3 pump Lift Station 2

[tjmurf]

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

 

[itsmoked]

Hi tjmurf; You have it right.

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

 

[BRIS]

I agree with itsmokeed and vary only the speed of one pump. However: although in any case you should have a check valve on each pump all pumps are delivering the same head and it is no more needed in this situation than in a fixed speed installation.

 

[Artisi]

Are you discharging into a common main or does each unit have a dedicated discharge line which is not effected by the head from the other pumps.

Naresuan University
Phitsanulok
Thailand

 

[tjmurf]

Yes Artisi, all 3 pumps are discharging into a common line.

 

[Artisi]

If you are pumping to a common discharge line, the control of pump speed and flow becomes rather complex.
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

 

[BRIS]

One would first question why VFD drive pumps have been installed for pumping from a wet well?. The simple control solution is to dump the VFD and run the pumps at 100% controlled on well level in the normal way. Presumably the investment in VFD has been made for a reason?

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.

 

[itsmoked]

Hey BRIS; If the pumps all have the same head, (as is logical), and you are now actually just controlling flow, since you can't control via pressure do you just watch the well and make incremental increases as you mentioned based on the well level. Essentially inferring flow?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BRIS]

itsmoked - good point - I am more used to controlling pumps in irrigation systems (based on demand rather than supply) and my suggestion is probably much more complex than is needed . The pumps can be controlled on the wet well level simply by increasing/reducing the speed incrementally in response to high and low level sensors. There is no need to measure flow - only level is needed.

 

[Artisi]

If the pumps are to maintain the same head irespective of flow / speed, assuming they are being speed controlled via independent VFD's - you will need to monitor discharge pressure as well as inflow to the station otherwise, as you increase speed to suit inflow the head will increase as the square of the speed increase whereas, flow will only increase proportionlly - I think this means a a complex control system.You need to sit-down with someone experienced with pumping system and logic control systems.

Naresuan University
Phitsanulok
Thailand

 

[tjmurf]

itsmoked is right, flow rate is inferred by the level change in the wet well. Bris is right about responding to supply rather than demand. The question was however, to adjust the speed of the pumps together when more than one is required, or adjust the speed of one while the other(s) are at 100%

 

[itsmoked]

Thanks tjmurf; While you could adjust them all at once the standard is as stated 'one at a time'. You can probably imagine the nasty hunting, beating, and surging you could get into with all pumps varying.

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

 

[rcooper]

What is the purpose of making the pumps variable speed? The control system and starters for variable speed pumps are more expensive than fixed speed, so this must be justified.

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.

 

[itsmoked]

Nice points rcooper.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[katmar]

Many relevant points about the pumps and their control systems have been raised, but the pumps should not be regarded as a system on their own. You need to consider the variability of the inflow as well as the requirements of the outflow in terms of allowable variability. These factors are possibly more important than whether you attain the BEP on the pumps. The well volume and the hydraulic design of the piping must also be part of the mix.

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

 

[BRIS]

The control solutions are beating about the bush unless there is an understanding of what the system is trying to do and the reason for installing VFD pumps.
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.

 

[katmar]

BRIS, I think you and I are very much on the same wavelength here. I would not run three pumps at 50% each - as you say we would then shut one pump off and run 2 at 75% each. This would bring the pumps closer to the BEP. The difficult part is selecting the criteria that will be used to decide when to bring in or take out individual pumps. Can it be done just on speed, or do we need to combine speed with well level and maybe flow too?

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

 

[BigInch]

The problem is one pump at BEP opearating rpm produces BEP flow and pressure, a second or third pump operating at a lower speed produces a lower flow and higher pressure. The higher pressure output of the 2nd pump causes the flow in the 1st pump to drop and the lower pressure output of the first pump causes the 2nd pump flow to increase. Now add a third. The net result is that it doesn't make much sense to vary the speed of one pump alone, because the net output of all the units will tend to track the unit running on variable speed. You might as well have all units running at equal speed. That's what they try to give you anyway.

 

[itsmoked]

Does make some sense but you can't have a low enough flow with all three running cause you can't run them all at 10%.

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

 

[BigInch]

Take my word for it, this one ain't too interesting. My speciality is transient hydraulic and thermal analysis of pump and pipeline networks of crude oil, hot heavy crude and multiproduct pipelines and gas pipeline transient analysis. I have designed many configurations of pump and compressor stations. One was for the Venezuela Extra Heavy Oil Pipeine project where we had a oilfield treating and pumping station with 3 booster pumps and 3 mainline shipping pumps. Each group of both the boosters and the shipping pumps could be configured on the fly to run any number in series or parallel. As the oil, the pipeline and the dirt around it heated up, the viscosity went from somewhere around 2000 to 180 cP and the flow changed from laminar, laminar with a warm center, turbulent centerline flow with laminar annular flow to full turbulent. As the pipeline heated up, we had turbulent at the hot end and laminar at the marine terminal and everything in between, inbetween. To get a small cold amount flowing in the pipeline required 2 boosters in series and two shipping pumps in series. As the pipeline heated up we could slowly increase the flow over a period of a week by changing the pump configurations through various combinations of boosters and shipping pumps in and out of series and parallel configurations and adjusting the variable speed drives to fine tune the flowrate target for the runup. That one was interesting.

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.