Gas blowby is an undesirable operation scenario for pressure vessels handling gas and liquid phase flows, often two phase separator vessels. In case of a gas blowby, the liquid is completely drained from the vessel thus gas flow from the pressure vessel can freely flow to the downstream equipments which are designed to handle liquid. This may result into overpressure in the downstream equipments.
How a gas blowby occurs
Two phase flow handling vessels, generally gas liquid separator vessels, are equipped with liquid level control on the liquid outlet line. This liquid level control valves maintain a certain liquid level in the separator vessel during normal operation. Liquid flow is taken out from the bottom of the vessel under level control while gas flow goes out from the top of the vessel under pressure control. Refer to typical P&ID arrangement of separator vessels, for details of the control loops.
If by some reason, the liquid level control valves on the bottom outlet fail open, there is a possibility to drain the liquid inventory to downstream equipments. In this scenario the high pressure gas flow has a free access to the downstream equipments without any pressure control loop on the bottom outlet line. Uncontrolled gas flow to the downstream equipments which are designed to handle normal liquid flow, can lead to overpressure and other possible damages depending on the type of downstream equipments.
Safeguards against gas blowby
Gas blowby can occur upon possible inadvertent failing and opening of liquid level control valves. When level control drops down below the normal level set point, low level alarms or a low low liquid level trip can be provided on the vessel to avoid gas blowby. On a low liquid level alarm, operator must act to prevent further drop in liquid level. If the liquid level still drops, at a low low liquid level a process shutdown should be activated by closing the automatic on-off valve on the bottom liquid outlet line.
To protect the downstream liquid handling equipments against gas blowby, appropriate relief devices should be installed on these equipments. These relief devices should be sized to handle the gas flow in case of gas blowby if sufficient safeguards are not present on the liquid line from separator, against gas blowby.