23 replies to this topic

[sunny0]

I need an answer:

 

Suction pressure of pump is higher than discharge pressure of pump?

 

OR:

 

Suction Pressure of pump is lower than discharge pressure of pump ?

 

Please provide reasons.

Kind Regards,

Sunny.

Edited by sunny0, 15 October 2013 - 01:40 PM.

[Art Montemayor]

First of all, please clarify that this is a CENTRIFUGAL PUMP you are writing about.  This is an important point to identify and specify.

 

Secondly, this is quite a common situation in some industrial applications.  I've used this Unit Operation twice in my career and seen it applied many times.

 

What you write is not a question - even if you put a question mark after the statement.  What is it that you question - the feasibility, the application, or the credibility?

[isbarqi]

Suction Pressure of pump is lower than discharge pressure of pump! EXACTLY.

[sunny0]

Hi,

 

I need to know about centrifugal pump & PD pump (Both Case).

This is a general question for understanding the phenomenon of pressure games in pump.

 

Please explain why suction pressure will be lower than discharge pressure in Centrifugal Pump ?

And what about PD pump?

 

Kind Regards,

Sunny.

[Raj Mehta]

Definitely if its a centrifugal pump, the discharge pressure has to be greater than the suction pressure. The pump develops this difference. 

 

Other way to see this is from Head perspective. The differential head is Discharge head - suction head. So the discharge pressure has to be greater than the suction.

 

Suction pressure diameter is usually 1 size higher than discharge, because we need minimum resistance in the suction side to reduce the losses and increase the NPSHa. 

 

Thanks. 

[breizh]

sunny0 ,

 

it might be interesting for you to read this resource .

 

Hope this helps you out.

Breizh

[sunny0]

To Raj Mehta: if suction dia is less than discharge dia than velocity will be higher at discharge means low pressure at discharge.

Please comment.

 

And thanks to Breizh

[Art Montemayor]

Sunny:

 

Why do you speculate that the suction side diameter of a centrifugal pump could be less that the discharge side diameter?   That is a case that I have never seen in the last 53 years!   And it probably doesn't exist.  Why worry about it?

 

Spend more time worrying about how to understand how a centrifugal and positive displacement pump works and how does an engineer determine their performance and proper operation.  The system's resistance is what determines the pressures, not the diameter of the suction and discharge pipes.

[Raj Mehta]

Sunny,

 

Look it this way, 

 

Suction side velocity is first selected and that too on a lower side <1m/s, In order to avoid the friction losses in the suction side.

 

Why friction loss is considered (given so much importance) only in the suction side?

Friction loss is given more importance in suction side (Note: friction loss is always kept to minimum, unless desired)for the reason that it is directly impacts your NPSHa, consequence of which would be Cavitation, which has to be ensured by a process engineer that it does not take place. Thus select a lower velocity (higher diameter suction piping relative to discharge piping) so that losses would reduce and life of the pump and suction piping will increase. 

 

The case you mentioned Sunny, as also explained by Mr. Art, does not take place. It would be a suicide designing of a pump by a process engineer. 

 

Thanks. 

[sunny0]

Hi Mr. Art & Mr. Raj,

 

I really liked your reply, but need to discuss more.

I hope you will not mind, just clearing my concepts.

 

Please compare friction parameters on inlet of pump & outlet of pump ?

If Friction is there in Suction than cavitation may occur hence we use to have a high dia.

But why we have low dia at discharge of pump ?

 

In double Centrifugal pump, why we use Pressure Safety Valve at discharge ?

 

Regards,

Sunny.

[isbarqi]

understanding a pump system ain't that difficult, basic principles: fluid move from high pressure to low pressure, if you want to move it the contrary, all you need is Work/energy, Pump, whatever it's type, give you this work so you can move a fluid from low pressure to high pressure. moreover, it's not just about pressure, but also energy state, the very basic equation you should understand is simple mechanical energy balance. you can read it through many book or other source, such as Transport Processes And Unit Operations - Geankoplis.

(sorry, I don't know how to attach a file? or picture?)

[kkala]

I think we have to speak beyond formal definitions, just trying to express the core, even not precisely.

1. A centrifugal pump "pushes", that is it creates ΔP. Thus pressure is lower at suction than at discharge (*)..

  - A centrifugal pump cannot "suck", so the liquid needs to have some absolute  pressure at suction to enter the pump.

  - Suction diameter is rather big to minimize friction, thus ensuring high absolute pressure at pump entrance (to ensure NPSHa).

  - Discharge diameter is smaller than suction diameter, usually for economic reasons. Total expenses will be less in the long term, even though a pump of higher head would have a higher purchase cost.

  -concerning post no 7: pressure+0.5*density*velocity2 = constant is not valid in real fluids, much more between suction and discharge of a pump, <http://www.cheresour...de-storage-tank>.

 - Double centrifugal pump may mean two pumps in series. Even at discharge of one pump, PSV is needed only in the case that maximum discharge pressure can be so high as to be unsafe for the discharge  piping itselp. Max discharge pressure occurs at shutoff (discharge valve completely closed) and should be ≤ design pressure of discharge piping.

2. On the other head a positive displacement pump (PDP) creates flow, discharge pressure is the consequence of pressure drop (due to friction, elevation, etc) along its whole discharge line and destination pressure. In theory pressure at discharge can be lower than suction, but this is an odd case; much odder than when I use bicycle brakes on a downhill road.

Along this downhill road suppose a water pipeline ending to a PDP. Suction pressure can be high, discharge pressure is practically 0 barg. The pump actually regulates flow, but this can be also obtained through a valve. It is not necessary to use a pump.

3. It is probably a good idea to study the Chapter of fluid dynamics from your book (e.g. of Unit Operations) systematically, solving numerous exercises. Avoid advanced subjects (e.g. compressible fluids), concentrate on basics. Your queries will be clarified. The exercises will help you clarifying the theory. I found this method effective, as a student and later on. Engineering wonderfully combines theory and exercises for mastering knowledge at the practical level needed.

4. Do not think, as an old professor used to say, that elders have understood all the basics. But one can proceed even so, trying to clarify them to the extent possible. Consequently any comment on the above is welcomed.

 

(*) that "pushes" and "sucks" are reported in Lieberman's book "a working guide to Process equipment", with practical definitions at the beginning. But I think it is not yet time to study this book.

Edited by kkala, 14 March 2013 - 09:00 AM.

[sukanta87]

Hi All,

 

Regarding this discussion and my query would you please suggest the velocity limit for suction and discharge piping of a centrifugal pump? Usually I use 3 - 6 ft/s at suction and 9 - 12ft/s at discharge line. Strong references are needed to specify the limit.

 

References in my hand: 3-6 ft/s (suction), 9-12 ft/s (discharge) @ pumpfundamentals.com

                                       6 m/s (discharge)                                  @ NORSOK P-001

                                       1-4 ft/s (suction), 3-5 ft/s (discharge)   @ Pipeline rules of thumb handbook (attached file)

                                       3-15 ft/s for liquid line                          @ API 14E

                                       etc ..................

 

In a plant it was seen that suction line is 1" and discharge line is 2" NPS for several pumps which seems reverse of the centrifugal pump basics! Would you please clarify the design?

 

 

                                         

 

Attached Files

•  Untitled.png   257.79KB   36 downloads

Edited by sukanta87, 10 June 2014 - 11:56 PM.

[Art Montemayor]

 

There is no such thing as a “velocity limit” that can be applied to the suction or discharge of a centrifugal pump unless you specify the application in detail.  Fluid velocity within a pipe is determined from a set of various conditions:

• Does the fluid cause erosion? (a chief concern of API 14E)
• Is the NPSHa affected in a negative manner? (i.e., in a centrifugal pump’s suction line)
• Does a slow velocity promote settling or precipitation of solids in the fluid? (a concern in flowlines from oil wells)
• Is there a pressure drop limitation in the fluid’s system?
• Is the fluid’s viscosity cause for an unacceptable pressure drop?
• Is the pipe size fixed for reasons other than the systems hydraulic needs? (like a minimum size that can be supported)

 I would never use a general velocity for the design diameter of the suction side of a pump.  I always insist on a detailed, calculated NPSHa definition for the specific pump in question.  Depending on the application, the type of pump, and other needs, the velocity is determined accordingly.

 

I have found the NORSOK fluid velocity specifications in their process standard P-001 to be totally ridiculously high – something that is unusual and out of character for a respected, conservative organization such as NORSOK.  I have often rejected NORSOK recommended velocities because they wouldn't stand up to the API 14E design criteria on offshore systems.

 

One of the best methods to determine the appropriate velocity for a fluid in a pipe is the method that Steve Hall employs in his excellent PIPESIZE program.  I highly recommend you employ his spreadsheet program when sizing your piping systems.  It is accurate, detailed, documented, and done in a professional manner - plus it is inexpensive and can be bought through our Forum.   I have used this program on many offshore and onshore projects to check pipe sizes and it works very well.

 

One reason for your centrifugal pump’s discharge being 2” rather than the obvious 1” is probably the fact that it is rather difficult (and expensive) to try to support a 1” line on a pipe rack.  Sometimes pipe sizes are determined because of common sense rather than velocities.

[ronny_fernandes]

Dear Art,

 

How are you? Hope you are in good health.

 

The below query is not related to the above topic but it is related to the velocity at the suction of the pump. 

 

In our project we have a Fire Water Pump and the velocity considered at the suction of the pump is 4 m/s. The Safety Department has done the same to reduce the pipe size to save cost :-). 

 

My question is will this high velocity hamper the performance & life of the pump and/or will it cause cavitation of the pump.

 

Thank you in advance

 

 

Regards,

Ronny

[breizh]

Ronny-fernandes .

 

Consider reading the documents attached .

 

Hope this helps

 

Breizh

[shan]

If the suction pressure is higher the discharge pressure, what you need is a valve instead of a pump.

[mayuresh]

Hi All,

I am a novice here. currently employed as Junior engg.

My question is,

 

"What happens ( or what is the direct effect) if the pump suction line size is kept lower than the Pump discharge size. This question is with regard to a centrifugal pump.

 

 I completely agree with the earlier posts that it can never be the case in actual practice.

 

i just need an answer as to what will happen (theoritically) if such a pump is designed and put into operation.

 

Again the purpose of asking this question is simply because I just don't want to be stumped on this question again at my workplace.

 

Best,

Mayuresh.

 

 

[fallah]

 

"What happens ( or what is the direct effect) if the pump suction line size is kept lower than the Pump discharge size. This question is with regard to a centrifugal pump.

 

 

Mayuresh,

 

Relevant line sizing criterias and required NPSH dictate the sizes of pump's suction/discharge line which mostly will lead to higher size for suction line...

Then if the suction line size is kept lower than that of discharge line it might lead to lower pump performance, unstable pump operation and pump cavitation...
 

[mayuresh]

Thank you. Appreciate it.

 

Best,

Mayuresh.

[samayaraj]

 

hi mayuresh,

 

For pump to work satisfactorily, you have to arrange the suction piping such that the required amount of liquid is coming to pump suction with less hindrance. So, bringing the liquid to suction is not pump's job. Once the liquid comes to pump, then its pump's duty to deliver.

 

The liquid has to be transferred to the pump's suction by using the pressure acting on the liquid surface. If the pressure acting on the liquid is of very less, or pump is fixed in negative suction or the pumping liquid is of near boiling/ saturation temperature, it will be difficult to bring the liquid up to the pump suction. In this case, NPSH will be very less. For NPSH to be positive and above the the required value, you have to reduce the pump suction (for negative suction) and reduce the pressure drop. If the suction length is fixed, the only variable is pressure drop. Less the velocity, less is the pressure drop and hence the bigger line size. If the suction size is not sized properly, pump will starve, cavitate due to vapor formation, flow reduction etc will happen.

[mayuresh]

hello Samayaraj,

 

Thank you, for the explanation. the concept seems very easy now. really appreciate your help.

Thanks again.

 

Best,

Mayuresh.

[Jaskar]

Hello.

 

I am a new member, first comment.

 

I am working with some new pump designs - I am interested in placing a high-capacity PD rotary pump at the intake of a centrifugal pump.  

 

The PD design enables the centrifugal pump to deal with maximum negative NPSH.  The first design is really a push from almost pure vacuum to about 20 psi through the PD pump to the centrifugal.

 

The PD design can freewheel or be bypassed, and would only activate on low NPSH conditions.  It can also be tied to a VFD for rpm control, versus proportioned from the rpm of the centrifugal pump motor through belts or gears (the PD is a high capacity, but low rpm combination).

 

What I am not sure of is the impact on the centrifugal pump if the PD is left on, but the centrifugal is essentially shut off.  The centrifugal could be freewheeled using a clutch, but it would be easier just to flow the PD through the centrifugal, and use a maximum pressure trip of 20 psi or lower, probably right at the outflow from the PD pump.

 

Does this seem like it is viable?  Is there any impact of pushing the fluid to the centrifugal pump in this way?

 

Thanks for any comments.

 

 

 

 

 

[samayaraj]

Hi Jaskar,

 

Seems to be a new topic. You have to start a new thread. Start a new topic and we will help you.