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Positive Displacement Pumps

pd pumps# reverse flow# max.h

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#1 Javidkocharli1

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Posted 15 May 2019 - 11:44 PM

Hi All,

 

I have got 2 questions to ask. I'd glad if you can help me on this..

 

1. I have got a PD pump, and is there any way of finding maximum pressure developed for closed outlet case ? I know that there is no real limit to the pressure that can be developed by a reciprocating pump under blocked-in condition, apart from the input power available and the actual mechanical strength of the pump components. So my question is how can I convert power to the head ?

 

2. If there is reverse flow from the injection point to the source, will there be a reverse flow through PD pump itself ?  is there any way of finding what percent of area of PD pump would result in leakage ?

 

 

Many thanks,

 

Javid



#2 Chemitofreak

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Posted 16 May 2019 - 01:32 AM

Hi Javid,

 

Here are my two cents.

 

1) Theoretically, the PD pump can develop infinite head, hence there is always a PSV provided at the discharge piping to protect the downstream system during a blocked outlet case, because of the PSV the pump will not develop a discharge pressure more than the set of the relief valve.

 

2) Yes, reverse flow is very much possible, but this is a PD pump, it has NRVs built in, that protect it from reverse flow or surge.

 

Hope this helps. Revert in case of any query.

 

Regards   



#3 Art Montemayor

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Posted 16 May 2019 - 12:29 PM

Javid,

 

You fail to outright specify what type of Positive Displacement pump you are writing about.  There are basically two types: rotary and reciprocating piston.  I have to assume you are alluding to the latter type.

Your query about how high can the discharge head of a reciprocating piston pump get is academic if we assume - like Chemitofreak - that you have a pressure relief device in the discharge side.  If that is NOT THE CASE than the discharge pressure will rise  - as you insinuate - to a level where one of two things happens:

  • The pump driver “burns” out and stops furnishing power; or
  • The weakest mechanical component in the assembly ruptures or fails.

 

Your second query is actually 2 questions:

  1. Chemitofreak is wrong in asserting that reverse flow is possible in a reciprocating piston pump.  There can be no “reverse flow through PD pump itself” simply because - as he/she describes - there are two (2) check valves (“NRVs”) built into the pump that allow it to operate.  These are the inlet valve and the discharge valve - the “heart of the pump” and simple check valves working in series.  This issue has arisen in many Hazops that I’ve attended and led and it has been questioned by persons unfamiliar with the simple working parts of such a pump.  It would be double jeopardy if both check valves failed at the same time.
  2. Why do you want to know the “percent of area of PD pump would result in leakage”?  Again I can refer you to Hazop procedures in asserting that under no conditions will I accept pump discharge leakage from a PD pump.  This is not only unacceptable, but is aggressively designed to be prevented at all times.  The same query could be raised for all sections, components, or parts of any unit operation in a process plant.  I would never be interested in the amount of area subject to leakage.  I am only interested - for safety’s sake - in whether there can be leakage, and if so, then attack those sites with appropriate engineering design to avoid ALL leakage where possible.  Of course, if the pumped fluid is hazardous, toxic, or presents a potential harm to humans or the environment, there are other pumping solutions to consider and apply, but we can’t go into that subject because you haven’t furnished us sufficient Basic Data.  We don’t even know the fluid involved and its pressures.  Experience and common sense indicates that the first site to be considered as a potential leak in a reciprocating positive displacement pump is the piston rod’s packing gland.

I highly recommend you study and make yourself familiar with all the inner workings and mechanical components of a reciprocating piston pump.



#4 Chemitofreak

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Posted 16 May 2019 - 01:37 PM

Hi Art,

When I say reverse flow is possible, I meant, flow from the destination up to the pump discharge, that is why I mentioned the NRV.

I Should have had elaborated

#5 Art Montemayor

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Posted 16 May 2019 - 11:35 PM

Chemitofreak:

 

No problem.  I felt it important to address Javid's basic concern that showed he/she has little knowledge of how a reciprocating pump works:  "will there be a reverse flow through PD pump itself?".   The answer is obvious and your comment seemed to contradict it.

 

I have always seen the PD reciprocating pump as the simplest device to pump fluids and regard it as a shame that more Chemical Engineers don't seem to understand its simplicity and main working parts.  You are correct when you explain your comment's basis.



#6 Javidkocharli1

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Posted 17 May 2019 - 05:49 AM

Hi ALL, 

 

Thanks for your answers.

 Basically, I am aware of how PD pump works. However, some companies have a regulation to consider reverse flow( probably very small to be quantified) credible, that's why I asked...

 

I will rule out reverse flow through PD pump, and use PV valve (in the pump discharge line) for PSV sizing purpose..

 

Regarding maximum expected pressure for closed outlet case for PD pumps , (which is called stall pressure) I found some formulas recently in API 674 .

 

Many thanks

 

 

Javid



#7 Art Montemayor

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Posted 17 May 2019 - 07:45 AM

What is a "PV" valve?



#8 Javidkocharli1

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Posted 21 May 2019 - 02:20 AM

Hi,

 

I meant Pressure control valve..

 

 

BRegards,

 

Javid






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