WTP VFD Question

[SliderR1]

I’ll start off by saying that this forum has some great information and thanks to everyone for sharing their thoughts. I’ve ran a few searches, but haven’t found specifically what I’m looking for.

I have been tasked with issuing a statement of work to replace four existing VFDs on high service pumps for a water treatment plant. After doing a little background investigation, I’m not so sure that VFDs are the best fit for this system. I don’t have a lot of experience in this sector of engineering and I’m basically limited to ‘free’ help from other engineers. If you guys could point me in the right direction, I can probably take the ball and run with it.

Here is some background on the system. The four main pumps are horizontal split case type rated at 500 gpm. They are driven by 50hp motors, controlled by old Square D Econoflex drives. I’m told the drives are obsolete and parts and service on them are no longer available. There are also two 350 gpm horizontal split case pumps driven by 40hp motors, no drives on these. These two pumps are rarely used because they have no deceleration capabilities on shutdown and if not valved off properly can cause line breaks out in the system – it has lots of 60 year old transite piping (basically these pumps are a pain to cycle compared to the other four and never get used).

The system is dominated by static head, approximately 200’. The 500 gpm pumps were originally designed for a TDH of 255’ and the 350 gpm pumps were designed for a TDH of 231’.
The lower limit on the PLCs for the big pumps is 80%, which I believe is set at the minimum to overcome static head. Currently at 80% the pumps are pumping roughly 200 gpm; which based on the pump curve, leads me to believe the impellers may be worn. At 100%, the four average around 460 gpm.
The utility would like to be able to supply water in the range of 150-1200 gpm. The average would be much closer to 300 gpm, with the upper limit used during peak demand and a major break occurring. All six pumps are plumbed to run in parallel, if needed.

I don’t believe that the two 350 gpm pumps’ curve is suited to VFD use based on the minimum head required for the system. Another issue with the smaller pumps is that the existing motors are not inverter rated and would need to be re-wound or replaced if coupled with a drive.

Would this be a good scenario to look at throttling valves, due to the high static head? It would need to be some type that could be controlled with the existing PLCs. Ideally the operator could dial up the required gpm on his screen and the valve would change as desired. I’d like to keep the system as cost effective and simple as possible.

 

[bimr]

You need to provide a schematic of some sort for the discharge piping. Is the pump station pumping to a reservoir, storage tank, or whatever? Your query can not be answered without an understanding of the system that the pump station is pumping into. Is there no friction loss when the pump station is operating?

 

[djs]

Many utilities are using a mixture of fixed speed and variable speed pumps. The caveat is that you must have at least one variable speed pump running at all times. Also the size of the variable speed pump must be at least as large as the largest fixed speed pump. So it should be possible to used the two fixed speed 3500 gpm pumps in conjunction with the four variable speed 500 gpm pumps. Also you do not need to replace all four variable frequency drives. You can go to say two variable speed drives and two soft starts. You will need a good pressure controller, a PLC or DCS system.

I do not advise using any type of throttling valve. It also sounds like the current system of staging pumps on and off can be improved. For example it is possible to stage a fixed speed pump on and off without experiencing water hammer.

I recommend talking with an experienced systems integrator or consulting engineer with ample experience in high service pumping. My preference is with a system integrator as many consulting engineers lack the practical experience of the systems integrators. Many of the better system integrators can provide professional engineering services.

 

[BigInch]

Get rid of the VFDs. There are so many reasons not to use them.
To make it short, and as you have already presumed, this is a perfect scenario for throttling valves, if you even need them. With so many pumps you can pretty much hit any reasonable flowrate you need without a throttling valve. Just turn the right number of pumps on or off. How critical is it that you flow 500, or 530 gpm?

Independent events are seldomly independent.

 

[SliderR1]

Pumping into four elevated 100k gal towers. Friction losses vary up to 55' at 500 gpm.

The average daily flowrate is about 250 gpm. Demand has lessened over the years from when the plant was originally built. Being able to pinpont a specific flowrate is not critical, the operators are only concerned with keeping the tanks at an acceptable level.

When breaks occur, the system experiences very fast losses - the four tanks can drain in less than two hours, hence the need to increase flowrate.

 

[LittleInch]

See this thread.

http://www.eng-tips.com/viewthread.cfm?qid=345887

go down a bit to my reply and look up the attached presentation which will give you a lot of backgorund info.

From your description you need to be able to draw a system curve as well as a pump curve, with your flow at 0 starting at 200 feet. Hence in reality your system curve is actually very flat and with centrifugals in parallel actually quite difficult to control accurately. Whilst I normally try not to use throottling valves due to wastage of energy, in this instance the actual loss will be low and if you need precise flow capability or rapid response to change in system curve then fixed speed motors will provide your course control and the valve, controlled by flow or pressure, will provide the fine control.

From the limited info ( apump curve would be good) it would appear that your pumps are actually OK as at 80% of speed?? you are probably barely making 200 feet so it's not surprising that your flow is really quite low. If flat out they're doing nearly their rated duty it is probbly OK, but a bit of wear is inevitable.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[bimr]

I will agree with the previous posts that a VFD is not warranted for this application. The VFD will not save any money. VFD's do not save money when you are pumping against a large static head.

You can obtain all of the flexibility that you require if you specify two (2) 150 gpm pumps, one (1) 300 gpm pump and two (2) 600 gpm pumps for a total of five (5) pumps. This pump arrangement will provide a rated plant capacity of 1200 gpm with the largest pump having a spare. The pumps should be designed to operate against the friction head when pumping at 1200 gpm.

I don't believe that there is any reason to use throttling valves. Since most of the electrical equipment appears to be old, it will be a wise investment to replace the electrical equipment.

 

[djs]

If you are pumping into elevated tanks, then no you do not need VFDs. Here in Florida most systems do not have elevated tanks, so need VFDs to control pressure.

You can stage the pumps on and off according to the water height in the tanks.

 

[hydrae]

Do the towers have just single pipe risers? (same pipe for both 'in' and 'out')
If so, that may be the reason for the VFDs, and the operators not wanting to use the fixed speed pumps.
There will be pressure waves in the entire system during pump start up and shutdown that will lead to damage on those 60 year transite pipes with fixed speed pumps lacking control valves.
If you want to get rid of any need for pressure control, either VFD or control valves, install piping that feeds the storage directly without direct connection to the distribution system and can handle the full value of the pressure waves during transients. Then the 5 pump system suggested by Bimr will work just fine. Otherwise, expect more overtime to repair the old brittle piping.

Hydrae