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Multiple Centrifugal Pumps In Series And Parallel




Multiple Centrifugal Pumps In Series And Parallel For many young process engineers the concept of multiple centrifugal pumps in series or parallel is often confusing. Today's blog entry tries to explain the concept of centrifugal pumps in series and parallel.

First of all let us understand what it means by pumps in series and parallel.

When we say multiple pumps are in series, it means that the discharge of the first pump provides the suction to the second pump and the discharge of the second pump provides the suction to the third pump and so on and so forth depending on the number of pumps in series.

When we say multiple pumps are in parallel, the flow to the suction is split depending on the number of pumps in parallel and when the individual discharge connections from the individual pumps in parallel unite to form a common discharge pipe or discharge header the flow recombines as a summation of the individual pump flows.

Let us try to understand the concept of series and parallel in terms of the flow Q and head H for the pumps.

To begin with let us start with pumps in series. In a series arrangement, each pump handles the same flow rate, but the total head produced by the combination of pumps will be additive. Since each pump generates a head H corresponding to a flow Q, when configured in series, the total head developed is HT = H1 + H2, where H1, H2 are the heads developed by the pumps in series at the common flow rate Q. This is illustrated in the attached sketch 1, where pump A produces a head H1 at a capacity of Q, while Pump B produces a head of H2 at the same capacity Q. Being in series, the combined head is the sum of the two heads.
Attached Image
The attached sketch 2 shows the single pump head curve and the combined head curve for two identical pumps in series. Each pump produces a head of H = 1200 at a capacity of Q = 1000 gpm. The combination in series will generate a total head of HT = 2400 at a capacity of Q = 1000.
Attached Image
Suppose there are three identical pumps in series, each producing 1500 ft of head at 1000 gpm capacity. The total head generated by the three pumps in series at 1000 gpm is:

HT = 3*1500 = 4500 ft

If these pumps in series are not identical but instead have differing heads of 1500 ft, 1200 ft, and 1400 ft at 1000 gpm, as shown in attached sketch 3, the total head generated by these pumps in series at Q = 1000 gpm is:

HT = 1500 + 1200 + 1400 = 4100 ft
Attached Image
Let us now move on to pumps in parallel. As mentioned earlier, for pumps configured in parallel, the flow rate Q is split between the pumps at the inlet into Q1 and Q2 and after passing through the pumps on the discharge side, the flows recombine back to the flow rate of Q, as shown in attached sketch 4. Each pump develops the same head H at the corresponding capacity. Thus, the first pump at capacity Q1 develops the same head H as the second pump at capacity Q2. This commonality of head across parallel pumps is the most important feature of pumps installed in parallel. If the pump heads are not matched, pumps in parallel will not function properly.
Attached Image
Consider two identical pumps, each with the H-Q curve, as shown in sketch 5. The combined H-Q curve in parallel operation is labeled in the figure as two pumps in parallel. At a head of 1200 ft, the capacity of each pump is 1000 gpm. Therefore, in combination, the parallel pumps will be capable of pumping 2000 gpm, generating a common head of 1200 ft. Every point on the combined H-Q curve has a capacity double that of each pump at the same head.
Attached Image
Therefore, when installed in parallel, the flow rates are additive, while the head across each pump is the same. Suppose there are three identical pumps, each developing 800-ft head at a capacity of 400 gpm. When configured in parallel, the flow rate of 1200 gpm is split equally through each pump (400 gpm each), and each pump develops a head of 800 ft. Thus, the total flow is:

QT = 400 + 400 + 400=1200 gpm

And the common head across each pump is H1 = H2 = H3 = 800 ft

To conclude, pumps in series have a common flow rate with heads being additive. With pumps in parallel, the flow rates are additive with a common head.

Hope the readers of my blog like this brief explanation on behavior of centrifugal pumps in series and parallel.

Let me have your opinion in the form of comments on this blog entry.

Regards,
Ankur.

Reference: Working Guide to Pumps and Pumping Stations - Calculations and Simulations by E. Shashi Menon and Pramila S. Menon




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meetyourmaker
Dec 14 2012 09:26 PM
Very nice article sir But shouldnt the output flowrate of a two pumps in parallel configuration depend on where the system curve intersects the new combined pump curve. So in theory for two pumps(rated at say H,Q) in parallel the total flowrate would be greater than Q but lower than 2Q and each pump would operate at a point to the left of the rated flow on its curve at head greater than H and flow less than Q

Very nice article sir But shouldnt the output flowrate of a two pumps in parallel configuration depend on where the system curve intersects the new combined pump curve. So in theory for two pumps(rated at say H,Q) in parallel the total flowrate would be greater than Q but lower than 2Q and each pump would operate at a point to the left of the rated flow on its curve at head greater than H and flow less than Q


When you design a pump system considering 2 or 3 parallel pumps, the connected piping (common suction, individual suction, individual discharge and common discharge) is designed with 2 or 3 parallel pumps. In such a case the pump manufacurer will ensure that the system curve intersects the combined pump curve at the rated point providing the rated flow for the required head. It means that the two or three pumps are exactly identical producing half the rated flow when run individually for the required head.

Essentially it means that the system curve itself will not remain the same if you run a single pump instead of two or three pumps for a pumping system designed for two or three pumps running in parallel.

Regards,
Ankur.
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Anjaney Shukla
Dec 17 2012 03:44 AM
Nice post sir,

Suppose we need 1400 gpm flow at 100 mt head.
1) two pump in series with 50 mt head and 1400 gpm flow
2) two pump in parallel with 700 flow and 100 mt head.

Which one will be better and what factor will decide it

Nice post sir,

Suppose we need 1400 gpm flow at 100 mt head.
1) two pump in series with 50 mt head and 1400 gpm flow
2) two pump in parallel with 700 flow and 100 mt head.

Which one will be better and what factor will decide it


For a new installation you do not decide in the manner mentioned by you. It is any time preferable to have one pump able to meet the differential head as well as flow requirements. In such a new installation a parallel connected pump(s )is provided only for to act as a standby for the operating pump as a operation and maintenance philosophy (1 operating + 1 standby if 2 parallel pumps are installed)

It is only when the following conditions occur that you go for series or parallel pumps.

Series configuration:

1. When pumps with extremely high heads for a given flow are required and such pumps are difficult to get (high cost plus availaibility plus maintenance). This is especially true where both flow rates and heads are very high.

2. When it is required to boost the head of an existing pump for new process requirement during a plant revamp or debottlenecking. In such a case you would provide another pump in series to the existing pump with the same flow rate to provide the increased head.

Parallel configuration:

1. When it is difficult to get (very high cost plus maintance issues) a single pump for very high flow rates. An example for parallel pumps would be sea water as cooling water pumping for an entire refining or petrochemical complex where flow rates required could be of the order of tens-of-thousands of cubic meters per hour. A single pump would be totally impractical in such a case and you would require multiple pumps in parallel to provide such large flow rates.

2. When for an existing plant undergoing a revamp or debottlenecking the existing pump is insufficient in terms of flow rate for the new process conditions required for the revamped / debottlenecked plant. In such a case a parallel pump would be required along with the existing one to cater for the increased flow rate.

Hope this helps.

Regards,
Ankur.
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Anjaney Shukla
Dec 17 2012 06:12 AM
Thanks Ankur
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abhi_agrawa
Dec 30 2012 10:52 PM
Ankur,

Very nicely written article. I have seen inconsistent approaches for the configuration of minimum continuous flow for pumps in parallel. I have seen people put a single MCF line, after the pump discharges meet and I have also seen people put individual MCF line on each pump. What are your thoughts?

Abhishek

Ankur,

Very nicely written article. I have seen inconsistent approaches for the configuration of minimum continuous flow for pumps in parallel. I have seen people put a single MCF line, after the pump discharges meet and I have also seen people put individual MCF line on each pump. What are your thoughts?

Abhishek


Abhishek,

Read my latest blog entry for minimum safe recycle flow schemes. My thoughts on MCF are clearly expressed in the attached sketches.

Regards,
Ankur.
Ankur, I saw an installation recently where the Owner discovered he had insufficient NPSH available so his pump cavitated. He says he solved the problem by installing a second pump in series. I didn't see it in action and can't confirm that the second pump actually reduced the NPSHR, but it sounds fishy to me. Your thoughts? Steve

Ankur, I saw an installation recently where the Owner discovered he had insufficient NPSH available so his pump cavitated. He says he solved the problem by installing a second pump in series. I didn't see it in action and can't confirm that the second pump actually reduced the NPSHR, but it sounds fishy to me. Your thoughts? Steve

 

Steve,

 

The trick here would be to select a (new) 'first' series pump of the pair to have a NPSHr that is adequate for the application.  For example, a submersible pump could work, or perhaps a larger, low-speed, low-head/high-flow pump.  Most pump curves will also show a NPSHr curve, so this is what you would need to examine carefully to ensure the new pump can handle the NPSHa.  Then, placing the new pump upstream of the existing pump will almost certainly eliminate the NPSHr shortfall of the existing pump on account of the head developed across the new pump.

 

The principles of Q vs H for series pumps as explained in this article will then apply to the new configuration.

 

As an example, I had this same problem recently when trying to suck a solution out of a tailings dam which was losing level through evaporation, and solved it in the manner described...

Ankur, I saw an installation recently where the Owner discovered he had insufficient NPSH available so his pump cavitated. He says he solved the problem by installing a second pump in series. I didn't see it in action and can't confirm that the second pump actually reduced the NPSHR, but it sounds fishy to me. Your thoughts? Steve

Steve,

 

jipjanneke has probably answered your query satisfactorily. The series application of centrifugal pumps that I have seen is related to pumping of liquids (petroleum products) through long-distance pipelines.

 

At storage terminals from where petroleum products need to be pumped through long-distance pipelines the concept of pumps in series makes a lot of economic sense.

 

The first pump in the series configuration is a high flow rate low-head pump which supplies to the suction of the second pump in series. The second pump in series is the high flow rate (same as the first pump) and again a low-head pump (different from the first pump and generally higher than the first pump). The first pump in series is called as a booster pump and the second pump as the main pump

 

Most of the times trying to buy a high flow rate and a high-head pump is difficult and also has the implication of a very high first cost. The solution is to split the head with two pumps in series. This solution has been used frequently since pump manufacturer's quote very high costs and long delivery schedules for high-flow and high-head pumps which are generally manufactured as tailor-made solutions thereby costing more and having longer delivery periods.

 

Regards,

Ankur.

Ankur,

 

Very good article indeed.

I have this configuration wherein the two Centrifugal pumps are in parallel.

 

Existing pump : 1000 m3/hr

Head :   162 m

Shut off head : 196.54 m

Motor Rating : 560 kW

RPM : 1800 

 

This existing pump will run in parallel to the new pump whose parameters i mentioned below:

 

New pump in parallel with above : 2200 m3/hr

Head : 107 m

Shut off head : 129 m

Motor Rating : 730 kw ( yet to be decided)

RPM : 1800 / 1200 (1 vendor quoted)

 

My question,  is it possible to run this pump in parallel with the existing pumps since nothing is matching with the existing pump except the liquid service.

Nor the shut off head, nor the flow, neither the rpm for 1 vendor.

 

What do you think, is it still possible to connect those pumps in parallel ?

Prasanjit,

 

Not a very great idea. Behavior will be unpredictable for both the pumps (the high capacity low head and low capacity higher head pump). It is possible that the low capacity pump (1000 m3/.h) may run at end of curve when the new higher capacity pump is hooked up in parallel, which is not desirable.

 

One overlooked aspect of fixed speed dissimilar pumps operating in parallel is the adverse effect on the pump motor which may overload or heat up leading to frequent motor trips or in a worst case scenario cause motor winding burn-out.

 

An alternative to connect your new pump with the existing dissimilar pump would be to provide a variable speed motor for the new pump to adjust the pump flow to match with the existing pump.

 

Hope this helps.

 

Regards,

Ankur.

Thank you Sir for your valuable input.

However, as per the project requirement, Variable Frequency Drive is  NOT envisaged.

 

As I am a mechanical engineer, could you please let me know what should I check in the new pump offers by Vendors looking from parallel operation perspective.

 

Thank you Sir.

Dear @ ankur2061

 

Would you like to suggest a de watering pump for highway road tunnel at very high altitude?

 

The requirements are

 Head -468 m

Discharge - 1400 LPM

Dear @ ankur2061

 

Would you like to suggest a de watering pump for highway road tunnel at very high altitude?

 

The requirements are

 Head -468 m

Discharge - 1400 LPM

Refer the links below:

 

http://www.jeepumpsi...entrifugal-pump

 

http://www.pumpsgear...monoblock-pumps

 

https://www.indiamar...entrifugal-pump

 

https://www.indiamar...8370207162.html

 

Regards,

Ankur

Thank you sir for your valuable inputs

 

I want to know which type of pump will be suitable for the functional requirement? 

Thank you sir for your valuable inputs

 

I want to know which type of pump will be suitable for the functional requirement? 

The links I have provided are for multi-stage centrifugal pumps.

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arunalcoholtech
Apr 14 2017 05:13 AM

VERY VERY NICE EXPLANATION FOR PUMPS IN SERIES AND PUMPS IN PARALLEL.

 

THANKS

Great write-up. If you are running three pumps in series, can they be run flange to flange? Or should they be spaced a specific distance to assure uniform velocity and prevent any cavitation?

Great write-up. If you are running three pumps in series, can they be run flange to flange? Or should they be spaced a specific distance to assure uniform velocity and prevent any cavitation?


Never seen series pumps with flange to flange. The discharge pressure of the first pump is the suction pressure of the subsequent pump in series helping to reduce the differential head of the subsequent pump in series. Also have never encountered three pumps in series. I guess you need to discuss this with the pump supplier and his pump specialist to decide whether such a configuration is practically applicable or not.

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