# Pressure Relief Valve Sizing Calculations – Steam Service

### Sample Problem Statement

Design a pressure relief valve for the following service.

Protected equipment : Reboiler
Relief service : Steam
Reason for relief : Blocked steam discharge
Relieving Rate : 10,000 kg/hr
Gas Density : 2.5 kg/m3
Ratio of specific heats for the gas (CP/CV) : 1.3
Compressibility factor of gas = 1.1
Molecular weight of gas = 18 gm/mole
Relieving temperature = 2500C
Set pressure : 5.0 barg
Accumulation : 10%
Back pressure at relief valve discharge : 0.5 barg
Type of relief valve : Conventional pressure relief valve

### Solution

This sample problem for relief valve sizing calculation can be solved in following steps, which are based on pressure relief valve sizing procedures described in API RP 520 Part I.

### Step1

First step is to use the relief valve sizing procedure given in the API 520 part I for steam relief valves. The following equation is used,

... (1)

A : orifice are requirement in mm2

W : flowrate in kg/hr
W = 10,000 kg/hr

P1 : Relieving pressure upstream to relief valve in bara
P1 = 6.51 bara (including 10% accumulation)

Kd : Coefficient of discharge
Kd = 0.975 for gas service

KC : combination correction factor for use of rupture discs
KC = 1 (for absence of rupture disc upstream to the valve)
(If a rupture disc is present, KC = 0.9)

Kb = Capacity correction factor due to back pressure
Kb = 1 (for conventional relief valves with near atmospheric backpressure)

KSH = Superheated steam correction factor
From Table 9 of API 520 Part I,
KSH = 0.9786 (@ 6.0 bara & 250 0C by liner interpolation)

Plugging this data into equation (1), the minimum required orifice area for the relief valve is calculated to be,

A = 32.24 cm2

### Step2

The next higher available orifice area should be selected for the pressure relief valve to be actually installed. Referring to the table of standard orifice sizes given in EnggCyclopedia's relief valve sizing calculator, next high available orifice area is selected to be A = 41.20 cm2 and the corresponding designating letter is 'P'. Available valve size is '4P6', which corresponds to different nozzle sizes for suction and discharge nozzles of the pressure relief valve. For a '4P6' valve suction nozzle is of 4" size and discharge nozzle of 6" size.