Kohasa Engineering Company Limited
Kohasa Engineering Company Limited
Kohasa Engineering Company Limited
MARCH, 2012
By
AYAH AYIBAIFIE R.
RULES OF ENGAGEMENT
INCLUDE Its up to you to include the whole team
L Learn by listening
The designer can then use API Std 526, Flanged Steel Pressure Relief
Valves, to select a pressure relief valve. API Std 526 is a purchase
specification for steel flanged valves.
BASIC DEFINITIONS
RELIEVING PRESSURE
Relieving pressure, shown as P1 in the various sizing equations, is the
inlet pressure of the pressure relief device at relieving conditions. The
relieving pressure is the total of set pressure plus overpressure.
BACK PRESSURE
Back pressure may affect the operation of the pressure relief valve in the
following ways: (a) variations in opening pressure, (b) reduction in flow
capacity, (c) instability, or a combination of all three.
Superimposed Back Pressure
Back pressure which is present at the outlet of a pressure relief valve when it
is required to operate is defined as superimposed back pressure.
This back pressure can be constant if the valve outlet is connected to a
process vessel or system which is held at a constant pressure.
Built-up back pressure occurs due to pressure drop in the discharge system
as a result of flow from the pressure relief valve. Short tailpipes that vent
directly to the atmosphere typically result in lower built-up back pressures
than long discharge systems.
SIZING FOR GAS OR VAPOR RELIEF
Flow through the pressure relief valve can either be critical or sub-critical
The maximum flow occurs at sonic velocity, which exists as long as the
pressure drop through the orifice is greater than the critical pressure drop
given by:
where
Pcf = critical flow nozzle pressure, in psia,
P1 = upstream relieving pressure, in psia,
k = specific heat ratio Cp/Cv
Note:
If the pressure downstream of the nozzle is less than, or equal to, the
critical flow pressure, Pcf, then critical flow will occur. If the downstream
pressure exceeds the critical flow pressure, Pcf, then subcritical flow
will occur
Pressure relief devices in gas or vapor service that operate at critical flow
conditions may be sized using Equation below:
Where:
W - Required flow through the device (kg/hr)
C - Coefficient determined from an expression of the ratio of the
specific heats (k = CP/Cv) of the gas or vapor at inlet relieving
conditions. This can be obtained from Figure 1 or Table 1.
Where k cannot be determined, it is suggested that a value of C
equal to 315 be used.
P1 upstream relieving pressure, kPaa
(= set pressure + allowable overpressure + atm pressure)
Kb - capacity correction factor due to back pressure.
The back pressure correction factor applies to balanced bellows
valves only. For conventional and pilot operated valves, use a value
for Kb equal to 1.0
Kc - combination correction factor for installations with a rupture disk
upstream of the pressure relief valve
= 1.0 when a rupture disk is not installed,
= 0.9 when a rupture disk is installed in combination with a pressure
relief valve
T - relieving temperature of the inlet gas or vapor , oK
Z - compressibility factor
M - Molecular weight of the gas or vapor at inlet relieving conditions.
Kd - Effective coefficient of discharge. For preliminary sizing, use the
following values:
= 0.975 when a pressure relief valve is installed with or
without a rupture disk in combination,
= 0.62 when a pressure relief valve is not installed and sizing
is for a rupture disk
V- required flow through the device, Nm3/min at 101.325 kPaa and 0C
G- specific gravity of gas at standard conditions referred to air at
standard conditions (normal conditions).
Figure 1: Curve for Evaluating Coefficient C in the Flow Equation from the Specific Heat Ratio, Assuming
Ideal Gas Behavior
Table 1: Values of Coefficient C
Figure 2: Back Pressure Correction Factor, Kb, for Balanced-Bellows Pressure Relief Valve
Figure 3: Correction factor due to back pressure
Sizing for Subcritical Flow: Gas or Vapor
When the ratio of back pressure to inlet pressure exceeds the critical
pressure ratio Pcf /P1, the flow through the pressure relief device is
subcritical.
Where;
F2 - Coeffficient of subcritical flow. See figure 4 for values or refer to API 520
for formula
P2 - back pressure, kPaa
Figure 4: Values of F2 for Subcritical Flow
SIZING FOR LIQUID RELIEF:
Where;
Q - flow rate, liters/min
p1 - upstream relieving pressure, kPag.
p2 - back pressure, kPag.