文档介绍：DESCRIPTION
This document briefly describes the one phase relief valve calculation spreadsheet, which will be the standard relief valve calculation tool of LGN. The calculation method used in this spreadsheet is based on calculation methods as given in API 520 part 1 (7th ed., 2000, page 41-56). It is strongly recommended to read these pages before one starts with the calculations. All calculations are done in SI units.
FORMULAS
GAS AND VAPOR FLOW
Flow through the relief valve is called critical if
where:
Pb
Maximum back pressure
kPa a
P
Upstream relieving pressure. This is the set pressure plus the allowable overpressure plus atmospheric pressure.
kPa a
ki
Ideal gas heat capacity ratio or Cp,ig / Cv,ig
-
Cp,ig
Ideal gas heat capacity at constant pressure
kJ/
Cv,ig
Ideal gas heat capacity at constant volume
kJ/
Although this equation is only valid for ideal gases, it may be used to calculate critical pressure ratio. ki can be obtained from table 7 in API 520 part 1. Otherwise, the following formula can be used:
where:
ki
Ideal gas heat capacity ratio
-
Cp,ig
Ideal gas heat capacity at constant pressure
kJ/
MW
Molecular weight
-
R
Gas constant
KJ/
If the ideal gas heat capacity is not known, use the actual heat capacity in the same equation. Errors are found to be only small.
Critical Flow (Gas or Vapor):
The following