Help needed for VFD proposal making for a pump 3

[coolbug]

I have been given the task of preparing proposal for VFD on few pumps to save energy. Now what information I can get is all pump & connected motor specifications and the actual running current. Kindly suggest me how to evaluate an appox. saving in energy by VFD installation???

 

[Artisi]

There is a wealth of information in this forum on VFD drives, I suggest you search out the information and educate yourself a little before coming back with questions that can be answered,not assumptions based on unknown information.
One of the first things you need to clear up, is there really any savings to be had by using VFD.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[stanier]

Check out the attached

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[coolbug]

Even if, on a precise level, there is on savings coz of installing VFD, i have to prepare a proposal for that.
Exact prob for me: 1. I don't have the exact discharge the pump is delivering.
2. I don't have the pump characteristic curve.
3. I don't have much people around to guide me.
I only have the running current and Rated current. How with help of it, I can estimate the energy consumption?

 

[LittleInch]

Coolbug,

The presentation by Stanier is an excellent summary of the whys and wherefalls of VFD design, especially slide 12, which summarises your issues very well. Also take account of Artisi's strapline - without proper data you cannot undertake this task properly.

VFDs are excellent bits of equipment and can save considerable amounts of money in the right location and usage, but cost more and have higher running costs in the wrong location.

VFDs are good for pumps which run at different flows and head duties for considerable periods, e.g. 10% @ full flow, 30% @ 75%, 40% at 60% and 20% at 50%. If you have this sort of data history for a "typical" day, week, month, then you can do some calculations.

Another key aspect where you might install VFDs is where you have issues over starting current. Although there are other ways of oing it, VFds provide a much smoother start current, avoiding the 5 - 8 times Full load cuurrent you get with a DOL starter.

One aspect to obtain from the manaufacturuer of the VFD is his system losses at different powers so that you can add those to the running current of the motor at the reduced speed.

with rated current and running current you can establish the maximum power of the motor and its normal running power. However without knowing the system and pump curve,it is not possible to establish what power saving could apply. Alternatively you could request the pump discharge pressure, flow rate, density of fluid and then pressure downstream of whatever is currently throttling your pump (probably some sort of control valve). Then you can work out the energy being "lost" which has been generated by the pump unneccasarily.

If you need an analogy to present to whoever has asked you to do this without giving you access to the data by the sound of it, then you could use something like - A car is towing a trailer up a hill of steady incline. What you are being asked to do is work out what the fuel consumtion would be without the trailer, but you are not being told the size and weight of the trailer. Therefore not possible to compute.

If you ever get the info, there are alternatives as ably stated by Stanier - such as reduced impellor diamter, control valves or a gearbox / fluid drive. In some cases such as high static head loads, VFDs are not suitable.

So in summary - No data - No calculation

My motto: Learn something new every day

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

 

[BigInch]

To have any chance to save money with VFDs, you must first meet 2 conditions,

1.) You must have a system curve H vs Q that follows a curve similar to H = K * Qⁿ, where K and n are constants, Q is flowrate, H is head.

2.) You must operate at variable flowrates outside the range of 80% to 110% of your pump's BEP flow for more than 50% of the total pump operating time, with half of that time spent operating between 50% to 70% of BEP flowrate.

If you don't meet those two criteria, the chances you will save any money at all are very, very small.

If you are still continuing after that, calculate the savings at each flowrate you will have if you operate with a VFD verses a control valve, (including constant efficiency loss of 5% for operating a VFD, and cutting both VFD and motor efficiency for partial load efficiency losses from 95% to zero between flowrates of 50% to 0% BEP, respectively) in $/hr, then multiply the savings at each flowrate by the percentage of total pump operating time you will spend operating at that flowrate. Sum them up and multiply by the number of hours you will operate this year. That's your yearly savings. Add the cost of a control valve/year and subtract the cost of VFD/year. For that I use a simple cost of 1/2 the valve cost/10, or for the VFD, the total cost of the VFD divided by 10 years.

Still going for VFD now?



Independent events are seldomly independent.

 

[BigInch]

If you don't meet condition 1, forget this exercise.
If you meet conditions 1 & 2, and the VFD does not save you money, then reduce the system's maximum design flowrate capacity and start again.

Independent events are seldomly independent.