Use of VFD's with a large pump

[npate9]

Hi all,

Through out my career experience, I've learned to operate the pump at its Best Efficiency point and that's how I size my pump. I have also come to learn that VFD's are great if you want to shift the pump curve down and save on energy. So with the help of VFD is it ok to run a large pump at a small flow (more description in next paragraph)?

Recently, my boss have assigned me a new project to me and that new process uses same raw material as one of our existing process. For the existing process, DCS system turns on the pump and supplies the raw material to the reactor. The reactor is charged with that raw material 4 times a day only for 20 minutes (at full production schedule). That pump is designed for 66 gpm at 131 ft. We operate that pump at its Best efficiency point which allows us to charge the reactor at 54 gpm. The new process uses the same raw material but different reactor. It requires us to charge the raw material at only 8 gpm. So if run both the reactor at same time, the max flow would be 62 gpm. According the pump curve 62 gpm is still with best efficiency point. If run only new process then flow rate is only 8 gpm. So I was wondering whether is it ok to put a VFD on that same existing pump and move the pump curve down so we can use same large size pump for a new process?

Just an FYI, I have considered adding a recirculation line from the pump discharge to the suction with solenoid valve but I don't want to run the pump at its full capacity if I don't have to because running the pump at its full capacity when its not need means waste of money.

I'm attaching rough drawing to better explain the situation. I want to thanks everyone in advance for their help.

 

[bimr]

A 60 gpm pump is not considered to be a large pump.

VFD's will not be able to reduce the pump capacity to less than approximately 50% without the risk of damage to the pump motor. A rule of thumb is 60-70% flow reduction with a VFD.

 

[MortenA]

Its unclear from your drawing how much of your head is the static head?

If the static head is a significant part of your total head there a risk that your saving will amount to very little, not justifying the reduced efficiency in the VFD and the motor+additional CAPEX. On the other hand if you situation is that you initially have a high level in your feed and at the end very low level - and this difference is big compared to the 131 ft - then it may be a good investment.

A switch from 62->8 gpm is a large reduction. As stated before by bimr this may not be achieable by use of VFD alone.

Another thing, its not a given thing that you can just add a VFD to an existing pump. The motor may not be able to cool sufficiencyly if the cooling fan of the motor run on the main shaft. Motor prepared for VDF may come with a cooling fan with its own motor if required.

 

[sshep]

Hey npate9,

The energy savings of a vsd is mainly the elimination (or reduction) of the control valve pressure drop. Not sure of your reactor pressure, so we dont know the control valve dP.

Calculate the annual operating cost via your electricity price and hydraulic horsepower/eff at 8gpm for 60 min/day (or whatever case), that is the basecase of your analysis. Compare to the cost of using minimum disharge pressure via vsd. Do an economic analysis for 10 Yr project life, this is your easiest route to decision.

It may be possible to trim the impeller for easy savings if your FCV is taking a high dP (low %open).

Do you run through the steam heater with the same pump as feeding the reactor?

Best wishes,
Sean