Pressure Relief Valve Sizing – Blocked Steam Discharge

In industrial processes, steam is commonly used as a heating medium or as a source of energy. However, steam systems can also pose a significant risk of overpressure in case of blocked steam discharge lines. In such scenarios, the pressure inside the equipment can rapidly increase, potentially causing equipment failure, leaks, and even explosions. In such scenarios, pressure relief valves are designed to automatically relieve excess pressure when it reaches a predetermined setpoint.

Table of content:
Overpressure due to blocked steam discharge
Pressure relief valve sizing calculation for blocked steam discharge
PSV sizing calculator – blocked gas discharge

Overpressure due to blocked steam discharge

When a steam discharge line is blocked, the steam pressure in the line can increase rapidly, leading to an overpressure scenario. This can be caused by factors such as valve closure or blockage in the downstream equipment. If the pressure in the steam line exceeds the design pressure, it can lead to equipment failure or even catastrophic accidents.

To prevent this, a pressure relief valve (PRV) can be installed to automatically relieve excess pressure when the pressure inside the system reaches a predetermined setpoint. The PRV should be selected and sized based on the specific properties of the steam, such as its temperature, pressure, and flow rate. The required capacity of the PRV should be determined by considering the maximum possible flow rate and the pressure drop across the valve.

Pressure relief valve sizing calculation for blocked steam discharge

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 (C_P/C_V) : 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)

K_d : Coefficient of discharge
K_d = 0.975 for gas service

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

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

K_SH = Superheated steam correction factor
From Table 9 of API 520 Part I,
K_SH = 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.