Jump to content

Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog


Liquid Pressure

liquid pressure

8 replies to this topic
Share this topic:
| More

#1 dhindsa_mexican


    Brand New Member

  • Members
  • 8 posts

Posted 17 August 2018 - 07:54 PM

Hi, I wanted to understand how liquid pressure works. For e.g. let's say we are pumping LPG into a metering skid and on high high pressure from the pump we isolate this metering skid (shutdown on inlet and outlet close). So, now there is some trapped liquid LPG inside the skid. What is the pressure of this liquid?? is it same as pump discharge pressure..i am not sure. kindly help. 

#2 Bobby Strain

Bobby Strain

    Gold Member

  • Members
  • 3,529 posts

Posted 17 August 2018 - 08:04 PM

Seems like we saw this before.



#3 dhindsa_mexican


    Brand New Member

  • Members
  • 8 posts

Posted 17 August 2018 - 08:20 PM

Hi Bobby, 


I am posting this question as I genuinely want to resolve a query.


I do not understand your reply and trying to kill my threads. Is there any issue with posting queries in the forum?

You are senior gold member and out of respect, I did not say anything last time you tried to post irrelevant responses. 

When you have an irrelevant response last time I could not get any further responses. 

#4 Technical Bard

Technical Bard

    Gold Member

  • ChE Plus Subscriber
  • 407 posts

Posted 18 August 2018 - 09:01 AM

If you isolates the OUTLET of the meeting skid first, then the skid will be pressurized to the pump condition.  If you isolate the skid INLET first, the skid would be pressurized based on the profile from the skid to the destination.  In either case, YES, the liquid is under pressure in the skid.  If it is LPG - it must be, or it won't stay liquid.  This is also why skids like this that can be isolated should have a thermal relief valve (because if it is isolated and then heats up (from the sun, for example), the pressure will rise VERY quickly to potentially very high pressures)

#5 breizh


    Gold Member

  • Admin
  • 6,374 posts

Posted 18 August 2018 - 09:19 PM




Is your system similar to the document attached ? To me without a proper schematic it's difficult to visualize the issue . I believe the answer given by T.B is right .


Most probably your metering skid is equipped with P gauge .

Note : You did not give any info about the pump !


To help you you need to help us first with sufficient information .


My 2 cents


Edited by breizh, 18 August 2018 - 09:29 PM.

#6 Art Montemayor

Art Montemayor

    Gold Member

  • Admin
  • 5,780 posts

Posted 19 August 2018 - 06:28 PM



Allow me to address some of the difficulties you are having in obtaining useful responses from our members:

  • You post the same topic in two totally different Forums - the Student Forum and the Industrial Professional Forum.  You identify yourself as a student or professor in your personal profile.  So, which are you - a student or a practicing professional engineer?  As stated in the “Guide lines for posting in the Forums”, Students should post in the student forum, while professionals should use the Industrial Professional and other forums.  There is a strong reason for having two different Forums.  Your posting the same topic in multiple Forums is not allowed and I will delete any excess postings of the same topic.  Please tell me your position so I can delete the post residing in the wrong Forum.
  • I do not believe that Bobby Strain is "out of respect" as you claim.  His point is a valid, rational, and logical one.  We cannot tolerate multiple postings of the same topic in different Forums.  Our serious members who are trying to help you cannot be responding to multiple postings for the simple reason of confusion created by a lack of knowledge as to which posting is the valid one and how to follow the various responses to all postings.  True engineers are trained to avoid nor accept confusion in order to solve a problem.  Multiple posting of the same topic create a madhouse.
  • Please read the comments and recommendations given by Breizh.  If you follow the hints that he gives you will find our members will go out of their way to help you reach a correct and reasonable answer to your query.

The many years that our Forums have been helping engineers - both students and practicing professionals - prove the useful help and solutions our expert members have given out freely, without financial compensation.
I am sure if you follow the guidelines given complete with a proper schematic, you will receive proper and expert responses to your query.

Await your response.

#7 dhindsa_mexican


    Brand New Member

  • Members
  • 8 posts

Posted 21 August 2018 - 07:06 PM

Hello Art, 


I agree with you on all your points. Kindly delete my post in the 'student' forum. 

When I got the following reply from Bobby in the student forum, I thought it was better to post my question here:


"This does not sound like a question that a student would ask."


I am a student and currently trying to learn the concepts of chemical engineering.

Anyways, I have attached a small sketch showing my query. I hope this will enable me to get responses from more learned members of this forum. 


Appreciate your comments. 



Attached Files

Edited by dhindsa_mexican, 21 August 2018 - 07:07 PM.

#8 gegio1960


    Gold Member

  • Members
  • 518 posts

Posted 22 August 2018 - 03:15 AM

the answer of TB is exhaustive.

this system, after the isolation, will go the equilibrium pressure of the trapped material at the external temperature.

it's very important the protection of a thermal relief valve set at that pressure (if the system is not already designed at a higher pressure).

good luck!

#9 Art Montemayor

Art Montemayor

    Gold Member

  • Admin
  • 5,780 posts

Posted 22 August 2018 - 10:02 AM



thank you for your response.  I have deleted your original posting and have moved this thread to the Student Forum.  There is a reason why we segregate the Student Forum from the Industrial Professionals Forum.  The reason is not due to bias or to relegate students to a lower standing in engineering knowledge.  Al contrario.  Our members strive to introduce students to the actual and real world of practical and important engineering concepts and experiences - something that is often left out of the usual University curriculum.  By identifying as a student you allow us to direct and guide you to the important basics that we, as old and experienced engineers, know to be the fundamental and critical learnings that should be applied to many engineering solutions.  We don’t want to talk over your head but rather lead you to the engineering fundamentals that can be applied to resolve key and critical problems - especially those that are potentially hazardous.  This query is one of those and deserves a full and detailed discussion.


Technical Bard and gegio 1960 have indicated the hazardous nature of the scenario and Bobby Strain and Breizh have advised that there are basic engineering rules and communications that should be applied.  To obtain the correct and applicable help, one has to communicate the correct, detailed, and full basic data.  Note how once you furnish a sketch of the scenario, the full impact of the potential hazard is revealed:


You are dealing with LPG - Liquefied Petroleum Gas - that, for industrial purposes, is a saturated liquid existing as a pressurized substance with adequate liquid expansion capability due to a vapor space always allowed in its storage vessels.  This is an important feature discussed in phase equilibria.  Gegio 1960 identifies it as such in pointing out that the blocked-in system must be protected by a thermal expansion safety valve.  It is assumed that your pump piping - both suction and discharge - is 100% liquid filled prior to being blocked in by the shutdown valves.  With nowhere to go to or no freedom to expand, the LPG liquid will try to reach its equilibrium vapor pressure when it is exposed to expanding due to heat transfer from the ambient surroundings.  Without a vapor space that can tolerate the slight liquid expansion, the system will convert itself to a hydraulic expansion one - one that can reach exponential pressures if allowed to do so.  Your system - the pump, piping, valves, fittings, meters, etc. will all be exposed to this increased liquid pressure caused by thermal effects and will fail at the weakest point, causing a very hazardous event.  That is why a thermal expansion valve is always installed in such systems in order to relieve the built-up hydraulic pressure developed when there is a phase-sensitive liquid trapped by a blocked-in condition.  This valve relieves the built-up, high pressure liquid to a safe environment.  


If you are unfamiliar with a thermal expansion valve, we can give you examples and explanations of it and how it works.

Similar Topics