VFD vs. recirc line
VFD vs. recirc line
(OP)
I have a low flow pump application with a centrifugal pump discharging 5 GPM at 100' head. Currently, the flow downstream of the pump is metered by a control valve that adjusts its position controlled by the level inside the tank feeding the pump. The suction head to the pump is slightly above atmospheric.
The problem we are currently having is that the control valve cycles between completely closed and part to full open, causing the pump casing to overpressure and blow seals out. The pump is constantly run.
One of the ways to remedy this is to install a VFD to control the discharge head and flow rate, and leave the control valve full open in manual. The easier way is to install a recirc line with a smaller diameter than the discharge piping on the pump discharge to the suction line of the pump. The control valve will still operate the way it does now.
What are the drawbacks to a recirc line?
Also, is there a possibility of creating a feedback loop and blowing out the pump with a recirc line? For example, the control valve is closed and the pressure is all going to the recirc line, therefore the pump suction pressure is the same as the discharge pressure and the pressure keeps building as a feedback loop until it breaks.
The problem we are currently having is that the control valve cycles between completely closed and part to full open, causing the pump casing to overpressure and blow seals out. The pump is constantly run.
One of the ways to remedy this is to install a VFD to control the discharge head and flow rate, and leave the control valve full open in manual. The easier way is to install a recirc line with a smaller diameter than the discharge piping on the pump discharge to the suction line of the pump. The control valve will still operate the way it does now.
What are the drawbacks to a recirc line?
Also, is there a possibility of creating a feedback loop and blowing out the pump with a recirc line? For example, the control valve is closed and the pressure is all going to the recirc line, therefore the pump suction pressure is the same as the discharge pressure and the pressure keeps building as a feedback loop until it breaks.
-Mike





RE: VFD vs. recirc line
Is it a positive displacement pump? Why/When do the pump seals blow now, when the CV is closed?
The recirculation line method may use more energy when the pump is at lower flow rates. If running that 5 HP motor costs you a lot, and you run low flowrates a lot of the time, you might consider that VFD solution. The VFD will tend to save energy at lower flows, but it will not give you your 100 ft head at those lower flowrates. At 50% flow, it will only give you 100 ft / 1^2 * 0.5^2 = 25 ft of head. At 25% flow, it will be down to 6 ft. So a VFD might not work at all.
A recirculation line might be used down to about 20% flow and still give 100 ft head. Below 20% flow expect to begin running into overheating trouble.
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: VFD vs. recirc line
The relief valve you are talking about...do you mean a relief valve that would dump back to the tank?
-Mike
RE: VFD vs. recirc line
Sounds like this is not a well designed pump, if it can't even handle its own shutoff head pressure.
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: VFD vs. recirc line
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: VFD vs. recirc line
I don't see why the same scheme would not work for a centrifugal pump that requires 100 ft to deliver liquid into the next process. As BigInch says, a VFD will lower the available head with the pump speed. That is often not a problem, but it can be in some applications.
David
RE: VFD vs. recirc line
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: VFD vs. recirc line
The supply tank pressure is 15 psig, so I don't believe that is much of an issue as the throttling is.
Just talked to the planner and it looks like the VFD is out of the question for now (with all the red tape, cost approvals, etc.) to get it installed. I'll take a look at that Kimray valve, maybe something similar will help.
Also, in my OP I wrote "Also, is there a possibility of creating a feedback loop and blowing out the pump with a recirc line? For example, the control valve is closed and the pressure is all going to the recirc line, therefore the pump suction pressure is the same as the discharge pressure and the pressure keeps building as a feedback loop until it breaks." I always wondered about this. Can this happen?
-Mike
RE: VFD vs. recirc line
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: VFD vs. recirc line
since the flow regulating valve cycles, does it operate correctly?what type of level sensor you are using to control the flow valve?Is it a pneumatic proportional
system or just open/close?
the seals blowing out is caused by chock wave, rather than cut off pressure.
I think you should take a close look to the flow valve controlling mechanism and at least dampen its respond time.
RE: VFD vs. recirc line
You can either size the orifice by calculation of by experimenting.
RE: VFD vs. recirc line
I'm thinking about putting a 1" line off the pump discharge with a check valve and a backpressure regulator set a few psi below the shutoff head back into the tank.
-Mike
RE: VFD vs. recirc line
For this small amount of flow,(5 gpm) and the need for delivery over a range of conditions, I believe that the replacement pump should be a PD type.
It is possible to get a DC motor driven pump in this small size and get the full range of floew conditions.
Alternately, if you can afford the cost of compressed air, a larger Air driven Diaphragm pump can meet the 5 gpm at 100 TDH requirements.
My opinion only
-MJC