Critical - Subcritical

Hi my friend, how are you today? I am sorry, It’s been a while my blog has not been updated. I was very busy this week; therefore I had no time to write a new article. Ok, Let's continue the discussion about back pressure. Now, by this article I will explain you that the critical and subcritical condition depend on the back pressure.

Before I make further explanation, please note, explanation in this article will be limited for gas vapor phase application only.

API 520 part 1 provides calculation procedure for sizing of effective area. The procedure is divided to critical and subcritical condition that depends on the back pressure that exists at outlet PSV. Critical condition is considered when the back pressure is lower than the critical flow pressure.

Critical Flow Rate

How critical condition occurred? If a compressible gas is expanded across a nozzle or orifice, at constant upstream condition, its velocity increases with the decreasing downstream pressure. The increasing velocity means that the mass flow rate increases. The flow rate will increase until a limiting value.

At sonic velocity, the flow rate could never increase anymore even though the downstream pressure is much lower. This maximum flow rate at sonic velocity is known as critical flow rate.

see this picture.

Critical Flow Pressure

To avoid confusion, terminology of critical flow pressure is used instead of critical pressure. Critical flow pressure is defined as an absolute pressure at nozzle exit at critical flow rate. The actual pressure at nozzle exit cannot fall below the critical flow pressure even though the downstream pressure is much lower

Critical flow pressure can be calculated using the ideal gas relationship.

See above figure. For PSV, pressure at outlet is known as back pressure. Based on the back pressure value, we can determine whether the condition is critical or subcritical.

Critical condition is considered if the back pressure is less than the critical flow pressure. In other word, if the back pressure is greater than the critical flow pressure, it will be considered as subcritical condition.

The orifice calculation procedures both for critical and sub critical condition shall be used in each appropriate condition.

My friend, that’s all, I can share to you. Critical and subcritical condition depend on the back pressure. Hopefully, this topic reminds us that the back pressure is something very important to be considered when we conduct PSV sizing.

Thank you for your attention.