安全阀计算手册样本

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., , 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

1

12-⎪⎪⎭

⎫

⎝⎛+≤i i k k i b k P P

where:

P b

Maximum back pressure

kPa a P Upstream relieving pressure. This is the set pressure plus the allowable overpressure plus atmospheric pressure.

kPa a k i Ideal gas heat capacity ratio or C p,ig / C v,ig -

C p,ig Ideal gas heat capacity at constant pressure kJ/kg.K C v,ig Ideal gas heat capacity at constant volume

kJ/kg.K

Although this equation is only valid for ideal gases, it may be used to calculate critical pressure ratio. k i can be obtained from table 7 in API 520 part 1. Otherwise, the following formula can be used:

R

MW c MW c k ig p ig p i -⋅⋅=

,,

where:

k i

Ideal gas heat capacity ratio

-

C p,ig Ideal gas heat capacity at constant pressure kJ/kg.K

MW Molecular weight -

R Gas constant KJ/kmol.K

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 equation can be used to size a pressure relief device in gas or vapor service that operates at critical flow conditions.

MW

Z

T K K P K C W A c

b d ⋅⋅⋅⋅⋅⋅=

160,13

where:

A Required discharge area (for standard types see table 1) mm 2 W Required mass flow through the device. kg/hr C

Coefficient determined from an expression of the ratio of the specific heats (k = C p /C v ) of the gas or vapor at inlet relieving conditions. This can be obtained from figure 32

in API 520 or with the following formulae:

1

112520-+⎪

⎭

⎫

⎝⎛+⋅⋅=k k k k C

hr lb R lb lb f mole m ⋅⋅⋅

C p Heat capacity at constant pressure at relieving conditions

kJ/kg.K C v

Heat capacity at constant volume at relieving conditions kJ/kg.K K d

Effective coefficient of discharge. For preliminary sizing, use the following values:

• For relief valve with or without rupture disk: 0.975 • For installation of rupture disk only: 0.62

-

P Upstream relieving pressure. This is the set pressure plus the allowable overpressure plus atmospheric pressure.

kPa a K b

Capacity correction factor due to back pressure, to be applied for balanced type relief valves only; For

preliminary sizing, this can be estimated from figure 30 in API 520 (see also table 2).

For conventional and piloted operated valves K b =1 -

K c

Combination correction factor for installations with a rupture disk upstream of the PSV.

• K c = 1.0 when a rupture disk is not installed

• K c = 0.9 when a rupture disk is installed upstream

the PSV and does not have a published value -

T Relieving temperature at inlet of the relief valve. K Z Compressibility factor at inlet relieving conditions -

MW

Molecular weight

Kg/kmol

The above equation should also to be used to size balanced pressure relief valves in subcritical flow (in this case K b should be obtained from the manufacturer).