Built Up and Superimpossed Back Pressure

Back pressure is defined as pressure that exists at PSV’s outlet. It has impact to performance of PSV, hence its value should be informed to vendor therefore shall be stated in the datasheet. Good concept understanding of the back pressure is helpful when calculates its value.

In this article, the following term is used for easier understanding
1. PSV in Ready Mode - PSV still in closed position - system pressure less than PSV’s set pressure - so that the PSV is not open yet
2. PSV in Open Mode - PSV in open position - system pressure higher than PSV’s set pressure - so that PSV is open

Superimposed back pressure is pressure coming from other sources when PSV in ready mode. It may be constant or variable depends on the sources.
Built up back pressure is pressure generated as a result of fluid flowing through PSV in open mode.
I hope this picture will give you better explanation

The picture above seems very theoretical. I do very understood that you prefer something more practical.
See this picture. It will help you get better understanding of the constant back pressure.

When blocked outlet case at downstream occurred, the PSV will open and built up back pressure is generated. See this picture.

The other important point is the spring PSV shall be capable for handling pressure not more than 7.5 psig, so that PSV will start to open at 10 psig. For this case, let your vendor know with the system and back pressure value.

Here is the other system has variable superimposed back pressure.

Back pressure can be calculated by means of hydraulic calculation, Flarenet or HYSYS simulator. I suggest you to use FlareNet for project and critical system where accurate calculation is required. FlareNet will give a better result.

Based on the back pressure, the PSV type can be selected. For built up back pressure less than 10% of PSV’s set pressure, a conventional type can be used. Balance below type can be used for back pressure up to 30%, and pilot type shall be used for higher back pressure.

Please note, even though back pressure is very important, it is not the only consideration in selection of PSV type. For instance, when reseat pressure of conventional PSV is lower than the operating pressure, it can’t be used even though the back pressure is lower than 10% of PSV’s set pressure. Pilot type is required for this case.

Refer to API 520 part I for more detail about back pressure. Hopefully, this article, at least help you get better understanding of the back pressure and realize the importance of it. I think, we still need more discussion how back pressure has impact to PSV performance, how to simulate it etc. Other time, I will make it as dedicated posting for you.

My friends, I feel that’s all I can share to you for this week. Hopefully, someday we will be a good process engineer. Even though we don’t have many experience recently, we still have the most important; it’s a spirit to grow better. We should take care of it with our life.

Thanks you.
See you next week.