6 replies to this topic

[Processmen]

Dear member,

I saw a sketch that was depicting two pumps connected to a single tank. The flow rate of the pumps were 15 and 50 m3/hr receptively. A single line was coming out of the tank and it was split in to two lines identical which were the suction lines of the two pumps. The pumps operate simultaneously and feed different destinations (not stand by).
My question is how can these two pumps operate in the designated flow rate as long as their suction lines are the same size and there is no flow control.
I think this configuration will not work, Am I right?

[jrtailor09]

Uottawa,

Yes, You can operate two pump simulteneously. The maximum allowable velocity for the suction lines around 1-2 m/sec. The pump flow rate of 15 m3/hr having suction velocity shall be lesser than 50 m3/hr which is acceptable.

Kindly ensure that suction velcoity for higher flow rate pump shall be in the operating range .

Regards,

jatin

[ankur2061]

Uottowa,

I am assuming that the pumps you are mentioning are centrifugal pumps.

The advantage of a bigger line on the suction side for the 15 m3/h pump is that your NPSHa for that is higher than that for the 50 m3/h pump considering the same source vessel and common outlet nozzle connection for the source vessel. This means that your suction head is higher for the smaller capacity pump.

The flowrate for any centrifugal pump is a function of the system resistance on the discharge side. The system resistance is indicated as a plot of the flow rate versus the differential head which is also known as the pump curve. If you change the system resistance your flow will change. Increase in system resistance will decrease the flow and vice versa. In other words, increase in the differential head will decrease the flow and vice versa.

If you are not changing the system resistance the pump will run at its rated flow for the rated differential head as per the pump curve.

However, if you change the system resistance for either of the pumps it will effect the flow since the pumps are connected in parallel at the suction.

Refer Section 5 about parallel flow for centrifugal pumps in the link below:

http://www.google.co...l2Gfu_A&cad=rja

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 15 December 2011 - 01:51 PM.

[kkala]

A few additional notes on the subject, assuming centrifugal pumps.
1. Arrangement of common suction, even to some extent only, is not uncommon in industry, just to avoid making another nozzle on the tank. This "saving" of nozzles is understood well for an existing tank, but I have seen this applied even for tanks to be constructed.
2.1 If the system is under design, first determine whether both pumps shall operate simultaneously. If so, check NPSHa for each of the two pumps at tank LLLL, assuming that both are operating at rated flow (i.e. at maximum flow specified in the design). If NPSHa is found inadequate in any case, estimate necessary tank level for it to be enough. This level had better have an alarm signal at least.
2.2 If pumps are to operate separately, check NPSHa for the 50 m3/h pump operating alone at tank LLLL. Also check NPSHa for the 15 m3/h pump, even though in the majority of cases the previous case will cover it. Necessary tank level for any NPSHa to be enough had better be specified.
3. Friction in suction line will be increased in case of two pumps, compared to single pump operation. However this will not practically change flow rate of each pump, mainly depending on friction in their discharge lines, on the ground that NPSHa for each pump is enough.
4. A disadvantage of common suction arrangement is the fact that both pumps shall be out of operation, if the common suction piping needs maintenance. I think this is not a frequent case, especially if the tank does not contain some sort of slurry. In case of slurry creating hard deposits, independent suction lines seem appropriate.

[Processmen]

thank you all, All the replies were so helpful.

[sheiko]

The system resistance is indicated as a plot of the flow rate versus the differential head which is also known as the pump curve.

Well, the above mentionned curve is rather the system curve, wich indeed intersects the pump curve.

Edited by sheiko, 17 December 2011 - 07:05 AM.

[ankur2061]

Sheiko,

Thanks for the correction. The pump curve is by the pump vendor and the system resistance curve is based on the calculated resistance to the flow and the intersection point of system resistance and the pump curve is the best efficiency point of the pump.

Regards,
Ankur.