Pump station Optimization

[MJFerreira]

Hi

I am trying to determine if an agricultural pumping scheme is running inefficiently and if there are energy saving design changes which can be implemented. I don't have much experience with pumping stations and if someone can point me in a direction to where I can learn more it would be greatly appreciated and possibly answer some of my questions.

The pump scheme is as follows:

9-off 330kW Pumps approx. 10 meters above the source (river) pump up to a reservoir 68m higher. Each 330kW pump is fed by 3 smaller 37kW pumps which are in parallel and are positioned closer to the river (approx 4 meters above the river). There are 27 37kW pumps in total. The 9-330kW pumps then pump into one pipe line up 68m to the reservoir. The river elevation is 1046m above sea level.

I have the pump curves for the existing pumps and I am trying to understand how the NPSHR is achieved for the 37kW motors. I am using a free modeler (PSIM) and keep coming up with an error on insufficient NPSH on the 37kW motor. The only time the 37kW pumps has sufficient NPSH is when the pumps are not more than 1m above the river which is not as per the installation. There doesn't appear to be any kind of flow control besides the shutoff valves. I haven't been able to do a site inspection so my information is a little bit limited. I want to understand the system so I know what information I need to collect on site. I also believe there are some undersized pipes etc.

I am wondering if a VSD, throttling valve or some other form of flow control needs to be implemented to make the pumps operate on their BEP but frankly my understanding of these systems is quite limited and I just get more confused when there are parallel and series pumps all in one pumping scheme.

 

[Artisi]

Sounds like a complex problem as it is presented and is missing any real hydraulic information.
For a meaningful analysis you will need to post drawings of the pump station / set-up with levels, pipe data, flow rates, pump curves, etc

There seems to be some disparity between 3 x 37kW supplying enough water for 1 x 330kW pump.



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.)

 

[BigInch]

Don't even think of VSD, or for that matter installing control valves either, until you have first determined
1.) if the pumps are not running at BEP.
2.) if installing VSDs or CV will make the system more efficient, or less.
3.) analysis of the time that you run each flowrate in the range of all your operational flowrates.

There can also be many operational changes you can attempt before resorting to adding more equipment. It is equally quite possible that your best option right now is a tank or two.

Unless you want to post the pump curves, NPSHR curves and a configuration diagram, pipe diameters and lengths, as well as the flowrates you are running, you probably won't get any useful answers. Nobody can tell what's going on with your system without that data.

Independent events are seldomly independent.

 

[petrochemical87]

you have 9 pumps in parallel(330KW). They all should produce the same head and flowrate in a parallel configuration. Determine the operating point of these pumps and check whether it is close to BEP .You use the suction and discharge pressures, amps drawn by the pumps to find the operating points. Are you considering VFD's because the pumped flow is more than you require? I dont think a parallel pumping station should have VFD's at all! you have 9 pumps to play with. you could try running fewer pumps to get them running closer to BEP this will be less expensive than installing VFDs.
This is a general answer for an ambiguous question.you need to provide more detail to get a better response.

 

[bimr]

If you are interested in learning the practical aspects of pump station design, I would highly recommend that you obtain the book titled Pumping Station Design. It includes answers to the preliminary questions that you mentioned.

http://www.amazon.com/Pumping-Station-Design-Garr-Jones/dp/1856175138

"Pumping Station Design, 3rd edition is an essential reference for all professionals. From the expert city engineer to the new design officer, this book assists those who need to apply the fundamentals of various disciplines and subjects in order to produce a well-integrated pumping station which is reliable, easy to operate and maintain, and free from design mistakes. The depth of experience and expertise of the authors, contributors, and peers reviewing the content, as well as the breadth of information, is unparalleled--making this the only book of its kind."

 

[MJFerreira]

Thanks for the feedback.

I have been able to establish that the large pumps are running at approx 322kW. I have attached the pump curve for the 330kW pump (SDC 250), the pump curve for 37kW (ETA150-315) as well as a sketch of the layout. (A bit crude... apologies...elevations are indicated as meters above sea level)

Unfortunately flows are a bit of an unknown for there are no flow meters or pressure gauges installed. Based on the pump curve (SDC) and power reading can it be assumed flow is approx 1050m3/hr? Which would require 350m3/hr from each 37kW pump, which looks to be close to the BEP for the 37kW pumps.

BigInch can you elaborate on the tanks. I had the same thought but didn't know if it was common practice. I assume a tank would go between the 37kw pumps and the 330kW pump.

I am still not sure if the NPSHr is being provided to the 37kW pumps.

Pipe sizes as indicated on the drawing still need to be confirmed for they are based of photos and selection tables only. Assumptions will be verified. The 750m long pipe appears to be about a diam. 800 and the 60m long pipe between the 37kW and 330kW pumps appear to be a diam 400mm.

There is limited variation on the pumping demand, the system is running full out and just keeping ahead of the farms water demand.

The purpose for this exercise is to be able to supply the pumps with solar power, however the system needs to be running optimally for the best ROI on the PV panels.

 

[BigInch]

Go worry about something else. The max you can save is 5% efficiency going from 350 to 250 m3/h. A VFD will cost at least 3-5% just being there doing nothing.
You must have flow variations or large swings in pressure requirements to be able to make economic use of a VFD or require control valves, respectively, or need tanks.
You don't know anything about the flow variations you need, so until you do, there's no way to rationally decide what you need and what you don't.


Independent events are seldomly independent.

 

[bimr]

If your current pumping need is to run full out to supply existing farmers, that is not opportunity to save energy with a VFD.

Motors for VFD us are called inverter duty and are a little higher quality than standard motors. Your motors are probably not inverter duty and should be changed.

Solar power will be a large expense for a system of this size and it will be difficult to justify.

Take BigInch's advice.

 

[Artisi]

After searching the net I came up with the curve for your ETA 150x350 (KSB)pumps, would have been nice for you to post a copy, but anyway at 1450 RPM this is an extremely poor pump selection for the 350m3/h duty- too far right on the curve and poor efficiency - at 350m3/h NPSHa looks to be about 3.5m, again maybe marginal if you have 4 metre + losses on the inlet side.

You should be looking a bigger pumps as the primaries - maybe 200x xxx or even 250x xxxx running further back on their curves at lower NPSH and better efficiency.

Time to employ someone who knows what they are doing to make a complete study of the installation.

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.)

 

[QualityTime]

1) The river elevation is 1046m above sea level. This means you have 30 ft of atmospheric pressure
2) Suction lift = 4m which is about 13 ft. The historical river level will be different for different times of the year. This has to be taken into account)
3) At BEP, the pump curve says 3.6m NPSHr which is equal to 12 ft. If you operate to the right of the BEP the NPSHr will be greater
4) Suction pipe friction loss approx 5.6 ft. This does not include fittings, entrance loss etc. which will make the friction loss worse
5) NPSHa = 30-13-12-5.6 = -0.6 ft. This simple calculation shows that you will cavitate locating the pumps 4 m above the river

 

[Artisi]

Correcting typing in my previous post - "at 350m3/h NPSHa looks to be about 3.5m," this should read NPSHr 3.5 m.

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.)

 

[QualityTime]

I will redo the calculation to include the vapor pressure effect of warm water:

1) The river elevation is 1046m above sea level. This means you have 30 ft of atmospheric pressure
2) Suction lift = 4m which is about 13 ft. The historical river level will be different for different times of the year. This has to be taken into account)
3) At BEP, the pump curve says 3.6m NPSHr which is equal to 12 ft. If you operate to the right of the BEP the NPSHr will be greater
4) Suction pipe friction loss approx 5.6 ft. This does not include fittings, entrance loss etc. which will make the friction loss worse
5) Also, warm water vapor pressure will make the NPSHa calculations even worse. There are tables for that. At 80 oF the vapor pressure = 1.2 ft
6) NPSHa = 30-13-12-5.6-1.2 = -1.8 ft. This simple calculation shows that you will cavitate locating the pumps 4 m above the river

 

[QualityTime]

I have an electronic version of Sanks' Pumping Station Design 2nd Edition

 

[MJFerreira]

Hi Quality Time

Where were you able to download the electronic version. I am based in South Africa so ordering books from oversees isn't as straight forward with our unreliable postal services.