Calculating NPSHa for booster pumps 3

[davincigee]

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

 

[TD2K]

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?

 

[davincigee]

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.

 

[TD2K]

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.

 

[Artisi]

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.)

 

[davincigee]

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

 

[davincigee]

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

 

[TD2K]

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.

 

[davincigee]

Thanks TD2K...... I am mightily grateful.

 

[Artisi]

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.)

 

[davincigee]

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).

 

[Artisi]

"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.)

 

[Artisi]

"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.)

 

[davincigee]

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!

 

[Latexman]

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

Good luck,
Latexman

 

[davincigee]

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.

 

[EnergyMix]

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.

 

[davincigee]

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.

 

[Artisi]

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.)

 

[davincigee]

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