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Booster pumping in series questions
2

Booster pumping in series questions

Booster pumping in series questions

(OP)
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.

RE: Booster pumping in series questions

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

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

(OP)
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?

RE: Booster pumping in series questions

That's correct, see the curve above.

RE: Booster pumping in series questions

(OP)
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!

RE: Booster pumping in series questions

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.)

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

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."

RE: Booster pumping in series questions

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

RE: Booster pumping in series questions

(OP)
Great answers! Helps alot! Thanks guys!

RE: Booster pumping in series questions

@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.

RE: Booster pumping in series questions

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

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

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

RE: Booster pumping in series questions

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#.

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

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.

RE: Booster pumping in series questions

What is incorrect is when a pumping system is designed to operate with 2 pumps in series, presumably, the pumping system will not work at all with a single pump. The flow from a single pump will not have enough pressure, but your diagram shows point 1 is where the system operates with one pump running.

For example, if you needed two 1,000 gpm pumps in series to pump to the top of the hill where the total pressure required is Htotal, a single pump with 1/2 the pressure will not produce any flow.



RE: Booster pumping in series questions

Lets all step back a bit and look at what is actually being pointed out. LittleInch 10Dec., this is a correct case if the flow thru a particular system need to be increased, adding a pump will increase flow with a corresponding increase in head.
Bimr 15 Dec. is also a correct case, if the TH is greater than 1 or 2 pumps can achieve in series, it will require additional pump/s.

Horse for course, depends what, where and why there is a need for series pumping.

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.)

RE: Booster pumping in series questions

Artisi - thank you. The other time you may need to run two in series is when the system curve changes, e.g. the end point of the system has a floating pressure. Sometimes one pump will suffice but at other times you need two in series to maintain the same flow or indeed any flow at all if the end pressure rises higher than the first pump head. Many many possibilities.

Pipeline booster pumps when the pumps are a long way apart can operate in both cases, e.g. you have a large hill in the middle, one pump generates enough head to get arrive at the base of the hill (but not enough to get over the hill) with a low pressure, enough to feed a larger head pump to generate enough head to get over the hill.

A different profile would mean that one pump at the start would be able to pump some liquid from one end to the other, but to get a higher flow you need a second pump half way along the line.

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

RE: Booster pumping in series questions

With an extreme case, my favourite example, pumping hot, heavy oil, two pumps in series might be needed, as a cold pipeline starts up with hot oil being injected at the inlet. As the hot oil makes its way down the pipeline, its lower viscosity begins to have a major effect in reducing system head losses even while flow through the system is actually increasing. That effect may continue to the point where the pump configuration might eventually need to be changed to parallel arrangement.

RE: Booster pumping in series questions

Of course if you use an insulated heated pipeline you don't have that effect 2thumbsup(in joke)

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

RE: Booster pumping in series questions

Who would want one of those???

RE: Booster pumping in series questions

We run two pumps in series, the second pump on a VFD for flow control, works great.

RE: Booster pumping in series questions

And so it should

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.)

RE: Booster pumping in series questions

Actually flow control VFD shouldn't be used for pumps. That's for gas systems. How do you control flow in a liquid piping system. Open or close an inlet valve, or open or close an outlet valve. VFD flow control only works for pump systems when there are no valves in the system. How often is that. Pressure control VFD works for pumps with, or without valves. You know what NPSHR you have to maintain to keep the pumps running properly, so use pressure control to VFD to keep suction pressure >= NPSHR. Speed up pumps when suction pressure is greater than NPSHR, and slow down (to a stop) when NPSHA drops below NPSHR. Combine with a maximum pressure trip, for when pressures rise due to outlet valve closure. If you get too much flow at the discharge close down on the outlet valve and back the pump up on the curve anywhere, at least until it reaches the high pressure trip. You could do that by an outlet flow control valve, but why try to control the pump with that? The pressure control settings are already doing that for you.

RE: Booster pumping in series questions

I respectfully disagree BigInch. Flow control VFD works very well for pumps and is particularly useful when pumping abrasive solids which will destroy control valves and (potentially) downstream piping in short order.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

RE: Booster pumping in series questions

I don't think you disagree at all. I have no problem with VFD, only the part about the "flow control" method of controlling it. Put it on pressure control and any other problems can also be resolved one way or another.

RE: Booster pumping in series questions

But pressure control still requires a valve to be closed on the discharge to regulate the flow- unless I'm mis-reading your post. So instead of using a pump with VFD + Flowmeter- I now need two additional control elements (pressure transmitter and flow control valve).

Why install the additional equipment when I don't need it? (and in the case of slurries- the control valve becomes a wear point).

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

RE: Booster pumping in series questions

No. Back pressure at the end of a pipe is often supplied by static head alone. You don't always need a valve. In the case of a pressure controlled vfd without a pipeline end valve, the pumps will run at full speed, or whatever lower speed becomes might become the limiting speed such that suction pressure remains above NPSHR.

If you absolutely must control FLOW from the end of the pipeline (be sure you really do have to control flow), you might be able to do that with a pump somewhere within the system (this applies to flow or pressure control of that pump), but only if the system hydraulic characteristics allow it. If the pump's downstream pipeline goes over a high hill, then down again to a lower elevation, you will probably have to use a valve at the end of the pipeline, or your pipeline might not run full in the downhill segment. In such circumstances high flow rates might have enough head loss that the pressure loss per unit length acts the same as an end valve (kind of a distributed backpressure) and the design flow rate can flow without a valve holding backpressure at the pipeline's end, although at other lower flow rates some back pressure might be required at the end of the pipeline to keep the pipeline flowing full at any lower flow rate. If you don't mind pipelines not flowing full, then you wouldn't need a valve at the end of the pipeline.

In chemical plant work flow control is often required. That is not true in liquid pipelines where the business objective of the pipeline is to move as much product as possible in the fastest time possible. You have to first realize that flow control is inherently contrary to the business objectives of liquid pipelines. It is actually the same for gas pipelines, but due to the compressibility of gas, there is wide pressure variation with flow which can vary constantly with temperature and line pack so customer requirements are usually based on meeting specific flow rates at specific minimum delivery pressures. It is rarely necessary that liquid pipelines actually have a specific required delivery flow rate from the end of a pipeline. Most liquid transportation contracts are based on moving a specific volume (10,000,000 bbls of your oil and maybe 20,000,000 of somebody elses) within a given month. We are usually not interested in doing that at anything but the fastest flow rate possible within a given system. For that example a design flow rate might be 1,000,000 bbls/day, but if we could do it at 1,100,000, we just might want to do it. That flexibility to flow at higher capacity should not be limited by somebody's arbitrary flow rate setting. It should only be limited by the maximum flow rate when the pumps start hitting NPSHR. That is a pressure setting, not flow rate.

RE: Booster pumping in series questions

BigInch your latest posts have made it clear to me that we are talking from very different backgrounds. My background is in metal extraction and refining plants where flow control is an inherent requirement for most pumping systems. This is obviously different to your background in moving bulk volumes of product from A to B.

Where I have experienced the use of booster pumps has been where high pressure is required to inject liquids/slurries into chemical reactors or to pump viscous slurries longer distances to tailings/waste disposal lines. Nearly all of these systems are two phase (liquid/solids) with minimal levels of ullage/storage between stages. Running at full flow and then stopping results in bogged/plugged lines.

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

RE: Booster pumping in series questions

Certainly there are requirements for control of flow, I'm just saying that controlling the pump to achive that objective is not always the most suitable method, especially in transportation pipelines. Control of pumps is best accomplished based on pressure, because it keeps the system within operating limits, no matter what the flow and pressure relationship might be at any given time. IMO pressure control is better with two phase gas/liquid system flows. I think more often than not in two phase flow you will be lucky if flow control works at all, as flow and pressure relationships can vary wildly depending on if liquid is running up a hill and gas down the other side, or vice versa, or other liquid hold up and slugging characteristics of the system. It might work at certain flow rates where liquids are not held up, but if flow drops to the point where liquid holdup becomes dominant, flow control may not work at all. VFD-Pressure control also works for slurries, as the pump can be made to slow down, or speed up based on suction and discharge pressure limits. If you can't maintain minimum suction pressure, or if you exceed discharge pressure, you have no alternative but to slow down the pump, no matter what you are pumping, then try to solve the problem some other way.

I suspect that you are not quite as interested in actual flow control as you are in simply varying the flow rate by any method that might work. In processes where you must inject a specific amount of one chemical into another, I can readily see that flow control (by mass or volumetric rate) is useful. Heavy crude oil pipelines benefit by injecting a diluent oil into a heavy oil at a controlled rate such that a lower resulting viscosity is maintained, or from injecting a specific amount of corrosion inhibitor, quantity varying directly with flow rate. As those processes are independent of pressure, flow control, normally in addition to control of suction and discharge pressures to keep the pumps and piping system within pressure operating limits, is highly useful.

If we look at heating/cooling systems where two flows are mixed to maintain a certain temperature, you could do that with flow control, or pressure control, but depending on entering water temperature variations, those relationships may not always require mixing in constant proportions. In that case neither flow or pressure control would be appropriate to control the process, however pressure control might still be included to keep pumps and system within operating limits. Temperature control should be used to control the process itself.

I still think that controlling pumps and system safety is one thing and that can usually be based on pressure, sometimes including temperture as well. Controlling process can be (and usually is) another thing entirely and the two can only be combined where there are supplemental and direct predictable relationships between them. Otherwise provide pressure control for system and equipment safety and provide separate controls based on whatever other parameter you might need to monitor and control, such as flow, temperture, color, viscosity, or salt content as necessary, but not necessairily wired to VFD pump control.


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