Breakeven point between throttling and VFD 1

[Yobbo]

In our powerplants a lot of pumps are (still) being controlled by throttling valves. Apart from the cooling circuits where the system load is mainly due to friction and hardly any static head, most system loads have got a rather large static head component. Controlling with VFD instead of throttling valves saves money, but it seems, due to a load dependant efficiency of the VFD, that in some cases throttling is still better. I guess the total costs of investment and power consumption must be considered. Is it possible to state in general, where the breakeven point between both controlling methods lies?
Any hints or literature references are welcome too.

Karel Postulart, The Netherlands
Nuon Power Generation

 

[BigInch]

First let's correct this statement, "Controlling with VFD instead of throttling valves cansave money, if done properly and under the right conditions, otherwise it costs more money.

My rule of thumb for the right conditions is around 50% to 85% of BEP pump flow. This is more true with static heads, which may easily up the low end of the range to 75%, or more, when more significant. The larger more capital intensive VFDs may drop the higher end to 80%, assuming all goes well with system harmonics.

Note that you must also spend a significant amount of time at the lower flowrates in order to make the VFD pay off. Occasional use at 35 to 65% BEP flow won't usually do it. This is somewhat harder to quantify on a general basis, but offhand I would guess that you would have to spend 50% of your time between 60 and 80% BEP. The main problem that rears its head when designing a new system is, if you're going to spend that much time in those low flowrates, you've probably picked your BEP wrong and revising the BEP downward somewhat negates the advantages of considering a VFD in the first place.

VFDs may have the advantage when a very wide variation in flowrate (in a uniform distribution pattern) are required, but then that would also indicate to me that you've got a poor process design, probably needing some tanks somewhere.

I think, if you can use them at all, you're likely to find their better application in your cooling loops. BFW systems are not a good place to begin looking at all; if you put them on flow control, you don't usually get the pressure you need all the time and, if you put them on pressure control, you don't usually get the flowrate you need.

Widely varying near normally distributed flowrates and a pressure requirement that varies with the flowrate squared should tip you off as to where to begin looking for VFD apps. But then again, you could consider multiple parallel pumps too. A pump running at BEP (or near) is the most efficient system you can have.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

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

http://virtualpipeline.spaces.live.com

 

[jistre]

I predict this one's gonna get fun with the VFD vs. no VFD argument. Where's my popcorn?

My thought on the matter is that if you have a system that is being designed to run at a given set point and not vary, then it is far better to properly size the pump for the application so that you may use the smallest throttling valve possible with the most efficient pump that makes economic sense, as it will outperform a VFD.

However, if you have a system that varies like a hydronic cooling or heating loop that has differing demands due to differing loads, then a VFD MIGHT save you money, but a properly sized pump with three-way valves and recirculation might still end up being a better deal.

Short answer is that it has to be evaluated on a case by case basis, taking into account all kinds of things like first costs, current electrical costs, future costs, total electrical usage for each method, hidden costs of side effects of each method, etc., etc.

Then you can make a decision on your particular installation.

 

[stanier]

This has been discussed in other threads.

Refer attached presentation

 

[BigInch]

There's no argument. For the most part it's a calculated result. The subjectiveness and errors only show up when you're too lazy to do a complete calculation and start estimating certain effects on the basis of inexperience.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

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

http://virtualpipeline.spaces.live.com