duty/assist/standby rotation of pumps

[kchayfie]

We are making some modifications to the control of the high lift pumps at a water treatment works to fix some problems with the supply to the reservoir. There are 3 pumps operating on a duty / assist / standby basis and a part of the solution was to fit variable speed drives to all three pumps and control the speed of the pumps to maintain a level in the downstream reservoir. I've been out of touch with the project for a while and but now I'm needed again to answer questions from the sub-contractor.

Apparently, our client has decided to fit VSDs to just two pumps (I told you I was out of touch!) and I need to tell the sub-contractor which pump operates as duty/assist/standby/etc and when to rotate the duty between the pumps.

I've been thinking about this for a while now and it feels like I've hit a brick wall. Can anybody offer advice on how to set the duty rotation of these pumps? The closest I've got is to set the fixed speed pump as the assist pump and rotate the variable speed pumps between duty and standby but I've been staring at the problem for so long I think I'm still missing something. Will this work? Is there a simpler solution? Will there be problems operating the pumps like this?

Thanks for your help with this - I really need to clear the mental blockage!

 

[Pulpboy]

First off it makes logical sense to have the VFD on the Assist pump, since it will save you the most energy and be varied the most. I would suggest the other VFD doesn't really matter where it goes.

If the duty (100%) pump fails then the spare is kicked in at 100% No VFD needed.

If the assist pump craps out 2 options

1: Use the Duty as the new assist and the spare as the duty. (VFD on the duty)

2: Use the Spare as the new assist (VFD on the spare)

My personal choice would be option 2.1 (slightly modified). Put the VFD's on the Duty and Assist and run them both a little slower. Less speed = less wear and both pumps will last longer. (duty at 80% : assist at 60% instead of 100%/40% etc) The spare acts as a spare and runs at the 100% mark when needed.

That's my 2 bits.

 

[katmar]

What dosenbro has written makes good sense to me. But you must see this question as part of a bigger problem, i.e. how do you run your maintenance schedule?

Long before we had reliable and reasonably priced VSD's there was always debate over whether to run standby and spare pumps regularly, or leave them idle until needed. The problem is that many plants used to use a "break down" maintenance schedule - if it ain't broke don't fix it. In this case, if you leave them idle they may not work when you need them, and if you run the spare regularly it will wear out at the same time as the duty pump.

My personal preference in your case would be to run the 100% pump as the duty pump and alternate (perhaps weekly) the assist and the spare. And put them all on a predictive / scheduled maintenance scheme.