13 replies to this topic

[kopguy]

I have sort of debate going with my colleagues wherein we have a brown field application in which water has to be pumped from a feed tank to a tank at an elevation 600 meters. We have to use existing centrifugal pumps for this operation and would need three pumps in series to meet the required head.

Proposed scheme is as follows, the first pump P1 would be installed close to feed tank at the bottom, the second pump P2 will be installed at 200 m elevation. The discharge pipe coming from first pump will be connected to suction of second pump (no break). Similarly the third pump P3 will be installed at ~ 400 m elevation and discharge pipe coming from second pump will be connected to suction of third pump.

For a certain flow, pressure drop across each of the piping legs is say dp1, dp2, dp3.

 

In such a situation what will be the discharge pressure of first pump P1

 

Will it be just dp1 or dp1+dp2+dp3?

Thanks in advance.

 

[TS1979]

The minimum discharge pressure for the first pump is "dp1 + density of water*g*height of first section (200m)". You need to add the NPSH requirement for the second pump to your first pump discharge pressure.

 

Thanks

[S.AHMAD]

1. Since you are using existing pump in series you need to do simulation of the pumps curve and the system characteristics to determine the pumping volume such that no cavitation occurs.

2. The Pump-1 discharge pressure (let say Pd-1) can be correlated as below:

 

Pd-1 = P_TK-2 + density x g x ( Z_TK-2 - Z1) + DP -DH_P-3 - DH_P-2

 

where P_TK-2 is the Tank-2 pressure, Z_TK-2 and Z1 are the elevation of Tank-2 level and P-1 respectively, DP is the total line pressure drop between P-1 and TK-2, DH_P-3 and DH_P-2 are the pump head of P-3 and P-2 respectively

3. The pump performance curve (head) can be correlated using quadratic formula:

 

DH = A + BQ + CQ²

 

4. For each volumetric flow, check the pump's suction pressure to ensure that the system does no cavitate.

Edited by S.AHMAD, 27 February 2013 - 09:41 PM.

[Bobby Strain]

Simply expressed, the pump discharge pressure is the sum of friction loss + static head + delivery pressure + suction pressure.

 

Bobby

[ankur2061]

kopguy,

 

Refer the following link:

 

http://www.cheresour...s-and-parallel/

 

Regards,

Ankur

[Raj Mehta]

The discharge pressure of any pump must be sufficient to overcome the static head + frictional losses or pressure drop due to turns, bends valves & other fittings in the line. 

 

In your case this calculate the discharge pressure for the first pump and repeat the same process for the other two pumps.The suction pressure for the 2nd pump would be same as discharge pressure of the 1st & make sure that this head is enough to avoid cavitation in the 2nd pump. 

 

Also keep an eye on the cavitation phenomenon by comparing NPSHa & NPSHr. of each pump.  

 

Thanks

 

Regards,

Raj Mehta

[ankur2061]

Quote

The suction pressure for the 2nd pump would be same as discharge pressure of the 1st & make sure that this head is enough to avoid cavitation in the 2nd pump.

Raj,

 

A small correction please. Suction pressure of the 2nd pump would be the destination or termination pressure of the 1st pump and not the discharge pressure of the 1st pump.

 

Regards,

Ankur

[fallah]

Raj,

 

...And the termination pressure of 1st pump should be such value to supply NPSHR of 2nd pump plus a reasonable margin... 

[kkala]

Besides previous posts, a simplified arithmetic example could be also useful. One is attached as "pumps.doc". It indicates that discharge pressure of pumps 1, 2, 3 is arithmetically close to dp1, dp2, dp3.  This can be physically reasonable, each pump has to overcome upstream total pressure loss up to next pump (or destination, for pump 3). Details can be seen in the example.

Attached Files

•  pumps.doc   29.5KB   31 downloads

[shan]

You may need to install suction drums for your pumps to avoid possible rpm alignment problem due to flow rate up/down surge.

[S.AHMAD]

kopguy

 

You are not responding! Have you got the answer to your question?

[Raj Mehta]

ankur2061, on 28 Feb 2013 - 06:48 AM, said:

Quote

The suction pressure for the 2nd pump would be same as discharge pressure of the 1st & make sure that this head is enough to avoid cavitation in the 2nd pump.

Raj,

 

A small correction please. Suction pressure of the 2nd pump would be the destination or termination pressure of the 1st pump and not the discharge pressure of the 1st pump.

 

Regards,

Ankur

 

Thank you very much Mr. Ankur. 

 

I agree my mistake, your are right. 

 

Thanks for highlighting. 

[Raj Mehta]

fallah, on 28 Feb 2013 - 08:15 AM, said:

Raj,

 

...And the termination pressure of 1st pump should be such value to supply NPSHR of 2nd pump plus a reasonable margin... 

 

I agree Mr. Fallah, hence I have mentioned in the last to keep a watch on the cavitation, which would take place when NPSHr > NPSHa. This is a general thumb rule for any centrifugal pump selection, i.e. NPSHa = NPSHr + 2 (margin depends on the criticality of the operation & the pump cost)

 

Thanks for that information Mr. Fallah,

 

Regards,

Raj Mehta

[Raj Mehta]

shan, on 28 Feb 2013 - 7:57 PM, said:

You may need to install suction drums for your pumps to avoid possible rpm alignment problem due to flow rate up/down surge.

Hello Mr. Shan,

 

Can you please explain the concept of suction drums ? I haven't come across this term anytime before.

 

Thanks,

 

Regards,

Raj Mehta