GAS BLOW-BY
Gas blow-by is the discharge of gas from a process equipment through a liquid outlet.
AT NORMAL OPERATING
CONDITIONS
DP -25 BAR
Control Valve
DP -20 BAR
Diagram-1: At normal Operating
WHEN CONTROL VALVE
STUCK OPEN, GAS WILL GO
THROUGH LIQUID LINE
DP -25 BAR
Control Valve
DP -20 BAR
Diagram-2: During control valve failure.
Cause
Gas blow-by can be caused by failure of a liquid level control system or inadvertent opening of a
bypass valve around a level control valve.
Effect
The effect of gas blow-by can be overpressure in a downstream equipment. Low level is the
detectable abnormal condition that indicates gas blow-by may occur.
Primary Protection:
Primary protection from gas blow-by should be provided by an LSL sensor to shut off the liquid
outlet or shut off inflow when closure of the inflow valve does not exceed the downstream vessel
MAWP caused by gas blow-by.
Secondary Protection:
Secondary protection from gas blow-by to a downstream component should be provided by over
pressure protection on the downstream component.
Location of Safety Devices:
The LSL sensor should be located an enough distance below the lowest operating liquid level to
avoid nuisance shutdowns, but with an adequate volume between the LSL sensor and liquid outlet
to prevent gas blow-by before shutdown is accomplished.
Relief valve sizing on downstream equipment’s:
To determine whether gas blowby case applicable or not, check design pressure of the equipment
in downstream control valve. If the design pressure is equal, the gas blowby will not cause
overpressure in the downstream equipment. If the design pressure of the downstream equipment
is lower than the upstream equipment, when control valve fails, the gas will flow to the downstream
equipment and cause overpressure.
Please note, gas blowby case is applicable if only the design pressure of downstream equipment
lower than the upstream pressure.
For a control valve failure scenario, the required relief load is the maximum flow through the
control valve at full open.
Sizing Equation:
The critical pressure drop for gas or vapor flow across a control valve is defined as below, where
P1 is the upstream pressure in psia, ΔP* is the critical limit in psi, and Cf is a characteristic
parameter of the control valve.
Eq. 1
∆P∗ = 0.5*Cf2*P1
If the pressure drop across the control valve exceeds the critical limit, then the mass flow rate
through the valve is given by Equation 2 below.
Eq. 2
W =2.8*Cf * P1*Cv* Square (SG / Z)
Otherwise, the mass flow rate through the valve is given by Equation 3 below, where Pr is the
downstream (relieving) pressure in psia, SG is the specific gravity relative to air at upstream
conditions, Z is the compressibility of the stream at upstream conditions, and Cv is a characteristic
parameter of the control valve.
Eq.3
W = 3.22 Cv * Square (∆P∗ *(Pr + P1) SG/ Z)