3 pump Lift Station
3 pump Lift Station
(OP)
Gentlemen;
Please help me out here. What is the best operational sequence to operate 3 identical centrifugal pumps which are driven with VFD's.
This is in a sewage application so the load varies.
In single pump mode everything is straight forward. The speed will vary between two level points and a minimum and maximum speed. If the flow is too low the drive will shut down and wait for the well to fill.
In two or three pump mode, should all of the drives be operating at the same speed? For example, pump 1 is at 100% speed while pump 2 is varying speed to match the well level.
Or as in the previous example, should both pumps speeds vary and match the fill rate of the well.
Is one method preferred over the other?
Thanks in advance!
tjmurf
Please help me out here. What is the best operational sequence to operate 3 identical centrifugal pumps which are driven with VFD's.
This is in a sewage application so the load varies.
In single pump mode everything is straight forward. The speed will vary between two level points and a minimum and maximum speed. If the flow is too low the drive will shut down and wait for the well to fill.
In two or three pump mode, should all of the drives be operating at the same speed? For example, pump 1 is at 100% speed while pump 2 is varying speed to match the well level.
Or as in the previous example, should both pumps speeds vary and match the fill rate of the well.
Is one method preferred over the other?
Thanks in advance!
tjmurf





RE: 3 pump Lift Station
One pump varies.
One pump reaches 100% second comes on and varies.
Two pumps reach 100% third pump comes on and varies.
etc. etc.
Of course you need check valves for this method.
If you want all the pumps to wear out at the same time and you don't want one pump dying via non-use,(and they most assuredly do), then you may need some logic from a very small PLC or intelligent relay to swap around the pump order every time they are all off. Plus I would want some hour meters so a year down the road you know what the impeller and bearings should look like.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
RE: 3 pump Lift Station
Naresuan University
Phitsanulok
Thailand
RE: 3 pump Lift Station
RE: 3 pump Lift Station
For example, if No1 unit has reached its maximun speed and flow to meet the system head imposed on it and No2 unit is called to start it must ramp up to a speed capable of producing the same head as unit No1 so that flow can take place from this unit - if inflow increases further and unit No3 is called to start, it must also meet the same head as the other 2 units so pumping can commence from this unit, whether this is at the same speeds or at different speeds will depend on the sophistication of the VFD control system .
However, even if you have a sophisticated control system that can vary the individual pump speeds between low level and high level at varying inflows with 1, 2, or 3 pumps online and operating - all pumps need to run at a speed capable of producing the same discharge head for any duty requiring more than one unit.
I guess a VFD system can be designed that can monitor inflow to the station against the required pump speed to achieve the necessary discharge flows and discharge pressures across 3 units at all the variable conditions.
ITT-Flygt as well as other manufacturers have a lot of literature and possibly information which might address the problem of multi-pump sewage stations using VFD pump units.
Naresuan University
Phitsanulok
Thailand
RE: 3 pump Lift Station
The obvious means of control is to measure the inflow and outflow rate and control the pumps on flow. Providing the pump speed is sufficient to meet the discharge head (you need to set minimum speeds for one two and three pumps) all pumps will produce the same head regardless of speed. They will produce different flows dependent on speed. Since you have a wet well and sewage system I assume that you can permit some short term mismatch between inflow and outflow rate and can therefore control the pumps on flow. If the outflow is less than the inflow the controller simply incrementally increases the speed of the control pump. It does this until the pump is at 100%. It then starts the next pump at the minimum speed that as been pre set to produce a minimum flow through that pump. The controller then adjusts the speed incrementally to match the inflow. The main problem is a step in the flow at each pump start. The potential for hunting may be overcome by utilising the storage in the wet well or by steeping back the speed of the pumps from 100% to say 90% when the next pump is switched in so that there is a minimum flow through the control pump.
RE: 3 pump Lift Station
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
RE: 3 pump Lift Station
Naresuan University
Phitsanulok
Thailand
RE: 3 pump Lift Station
RE: 3 pump Lift Station
I rather like BRIS's incremental step changes. It really makes a lot of sense. It injects hysteresis which helps prevent hunting too. Ever noticed those stoopid computer fans that go rrrrrRRRRRrrrrrrrrrrrRRRrrrrrRRrRRRRR because they have no hysteresis? They drive you nuts just as a big pump constantly varying would cause extra noise and wear.
If you use step-wise changes you can also easily map the commands into something like a PLC. A direct mapping of the exact level verses what speed is sent to which pump, easily taking into account BRIS's other point about possibly needing to back off on the first pump a little once the second pump starts.
Example:
SUMP LEVEL(in) PUMP %SPEED PUMP %SPEED
1 1 10
2 1 20
3 1 30
...
10 1 100
11 1 90 2 10
12 1 100 2 10
13 1 100 2 20
etc.
This would be pretty easy to set up as a table in something like a PLC.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
1) At the sewage works, a constantly varying flow from a variable discharge pumping station is preferential to intermitent "slugs" of sewage arriving at the works. The constantly varying flow will tend to improve the performance of the primary settlement tanks and possibly the secondary treatment.
2) A variable speed pumping station can have a much smaller sump than a fixed speed pumping station. The reduced capital cost of the sump can outweigh the increased capital cost of the starters.
If the aim of the variable speed is 1) or 2) then running the pumps at different speeds will meet these objectives, but it is not easy.
3) Variable speed pumping stations can be more efficient than fixed speed pumping station for systems with high dynamic head losses and low static heads. If this is your aim, then the duty pumps must run at the same speed. Running one pump at 100% and a second at a slower speed will result in the pumps running at different parts of their curves, so it is impossible to optimise the efficiency of the pumping station. With all the duty pumps running at the same speed it is possible for all the pumps to run close to the Best Efficiency Point, so the operating cost of the pump station will be reduced.
If you do decide to run a pump up to 100%, then start a second at a low flow, BEWARE. The typical minimum flow of a pump for continuous operation is 50% of maximum flow. Well engineered pumps may go as low as 30% of maxiumum flow. If you are after a continuously varying flow, then you will bring the duty pump up to 100%. You will then need to slow the duty pump to 70% and start the first assist pump to provide 30% of the flow. The speed of the duty pump can then increase to 100%, but the assist may have to increase in speed to pump against the additional head generated by the duty pump. The assist pump can then pump at between 30% and 100%, but for the second assist pump to start, the flow from the first assist pump will have to fall to 70%, and then increase again to 100%. All in all this is far more complicated than making the pumps operate at the same speed, with the duty pump slowing down to a pre-determined speed on the start up of the assist pump, and the assist pump starting at the same speed as the duty pump.
RE: 3 pump Lift Station
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
For example, if the inflow is generally fairly steady around 20 to 30% of the maximum, and significant variations are allowable in the outflow then I would put a VFD on the first pump ONLY and control it on the level in the well. If the well level exceeds the next set point (say 60%) the first pump locks on 100% and the second pump comes in at full capacity and similarly when the final set point (say 75%) is exceeded the third pump comes in at 100%.
Remember that when you bring in a second pump at full speed you do not double the total flowrate. As the flow increases the system head increases rapidly and the pump moves up its curve to a higher head and lower flow. You need to check the actual curves to be able to predict exactly what will happen when the second pump comes in. But the point I am trying to make is that even though you double the number of pumps, you do not see a doubling of the flow and so the variability of the flow is not as extreme as you might first expect. The switch from 2 to 3 pumps is even less dramatic.
Keep Keith's comments about rotating the pump duties and recording their running hours in mind. These things are so much easier with modern systems.
As an example at the other extreme - if the inflow is quite variable, but you would like the outflow to vary as little and as slowly as possible then you may need to increase the well size and control all three pumps with VFD's. If I had more than one pump running in this case I would much prefer to run them at the same speed. I cannot imagine a workable system where two or more pumps are running at different speeds. Devising a strategy for selecting the number of pumps to run simultaneously would not be easy, and would probably require the outflow to be measured. You could possibly set the number of pumps according to the well level and the pump speed, but I suspect this would take very careful setup and tuning. As RCooper stated, the number of pumps running simultaneously should be set such that each pump is running between 50% and 100% of its maximum flow.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: 3 pump Lift Station
I was obviously having a bad day yesterday because I have always advised against trying to control pumps on flow and that is what I proposed above. The problem of trying to match inflow and outflow is that there are always inaccuracies in metering and the wet well will invariably rise or fall to a maximum or minimum and you end up controlling on level.
In water waste water applications I have never come across an application that has needed continues variation and always work speed variations incrementally. In this example if we allow 10% increments the pumps can be controlled on wet well level in the same method as fixed speed pumps but with 300 instead of 3 increments.
I not a pump expert but 50% minimum flow sounds high to me – we have gone much lower than this on some of our systems.
If 50% is a minimum and all 3 pumps are to be operated simultaneously at the same speed then the minimum flow will be 150% ??. I don’t see any option than starting pumps sequentially. Again if the minimum flow is to be in the order of 50% it would be necessary to run the first pump up to nearly 100% before starting the second pump and then bringing them both to 50%. They would then run up to say 75% before bringing in the 3rd pump and reducing all 3 to say 50%.
In general for specific applications we have invariably ended up varying only one control pump and keeping the others fixed: gives the simplest solution.
RE: 3 pump Lift Station
Most pumps will run very stable quite a bit below 50%, but my experience is that if this is done for extended periods the maintenance requirement goes up. But I must admit that I was thinking more of fixed speed pumps being throttled to below 50% with a discharge valve. As the speed decreases the BEP decreases as well and you could probably safely run quite a bit below 50% of max capacity.
If two pumps were running during a decreasing flow period they could be allowed to go down to say 40-45% before one was stopped and the remaining one picked up to 80-90%. As Keith said, we need a bit of hysteresis and this would allow the last pump to be ramped up to 100% before the second pump kicked in again.
Generally, simple is better. I agree with you that it would be best to have the lead pump on VFD and the others on 100%. See my first example above.
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: 3 pump Lift Station
RE: 3 pump Lift Station
Fun problem to say the least. Probably all these ways work. Some applications better than others.
If you could precisely model a pump with all inlet heads and all output pressures you could probably arrive at an optimum configuration.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
Obviously, assuming all pumps are equal and thus have the same rated BEP flow, you must run only one pump between a minimum of 50% of BEP flow to say 110%, two pumps running to yield flows between 100% of BEP (2 at 50% each) to 200-220% (2 at 100% each) and 3 between 150% to 300-330% BEP flow. You will have some flexibility as to how many pumps you want to run when you wish to have a flowrate between 150% (3 pumps running at 50% each) and 200% to 220% (2 pumps running at 100% each) and 200% to say as much as 330% BEP, but you can optimize the configuration at any time by using the pump head vs flow curves, and corresponding efficiency point and power consumption figures. Things get out of hand quickly if you try to run any boosters or shipping at different speeds.
RE: 3 pump Lift Station
I did a mapping of the Alaskan(Alyeska) Pipe Line for sonic flow noise for determining if the noise in the pipe would obscure the sound of the shock-wave caused by leak initiation. I was very impressed looking at the 18,000HP gas turbines that run the pumps at each of the 7(running) pump stations. They must have a lot of fun controlling that beast too. (4 pumps per station, 48" pipe, 1,200PSI, 60,000gpm/pump)
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
I like diesel drives better than electric, because you have built in variable speed if you want to use it (just step on the accelerator), not to mention that I'm an ME and would rather work with fuel supplies than power cables. Do you know if the TAPS pumps are variable speed?
I'm interested in your sonic leak detection work. Can you tell us a little more about your findings? I have not seen any claims from the pipeline sonic leak detection vendors that address the masking issue you mention. I assume that there are different sonic fingerprints for each type of leak, true? Are the frequencies readily distinguishable or must one use some form of AI waveform recognition routines differentiate between them, or is it easier (or harder) than that? Thx.
RE: 3 pump Lift Station
I'm not sure they throttle them, since they have put about half of the pump stations on standby because DRA (Drag Reducing Agent) has allowed them to pump more efficiently. They probably run them full out so they can not, run others at all.
Leaks:
Essentially a hole occurs.
You listen for the shockwave front to come by.
You monitor at several locations.
At each point you detect the wavefront.
You do the time-of-flight calcs.
This gives the origin of the hole.
Detriments:
The accuracy of the location is a function of your quality of time synchronization.
The magnitude of the signals to work with are a function of the hole size.
The discrimination ability of the system is directly related to the signal-to-noise ratio.
The noise is what you need to analyze so you can estimate sensor spacing and minimum detectable leak size.
Also the electrical noise of the transducer is important, some are pretty noisy.
We found that away from pumps the line was generally pretty quiet. We found we could hear leaks pretty well. Even across the hard-to-imagine slack line dropping down to Valdez.
Implementation problems:
Of course there is always the problem of what is a "small enough leak" to not detect?
How close together do the transducers need to be?
There is a serious communications and synchronism requirement.
The more noise the closer together you need your sensors.
Every line is different and hence you must survey them before any design can take place.
We didn't use AI though there was constant work on the actual algorithm relating to slope, magnitude, and shape of the shockfront.
Do keep in mind this method was not the type used on communications lines where a gas(N) leaking out of a tiny hole, permanently whistles in the ultrasonic region, and can be detected at anytime by just hearing it, and located by tracking down the noise source.
With respect to the Alyeska Pipeline their leak detection was/is basically; Transverse flow meters at each pump station coupled to a detailed flow model of the entire pipeline. If a leak occurs then the model will slowly drift from reality until it becomes obvious. So if a pin hole or more likely a 30-06 hole develops it may take a few hours possibly even 24 to detect the leak. A lot of the leaks are actually detected visually early on. The line is overflown often.. very often, exceptionally often! To be fair, there haven't been many in the line's life time.
They were considering our product in hopes of increasing detection speed. We did the study, they never went very far on it. I don't really know why. I was just the data collector and bear dodger and the guy to put the hardware together. I wasn't in the wheeling dealing, marketing aspect. It was all complicated because we were in bed with a large Japanese corporation that had a foot-in via their extremely sophisticated multimillion dollar pigs.
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
Ya, the end I'm familiar with is the transient/hydraulic analysis and attaching the hydraulic model to the SCADA system, cross checking the pressure/flow indicators, statistical analysis of the flow summations, etc.
I think that's par for the course. I "hear" a lot of good things about ultrasonics, but they don't seem to get much farther than the paper plans, at least for pipelines. Funny they are trying to sell ultrasonics AND instrument pigs. You'd think they'd be happy "pushing" their multimillion pigs all by themselves.
RE: 3 pump Lift Station
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: 3 pump Lift Station
Call before you dig.
RE: 3 pump Lift Station
The reason for the VFDs is to give a steady flow into the treatment plant. In addition the VFDs mitigate the very real surge pressures that create fatigue damage and increase maintenance of the pumps and other equipment.
The use of VFDs can be beneficial in reducing the overall power costs by managing the supply and demand cycles. This includes routine high flow requirements to deslime the rising mains.
Yes it is complex but it can be modelled using AFTs Fathom or Epanet. Or you can sit there with a large piece of paper and work through the logic and then go and commission it.
Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au
RE: 3 pump Lift Station
Please excuse me, but I don't understand your comment as it relates to the original thread, concerning recommended operating configurations for the range of flowrates possible over a pump station comprised of several equal pumps with VFD. I (we?) have no doubt that unequal pumps must be operated at different speeds (to yield a common discharge pressure) and that they could all be controlled with VFDs to do so. I think we were all wondering what advantage there would be to running similar pumps at different speeds when targeting a particular flowrate within the range of the pump station? Can you tell us your thoughts on that?
Just because you can simulate it, doesn't mean it will work.
RE: 3 pump Lift Station
A lot more information on this application would help to make any meaningful suggestions, for example what is the station minimum - maximum flows and the variables - are we looking at - 3 x 15kW units - why bother, or 3 x 500kW units - possibly worth considering based on other information like - what is the well size / retention time and the designers philosophy in selecting VFD's etc.
Naresuan University
Phitsanulok
Thailand
RE: 3 pump Lift Station
RE: 3 pump Lift Station
Interesting to note that from the cube law it is cheaper to run two pumps at 50% than one at 100%. This doesn't exactly pan out in practice, but not a bad goal to work to.
CinciMace
RE: 3 pump Lift Station