VFD Savings

[Ingenieur]

here's the issue based on what we know
valve drop ~40 psi, 100 ft hd, valve appears full open
hp = 40
meeting demand

if we slow the pump down we loose head and pressure
the valve can't open more
if you remove the valve you need to add controls

let's say we reduce pressure to 60/96 ~ 7 psi across valve
pressure is 60% of original
flow drops to 40% of original
demand not met

P = 0.6^^1.5 x 40 =19 hp
speed ~ flow ~ 60 x cubert(0.4)= 43 Hz
but pump eff will drop, so hp req may be like 0.7/0.6 x 19 ~ 23 hp or more

still only need less than 10 hp
get a pump sized for current flow at 60 psi, set prv ~ 50-55

I would do a real analysis before doing anything
you might make things worse

 

[ptonsparky]

I am guessing the water tanks are taped from the 12" & is distributed to the barns. At some point the 12" can feed the 4 flush tanks around the property and they do use it on occasion. Guessing again that once the valve to the flush tanks are opened we have very little pressure on the outlet side of the PRV valves causing them to go full open and the pump at full discharge or whatever the misc 6" and 800 - 1000 feet of 12" pipe and 30' tall tank would be for head.

Thanks guys.

I will suggest the VFD but also suggest an engineer be involved. (It won't happen.)

 

[Besoeker]

can't remove the valves, they regulate the pressure

You can and use a variable speed drive to regulate the pressure and eliminate the most of the 40hp that you say they waste.

 

[ptonsparky]

here's the issue based on what we know
valve drop ~40 psi, 100 ft hd, valve appears full open
hp = 40
meeting demand...

I see it as more closed than open. Full open would mean the flush is being used with large qty of water flowing. Until then the 40 Hp is more than enough motor to sustain the 50 lbs of pressure.

 

[Ingenieur]

You can and use a variable speed drive to regulate the pressure and eliminate the most of the 40hp that you say they waste.

it would be cheaper, capital, and running costs to leave the valves and size the pump properly
not to mention an easier to maintain and operate system

 

[ptonsparky]

it would be cheaper, capital, and running costs to leave the valves and size the pump properly
not to mention an easier to maintain and operate system

How would you size the pump differently?

 

[Ingenieur]

I see it as more closed than open. Full open would mean the flush is being used with large qty of water flowing. Until then the 40 Hp is more than enough motor to sustain the 50 lbs of pressure.

you may be right
total head ~ 98 psi, 225 ft
eff is low since pump is at 40% of rated HP, use 0.6

40 = (Q x 225 x 8.34)/(33000 x 0.6), Q = 420 GPM or 210 per valve
valve is linear with max flow of 4400 gpm at 100 psi, min 440 at 1 psi, Cv = 440 (prv suggested range is 10-1800 gpm, 1800 ~ 17 psi)
valve position ~ 210/4400 ~ 4.8% or Cv ~ 21
check: Q = 21 sqrt(98) ~ 210 gpm
you are correct, valve is essentially closed

so if he wants to maintain flow ~420 GPM and 12" pressure ~ 50psi, get a pump that puts out ~420 at 55 psi, valve drop 5 psi
the valves should be sequence, not set the same, one at 55 and one at 50, the 55 will control, the 50 closed, as demand increases press will drop to 50: 55 will be open, 50 will control
HP ~ 22, need 25 HP pump

 

[Ingenieur]

How would you size the pump differently?

see my previous post
caveat, based on limited info

 

[Besoeker]

it would be cheaper, capital, and running costs to leave the valves and size the pump properly
not to mention an easier to maintain and operate system

I think you knew that's simply not true when you mentioned wasted power.
When you're in a hole...

 

[Ingenieur]

I think you knew that's simply not true....

are you calling me a liar?
implying I'm saying something I know not to be true?
you have a lot of gall

you don't understand the mechanical system, and that's OK
you need to realize when you are wrong and in over your head

what does a 100 hp vfd cost, material and installation
and run at 20 HP, lol
vs.
a 25 HP pump that will only need a CB and OL

btw: more than a few said the best option is to replace the pump
a bunch of liars lol

 

[ptonsparky]

So I need 25 hp to maintain approx 50 psi during normal operation. I don’t think that is the argument. Will that same 25 give me the 1800 GPM, or whatever the 100 delivers, at flush?

 

[Besoeker]

are you calling me a liar?
implying I'm saying something I know not to be true?
you have a lot of gall

you don't understand the mechanical system, and that's OK
you need to realize when you are wrong and in over your head

what does a 100 hp vfd cost, material and installation
and run at 20 HP, lol
vs.
a 25 HP pump that will only need a CB and OL

btw: more than a few said the best option is to replace the pump
a bunch of liars lol

You have already pointed out that throttling valves waste energy. That's why variable speed drives gained such a foothold in pumping systems. To avoid that wasted energy.
Case closed.

 

[Ingenieur]

You have already pointed out that throttling valves waste energy. That's why variable speed drives gained such a foothold in pumping systems. To avoid that wasted energy.
Case closed.

wrong, for this application
as others have said

the drive will waste more
losses
moving pump to a lower eff

a new pump can be sized for max pf, eff and no vfd losses

vfd is not a good choice, bad on bad

 

[Ingenieur]

So I need 25 hp to maintain approx 50 psi during normal operation. I don’t think that is the argument. Will that same 25 give me the 1800 GPM, or whatever the 100 delivers, at flush?

so normal flow is ~420
and peak when flushing is 1800?
does pressure need maintained when flushing? or only flow and pressure can drop

1800 x 35 (15 psi) x 8.34 /(33000 x 0.7) ~ 23 hp
should be fine for flushing
pressure will drop and both valves will go full open (900 gpm per valve press drop ~ 4.5 psi, leaves 10 for the rest of the system, should be fine)

 

[Besoeker]

the drive will waste more
losses

Almost 40hp?
Get real.

 

[Ingenieur]

Almost 40hp?
Get real.

simmer down lil' buck-a-roo :lol:
who said 40 HP?
that is where it is running now (with the valve loss)
vfd will require ~10% more power than a properly sized pump
2-3 HP

even a broken clock is right twice a day...still waiting on you

 

[Besoeker]

simmer down lil' buck-a-roo :lol:
who said 40 HP?

You did.

40 hp load , most of it wasted on valve drop

Post #53.
40 hp losses is about 30kW.
Lose the throttling valve, lose that 30kW of wasted energy, fit a VFD. Even operating at 100HP, its losses would be less than one tenth of that.

 

[winnie]

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

 

[Besoeker]

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

Well balanced post if I may say so.

 

[Ingenieur]

You did.

Post #53.
40 hp losses is about 30kW.
Lose the throttling valve, lose that 30kW of wasted energy, fit a VFD. Even operating at 100HP, its losses would be less than one tenth of that.

most as in 20-25 as calculated
you need to read AND understand