SIST EN 13136:2014
(Main)Refrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation
Refrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation
This European Standard describes the calculation of mass flow for sizing pressure relief devices for components of refrigerating systems. This European Standard describes the calculation of discharge capacities for pressure relief valves and other pressure relief devices in refrigerating systems including the necessary data for sizing these when relieving to atmosphere or to components within the system at lower pressure. This European Standard specifies the requirements for selection of pressure relief devices to prevent excessive pressure due to internal and external heat sources, the sources of increasing pressure (e.g. compressor, heaters, etc.) and thermal expansion of trapped liquid. This European Standard describes the calculation of the pressure loss in the upstream and downstream line of pressure relief valves and other pressure relief devices and includes the necessary data. This European Standard refers to other relevant standards in Clause 5.
Kälteanlagen und Wärmepumpen - Druckentlastungseinrichtungen und zugehörige Leitungen - Berechnungsverfahren
Diese Europäische Norm beschreibt die Berechnung der Massenströme zur Größenbestimmung von Druckentlastungseinrichtungen für Teile von Kälteanlagen.
ANMERKUNG Der in dieser Norm verwendete Begriff „Kälteanlage“ schließt Wärmepumpen mit ein.
1.2 Diese Europäische Norm beschreibt die Berechnung der Abblaseleistungen von Druckentlastungsventilen und anderen Druckentlastungseinrichtungen in Kälteanlagen einschließlich der hierzu erforderlichen Daten zur Größenbestimmung dieser Einrichtungen, wenn sie ins Freie oder in Bauteile der Kälteanlage mit niedrigerem Druck abblasen.
1.3 Diese Europäische Norm legt die Anforderungen an die Auswahl von Druckentlastungseinrichtungen zur Vermeidung von unzulässigen Drücken, hervorgerufen durch innere und äußere Wärmequellen, Druckerzeuger (z. B. Verdichter, Erhitzer, usw.) und die Wärmeausdehnung eingesperrter Flüssigkeit fest.
1.4 Diese Europäische Norm beschreibt die Berechnung des Druckabfalls in den zu- und abführenden Leitungen von Druckentlastungsventilen und anderen Druckentlastungseinrichtungen einschließlich der erforderlichen Daten.
1.5 Diese Europäische Norm verweist in Abschnitt 5 Allgemeines auf weitere einschlägige Normen.
Systèmes de réfrigération et pompes à chaleur - Dispositifs limiteurs de pression et tuyauteries associées - Méthodes de calcul
La présente Norme européenne décrit le mode de calcul du débit-masse pour le dimensionnement des dispositifs limiteurs de pression pour les composants des systèmes de réfrigération.
NOTE Le terme « système de réfrigération » utilisé dans la présente norme inclut les pompes à chaleur.
1.2 La présente Norme européenne décrit le mode de calcul des capacités de décharge des soupapes de sûreté et autres dispositifs limiteurs de pression des systèmes de réfrigération, y compris les données nécessaires à leur dimensionnement lors de leur décharge dans l’atmosphère ou dans les composants du système à basse pression.
1.3 La présente Norme européenne spécifie les exigences pour la sélection des dispositifs limiteurs de pression pour éviter une pression excessive due aux sources de chaleur internes et externes, aux autres sources de génération de pression (par exemple compresseurs, chauffages, etc.) et à la dilatation du fluide piégé.
1.4 La présente Norme européenne décrit le mode de calcul de la perte de charge dans la tubulure en amont et en aval des soupapes de sûreté et autres dispositifs limiteurs de pression, et inclut les données nécessaires.
1.5 La présente Norme européenne renvoie à d’autres normes pertinentes à l’Article 5, Généralités.
Hladilni sistemi in toplotne črpalke - Tlačne varnostne naprave in njihove napeljave - Metode za izračun
General Information
Relations
Standards Content (Sample)
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.Kälteanlagen und Wärmepumpen - Druckentlastungseinrichtungen und zugehörige Leitungen - BerechnungsverfahrenSystèmes de réfrigération et pompes à chaleur - Dispositifs limiteurs de pression et tuyauteries associées - Méthodes de calculRefrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation27.200Hladilna tehnologijaRefrigerating technology27.080Heat pumpsICS:Ta slovenski standard je istoveten z:EN 13136:2013SIST EN 13136:2014en01-februar-2014SIST EN 13136:2014SLOVENSKI
STANDARDSIST EN 13136:2002/A1:2005SIST EN 13136:20021DGRPHãþD
SIST EN 13136:2014
EUROPEAN STANDARD NORME EUROPÉENNE EUROPÄISCHE NORM
EN 13136
October 2013 ICS 27.080; 27.200 Supersedes EN 13136:2001English Version
Refrigerating systems and heat pumps - Pressure relief devices and their associated piping - Methods for calculation
Systèmes frigorifiques et pompes à chaleur - Dispositifs de limitation de pression et tuyauteries associées - Méthodes de calcul
Kälteanlagen und Wärmepumpen - Druckentlastungseinrichtungen und zugehörige Leitungen -Berechnungsverfahren This European Standard was approved by CEN on 24 August 2013.
CEN members are bound to comply with the CEN/CENELEC Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration. Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC 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 translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC Management Centre has the same status as the official versions.
CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre:
Avenue Marnix 17,
B-1000 Brussels © 2013 CEN All rights of exploitation in any form and by any means reserved worldwide for CEN national Members. Ref. No. EN 13136:2013: ESIST EN 13136:2014
EN 13136:2013 (E) 2 Contents Page Foreword . 3 Introduction . 4 1 Scope . 5 2 Normative references . 5 3 Terms and definitions . 5 4 Symbols . 6 5 General . 8 6 Pressure relief devices for protection of system components . 8 6.1 General . 8 6.2 Excessive pressure caused by heat sources . 9 6.2.1 External heat sources . 9 6.2.2 Internal heat sources . 10 6.3 Excessive pressure caused by compressors . 11 6.4 Excessive pressure caused by liquid expansion . 11 7 Discharge capacities of pressure relief devices . 12 7.1 General . 12 7.2 Determination of pressure relief valve performance . 12 7.2.1 Determination of coefficient of discharge . 12 7.2.2 Critical and sub-critical flow . 12 7.2.3 Function of the isentropic exponent (C) . 12 7.2.4 Correction factor for sub-critical flow . 13 7.2.5 Discharge capacity of pressure relief valves . 13 7.3 Calculation of capacity and flow area of bursting discs or fusible plugs . 14 7.4 Pressure loss in upstream/downstream lines. 14 7.4.1 General . 14 7.4.2 Pressure loss in components . 14 7.4.3 Pressure loss in the upsteam line . 15 7.4.4 Pressure loss in the downstream line . 15 Annex A (normative)
Values of functions, factors and properties of refrigerants . 17 Annex B (informative)
Calculation of flow areas for non-evaporating and evaporating liquids . 24 B.1 Calculation of the flow area for non-evaporating liquids . 24 B.2 Calculation of the flow area for evaporating liquids . 24 Annex C (informative)
Example of calculation for sizing pressure relief devices with the corresponding pipes . 26 C.1 Assumptions for the calculation example. 27 C.2 Calculation of the required minimum discharge capacity, Qmd
at standard heat flow rate . 27 C.3 Calculation of the required minimum discharge capacity Qmd at reduced heat flow rate . 28 C.4 Calculation of flow area Ac, selection of pressure relief valve . 28 C.5 Pressure loss in upstream line (from vessel to pressure relief valve). 29 C.6 Pressure loss in downstream line (from pressure relief valve to atmosphere) . 30 Annex ZA (informative)
Clauses of this European Standard addressing essential requirements or other provisions of EU Directives . 32 Bibliography. 33
SIST EN 13136:2014
EN 13136:2013 (E) 3 Foreword This document (EN 13136:2013) has been prepared by Technical Committee CEN/TC 182 “Refrigerating systems, safety and environmental requirements”, 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 April 2014, and conflicting national standards shall be withdrawn at the latest by April 2014. Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. CEN [and/or CENELEC] shall not be held responsible for identifying any or all such patent rights. This document supersedes EN 13136:2001. 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. Compared to EN 13136:2001, EN 13136:2013 takes into account the application of CO2 and the amendment A1, published in 2005. According to the CEN-CENELEC Internal Regulations, the national standards organisations of the following countries are bound to implement this European Standard: Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and the United Kingdom. SIST EN 13136:2014
EN 13136:2013 (E) 4
Introduction This European Standard is based on applicable parts of EN ISO 4126-1:2013, EN ISO 4126-2:2003 and EN 12284. It is suited to the specific requirements, and includes the data, of refrigerating systems. It provides means of satisfying the pressure relief devices requirements of EN 378-2:2008+A2:2012. SIST EN 13136:2014
EN 13136:2013 (E) 5 1 Scope 1.1 This European Standard describes the calculation of mass flow for sizing pressure relief devices for components of refrigerating systems. NOTE The term "refrigerating system" used in this European Standard includes heat pumps. 1.2 This European Standard describes the calculation of discharge capacities for pressure relief valves and other pressure relief devices in refrigerating systems including the necessary data for sizing these when relieving to atmosphere or to components within the system at lower pressure. 1.3 This European Standard specifies the requirements for selection of pressure relief devices to prevent excessive pressure due to internal and external heat sources, the sources of increasing pressure (e.g. compressor, heaters, etc.) and thermal expansion of trapped liquid.
1.4 This European Standard describes the calculation of the pressure loss in the upstream and downstream line of pressure relief valves and other pressure relief devices and includes the necessary data. 1.5 This European Standard refers to other relevant standards in Clause 5. 2 Normative references The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies. EN 378-1:2008+A2:2012, Refrigerating systems and heat pumps — Safety and environmental requirements — Part 1: Basic requirements, definitions, classification and selection criteria EN 378-2:2008+A2:2012, Refrigerating systems and heat pumps — Safety and environmental requirements — Part 2: Design, construction, testing, marking and documentation EN 764-1:2004, Pressure equipment — Part 1: Terminology — Pressure, temperature, volume, nominal size EN 764-2:2012, Pressure equipment — Part 2: Quantities, symbols and units EN 12284:2003, Refrigerating systems and heat pumps — Valves — Requirements, testing and marking EN ISO 4126-1:2013, Safety devices for protection against excessive pressure — Part 1: Safety valves (ISO 4126-1:2013) EN ISO 4126-2:2003, Safety devices for protection against excessive pressure — Part 2: Bursting disc safety devices (ISO 4126-2:2003) ISO 817, Refrigerants — Designation system 3 Terms and definitions For the purposes of this document, the terms and definitions given in EN 378-1:2008+A2:2012, EN 12284:2003, EN ISO 4126-1:2013, EN ISO 4126-2:2003 and EN 764-1:2004 apply. SIST EN 13136:2014
EN 13136:2013 (E) 6 4 Symbols For the purposes of this document, the symbols given in EN 764-2:2012 and the following apply: Symbol Designation Unit A Flow area of the pressure relief valve ×=42dAπ mm2 Ac Calculated flow area mm2 ADN Valve cross section related to DN mm2 Ain Inside area of inlet tube mm2 Aliq Calculated flow area of liquid after expansion mm2 Aout Inside area of outlet tube mm2 AR Inside area of tube mm2 Asurf External surface area of the vessel m2 Avap Calculated flow area of vapour after expansion mm2 C Function of the isentropic exponents (Table A.2)
– DN Nominal size (see EN ISO 6708:1995) – d Actual most narrow flow diameter of the pressure relief valve mm dc Calculated flow diameter of the pressure relief valve mm din Inside diameter of inlet tube mm dout Inside diameter of outlet tube mm DR Outside diameter of tube (Table A.4)
mm dR Inside diameter of tube mm hvap Heat of vaporisation calculated at 1,1 times the set pressure of the pressure relief device (for super critical or superheated conditions see 6.1)
kJ/kg Kb Theoretical capacity correction factor for sub-critical flow (Table A.3) – Kd Certified coefficient of discharge taking into account the backpressure ratio pb/po and the possible reduced stroke of the pressure relief valve – Kdr De-rated coefficient of discharge [ζ9,0×=ddrKK – Kdrl De-rated coefficient of discharge for liquid [ζ8,0×≈drdrlKK – Kvs Valve constant (the rate of water flow for a differential pressure ûp of 1 bar at the rated full opening) m3/h Kv Viscosity correction factor – SIST EN 13136:2014
EN 13136:2013 (E) 7
K Isentropic exponent of the refrigerant; for calculation, the value of K shall be as measured at 25 °C and 1,013 bar – L Length of tube mm Lin Length of inlet tube mm Lout Length of outlet tube mm n Rotational frequency min- 1 patm Atmospheric pressure (1 bar) bar pb Back pressure at outlet of pressure relief device, absolute
bar pc Critical absolute pressure bar po Actual relieving pressure po = 1,1 pset + patm bar ps Maximum allowable pressure of a component, gaugea bar pset Set pressure, gauge (the pre-determined pressure at which a pressure relief valve under operation starts to open) bar P1 Pressure at the inlet to downstream line absolute (in practice = pb) bar P2 Pressure at the outlet of downstream line absolute bar ûp Differential pressure bar ûpin Pressure loss in the upstream line of pressure relief valve bar ûpout Pressure loss in the downstream line of pressure relief valve bar Qh Rate of heat production, internal heat source kW Qliq Flow of liquid after expansion kg/h Qm Calculated refrigerant mass flow rate of the pressure relief device kg/h qm Theoretical discharge capacity kg/h ⋅ mm2 q’m Actual discharge capacity determined by tests kg/h ⋅ mm2 Qmd Minimum required discharge capacity, of refrigerant, of the pressure relief device kg/h Qmd’ Adjusted discharge capacity of refrigerant, of the pressure relief device, used for pressure drop calculation kg/h Qvap Flow of vapour after expansion kg/h R Bending radius of tube (Table A.4) mm Re Reynolds number – s Thickness of insulation m V Theoretical displacement
m3 vo Specific volume of vapour or liquid m3/kg w0 Actual flow speed of liquid in the smallest section of pressure relief valve m/s w1 Speed at the inlet into the downstream line m/s x Vapour fraction of refrigerant at pC – SIST EN 13136:2014
EN 13136:2013 (E) 8
. Flush connection angle (Table A.4) °
Pressure loss coefficient ∑==nnn1ζζ – DN Pressure loss coefficient related to DN – n Pressure loss coefficient of a single component – v Volumetric efficiency estimated at suction pressure and discharge pressure equivalent to the pressure relief device setting –
≈ 0,02) –
Kinematic viscosity m2/s
Density of vapour or liquid ( = 1/vo) kg/m3 10 Vapour density at refrigerant saturation pressure/dew point at 10 °C kg/m3 3 Density of heat flow rate kW/m2 3red Reduced density of heat flow rate kW/m2 a The Pressure Equipment Directive 97/23/EC identifies the maximum allowable pressure by the symbol "PS". 5 General Requirements for protection against excessive pressure in refrigeration systems and heat pumps are given in EN 378-2. For design and manufacturing of bodies, bonnets and bolts for pressure relief devices — safety valves and bursting discs — specification of strength pressure test, EN 12284 applies.
For other aspects, the requirements of EN ISO 4126-1:2013 Safety Valves, Clause 3, Terms and definitions, Clause 5, Design, Clause 7, Type tests and Clause 10, Marking and sealing and EN ISO 4126-2:2003 Bursting Disc Safety Devices, Clause 17 Marking, 17.2 Bursting discs/bursting disc assemblies and 17.3 Bursting disc holders apply. NOTE Calculations for flow areas for non-evaporating and evaporating liquids are given in Annex B. Calculations for a pressure relief device with the corresponding pipes are given in Annex C. 6 Pressure relief devices for protection of system components 6.1 General Calculations shall be based on known or assumed processes which result in increases in pressure. All foreseeable processes shall be considered including those covered in 6.2, 6.3 and 6.4.
For the general purposes of this European Standard, hvap is calculated at 1,1 times the set pressure of the pressure relief device. If the set pressure of the pressure relief valve times 1,1, is higher than the saturated pressure of the refrigerant at (critical temperature minus 5 [K]) then hvap and vo shall be taken at critical temperature minus 5 [K]. If the temperature, at 1,1 times the set pressure of the pressure relief device, is higher than the saturated temperature (superheated gas), then hvap shall be taken at saturated condition.
SIST EN 13136:2014
EN 13136:2013 (E) 9 In case of relieving CO2 to a pressure below the triple point (e.g. atmospheric pressure), there is a possibility to create solid CO2. Necessary precautions shall be taken to ensure a safe operation. Vessels operating normally in the gas phase may however contain liquid refrigerant, which may evaporate under an external heat impact. NOTE Vessels only containing refrigerant in the gas phase do not produce a continuous mass flow under an external heat impact. In case of supercritical pressure, the valve shall be suitable for both gas and liquid. 6.2 Excessive pressure caused by heat sources 6.2.1 External heat sources Where necessary the minimum required discharge capacity of the pressure relief device for pressure vessels shall be determined by the following: vapsurfmd3600hAQ××=ϕ
[kg/h] (1) For those pressure vessels in this European Standard, the density of heat flow rate is assumed to be 2kW/m 10 =ϕ (2)
but a higher value shall be used if necessary. Where the thickness(s) of the insulation of the pressure vessel is bigger than 0,04 [m] and the insulation is tested according to reaction of fire as described in EN 13501-1 and classified better than class C, a reduced density of heat flow rate can be used and determined as follows: ]kW/m[04,02reds×=ϕϕ (3) The sizing of the pressure relief device and calculating of pressure loss are carried out in accordance with Clause 7. For pressure vessels the total external surface area of the vessel shall be taken as Asurf. SIST EN 13136:2014
EN 13136:2013 (E) 10
Figure 1 — Plate Heat Exchanger (PHE) Figure 2 — Plate and Shell Heat Exchanger (PSHE) surfA of Plate Heat Exchanger will be calculated as follows:
122313ALLLLLL=××+×+×surf2() [m2] (4) surfA for Plate and Shell Heat Exchanger will be calculated as follows: ()1121d)4/(2LdA××+××=ππsurf [m2] (5) Heat exchangers are generally considered to be vessels. Due to its unique design, some fin and tube heat exchangers in refrigeration systems may be classified according to Article 1 paragraph 2.1.2 last sentence. For further details, see guideline 2/4 of PED 97/23/EC. Higher values for density of heat flow rate than 10 kW/m2 may be necessary where in case of fire full engulfment for the pressure vessel is to be expected and/or in the case the pressure vessel is insulated with a flammable insulation. Other calculation methods could be necessary in case of heat radiation with a higher heat flow directed to one side of the vessel. Where pressure vessels of a refrigerating system are protected against excessive pressure according to EN 378-2:2008+A2:2012, 6.2 and monitored according to EN 378-3:2008+A1:2012, Clause 7 and installed in
special machinery rooms as specified in EN 378-3:2008+A1:2012, Clause 5, no external heat sources for sizing the pressure relief devices used for those vessels themselves may be considered. But, nevertheless, for the sizing of those pressure relief devices on the low pressure side of the refrigerating system all connected pressure vessels, compressors and pumps should be taken into account (EN 378-2:2008+A2:2012, 6.2.6.3). Combustion heat potential of insulations in case of fire is not part of the calculations in this European Standard. Care should be taken at welding activities near insulated vessels and pipes. Electric equipment inside of the flammable insulation should be carried out according to EN 60204-1. 6.2.2 Internal heat sources The minimum required discharge capacity of the pressure relief device for conditions which arise due to an internal source of excessive heat shall be determined by the following: vaphmd3600hQQ×=
[kg/h] (6) SIST EN 13136:2014
EN 13136:2013 (E) 11 The sizing of the pressure relief device and calculating of pressure loss are carried out in accordance with Clause 7. 6.3 Excessive pressure caused by compressors The minimum required discharge capacity of the pressure relief device for excessive pressure caused by compressors shall be determined by the following: v10md60ηρ××××=nVQ
[kg/h] (7) For low temperature operations, where it can be established that the compressor motor cannot run with the suction pressure corresponding to 10 °C saturated conditions, then the value at the highest suction pressure shall be used in the calculation. NOTE 1 In cases where discharge shut-off valves are not fitted, a high pressure relief device will suffice, providing there are no intermediate shut-off valves. NOTE 2 Non-positive displacement compressors need not have a pressure relief device providing it is not possible to exceed the maximum allowable pressure. NOTE 3 Relieving to the low pressure side may cause compressor overheating and / or uncontrolled internal pressure in compressors (e.g. in screw compressors). EN 12693 covers compressors which can run against a closed discharge valve. EN 12693 should, therefore, be considered, especially the requirement covering conditions under which the allowable evaporating temperature exceeds the value of 10 °C by more than 5 K. The sizing of the pressure relief device and the calculation of the pressure loss shall be carried out in accordance with Clause 7. 6.4 Excessive pressure caused by liquid expansion The effective area [ζdrKA× of the pressure relief device to protect against excessive pressure caused by the expansion of trapped liquid shall be at least 0,02 mm2 per litre of trapped volume, except that the minimum diameter shall not be less than 1 mm. For refrigerants where the temperature difference between relieving temperature and critical temperature is less than 20 [K], then the expansion of trapped liquid shall be at least 0,04 mm2 per litre of trapped volume. NOTE Liquids having a temperature close to the critical temperature expand considerably. It is advisable to take into account the backpressure ratio pb/po and the possibly reduced stroke of the pressure relief valve. The possibility of contamination by dirt should be considered. Where practicable, the pressure relief device shall relieve to the low pressure side of the system and the pressure relief device shall meet the following requirements even at maximum back pressure: atmseto1,1ppp+×≤
[bar]
(8) SIST EN 13136:2014
EN 13136:2013 (E) 12 7 Discharge capacities of pressure relief devices 7.1 General When the operational characteristics have been satisfactorily established, it is acceptable to use steam, air or other gas of known characteristics as the fluid for flow characteristics tests except for valves designed for liquid service (see Annex B). When discharged quantities are being assessed, the valve disc shall be held at the minimum lift as determined by the operating characteristics test. 7.2 Determination of pressure relief valve performance 7.2.1 Determination of coefficient of discharge The coefficient of discharge is calculated from: mmd'qqK=
[-] (9) The de-rated coefficient of discharge is calculated from: ddr9,0KK×=
[-]
(10) 7.2.2 Critical and sub-critical flow The flow of a gas or vapour through an orifice, such as the flow areas of a pressure relief valve, increases as the outlet pressure is decreased until critical flow is achieved. Further decrease in outlet pressure will not result in any further increase in flow. Critical flow occurs when
)1/(ob12−+≤κκκpp [–]
(11) and sub-critical flow occurs when )1/(ob12−+[κκκpp [–] (12) assuming the validity of Rankine's law. If the flow is critical Kb = 1, and if the flow is sub-critical, the correction factor shall be calculated according to Formula (14) in 7.2.4 or taken from Table A.3. 7.2.3 Function of the isentropic exponent (C) The function (C) of the isentropic exponent is calculated from: )1/()1(12948,3−++××=κκκκC
[–] (13) For this calculation, the value of K shall be as measured at 25 °C and 1,013 bar. Values of K and calculated values of C for some refrigerants are given in Table A.1, and values of C as a function of K are given in Table A.2. SIST EN 13136:2014
EN 13136:2013 (E) 13 7.2.4 Correction factor for sub-critical flow The correction factor for sub-critical flow is calculated from: )1/()1(/)1(0b/20bb1212−+++×−×−⋅=κκκκκκκκκppppK
[–] (14) For this calculation the value of
shall be as measured at 25 °C and 1,013 bar. Values of Kb as a function of pb/po are given in Table A.3 for various values of . 7.2.5 Discharge capacity of pressure relief valves 7.2.5.1 General For the most common use of pressure relief valves in refrigerating systems, the back pressure is lower than approximately 0,5 x relieving pressure (pb ≤
0,5 po) and Kb = 1, which indicates that the flow through the pressure relief valve is "critical flow".
For valves where the lift is a function of back pressure, the manufacturer shall state the maximum permissible back pressure ratio pb/po and the relating certified coefficient of discharge taking into account the possibly reduced stroke of the pressure relief valve. 7.2.5.2 Calculation of the mass flow The mass flow for critical and sub-critical flow is calculated from: oobdrm2883,0vpKKACQ×××××=
[kg/h]
(15) mdmmdmd1,25QQQQQ<<×=[=md' mmdm1,25/1,25QQQQ≥×=[=md' The flow area Ac is calculated from the minimum required discharge capacity of refrigerant Qmd’ according to Formulae (1), (6) and (7) as follows: md'md'odrbo0drbo3,4690,2883cQQvACKKppCKKv==××××××××
[mm2]
(16) where for critical flow Kb = 1.
For the specific volume vo the value pertaining to pressure po is to be inserted. Values of C as a function of K are given in Table A.2. Values of Kb as a function of pb/po are given in Table A.3 for various values of K. SIST EN 13136:2014
EN 13136:2013 (E) 14 7.3 Calculation of capacity and flow area of bursting discs or fusible plugs Domed bursting discs shall be designed so that they burst due to tensile forces when the bursting pressure is applied to the concave side of the bursting disc. They shall be domed such that no further plastic flow will occur initially when the bursting disc is subject to its intended operating condition. The discharge capacity of a bursting disc or fusible plug shall be calculated from the formula given in 7.2.5.2. The following values for Kdr shall be the maximum used depending on how the pipe between the vessel and the bursting disc or fusible plug is mounted on the vessel: a) flush or flared connection (see Table A.4): Kdr = 0,70; b) inserted connection (see Table A.4): Kdr = 0,55. If the Kdr -value of the bursting disc or fusible plug itself is lower than the maximum value given above, then the smaller value shall be used in the calculation. 7.4 Pressure loss in upstream/downstream lines 7.4.1 General To ensure correct operation of pressure relief valves the pressure loss at minimum required capacity (i.e. po = 1,1 x pset + 1) in both upstream and downstream lines, including any changeover device shall not exceed the following: the values stated by the supplier of the pressure relief valve, or; upstream line (including changeover device): ûpin≤ 0,03 X po [bar]; downstream line (back pressure dependent): ûpout ≤ 0,10 X po [bar]; downstream line (back pressure independent): ûpout ≤ 0,20 X po [bar]. The velocity in the up- / downstream line shall not reach critic
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