Booster Pump feed Positive Displacement Pump in Series 2

[bmw318be]

Hi Sir,

Here's the situation: We retrofit a fuel system, We use centrifugal pumps that are sitting directly at the storage tanks to pump to existing day tank which is all at level 1, pressure is less than 2 bar maximum.

Now, our end user is build a new building with the day tank on the top level 4, 50 m vertical height. Pressure losses around 6 bar.

Assuming, distance between New Pump room of PD pumps is too far away, we would need to tie in to the existing centrifugal pump in order for the PD to have sufficient NPSHa as distance is a concern and it is not economical to build a new line.

Centrifugal flow is 600 LPM at shut off head of 50 m

Centrifugal discharge line is having a calculated frictional loss up to PD pump inlet is 22 m

PD pump rated flow is 200 LPM at 5 bar

Question 1:
If the centrifugal feed the PD pump which has a lower flow of 200 LPM, would the system pressure of centrifugal follow to 200 LPM since the maximum displacement of the PD is only 200 LPM ?

Question 2:

Assuming the centrifugal is able to operate at 200 LPM curve, can I say that the centrifugal pump able to feed well to the PD pump ?

Question 3:
Would the Inlet pressure before the PD pump inlet is 22 m, which is the frictional loss or it is close to 0 barg since PD pump would withdraw 200 LPM from 400 LPM at no pressure:

Question 4:
i see some discussion, for series, we need to have a balance flow, would the centrifugal on the inlet considered balance as it would adapt to the flow the PD pump displacing

Question 5:
Can I considered the inlet pressure as 0 barg so the NPSHa shall be around 10 m absolute. Assuming the inlet is packed


Question 6:

Is it recommend to install low pressure switch at 1 bar on the inlet of the PD pump so it would protect the PD pump in case no flow.

https://1drv.ms/b/s!AshU1On9Vy6fiasvViVIgVb4EWlrvA?e=tR84SG

 

[Nutzman]

Hi bmw318be,

I fail to see how the 3 pressure reducing valves are in working in series in your sketch? The 3 PRV's are on a "side leg" between the centrifugal pump and the PD pump. Meaning, there are no isolating valves between the centrifugal pump and the PD pump, meaning why would the fluid take the leg via the PRV's when it can take the path of least resistance and bypass the PRV's?
In terms of the differential pressure How can you minus a positive pressure (1.9bar). The 1.9 bar presents itself at the suction of the PD pump. The PD pump "add's to the pressure with every revolution. Agreed the PD pump will "push" against the system up to 5 bar. If the discharge pipe were to burst, the PD pump would still only produce 200LPM against no resistance.

Hopefully this clears the piping?

 

[bmw318be]

Hi Sir

Looking at the curve, for a flow of 200LPM, the head pressure is ~41m
You mention the friction loss's of 22m.
That means the inlet pressure at the PD pump is 41-22 = 19m
The PD pump can generate 200LPM @ 5bar.
That means the discharge pressure on the PD pump will be 5 + ~1.9 = ~6.9 bar.
If you restrict the discharge of the PD, it will keep pumping until something goes Pop!!
The increased pressure and "flow" of the centrifugal pump, will not cause the PD pump to pump more flow. NIMHO.


Is it possible this scenario:

for pd need to call high fuel, to have operating point at extreme right of 550 LPM with head 15 m and the frictional loss calculated is 40 m head at 550 LPM.

In other word inlet of PD is 15-40 m = -25 m (is this would not be cavitation ? ). Means the basis of this sizing, the frictional loss must be within right of the operating point of turbine pump so the positive displacement pump would have positive inlet pressure.

https://1drv.ms/u/s!AshU1On9Vy6fiawteUJkKlBxsDh6PQ?e=jXPi9b

 

[LittleInch]

bmw.

Your Pd pump will do 200 lpm +/- about 10 l/min. Full Stop. So long as the inlet pressure is 0m or maybe even -5m, it will do that flow. If the inlet head is 50m it will still do that flow.

It is, for intents and purposes a FIXED FLOWRATE DEVICE.

Therefore your question is not possible unless you add another pump (which you don't say you are doing). so therefore the scenario you describe can't happen.

But your pd pump will continue to try and take the fixed flowrate it is designed to. If you don't feed it with enough liquid then it will start to cavitate and flow less fluid than you want to.

Do you understand that?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bmw318be]

Littleinch,

Thanks i got it, I corrected my statement.

Is it possible this scenario:

for pd need to call high fuel, to have operating point at extreme right of 550 LPM with head 15 m and the frictional loss calculated is 40 m head at 550 LPM.

In other word inlet of PD is 15-40 m = -25 m (is this would not be cavitation ? ). Means the basis of this sizing, the frictional loss must be within right of the operating point of turbine pump so the positive displacement pump would have positive inlet pressure.

[Attached]

https://1drv.ms/u/s!AshU1On9Vy6fiawteUJkKlBxsDh6PQ?e=jXPi9b

Just curious the basis of the inlet pressure calculated before the pd pump at different scenario.

 

[Nutzman]

Hi bmw318be,

Please refer back to my earlier comment. The PD is the restriction/valve.
If the PD pump has belts and pulleys, it will run at a set speed. No faster, no slower. For every revolution of the PD, it will displace "X" LPM. Full stop.
If the centrifugal pump can pump 600LPM at 50m and "see's" a resistance and only 200LPM can flow, the centrifugal pump, will be pushed back on the curve to 200LPM.

The only way to increase the flow through the PD, is to install a VSD/VFD (depending were you in the world, as to which terms you would use)? But then again, you have to be careful and not go "off" the end of your Hertz curve. Power drops off somewhat when you exceed 50 or 60hZ.

 

[Artisi]

This post must now be a challenger for the longest 101 pump hydraulic discussion for the past 12 months.

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.)

 

[LittleInch]

bmw,

I don't think you have as you still keep saying pd pump calling for "high fuel".

Unless your pd pump is a variable speed device (you haven't told us that), your pd pump can only do 200 lpm +/- about 10 l/min

Yu need to undestand also that you can't get sort of negative head / pressure. It physically doesn't exist.

PLease do some reading around pump curves and system curves.

YOu have a pump curve
You have a system curve ( head loss over a range of flow rates.
Draw them o the same graph
Where the system curve overlaps that is your MAX FLOW. as you will have zero m available for your pd pump.
Anything to the left of the intersection point will result in a positive head at the inlet to the PD pump.

Look at fig 2 of the attached

https://www.michael-smith-engineers.co.uk/resources/useful-info/centrifugal-pump-selection

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[goutam_freelance]

Since a lot of good discussions have already taken place I do not wish to get into the same debate again. But I do have one observation:
1. The exact distance of centrifugal to PD pump is not clear. But you said that it is 'far off'
2. While you calculate NPSHA for PD pump please do consider the 'acceleration head' . It is the drop in head at PD suction produced due to sudden acceleration of liquid in suction piping. This head increases with increase in suction piping length.
3. As per Handbooks, too long a suction piping is not recommended. If absolutely necessary do all the calculations including NPSHA/NPSHR calculations thoroughly. It is recommended to have a pulsation dampener or accumulator in the suction side.

 

[bmw318be]

Hi Littleinch and guys,

Thanks for helping me understand this comcept of centrifugal. It is really meaningful and deep topic of system curve and although i can only comprehend the concept 20%, i would do more readings as per your auggestions

 

[bmw318be]

Hi Goutam,

I do not think pd pump such as gear types is having acceleration losses as this is for diapraghm pump conceot that need pulsation dampener

 

[goutam_freelance]

@bmw318be, you may be partially correct in that effect of flow variations in gear or rotary type pumps is less than from a reciprocating pump , but nevertheless flow being intermittent in character there will be pressure variations in suction (and discharge). So use of pulsation dampeners at suction, short suction line as well as consideration of 'acceleration head ' in NPSHA calculation has been recommended in Mark's Handbook (Chap-14).

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/