Booster pumping in series questions 2

[jacky89]

I have two theoretical questions:

1) What would hapen if an existing water line is flowing (say 5,000 gpm), and I place a centrifugal pump in the line with a pump curve that does not dexceed 1,000 gpm?

2) Can someone enlighten me why when you place two pumps in series, the outlet TDH and pressure is doubled but the flow rate is not? I cannot see how the equation works since if you increase the pressure, shouldn't flow rate also increase since nothing else changes in the piping system?

Thanks.

 

[bimr]

1. It will not work. The 5,000 gpm flow will be reduced to whatever is capable of passing through the smaller diameter casing of the 1,000 gpm pump.

2. Putting 2 pumps in series is similar to installing a single pump with higher discharge pressure. See the curve below. You are increasing the pressure, not the flow.

Note: A pump does not create pressure, it only creates flow! Pressure is a measurement of the resistance to flow.

The kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) which catches the liquid and slows it down. When the liquid slows down in the pump casing some of the kinetic energy is converted to pressure energy. It is the resistance to the pump's flow that is read on a pressure gauge attached to the discharge line.

SERIES OPERATION

Centrifugal pumps are connected in series if the discharge of one pump is connected to the suction side of a second pump. Two similar pumps, in series, operate in the same manner as a two-stage centrifugal pump.

Each of the pumps is putting energy into the pumping fluid, so the resultant head is the sum of the individual heads.

Some things to consider when you connect pumps in series:
•Both pumps must have the same width impeller or the difference in capacities (GPM or Cubic meters/hour.) could cause a cavitation problem if the first pump cannot supply enough liquid to the second pump.
•Both pumps must run at the same speed (same reason).
•Be sure the casing of the second pump is strong enough to resist the higher pressure. Higher strength material, ribbing, or extra bolting may be required.
•The stuffing box of the second pump will see the discharge pressure of the first pump. You may need a high-pressure mechanical seal.
•Be sure both pumps are filled with liquid during start-up and operation.
•Start the second pump after the first pump is running.

http://www.mcnallyinstitute.com/18-html/18-1.htm

 

[Compositepro]

TDH is total dead head, I assume. That means the flow is zero. It is hard to use equations correctly if you do not understand the fundamentals of how a centrifugal pump works. It spins water in a chamber to give it velocity (momentum). This velocity can then be converted to flow or pressure.

 

[jacky89]

Thanks for the detailed response. Does it mean if I have two pumps in series, if having one pump pushes 1,000 gpm, enabling the second identical pump will yield the same 1,000 gpm since having pumps in series doesn't change flow? But the pressure at the discharge of the second pump will double?

 

[bimr]

That's correct, see the curve above.

 

[jacky89]

Is it also true that whether the water level in the upstream tank is 10' above pump or 50' above pump, the flow rate will not change? Assuming single pump and the water level in the downstream tank is constant 100' above pump. Thanks!

 

[Artisi]

Think you need to sketch out what you have and what you are trying to achieve, too many what if's etc.

Back to your initial post:
1. -installing a second pump of lower capacity in the pipeline will result in an additional loss to the system which could be significant.

2. - the total head on the discharge of a pump is a product of static head plus friction in the pipework plus entry / exit losses etc. therefore adding a second pump in series to an existing pipeline will increase the discharge pressure and flowrate to meet the total head imposed by the system. (increasing flow increases the friction loss Q2/Q1^2)

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]

The thing missing from the graphs by bimr and in your thinking is the system curve. If you put two pumps in series then the flow through both pumps needs to be the same.

However what flow you get through your downstream pipe may well be variable depending on the outlet pressure. See below.

Normally you only use two pumps in series when you have a steep part of your system curve present hence in the picture above, point 3 is probably not much more than 15% more than points 1 and 2.

"since nothing else changes in the piping system" - errr, no, the pressure required to flow double the flow will generally be four times what you had before ( for a system with no static head) or at least a considerable increase

The same principle also applies in your last post, i.e. your pump output pressure/head will be higher as the inlet pressure/head is higher and hence if your system curve allows it the flow will be higher.

Composite Pro - TDH should be spelled out, but is usually Total Developed Head or total differential head. Bit confusing so usually best to use wrds, but means a flowing head in either case.

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

 

[BigInch]

LittleInch nailed it. You can't assume you only have pumps in the system. You also have pipes, valves, etc. which have resistance to flow. Change flow and you increase the resistance proportionally by the square of the flow. That gives a total flow through the system much lower than what you would get by summing the two pump curves by themselves alone.

 

[GameJudge]

TDH is total dynamic head, the "total equivalent height that a fluid is to be pumped, taking into account friction losses in the pipe. TDH = Static Height + Static Lift + Friction Loss."

 

[bimr]

The description is missing from the graph by LittleInch:

Centrifugal pumps in series are used to overcome larger system head loss than one pump can handle alone.

•for two identical pumps in series the head will be twice the head of a single pump at the same flow rate - as indicated in point 2.

With a constant flowrate the combined head moves from 1 to 2.

Note! In practice the combined head and flow rate moves along the system curve to point 3.
•point 3 is where the system operates with both pumps running
•point 1 is where the system operates with one pump running

 

[jacky89]

Great answers! Helps alot! Thanks guys!

 

[MortenA]

@jacky89,

dont think this ha sbeen mentioned: A common reason to add a "booster" pump is to "boost" the pressure for the "main" pump. Often high capacity/high head pumps have poor NPSH and to avoid the risk of cavitation the booster pump increases the pump at the inlet to the main pump.

 

[jacky89]

So the only time to put pumps in series is when you have NPSH problems?

 

[someguy79]

There are other reasons to pump in series. One example I deal with is wastewater pumping.

The stuff that's in the wastewater requires use of impellers with large openings. Most of the appropriate impellers will not make a lot of head.

So when there's a sewage pump station that needs high head, pumps in series get used.

 

[BigInch]

Pipelines can have many pumps in series and there can be many miles between pumps.

There's two reasons to have pumps in series
1) To feed another pump of relatively high NPSHR
2) To get fluid up to a very high elevation, or into another region of very much higher pressure whenever the one pump you have can't develop enough head on its own to do so.

 

[MortenA]

I agree with the two most resent posts (and that's why i added "a common" in the beginning.

 

[1gibson]

Adding to BigInch's second point, to keep pipeline below a certain pressure rating by having multiple pumps. Maybe you can find one pump to do it all, but if that means a 900# flanged line the whole way, its not economical. Instead use multiple pumps spaced along the line and maybe you can keep everything 600#.

 

[bimr]

The potential advantages of an in-line booster pumping station include (1) the pipeline on the suction side of the booster station can be designed for a low pressure rating, which thereby reduces the pipeline construction costs; (2) all of the water in the system need not be pumped at maximum system pressure, so energy costs are reduced; and (3) the primary pumping station (e.g., the source pumping station at a clear well or reservoir) need not be designed for the high pressure conditions that are necessary for only a part of the entire water system.

The potential disadvantages include (1) additional pumping station construction cost, unless the cost of the primary pumping station can be reduced; (2) additional pumping station O&M costs; (3) increase operational complexity; (4) additional electric power substation required; (5) complicated analysis and control of system hydraulic transients; and (6) the possible need for other facilities, such as access roads and power lines.

Note that the curve posted above by LittleInch is incorrect. Garr Jones' curve is more appropriate.

 

[LittleInch]

Why is it "incorrect"? Different from this curve yes, but incorrect?

What the curves above show is that there would be no flow unless both pumps were running. The other curve I posted above shows that flow would happen with one or two pumps, but the increase would be small when both pumps are running. Totally correct.

BTW your point 2 above does not apply when the overall pressure loss is the same. For liquid flow, the total frictional losses and hence energy used are the same whether you have one big pump or 10 little pumps in series for the same flow and same size pipe. Some systems will be more suited to series pumping than others. You can't generalise these things.

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