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]

Wow BigInch; sounds like way more fun!

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

 

[BigInch]

Smoke,

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.

 

[itsmoked]

Big; I'm not sure if they are variable. They seemed to me to be running full-out. I stood next to one that was running. It was in a fire cell. There was a standard black fuel meter/counter mounted on the turbine with big digits on it. Diesel 2 at a third of a gallon a second was clicking by. Twenty gallons a minute... 30,000 gallons/day.
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

 

[BigInch]

Its not the DRA. Half are on standby because they don't have the oil to flow through them.

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.

 

[itsmoked]

I was told by two separate Alyeska engineers that the DRA was specifically used to allow them to shut off multiple pump stations. That was the only reason to bother with it as it has some serious downsides. Made sense to me. I do believe that it let them shut down something like 3 and now they have shut down more like 6, so there is more to it then that, as you say, less flow needed. Though they have moved 124MBBL so far this year, almost double normal. Hmmm, wonder if the price gouging has something to do with it...

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[BigInch]

Their engineers were giving you their daydream stories. Check out their website. They have the entire history of the pipeline on it. Look at the historical flowrates. DRA.. it ain't, that helps you flow more, not less.

Call before you dig.

 

[stanier]

Cronulla STP in Sydney has up to 7 pumps of different sizes with VFDs. The system manages to handle flows up to 5000L/s without problem.

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

http://www.waterhammer.bigblog.com.au

 

[BigInch]

Hello Stanier,

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.

 

[Artisi]

I think it is feasible to run identical pumps at different speeds to balance the inflow but is it all that practical -and it means a complex control arrangement - at the end of the day, is it really worth it.

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

 

[BigInch]

Sorry, its not feasible, if the pumps connect to common headers, or the suction pressure is otherwise held equal. Remember Hardy-Cross and the electrical network analogy, both say the head loss or voltage drop in any loop must be equal to 0. Start at the suction node, add the diff head of one pump and arrive at the discharge node, turn around and subtract the differential head of the other pump and you wind up with zero at the suction node. Balanced! Two identical pumps, running at identical speed, with identical suction pressure (connected to a common suction), will have equal differential pressures and therefore the flow must also be equal, otherwise there is a major problem with Bernoulli, Hardy-Cross and Ohm's law, OR the pumps are in a transient condition. My contention is that, yes you can let your pumps hunt each other, and allow them to do so with a bad control system, but WHY? Why not run and control them correctly?

 

[KiwiMace]

I have done two sewage lift station refurbs, one at 2 x Constant and the other with 4 x VSD. VSD's are great for refurbishments, as you never know what you are going to get for downstream conditions. VSD's eliminate surge risk. Also you get an improved throughput for a given sump size and number of permissable starts/cycles.

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

 

[BigInch]

Let's don't go too far with the benefits of VSDs. They don't exactly do anything to minimize surge when a station ESD valve slams shut on an oil pipeline.