Gas Blowby Case

Gas Blowby is the discharge of gas from a process component through a liquid outlet. It can be caused;
- Failure of a liquid level control system.
- Or in advertent opening of the control valve by pass.

One of the effects of gas blow by can be overpressure in a downstream component. Of course, the low level is the detectable condition that indicates gas blowby may be occurred.

This picture below show gas blowby case

For instance in a separator system, to determine whether this 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. Conversely, If the design pressure of the downstream equipment is lower than the upstream equipment, when control valve fail, 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 upsteam pressure.

The required load capacity of PSV can be determined based on the mass balance in the system after control valve fail is occurred.

Here is just simple case.

The most often questions which is raised by new process engineer in calculation load capacity gas blowby is the formula for determining the maximum flow through the control valve

The maximum flow through control valve is depends on the control valve Cv and the pressure drop across the control valve. The following picture show an example formula from one of control valve Vendor; please note, each Vendor has their own formula that might different in each other. I give this formula to show you what the variable that has affect to flowrate through the control valve

My friend, for detail calculation in project stage, I suggest you to use INSTRUCAL software for determining the maximum flow through the control valve.

After the maximum flow through control valve is calculated, the required capacity of PSV at downstream equipment can be calculated (for simple, Load = Maximum flow stream 3 – Normal flow stream 4)

Thank you for reading my friend, that’s all I can share to you today. Correct me if I am wrong…

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