pumps in series over long distances

[dallas1977]

i'm new here but i have question about pumping over a long distance with no elevation change.. knowing the pressure drop over the length of pipe, how would you determine the number of (identical centrifugal) pumps necessary? if you wanted a specific flow rate at the end of length, would you just find the head that can be delivered by a single pump at that specific flow rate and divide the pressure drop over the length of pipe by the delivered head? this from the idea that two centrifugal pumps operating at the same speed and the same volumetric flow rate contribute the same pump head..

 

[dallas1977]

oh, and if that is correct, how would you determine spacing between pumps? would it be equidistant along the length of pipe?

 

[bulkhandling]

Normally we install the pumps in the same pump station. The distance between the pumps should be minimum with convenient piping connection and maintenance. We use a same controller if the pumps are variable driven to make sure that all the pumps are running at same speed and share same pressure.
We normally do not install the second, third pump3 far away at the midway of the pipeline because that will create extra investment and maintenance issue. But that does give you the advantage to reduce the pipe design pressure. In that case, you should calculate the pumps seperately.

 

[PUMPDESIGNER]

It would be easier to comment accurately if the design intent was explained more.

Do you have outlets on that long pipe between the inlet and end of the pipe?

Will sufficiently high pressure at the inlet to that long pipe, be high enough to produce the flow desired, and yet not produce unacceptable high pressures?

If possible, it is better to go with a multistage pump, each impeller having a specific speed of Ns = 2000-3000, resulting in the highest efficiency (Ns=2000-3000 produces highest efficiency). A single multistage pump is more efficient, simpler, less expensive, easier to repair, etc. If redundancy is needed, buy two pumps and alternate them.

Running pumps in series is in my mind a method of last resort or used only when there is some obvious advantage. The designer must remember that series controls are more complex and expensive both to build and to maintain and repair. There must be some dominant factor in the design intent that would dictate series pumps over multistage pumps, otherwise you would go with multistage.

PUMPDESIGNER

 

[dallas1977]

no outlets between inlet and end.. where would you find pump curves for a multistage pump? i am unfamiliar with how to design for pumping over long distances and have consulted many books on the topics of pipes and pumps with no real enlightening advice.. i know for one instance, that i want to pump 250 gpm over a couple of miles..

my (apparently bad) understanding was that if the pressure drop was say 100 ft of head over a distance and the flow rate that you wanted was such that the chosen pump could produce say 50 ft of head, then at half the distance you would want another pump.. i've never encountered this kind of problem though, and i hope that is not an embarassing idea..

what i have is the pipe length, the pressure drop, and the desired flow rate.. i'm just having difficulty designing for the pump(s) needed..

 

[stanier]

Pumping over a couple of miles is hardly a long distance. In the oil industry pumping over a 1000kms is achieved with one pump.

It issimplistic to divide the total head by two and install two pumps in series. There are other matters to consider. The location of the second pump in respect of the hydraiulic grade line. The net positive suction head at the suction of the second pump has to be engineered so that you do not create cavitation. The mechanical seal or stuffing box of the second pump has to be designed for the maximum pressure.

The system you describe may be subject to extreme surge pressures when one pump fails and the second keeps running. A detailed analysis is recommended. The controls on stopping and starting each pump may be more complex because of the need to avoid waterhammer.

Suggest yiou look at

http://www.mcnallyinstitute.com

or

http://www.pump-zone.com

for more information. ITT Flygt has some pump system design software available for free that will enable you to model your system in series pumping. Also you could download Epanet from the web and model such systems.

 

[tgreif]

stanier

Over 600 miles (1,000km) with one pump? What kind of discharge pressures are we talking about here. Most pipelines I'm aware of have some in-line booster stations along the pipeline to boost pressure.

 

[smckennz]

Hi Dallas
when possible, it is more convenient to have the pumps grouped at the start of the line. This reduces maintenance and energy reticulation costs (e.g. power lines)and also pump building and service road costs.
The price paid is heavier pipe walls and fittings and also final stage(s) pump construction as the discharge pressure will be higher than with the staged approach you describe.
A lifecycle cost analysis is a good way of deciding on the optimum spacing between pumps.
With staged pumps extra care, and often extra gear is required to handle transients (starting, stopping,water hammer, speed changes etc) as Pumpdesigner indicates.

Cheers

Steve

 

[stanier]

tgrief

I cant recall the design pressure of the pipelineas this was back in the 80's. It was designed to carry 55,000 bpd of crude oil from the Jackson field in SW Queensland Australia to Moonie. The crude was a waxy crude. I worked with a team to reduce the poour point of the crude so that one positive displacement pump could pump the crude 1100km. The wall thickness of the DN300 pipe wasnt that much . From memory X60 or X70 was used for the line.

 

[bulkhandling]

Dallas,
For only a few miles, if you have the right pipe size, one centrifugal pump is probably good enough. Give the data to a few vendors, they'll size the pump for you. You need to give them flowrate, total dynamic head, fluid properties.
Good luck