Using orifice plate (or ball valve) with VFD?

[floben]

Good Day,

My team is evaluating the use of an existing pumping system with new equpment that will require less pressure from the pump. The existing pumps are controlled by VFDs; the first page of the attached curves shows the design point of 632 gpm @ 618'TDH. With new equpment installed, the discharge pressure from the pumps needs to be approximately 208' (same flow). Using the manufacturer's software, it appears the pumps would need to run at I've been however, it appears that meeting the design condition of 632 gpm @ ____' TDH would result in the pump operating at 2,470 rpm to get to the required flow and discharge pressure. It looks as though this condition would cause the pump to run right at the end of it's curve (see attached). My team could use your help with the following questions and any other suggestions you may have:

1) What is likely to happen if the pump runs near the end of it's curve?
2) Does it make any sense to try to add an orifice plate on the discharge piping and to increase the pump speed to get the pump closer to a more efficient operating point?
3) For item 2 above, note that the discharge pressure will vary on a daily basis by about 35 psi or so.

I'm concerned both about operating the pump at the end of its curve, and am just as concerned about tying to mess with having a pressure inducing valve or orifice plate fighting the the VFDs.

Thanks for your time and any help you can offer.

 

[bimr]

The pump manufacturers recommend that you don't operate in that region. The pump is not designed to operate continuously at the end of the curve, and doing so can lead to vibration, higher operating temperature, surging, and high bearing loading. You will have poor energy efficiency and the NPSHr increases which may also cause problems.

An orifice plate is also not a good idea as it will give you poor efficiency. An orifice plate is sometimes used as a bandaid to correct an improper installation, it is not something that you want to design into your system.

How old is the pump? You are putting in a new system with an old pump, and operating in a region that may cause problems with the pump.

The preferred solution is a new pump sized for the application.

 

[QualityTime]

I would agree with bimr comments. It might be worth checking with Goulds if they can provide you with a suitable impellor that will fit in this pump casing. It may be a long shot though