Pressure control with VFD on existing Booster Turbine pumps? 1

[uranotek]

I am Electrical engineer looking for some hydraulic/mechanical advice on convenience of implementing pressure control by VFD in (3) 40 HP vertical turbine pumps.

The brand new booster Pumps station has never being in service for 10 years as city customers complain about the high rate/pressure releases a lot of sediment from the century old pipeline system.Other issues arise as potential leaks / ruptures in the pipeline. The older pumping system is still operating at a lower pressure.

The main contractor did not install any water supply pressure control devices such as VFDs, Backpressure valve, Recirculating control valve or reducing impellers diameter.

The motors are controlled through PLC and standard soft-starters/contactors I need to justify technical and economically the high cost of 3 VFDs

My questions are:

1- Considering the affinity laws (flow vs RPM ) and ( flow vs HP) What kind of consideration/limitations exist by implementing pressure control based on VFDs?

2- How to find out the min and max. permissible variation of Pump speed (rpm) from the pump's efficiency curve rated at 1780 rpm. In my research I found out some issues might appear on speed control affecting the pump and/or the motor efficiency, energy cost etc.

From the curve I checked that 2 pumps need to run (in parallel) to meet the target Head and GPM.

 

[cvg]

this has been discussed dozens of times in this forum. Review the existing threads and see if you find your answers.

http://eng-tips.com/search.cfm?action=Search&pid=407&requesttimeout=75

 

[BigInch]

Your minimum RPM2 must still develop the head you need to move Q2 flow at that minimum rpm.
H2 = H1/1780^2 * RPM2^2
Q2 = Q1/1780 * RPM2

Your maximum rpm must not overpressure the system and still deliver a flowrate at rates that your system can handle, ie must not cause waterhammer if a valve is closed, have a flow velocity too high, draw too much power, must not cavitate the pumps, etc.

If you have a static head, pay close attention to H2 at RPM2 and be sure you still get the flowrate you need. The pump efficiency may not be the same as what you see on the pump curve.

**********************
"Being GREEN isn't easy" ..Kermit the

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[uranotek]

Thanks cvg and BigInch for your feedback, I already did some research in these forums but I couldn't find advice on using the efficiency curves for predicting/limiting VFD min. and max. performance from an Electrical engineering perspective.

Apology if I write/ask thinking about some other Electrical engineers facing similar situations which could be boring or overstated for mechanical folks.

The pump manufacturer just confirmed what BigInch said, the efficieny curve will change as the VFD controls the pressure and the curve needs to be "modeled" for different RPMs. The one I got is only for 1780rpm, as I don't have a modeling software I will start using the formula BigInch just provided or team-up with a mechanical designer.

Questions:
A- Is it advisable to implement a combination of pressure control devices? i.e VFD speed control + installing a new 10" Back Pressure control valve + recycling loop?.
NOTE: The discharge header for the 3 pumps is buried, not easy or cheap to access in order to install a PCV.


B- If the answer is yes, then how the VFD and PCV pressure setpoints should work?
I would assume the VFD does the coarse control and PCV should be doing the trimming (by having a lower pressure setpoint and assuring recycling if pressure drops low)?

 

[cvg]

you mention the problem is that sediment in the water is a problem when the boosters are on. This is due to the higher pumping rate of your booster station. The higher pressure should not affect sediment, but may cause the older pipes to leak more.

If you can't increase the pressure for fear of leaks and can't increase the flow rate because sediment is a problem, then what's the point of running the boosters? (if you don't replace the 100-year old water mains first)

 

[uranotek]

Good points cvg. I am contractor, however some reasons by the City water company are:

1-Old pumping system has more than 50 years. Maintenance and operational costs, unmatched new standards plus parts availability are weighing in.

2- Lack of funding to retrofit pipelines, as projects' budget has shrunk and there is a regulated price on the city water, plus increasing agencies and government regulation.

3- Reduce operational costs, extend motors life etc. once the new booster system works by econmizing power consumption. We al know this topic has been widely discussed in eng-tips.

4- Optimize the engineering of the new booster system. It makes no sense a booster system with no pressure control even for a new pipeline system.

 

[BigInch]

I doubt it would make any sense at all to use a VFD and a control valve both controlling pressure.

Its possible that a recycle line could be needed, if for some reason the pump had to run a long time at relatively high rpm before it generated enough cooling flow when it finally opened the check valve into the downstream pipeline running up the side of some 6000 ft tall mountain. Otherwise I doubt that you need it. When flows are very low and rpms are high, is when you need some recycling ability.

**********************
"Being GREEN isn't easy" ..Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com/

 

[ccfowler]

Running pumps in parallel driven by VFD's is an invitation to stability problems. I would be much more inclined to drive the pumps with conventional induction motors and use the PLC to control a common control valve (and a common recirc loop, if needed). In nearly all cases (similar to what you have described), this arrangement can be expected to provide both better stability and less net energy usage than VFD's. There are applications where VFD's are an excellent choice, but this is not likely to be one of them.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[uranotek]

I agree a single PCV can simplify the pressure control.
I assume VFDs stability problems can be handled in Auto-stagging configuration when the lag pump can handle pressure control variations after the Lead pump has reached top rpms to meet demand.
I have done it before with Gas booster units running on VFDs. However, I would not imply that the same stability behaviour will occur here.

I appreciate your valuable comments

 

[ccfowler]

The pumps actual performance curves are very important. Each pump will always follow its curve including the adjustments for shaft speed. The seemingly minor shaft speed variations with load with a simple induction motor drive actually can be significant enough to contribute to stability with pumps in parallel if the pump curves are less than ideal:

A common situation would be where two (or more) seemingly identical pumps operate nicely together for a while and then some piping system transient gets one pump to hog load a bit. Its shaft speed slows a bit, and its head then drops a bit then another pump that had shed some load speeds up a bit and hogs some load, and the party get going!

Now add multiple VFD's trying to impress some sort of control on this circus, and it is most unlikely that stability will improve.

Careful choice of pump curve characteristics is important to having multiple pumps play nicely together. The old advice to have curves where the head rises continuously to shut-off is valid but can be asking a bit much. It is most important to make sure that all of the pump curves have suitable characteristics throughout the entire range of operating conditions that can reasonably be expected--including start-up and shut-down.

Pumps operating in parallel with well chosen curves and driven by simple induction motors can be a model of inherent stability. Add VFD's to these same pumps and motors, and stability can quickly become a very precious commodity.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[uranotek]

Thanks all of you. I got a solid perspective of different scenarios to be discussed with the water company.

 

[stanier]

Check out the attached re VFDs and the falacy of saving energy. The notes may lead you to some other papers of interest.

 

[uranotek]

Stainer, thanks for sharing the presentation about myths and facts on VFDs & Pumps.