAN COMMITTEE FOR STANDARDIZATIONCOMITÉ EUROPÉEN DE NORMALISATIONEUROPÄISCHES KOMITEE FÜR NORMUNGManagement Centre: rue de Stassart, 36

B-1050 Brussels© 2007 CENAll rights of exploitation in any form and by any means reservedworldwide for CEN national Members.Ref. No. EN 12952-11:2007: ESIST EN 12952-11:2007

EN 12952-11:2007 (E) 2 Contents Page Foreword..............................................................................................................................................................3 Introduction.........................................................................................................................................................4 1 Scope......................................................................................................................................................5 2 Normative references............................................................................................................................5 3 Terms and definitions...........................................................................................................................6 4 Requirements for limiters.....................................................................................................................8 4.1 General....................................................................................................................................................8 4.2 Materials and design.............................................................................................................................8 4.3 Electrical equipment..............................................................................................................................9 4.4 Fault assessment...................................................................................................................................9 4.5 Marking.................................................................................................................................................11 5 Special requirements for water level limiters...................................................................................13 5.1 Components.........................................................................................................................................13 5.2 Design...................................................................................................................................................13 5.3 Floating devices...................................................................................................................................14 5.4 Level electrode devices......................................................................................................................14 5.5 Examination of functional capability.................................................................................................15 5.6 Fault detection.....................................................................................................................................18 6 Special requirements for pressure limiters......................................................................................18 6.1 Components.........................................................................................................................................18 6.2 Additional fault assessment requirements.......................................................................................18 6.3 Design...................................................................................................................................................18 6.4 Electrical equipment............................................................................................................................19 6.5 Examination of functional capability.................................................................................................19 6.6 Fault detection.....................................................................................................................................21 7 Special requirements for temperature limiters.................................................................................21 7.1 Components.........................................................................................................................................21 7.2 Design...................................................................................................................................................21 7.3 Electrical equipment............................................................................................................................23 7.4 Examination of functional capability.................................................................................................23 8 Special requirements for flow limiters...............................................................................................24 8.1 Components.........................................................................................................................................24 8.2 Design...................................................................................................................................................24 8.3 Electrical equipment............................................................................................................................24 8.4 Examination of functional capability.................................................................................................24 Annex A (informative)

Limiting device............................................................................................................25 Annex B (informative)

Example of an examination plan...............................................................................26 Annex C (informative)

Marking of limiters......................................................................................................28 Annex D (normative)

Immunity against electrical and electromagnetic influences —

Requirements and testing...................................................................................................................31 Annex ZA (informative)

Relationship between this European Standard and the Essential

Requirements of EU Directive 97/23/EC............................................................................................35 Bibliography......................................................................................................................................................36

SIST EN 12952-11:2007

EN 12952-11:2007 (E) 3 Foreword This document (EN 12952-11:2007) has been prepared by Technical Committee CEN/TC 269 “Shell and water-tube boilers”, the secretariat of which is held by DIN. This European Standard shall be given the status of a national standard, either by publication of an identical text or by endorsement, at the latest by January 2008, and conflicting national standards shall be withdrawn at the latest by January 2008. This document has been prepared under a mandate given to CEN by the European Commission and the European Free Trade Association, and supports essential requirements of EU Directive(s). For relationship with EU Directive(s), see informative Annex ZA, which is an integral part of this document. The European Standard EN 12952 concerning water-tube boilers and auxiliary installations consists of the following Parts:  Part 1: General.  Part 2: Materials for pressure parts of boilers and accessories.  Part 3: Design and calculation for pressure parts.  Part 4: In-service boiler life expectancy calculations.  Part 5: Workmanship and construction of pressure parts of the boiler.  Part 6: Inspection during construction, documentation and marking of pressure parts of the boiler.  Part 7: Requirements for equipment for the boiler.  Part 8: Requirements for firing systems for liquid and gaseous fuels for the boiler.  Part 9: Requirements for firing systems for pulverized solid fuels for the boiler.  Part 10: Requirements for safeguards against excessive pressure.  Part 11: Requirements for limiting devices of the boiler and accessories.  Part 12: Requirements for boiler feedwater and boiler water quality.  Part 13: Requirements for flue gas cleaning systems.  Part 14: Requirements for flue gas DENOX-systems using liquefied pressurized ammonia and ammonia water solution.  Part 15: Acceptance tests.  Part 16: Requirements for grate and fluidized bed firing systems for solid fuels for the boiler. CR 12952 Part 17: Guideline for the involvement of an inspection body independent of the manufacturer. Although these Parts can be obtained separately, it should be recognised that the Parts are inter-dependent. As such, the design and manufacture of water-tube boilers requires the application of more than one Part in order for the requirements of the standard to be satisfactorily fulfilled. NOTE Parts 4 and 15 are not applicable during the design, construction and installation stages. According to the CEN/CENELEC Internal Regulations, the national standards organizations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Cyprus, Czech Repub-lic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom. SIST EN 12952-11:2007

EN 12952-11:2007 (E) 4 Introduction The types of limiters which shall be fitted to boilers are specified in EN 12952-7 and the design of the safety systems are specified in EN 50156-1. A limiter (or limiting device) is one element of a water-tube boiler safety system. It comprises a sensor and monitoring elements to achieve the desired level of reliability. In order to provide the necessary safety function, for example, to cut off the heat supply to the boiler in the event of a low water fault, the limiter is connected to other elements in the safety system such as actuators and safety logic circuits.

SIST EN 12952-11:2007

EN 12952-11:2007 (E) 5 1 Scope This European Standard specifies requirements for limiters (or limiting devices) which are incorporated into safety systems for water-tube boilers as defined in EN 12952-1. A limiter (or limiting device) can be either:  a safety accessory as defined in the Pressure Equipment Directive, Article 1, clause 2.1.3, and needs to include the safety logic and final actuator, or  one element of a safety system, for example, a self-monitoring water level sensor used as part of a safety accessory as defined in the Pressure Equipment Directive, Article 1, clause 2.1.3. The overall boiler pro-tection function shall be provided in association with additional safety logic (where appropriate) and a final actuator. The design requirements and examination of functional capability for the limiters are covered in this European Standard. For an explanation of the extent of the limiter (or limiting device) see Figure A.1. 2 Normative references The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 298:2003, Automatic gas burner control systems for gas burners and gas burning appliances with or with-out fans EN 50156-1:2004, Electrical equipment for furnaces and ancillary equipment — Part 1: Requirements for ap-plication design and installation EN 60529:1991, Degrees of protection provided by enclosures (IP code) (IEC 60529:1989) EN 60730-1:2000, Automatic electrical controls for household and similar use — Part 1: General requirements (IEC 60730-1:1999, modified) EN 61000-4-2:1995, Electromagnetic compatibility (EMC) — Part 4: Testing and measurement techniques — Section 2: Electrostatic discharge immunity test — Basic EMC publication (IEC 61000-4-2:1995) EN 61000-4-3:2006, Electromagnetic compatibility (EMC) — Part 4-3: Testing and measurement techniques — Radiated, radio-frequency, electromagnetic field immunity test (IEC 61000-4-3:2006) EN 61000-4-4:2004, Electromagnetic compatibility (EMC) — Part 4-4: Testing and measurement techniques — Electrical fast transient/burst immunity test (IEC 61000-4-4:2004) EN 61000-4-5:2006, Electromagnetic compatibility (EMC) — Part 4-5: Testing and measurement techniques — Surge immunity test (IEC 61000-4-5:2005) EN 61000-4-6:1996, Electromagnetic compatibility (EMC) — Part 4: Testing and measurement techniques — Section 6: Immunity to conducted disturbances, induced by radio-frequency fields (IEC 61000-4-6:1996) EN 61000-4-8:1993, Electromagnetic compatibility (EMC) — Part 4: Testing and measurement techniques — Section 8: Power frequency magnetic field immunity test; basic EMC publication (IEC 61000-4-8:1993) SIST EN 12952-11:2007

EN 12952-11:2007 (E) 6 EN 61000-4-11:2004, Electromagnetic compatibility (EMC) — Part 4-11: Testing and measurement

techniques — Voltage dips, short interruptions and voltage variations immunity tests (IEC 61000-4-11:2004) EN 61000-6-2:2005, Electromagnetic compatibility (EMC) — Part 6-2: Generic standards — Immunity for in-dustrial environments (IEC 61000-6-2:2005) EN 61508-3:2001, Functional safety of electrical/electronic/programmable electronic safety-related systems — Part 3: Software requirements (IEC 61508-3:1998 + Corrigendum 1999) 3 Terms and definitions For the purposes of this document, the following terms and definitions apply. 3.1 limiter limiting device that, on reaching a fixed value (e.g. pressure, temperature, flow, water level) is used to inter-rupt and lock-out the energy supply NOTE Limiting device comprises:  a measuring or detection function and  an activation function for correction, or shutdown, or shutdown and lockout, and which is used to carry out safety related functions as defined in the PED, as on its own or as part of safety (protective) system (e.g. sensors, limiters) (see also Figure 3.1). If this is achieved by multi channel systems, then all items or limit-ers for safety purposes are included within the safety (protective) system.

Figure 1 — Protective devices and safety accessories according to Directive 97/23/EC (PED) 3.2 actuating element component which produces changes in other electrical circuits or volume flows (e.g. fuel, air) as a result of the effect of changes in signal NOTE For example, a gas shut off valve is not an actuating element. 3.3 fail-safe limiter is fail-safe if it possesses the capability of remaining in a safe condition or transferring immediately to another safe condition in the event of certain faults occurring 3.4 self-monitoring regular and automatic determination that all chosen components of a safety system are capable of functioning as required SIST EN 12952-11:2007

EN 12952-11:2007 (E) 7 3.5 redundancy provision of more than one device or system which, in the event of a fault, will still be provided by the

necessary facilities 3.6 diversity provision of different means of performing the required function, e.g. other physical principles or other ways of solving the same problem 3.7 complex electronics assemblies which use electronic components with more than one functional output 3.8 safety shut-down process which is effected immediately following the detection of a fault within the limiter or caused by exceeding the threshold of the process value limit resulting in a defined state with deactivated terminals of the safety

output(s) 3.9 lock-out safety shut-down condition of the limiter, such that a restart can only be accomplished by a manual reset of the limiter or by a manual reset of the safety logic and by no other means. This will be achieved by a

competent operator taking account of the physical situation 3.10 sensor transducer which, on reaching a defined limit value, outputs a signal and/or cuts out and only reverses the output signal in the event of a specific change in the performance quantity (e.g. pressure, temperature, flow, level) NOTE Sensors are used for signalling or for triggering control processes. SIST EN 12952-11:2007

EN 12952-11:2007 (E) 8 4 Requirements for limiters 4.1

General 4.1.1 The requirements set out below shall be followed to ensure uniform assessment of different devices. A limiter shall be such that a single fault in any related part shall not lead to a loss of the safety function. This shall be achieved by fault avoidance techniques such as self-monitoring with redundancy, diversity or a com-bination of these methods. Fault assessment for the electrical components shall be in accordance with 4.4. The fault assessment chart, see Figure 2 shall also be applied for faults in hydraulic, pneumatic and

mechanical components. NOTE The various elements of limiters are given in Annex A. An example of an examination plan is given in Annex B. 4.1.2 Limiters shall function independently of each other and of controls unless their safety function cannot be affected by other such functions. Manual resetting can be realised as a part of the limiter or as a part of the safety logic. Instructions shall be delivered together with the limiter including necessary precautions for a safe installation of it. 4.2

Materials and design 4.2.1 The use of materials with significant differences in their electrochemical potential shall be avoided in order to prevent corrosion which could affect the function of the limiter. 4.2.2 Care shall be taken that if magnetic materials are chosen, they do not adversely affect the working of the limiter. 4.2.3 Parts of the limiter shall be designed to comply with the applicable European Standards. 4.2.4 Limiters shall be capable of withstanding the thermal, mechanical, chemical and electrical loads that can occur during operation. 4.2.5 Limiters shall be designed such that changes in critical circuit component values (such as those

affecting timing) within the component manufacturer's declared worst case tolerances, including the long term stability, shall result in the system continuing to function in accordance with this standard. Compliance shall be checked by worst case analysis. 4.2.6 Limiters using complex electronics For limiters using complex electronics the following requirements apply additionally: • General Systematic errors (built into the design) shall be avoided and random faults (component faults) shall be

controlled by techniques such as self-monitoring with redundancy, diversity or a combination of these methods. • Fault avoidance and fault tolerance The design of the software and hardware shall be based on the functional analysis of the limiter resulting in a structured design explicitly incorporating the control flow, data flow and time related functions required by the application. In the case of custom-chips special attention is required with regard to measures taken to

minimise systematic errors. Software shall be designed using EN 61508-3 to a SIL level (Safety Integrity Level) as determined by analysis according to EN 50156-1. SIST EN 12952-11:2007

EN 12952-11:2007 (E) 9 4.3

Electrical equipment 4.3.1 All wiring and electrical equipment in connection with the limiter shall be adequately protected against the ingress of moisture and the effect of temperature (see [2], [3]). 4.3.2 The function of the limiter and the associated electrical circuit responsible for shutting down and

locking out the heat supply system shall not be affected by other electrical circuits in their proximity. Screened

cables shall be used where necessary (see [2], [3]). 4.3.3 Electrical components within units directly attached to the boiler shall be capable of withstanding a temperature environment resulting from surrounding temperatures of up to 70 °C. Components within units not directly attached to the boiler shall be capable of withstanding an ambient temperature of up to 55 °C. Any equipment that is in contact with parts carrying steam or hot water shall be capable of withstanding the

temperature of those parts. 4.3.4 Devices shall have, as a minimum, a protection rating to IP 54 in accordance with EN 60529. When units are installed inside an enclosure or control box, the IP rating required for the box shall be considered adequate. 4.3.5 All mechanical output contacts of the device shall be of the snap action type. Semi-conductor switches shall have similar characteristics. 4.3.6 The limiter shall tolerate electrical and electromagnetical influences as defined in Annex D. 4.4

Fault assessment 4.4.1 General The limiter, excluding the stored programme section, shall be so constructed that the fault assessment analysis

results in termination. Power failure, breaks in connecting cables and short circuits shall also be considered and

included in the fault assessment analysis. 4.4.2 Fault models and exclusions 4.4.2.1 General With fault assessment in accordance with Figure 2, it shall be assumed that certain faults do not occur. Such assumptions are justified by describing the failure mechanism as well as by stating the conditions

relating to design, construction, environment etc. for the conductors, components and equipment. Faults which shall be taken into account are based on in EN 298:2003, Annex A with consideration of the following faults which may be excluded without further justification: 4.4.2.2 Conductor-to-conductor short circuit fault This fault may be excluded if: a) cables and conductors as specified in EN 50156-1 are used; b) components are encapsulated so that they are moisture resistant or, if they are hermetically sealed and they are capable of withstanding the test specified in EN 50156-1; c) the clearance between live parts shall be designed according to overvoltage category III and pollution degree 3 and the creepage distance shall be designed according to pollution degree 3 but at least for the nominal voltage of 63 V as specified in EN 60664-1; SIST EN 12952-11:2007

EN 12952-11:2007 (E) 10 d) printed conductors (tracks) shall be varnished so that they are resistant to ageing by virtue of the distance between printed conductors being equivalent to at least the values specified in EN 60664-1:2003, Table 4 for pollution degree 1, and with a minimum nominal voltage of 32 V (minimum creepage distance of 0,14 mm). 4.4.2.3 Short circuit in wound film resistors This fault may be excluded if the wound film resistors shall be used with a varnished or encapsulated resistive layer and axial terminations. The possibility of condensation shall be excluded during operation. The limits, e.g. voltage limit, power, shall not be exceeded even under worst case conditions. 4.4.2.4 Short circuit in wire-wound resistors This fault may be excluded if the winding shall be a single layer winding and is secured by means of a glaze or embedded in a sealing compound. 4.4.2.5 Non-opening of contact elements due to permanent welding This fault may be excluded if contactors, relays or auxiliary switches for example, shall be protected against the effects of short circuits by the appropriate overcurrent protective or current limiting devices. In rating the overcurrent protective device, the nominal current of the device as stated by the manufacturer, shall be

multiplied by a safety factor of 0,6. Fault exclusion shall also be permissible if the prospective short circuit

current is less than the nominal current for the contact element concerned. Where contact elements are

connected in series, the contact element with the lowest overcurrent strength shall be the deciding factor. Reed contacts shall not be used. 4.4.2.6 Mechanical failure of switching devices This fault may be excluded if the switching devices are type tested to demonstrate they shall still be operative after at least 250 000 switching cycles under conditions similar to operating conditions. Contactors and relays shall, in addition, be capable of a mechanical endurance of 3 000 000 switching cycles, except for pressure limiters, see Table 2. NOTE The term "conditions similar to operating conditions" covers chemical and climatic influences as well as

electrical and mechanical stresses. 4.4.2.7 Faults in components for safe isolation Faults in components which are provided for safe isolation of electrical circuits (e.g. power circuits and

telecommunications circuits) in accordance with EN 61140 may be excluded. These include: a) inter-winding short circuits in transformers (e.g. primary-secondary). Transformers shall comply with the electrical and mechanical requirements of EN 60742. Except that transformers with working voltages up to 200 V, insulation between windings and insulation against the core shall be designed for a test voltage of 2 kV rms. Transformers shall as a minimum be short-circuit proof. Displacement of windings, turns and connection lines shall be prevented, e.g. by vacuum

impregnation or encapsulation; b) transient voltage of switching devices like relays, contactors or auxiliary contacts between contacts and between coil and contacts. The insulation between contacts or between coil and contact shall be designed for nominal voltages Ub up to 200 V for a test voltage of 2 kV rms and at nominal voltages 200 V < Ub < 500 V for a test voltage of 3,75 kV rms. By special design features (e.g. caps, ribs, encapsulation, banding) at contacts and coils, safe isolation shall also be guaranteed against faults such as spring breakage; c) short-circuiting of isolating distances in optocouplers. SIST EN 12952-11:2007

EN 12952-11:2007 (E) 11 The clearance and creepage distances of the optocoupler in its installed position shall fulfil the relevant conditions of EN 60664-1:2003, 3.1 and 3.2. 4.5

Marking The limiters shall be marked with the following:  manufacturer´s name and/or trademark;  year of manufacture;  maximum/minimum allowable design limits;  unique type reference. NOTE Other markings may be added by the limiter manufacturer or placed in the operating instructions, see

Annex C. SIST EN 12952-11:2007
EN 12952-11:2007 (E) 12

Key 1 reassessment Figure 2 — Fault assessment chart for limiters excluding the stored programme section SIST EN 12952-11:2007

EN 12952-11:2007 (E) 13 5 Special requirements for water level limiters 5.1

Components This limiter shall consist of one or more units needed to provide the necessary safety function. The limiter shall comprise of the following elements where applicable: sensor, protection tube or external chamber (see NOTE), timing element, testing devices and other associated equipment up to the terminals of the switching output contacts as shown in Annex A. NOTE Protection tubes and external chambers may be considered to be part of the boiler and in these cases it will be necessary for the limiter manufacturer and the boiler manufacturer to agree on the design and manufacturing

requirements to ensure that the limiter system performs as intended. Examples of water level sensor are: float level switch, electrode probe, conductivity sensor. 5.2

Design 5.2.1 General Chambers, connecting pipes and protection tubes shall be designed so that they:  allow free movement in the tube to equalise with the water level in the boiler;  can be cleaned and inspected;  prevent the build up of sludge in the tubes/chambers. 5.2.2 Internal protection tubes 5.2.2.1 Openings on the protection tube, which are necessary to ensure level equalisation, shall have a minimum diameter of 20 mm or equivalent area

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