process engineer_ built up and superimpossed back pressure

7/29/2019 Process Engineer_ Built Up and Superimpossed Back Pressure

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Back pressure is defined as pressure that exists at PSVs 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 PSVs set

pressure - so that the PSV is not open yet

2. PSV in Open Mode - PSV in open position - system pressure higher than PSVs 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.

Built Up and Superimpossed Back Pressure

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15/08/13 Process Engineer: Built Up and Superimpossed Back Pressure

process-eng.blogspot.com/2011/02/built-up-and-superimposed-back-pressure_2304.html

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.

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Built Up Back

Pressure

Calculation

Back Pressure

Effect

PSV Calculation -

Kd,Kc,Kb factor

Critical - Subcritical Blowdown PSV

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 PSVs 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 cant be used

even though the back pressure is lower than 10% of PSVs 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 thats all I can share to you for this week. Hopefully, someday we will be a good process engineer.

Even though we dont have many experience recently, we still have the most important; its a spirit to grow

better. We should take care of it with our life.

Thanks you.

See you next week.

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7 comments:

Anonymous Aug 1, 2011, 5:58:00 PM

Thank you. Quite refresher and informative! Keep up the good work

Reply

Anonymous Mar 14, 2012, 11:21:00 PM

quite good. compact and easy understandable.

Reply

yale ye Mar 14, 2012, 11:28:00 PM

good. and how to estimate the variable superimposed back pressure for a flare system.

Reply

Triyanto SR Mar 19, 2012, 9:37:00 PM

Thank All,,

dear Yale ye..

a variable superimpossed back pressure is back pressure which may be caused by other sources that

continuously flows to the flare..

please see the posting 'built up back pressure calculation' for understanding the concept of back pressure

calculation.

Statistic
http://s12.shinystat.com/cgi-bin/shinystatv.cgi?USER=tsriyadi&NH=1&NHF=1http://process-eng.blogspot.com/2011/02/built-up-and-superimposed-back-pressure_2304.html?showComment=1332167853578#c8921117118792723384http://www.blogger.com/profile/02712074004038742182http://process-eng.blogspot.com/2011/02/built-up-and-superimposed-back-pressure_2304.html?showComment=1331742491480#c7020406837798332597http://www.blogger.com/profile/02074063132886425259http://process-eng.blogspot.com/2011/02/built-up-and-superimposed-back-pressure_2304.html?showComment=1331742104758#c3789625983911708659http://process-eng.blogspot.com/2011/02/built-up-and-superimposed-back-pressure_2304.html?showComment=1312196310727#c119680620984142217http://www.linkwithin.com/learn?ref=widgethttp://process-eng.blogspot.com/2012/04/blowdown.htmlhttp://process-eng.blogspot.com/2011/03/critical-subcritical.htmlhttp://process-eng.blogspot.com/2011/02/psv-calculation-kd-kc-kb-value.htmlhttp://process-eng.blogspot.com/2011/03/back-pressure-effect.htmlhttp://process-eng.blogspot.com/2012/03/built-up-back-pressure-calculation.html


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for more complex case/system,,I suggest you to use Flarenet simulator.

Thank

Reply

Anonymous Aug 2, 2012, 3:17:00 PM

thank you, your article is really good and give me undestanding regarding back pressure.

Reply

Anonymous Apr 13, 2013, 4:06:00 AM

thank you so much!! Now i can do my work =)

Reply

Anonymous May 10, 2013, 3:53:00 AM

Thanks a lot!!!

Reply

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