VFD Savings

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Ingenieur

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winnie said:
Hmm. So you have at least 2 operating states. Normal where you are pumping via the pressure regulating valves to another tank, and 'flush' where you are pumping considerably more volume of water via much longer pipes.

The pump and motor may be sized for the 'flush' operation. Then for normal operation the pressure regulating valves are used for control.

The PRVs are in the system to provide control. The PRVs are analogous to linear voltage regulators. They function by creating a _variable_ resistance which gets adjusted to maintain the desired output. As linear devices any power not delivered to the load must be dissipated as heat in the device.

As Ingenieur has calculated, if you replace the pump and motor appropriately, you could reduce the pressure 'upstream' of the PRVs, getting the same output simply with the valves more 'open' (lower resistance) dissipating less energy in the valves. This would save energy while keeping the PRVs for control.

As has been pointed out, a VFD controlled motor could entirely replace the PRVs in terms of control. VFDs can be manually adjusted to get the required flow rate, or can be implemented with flow or pressure sensors to respond to changing conditions.

The benefit to using a VFD is much greater flexibility. If you need higher pressure or flow for some operations, then you have that available. On the other hand, using the pump well below its design point might make the pump much less efficient. The losses in the VFD and motor will be small; the efficiency hit to the pump might be quite large.

There are lots of issues here which could impact the end result...but you are talking some 20K of electricity used per year; getting an engineer involved is well worth your customer's money.

-Jon
Click to expand...

the nice thing about the prv you can set them a few psi > desired output
essentially no loss
it does not sound like accuracy is critical

I would have used 1/3 2/3 valves
6" and 10" or 8"
dp much lower
set the 1/3 at 53 psi for control of the 420 gpm dp ~ 1 psi <1/3 hp
when flushing press will drop, 1/3 goes full open, and the 2/3 will flow and control the balance ~1400 dp 2 psi 2 hp

losses <2.5
the vfd losses will be greater due to loss of eff
how low freq for 420 gpm on a 2800 gpm pump?
 
Besoeker

Besoeker

Senior Member
Location
UK
Ingenieur said:
did you figure that out by youself?
the vfd will need 2-3 hp more than a properly sized pump
Click to expand...
But those pesky energy wasting valves would be gone without the need to buy and install a new motor, new pump, new pipework, new cabling, new switchgear, and new protection devices.
I'm sure you can figure it out
 
I

Ingenieur

Senior Member
Location
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Besoeker said:
But those pesky energy wasting valves would be gone.
Click to expand...

sized properly 2.5 hp max for pump and valve

and the vfd would lose at least 1/6 x 22 ~ 4 hp due to lower pump eff
and another 1.5 hp running at < 20 Hz(may not move water)
5-6 losses vs 2.5
do the math
 
I

Ingenieur

Senior Member
Location
Earth
Besoeker said:
But those pesky energy wasting valves would be gone without the need to buy and install a new motor, new pump, new pipework, new cabling, new switchgear, and new protection devices.
I'm sure you can figure it out
Click to expand...

no new valves (would be better though)
no new pipe
no wiring only ol and cb
no new swgr

vfd
wiring
pressure xmtr
pid controller

100 hp vfd driving 100 hp pump running at 20-25 hp lol

savings with pump vs vfd
power 2-3k per year, possible more
lower capital cost, 3-4k
easier to operate and maintain
 
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