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Calculating NPSHa for booster pumps
3

Calculating NPSHa for booster pumps

Calculating NPSHa for booster pumps

(OP)
Hi,
I have been tasked to perform a fluid mechanics analysis of the possibility of transferring gasoil and gasoline from our tank farm to another tank farm hundreds of km away via a booster pump. We are certain that the booster pump can send gasoil or gasoline to our desired destination. However what we are uncertain is whether our pumps (rated head is 62m) can actually provide enough suction pressure (npsha) to the booster pumps. Can anyone please help with an appropriate formula to crack this? I tried the usual formula of NPSHa= Ha+Hs-Hvp-Hf-Hi. (where Ha=atm head, Hs=static head, Hvp=vapor pressure, Hi=suction inlet head (usually with a safety factor of 2ft)) . But I am pretty not sure if it'll work. Please advise me if i am on the right track and if i am not what is the right approach.
Thanks

RE: Calculating NPSHa for booster pumps

I'm not following what your system is and what you are installing where. Are the pumps you refer to as "we are uncertain is whether our pumps (rated head is 62m) can actually provide enough suction pressure (npsha) to the booster pumps" upstream of existing pumps that you plan to use to pump the HC liquid to the distant tankfarm? That is, you'll have existing tanks, new booster pumps, existing pumps, pipeline, distant tank farm? Is your question about the new booster pumps NPSHA or the NSPHA the existing pumps will see once the new booster pumps are in service?

RE: Calculating NPSHa for booster pumps

(OP)
No,our pumps (rated 62m) are downstream the booster pumps. We intend to pump gasoil and gasoline from our tank farm to another tank farm several kms away. To be able to achieve this, we will first use our pumps (62m head) to deliver the liquid to the booster pump which resides in a booster station close to our tank farm. The booster pump, which of course is much powerful will send the liquid at a much higher pressure to our desired destination. What we need to know is if our pumps (they are 3 in number; rated head=62m, rated Q=145.2m3/hr each)can meet the NPSHa that must be achieved before the booster pumps (they are also 3 in number; rated Q=372m3/hr) can work effectively. I need to know if I am in the right track by using the formula NPSHa=Ha+Hs-Hvp-Hf-Hi.

RE: Calculating NPSHa for booster pumps

Thanks for the clarification.

Basically yes. NPSHA is the difference between the system pressure and the fluid's vapor pressure at the inlet to your booster pump by the time you rearrange the equation (source pressure +/- elevation changes - line losses). You should have ample head to the suction of your booster pumps. You'll need to estimate the line losses between the discharge of the new pumps and the suction of your existing pumps but if the piping has been designed using typical velocities, I wouldn't expect more than a few psi pressure drop if that. You should have 50m + suction head.

RE: Calculating NPSHa for booster pumps

Not clear, are the booster pumps rated 372m3/hr total for 3 units or is that each unit?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

(OP)
Thanks TD2K. Really much appreciate it. Artisi, the rated discharge is for each pump. I don't have the rated head of the booster pumps. But these are the few parameters I can give on both types of pumps;

Our pump (Sulzer) Booster pump

rated flow: 145.2m3/hr NPSHa = > 7m
differential head/pressure rated : 62.24m suction pressure (gasoil)= max: 10.5barg
Efficiency: 68.29% normal: 5.3barg
rated power: 38.62hp (gasoline)= max: 10.5barg
normal: 5.6barg
discharge pressure (gasoil)= 13.2barg
(gasoline)= 12.7barg
capacity of each pump = 372m3/h


So Artisi, wat is your take in this. Can I get your candid opinion on this? Thanks

RE: Calculating NPSHa for booster pumps

(OP)


Sorry Artisi, I am new to this forum so here is the corrected info...

Booster pumps (Sulzer)
NPSHa = > 7m
suction head(gasoline)= max: 10.5barg
normal: 5.6barg
(gasoil)= max: 10.5barg
normal: 5.6barg
discharge pressure (gasoil)= 13.2barg
(gasoline)= 12.7barg
capacity of each pump = 372m3/h

Our pumps (Sulzer)
rated flow: 145.2m3/hr
differential head/pressure rated : 62.24m
Efficiency: 68.29%
rated power: 38.62hp

RE: Calculating NPSHa for booster pumps

Pressure wise, I don't see a problem. Your pumps will provide the necessary NPSHA for the booster pumps. However, the flows don't seem to match. Your pumps are rated for 145 m3/hr while the booster pumps have a capacity of 372 m3/hr (I don't know if that is the design point or end of curve since you say capacity for one set of pumps and rated for the other).

You'll want to ensure you don't flow enough to run your pumps far enough out on their curves that their head decreases to the point where the booster pumps aren't being supplied with enough NPSHA. You could also run two or more of your pumps in parallel to feed a booster pump, that would reduce this issue relative to 1 of your pumps to 1 booster pump.

I hope that's clear with 'your' and 'booster' pumps.

RE: Calculating NPSHa for booster pumps

(OP)
Thanks TD2K...... I am mightily grateful.

RE: Calculating NPSHa for booster pumps

TD2K, beat me to the punch re the mismatch in flow rates.
My suggestion is to bring someone on board who understands pumping equipment / analysis of pumping systems to have a good look at the project. The way you have presented this with very little meaningful information it would appear that there may be a number of seperate issues to be reviewed, the NPSHa/r of the booster pumps being a very minor consideration in the overal scheme.

To give you an idea,
1. you need to analysis the flowrate / pressure at the booster pump inlets
2. you need to calculate the flowrate / pressure that the booster pumps will generate
3. you need to review if the pressure in the booster pumps is within the design capabilities.
4. seeing as how the tank farm is hundreds of km away are the booster pumps capable of developing the head required.
etc etc.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

(OP)
Well Artisi, I can't provide thorough details of the booster pump. But can you please tell me the steps u will take in ;
1. analysing the flowrate at the booster inlet.
2. analysing the pressure at the booster inlet.
From my earlier submissions, I stated that I needed to know if I was in the right track. Moreso, the booster pumps are operated by a separate entity who have on several occasions used the booster pump for transfer to the same desired tank farm destination. Thus, we neither need to worry about the flowrate/pressure that the booster pumps will generate nor review if the pressure in the booster pumps is within design capabilities because they have been used in such a situation before.
Let me explain this a bit. The booster station belongs to a company very close to our tank farm. They use their facilities to pump petroleum products from the Conventional Buoy Moor (CBM) to the tank farm that I earlier said was our desired destination. What we are seeking to do is present the possibiliy of transfering products from our tank farm to the desired destination.
Our main concern is to find out if the booster pumps can pick up the products that our pumps haved pumped, or if our pumps can meet the desired NPSH available that is specified by the manufacturer. I have done all the head analysis, friction, vapour pressure, atm. head, inlet suction loss, and the system head. It is possible, very possible from my ananalysis however I will also want to hear from you. How will you analyse the flowrate at the booster inlet given that you know the suction pressure (refer to my earlier posts).

RE: Calculating NPSHa for booster pumps

"I have done all the head analysis, friction, vapour pressure, atm. head, inlet suction loss, and the system head."

If all of the above has been done then you would know what pressure is available at the inlet to the booster pumps.

I think the NPSHa of 7M you have quoted for the Sulzer pumps is wrong or misleading -- should this be NPSHr.

What you need to know is the NSPSHr of the (Sulzer) booster pumps, is this 7M?

Provided the pressure at the inlet of the booster pumps is above the NPSHr then you don't have any problems, provided you don't exceed the max. inlet pressure of the booster pumps.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

"What you need to know is the NSPSHr of the (Sulzer) booster pumps, is this 7M?"

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

(OP)
We don't know the NPSHr of the booster pump. We only know the NPSHa. Shouldn't that be enough considering the fact that the booster station operatives gave us this info. Based on this information, ours is to meet the NPSHa that is all. If the NPSHr turns out to be greater than the NPSHa then we can't bear the consequences of pump failure. After all, it is not our responsibility to ensure that the pumps are within safe conditions! I only needed your perspective on how to approach this problem not a show-off of your engineering prowess!

RE: Calculating NPSHa for booster pumps

To finalize the analysis, one MUST know the NPSHr of the booster pumps.

Good luck,
Latexman

RE: Calculating NPSHa for booster pumps

(OP)
Thanks Latexman. As long as our NPSHa is greater than the figure the booster station operators gave us, we have no problem at all. Nonetheless, we shall make these enquiries eventually to ensure that our pumps can actually perform this operation. Thanks again and to all who contributed to this thread. I am really grateful.

RE: Calculating NPSHa for booster pumps

I realize I'm coming into this somewhat of a landmine of a conversation a bit lated, but do you have to consider any elevation differences? That will make a difference. Searching previous threads in this forum might give you an idea on how to tackle the problem as you appear to be intent on alienating some of the more helpful members.

RE: Calculating NPSHa for booster pumps

(OP)
Thanks Energymix. Indeed a thorough analysis to the NPSHa will not be complete without elevation differences. I have factored that into the analysis. Thanks tho'......
I however don't believe that I am alienating anyone. Yet I understand that I should have ignored certain statements and focused on getting the answer to ny problem. I believe Artisi is a good engineer and had the right intentions. Thanks for pointing that out.

RE: Calculating NPSHa for booster pumps

davincigee, that's ok, I've been criticised by experts - your comment was like water off a ducks back.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

(OP)
Hahahah..... that was funny Artisi. A little follow-up question: I am a bit confused,the formula, NPSHa= Ha+Hs-Hvp-Hf-Hi(+/-He) where Ha=atm. head; Hs=pump head; Hvp=vapour pressure head; Hf=friction head; Hi=inlet suction loss head,He=elevation differences head; has the atmospheric head factored into it. However, my concern is, should we factor atmospheric head into the equation because personally, I think that has already been taken care of (the pump head contains the atmospheric head). Thanks

RE: Calculating NPSHa for booster pumps

NPSH_R is an "absolute head".

To compare NPSH_A to NPSH_R, obviously both then need to be in terms of atmospheric pressure.

NPSHA = (atm absolute pressure, OR absolute tank pressure) + elevation head - vapor pressure - friction loss + velocity head

When the fluid is in contact with the atmospheric, or same as being pumped from a tank at atmospheric pressure, then add atm [absolute] pressure.

If the fluid is being pumped from a closed tank or closed pipe, then add the tank's or pipe's absolute pressure, which = tank or pipe press psig + atm pressure.

Where,
Elevation head is + when the pump Centerline is below the fluid level.
and
Velocity head is customarily ignored.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating NPSHa for booster pumps

Sorry. To compare NPSH_A to NPSH_R, obviously both then need to be in terms of absolute pressure.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating NPSHa for booster pumps

Trust you can stand a candid comment, which I will make anyway -- I think you are going over the top with this NPSHa/r calculation.

If it were me, I would initially draw a system curve for the pump discharge ie flow against head ( head consisting of friction loss for the pipeline from your farm to the booster pumps plus any ancillary valves and a bit of fudge factor -to this I would add the NPSHr of the booster pump/s or the NPSHa of 7M if that is all you have available + any elevation change) ignore all the other add-on's and deductions at this point. Draw this across the pump performance curve, if it intersect the pump curve and a meaningful flow rate then this will indicate that NPSH should be ok - if the head is above the HQ curve it will indicate that you could have insufficient NPSHa at the booster inlet.

From here it becomes more complicated and you now need to look at series pumping and the effect of the pumps working together in series which will entail further calculations etc. However, what is not clear and it is not possible to make any comment is the apparent difference in capacity of "your" pumps and the booster pumps.

Of course I and others have assumed that you are pumping direct into the booster pumps - Correct?

Take this for what it is worth, if you don't agree let me know one way or the other - either way is ok with me.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

(OP)
Wow.... I never thought of the analysis that way. That was really clever. Thank you Artisi. I'm going to try it and give you an anwer. To answer your questions; the rated capacity of our pumps are 145m3/hr and the rated capacity of the booster pumps are 372m3/hr. ....and yes you assumed right, we are pumping directly to the booster pumps. Thanks a million...!

RE: Calculating NPSHa for booster pumps

With the mismatch in flowrates, you will need to have a very detailed look at that. As it is now, with no real information, it is difficult to predict what will happen, other than both pumps will try to pump the same flowrate, no matter what their actual ratings are.

For example, say the pumps ranges are

372 m3/h rating has a range of 200 to 400, and the
145 m3/h rated pump has an actual range of 100 to 200 this system won't work very well, except at the 200 m3/h point, at which you will get the combined head of both pumps when they are flowing 200 m3/h. If that's at the far right end of the curve of the 145m3 pumps, then head from that pump is likely to be relatively low. Maybe even very low, thus they may not contribute to the effort much at all. Possibly nothing, while the 372 m3/h pumps are pumping away at 200 m3/h but at a very high discharge pressure. No load sharing, a complete mismatch in fact. Turn the 145m3/h pumps off and bypass them to achieve nearly the same effect. And the 372 m3/h pumps wind up operating at 200 m3/h, relatively far to the left on the curve, in a possibly very inefficient region of that curve, no matter what you do with the 145 m3/h pumps. That might amount to a very costly pumping exercise.

You must also know if you can control the flowrates, or if you will have to just let this system flat at the intersection of the natural pump and system curve. Probably not a good idea with this kind of a mismatch. Both pumps are likely to operate in very inefficient regions of their respective curves.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating NPSHa for booster pumps

BigInch thumbsup2 spot-on, correct. Next problem to review and remedy.
TD2K mentioned this way back suggesting 2 or 3 pumps in parallel to feed 1 of the booster pumps.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

Yes, but it seemed like you didn't wanted to ignore that topic back then.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating NPSHa for booster pumps

Always on the back-burner, but 1 problem at a time, plus the original question was about NPSH - so left it alone other than to suggest early in the discussion of getting someone to review the overall pumping system - but sigh - to no avail.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: Calculating NPSHa for booster pumps

Exactly. It does go that way more often than not. They think they have a problem, only to find that it is nothing next to the [elephant2] in the room.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

RE: Calculating NPSHa for booster pumps

(OP)
Come on Artisi! I thought we were over this. Anyway, I took TD2K's advice concerning the . I wish I could give more information especially that pertaining to the booster pumps but the organisation won't provide those info's for some wierd reasons. I hear they have some disputes with my company. I was lucky enough to obtain the NPSHa from test result document during the commisssioning of the booster pumps. Nonetheless, everyone's help has enlightened me more... Hey Artisi, like I admitted, I made an error and I apologise. All the same thanks. I am still trying to obtain the full pump data from the booster pumps so I can use your alternative procedure to obtain the results of my analysis i.e. if I manage to get it. But I will get back to you if I get some results...Thanks guys

RE: Calculating NPSHa for booster pumps

The performance of the booster pumps is really required, otherwise you have no way of knowing what flow rate you need to supply to the booster pumps. Series pumping is not a black art but you must have detail of both pump/s sets - the pumps interact with each and cannot be treated as seperate identities.

The info from BigInch re the difference in flow rates is the crux of a successful operation.

A simple discussion is attached to help in the understanding of series pumping.

http://www.doylepump.com.au/graph/calc/pump_school...

A rough and ready graphic of what will / could happen with the mis-match in flows.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

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