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This standard is drafted in accordance with the rules given in GB/T 1.1-2009.
This standard has been redrafted and modified adoption of International Standard ISO 4126-7:2013 Safety Devices for Protection against Excessive Pressure — Part 7: Common Data.
The technical deviations between this standard and the International Standard ISO 4126-7:2013, together with their justifications, is given below:
— adjustment on technical differences had been made in Scope of this standard so that the technical conditions and standard structure in China are met; the adjustment situations embodied a concentrated reflection in Clause 1 “Scope”, and the specific adjustments are as follows:
● the scope of Clause 1 of ISO 4126-7:2013 is changed to "This standard specifies common data for performance parameters related to safety valve protection devices." ;
● in the scope of Clause 1 of ISO 4126-7:2013, " it is not recommended to use the ideal gas formula presented in 6.3 when the relieving temperature is greater than 90% of the thermodynamic critical temperature and the relieving pressure is greater than 50% of the thermodynamic critical pressure. In addition, the method specified in 6.3 is not applicable in the case of gas condensation” is adjusted to 6.3 of this standard.
— the adjustments of technical deviations are made for the normative references in this standard so as to adapt to the technical conditions of China. The adjustments are mainly reflected in Clause 2 "Normative References", which are shown in the following:
● ISO 4126-1 is replaced by GB/T 12241, which is modified in relation to the international standard;
● IEC 4126-4 is replaced by GB/T 28778 which is modified in relation to the international standard (see Clause 3);
● ISO 4126-2 and ISO 4126-3 are deleted;
— Some terms in Clause 3 of ISO 4126-7:2013 have been deleted. Because these terms have been specified in GB/T 12241 and GB/T 28778, and their meanings are basically the same.
— The symbol content in Table 1 of Clause 4 of ISO 4126-7:2013 is adjusted as follows:
● the symbol PS is deleted. Because the symbol is not covered by this standard;
● the symbol Q ?_m is changed to Qm, because the international standard is not unified;
● for the purpose of consistent with other standards, the symbol μ0 is changed to μ.
— some formula numbers of ISO 4126-7:2013 are deleted. The formulas in ISO 4126-7:2013 explaining data sources are numbered, and such numbers have been deleted in accordance with Chinese practice.
— Table 2 of ISO 4126-7:2013 is divided into Tables 2 to 6 of this standard. The steam pressure coefficients under different conditions are listed in the same table in ISO 4126-7:2013. The steam pressure coefficients under different conditions are listed separately in this standard for the convenience of searching and in line with Chinese practice.
— the first paragraph in 6.3 of ISO 4126-7:2013 is adjusted to 6.3.1 of this standard to meet the requirements of China's standard structure.
— different gas parameters in 7.6 of ISO 4126-7:2013 are deleted, because such gas parameters are standard parameters.
— the technical requirements of different springs in Clauses 8 and 9 of ISO 4126-7:2013 are deleted, because China has corresponding national standards for springs and the technical requirements are basically the same.
For the purposes of this standard, the following editorial changes have also been made:
— US units and formulas in ISO 4126-7:2013 are converted to metric units and formulas.
— Example 2 in A.1 of Annex A of ISO 4126-7:2018 is deleted. Because Example 2 also an example to illustrate the capacity calculation of gas medium under critical flow.
This standard was proposed by the China Machinery Industry Federation.
This standard is under the jurisdiction of National Technical Committee 503 on Pressure Relief Devices of Standardization Administration of China (SAC/TC 503).
Safety Devices for Protection against Excessive Pressure — Common Data
1 Scope
This standard specifies common data for performance parameters related to safety valve protection devices.
This standard is applicable to non-flashing liquids or non-gas/liquid two-phase mixtures
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.
GB/T 12241 Safety Valves — General Requirements (GB/T 12241-2005, ISO 4126-1: 1991, MOD)
GB/T 28778 Pilot-operated Safety Valves (GB/T 28778-2012, ISO 4126-4:2004, MOD)
3 Terms and Definitions
For the purposes of this document, the terms and definitions given in GB/T 12241, GB/T 28778 and the following apply.
3.1
safety valve
valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored
3.2
set pressure
predetermined pressure at which a safety valve under operating conditions commences to open
Note: It is the gauge pressure measured at the valve inlet at which the pressure forces tending to open the valve for the specific service conditions are in equilibrium with the forces retaining the valve disc on its seat.
3.3
overpressure
pressure increase over set pressure, usually expressed as a percentage of the set pressure
3.4
relieving pressure
the sum of set pressure plus overpressure
3.5
built-up back pressure
pressure existing at the outlet of a safety valve caused by flow through the valve and the discharge system
3.6
superimposed back pressure
pressure existing at the outlet of a safety valve at the time when the device is required to operate, it is the result of pressure in the discharge system from other sources
3.7
flow area
minimum cross-sectional flow area (but not the smallest area between the disc and seat) between inlet and seat which is used to calculate the theoretical flow capacity, with no deduction for any obstruction
3.8
theoretical discharge capacity
calculated capacity expressed in mass or volumetric units of a theoretically perfect nozzle having a cross-sectional flow area equal to the flow area of a safety valve
3.9
coefficient of discharge
value of actual discharge capacity divided by the theoretical discharge capacity
3.10
certified discharge capacity
that portion of the measured capacity permitted to be used as a basis for the application of a safety valve. It can be calculated by one of the following:
a) measured capacity times the de-rating factor (0.9);
b) theoretical capacity times the coefficient of discharge times the de-rating factor (0.9);
c) theoretical capacity times the certified de-rated coefficient of discharge.
3.11
dryness fraction steam quality
mass percentage of dry saturated steam per kilogram of wet saturated steam
4 Symbols and Units
For the purposes of this document, the symbols and their descriptions and units listed in Table 1 apply.
Table 1 Symbols, descriptions and units
Symbol Description Unit
A Flow area of a safety valve (smallest area between the disc and seat) mm2
C Function of the isentropic exponent, k 1
Kb Theoretical capacity correction factor for subcritical flow 1
Kd Coefficient of discharge a 1
Kdr Certified de-rated coefficient of discharge (Kd × 0.9)a 1
Kv Viscosity correction factor 1
k Isentropic exponent 1
M Molar mass g/mol
n Number of tests 1
p0 Relieving pressure MPa (absolute)
pb Back pressure MPa (absolute)
pe Critical pressure MPa (absolute)
pr Reduced pressure 1
Qm Mass flow rate kg/ kg/h
qm Theoretical specific discharge capacity kg/(h·mm2)
q′m Specific discharge capacity determined by tests kg/(h·mm2)
R Universal gas constant 1
Re Reynolds number 1
T0 Relieving temperature K
Tc Thermodynamic critical temperature K
Tr Reduced temperature 1
m Dynamic viscosity Pa·s
v0 Specific volume at relieving pressure and temperature m3/kg
x0 Dryness fraction of wet steam at the valve inlet at relieving pressure and temperature b 1
ks Steam pressure coefficient h·mm2·MPa (absolute)/kg
Z Compressibility factor 1
a Kd and Kdr are expressed as 0.xxx.
b x0 is expressed as 0.xx.
5 Determination of Safety Valve Performance
5.1 Determination of coefficient of discharge
The coefficient of discharge, Kd, is calculated from Formula (1):
(1)
Kd shall be calculated up to three significant decimal places.
5.2 Critical and subcritical flow
The theoretical flow of a gas or vapour through an orifice, such as the flow area of a safety valve, increases as the downstream pressure is decreased to the critical pressure, until critical flow is achieved. Further decrease in downstream pressure will not result in any further increase in flow.
Critical flow occurs when meet the requirements of Formula (2):
(2)
Subcritical flow occurs when meet the requirements of Formula (3):
(3)
5.3 Discharge capacity at critical flow
5.3.1 Discharge capacity for steam is calculated from Formula (4):
(4)
Formula (4) allows the use of steam tables to obtain the specific volume of steam at various pressures and temperatures. The user is cautioned that the direct use of this equation can lead to an error of more than 20% as the temperature approaches the saturated or supercritical condition. An error of less than 1% can only be achieved at a steam temperature at least higher than 30°C above saturation condition, it can be calculated from Formula (5):
(5)
where, ks is the steam pressure coefficient, it can be calculated from Formula (6):
(6)
Where,
Values for the steam pressure coefficient, ks, can be obtained in Tables 2 to 6. See 6.3.1 for background on the development of Tables 2 to 6.
Formulas (4) and (5) are applicable to dry saturated and superheated steam. Dry saturated steam in this context refers to steam with a minimum dryness fraction of 98% where C is a function of the isentropic exponent at the relieving conditions, it can be calculated from Formula (7):
(7)
Where,
The value of k used to determine C shall be based on the actual flowing conditions at the pressure relief device inlet and shall be determined from Table 7.
5.3.2 Discharge capacity for any gas under critical flow conditions is calculated from Formula (8):
(8)
See Figure 1 for values of Z.
Foreword II
1 Scope
2 Normative References
3 Terms and Definitions
4 Symbols and Units
5 Determination of Safety Valve Performance
5.1 Determination of coefficient of discharge
5.2 Critical and subcritical flow
5.3 Discharge capacity at critical flow
5.4 Discharge capacity for any gas at subcritical flow
5.5 Discharge capacity in the turbulent zone where the Reynolds number Re is ≥ 80
6 Calculation of Rated Discharge and Determination of Flow Area
6.1 General
6.2 Valves for gas or vapour relief
6.3 Calculation of capacity
6.3.1 Description of capacity calculation
6.3.2 Capacity calculation for steam at critical flow
6.3.3 Capacity calculations for wet steam
6.3.4 Capacity calculations for gaseous media
7 Thermodynamic Properties
7.1 Steam pressure coefficient, ks
7.2 Value of C as a function of k
7.3 Theoretical capacity correction factors for sub-critical flow (Kb)
7.4 Compressibility factor
7.5 Correction factor for viscosity, Kv
Annex A (Informative) Examples of Capacity Calculations for Various Media
Bibliography
