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Originally Posted by don_resqcapt19
Ok, I assumed that the reduction of pressure on the discharge valve also resulted in a reduction in flow.
look at it like this
you have a valve partially closed, as you open it (reduce press loss) flow increases

you are correct if you consider an oriface
as the pump slows down, pressure drops, as does flow

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Agreeing with ingenieur here.

The pressure reducing valve has reduced the flow from what it would have been if it were not there. It is like a linear voltage regulator in a series circuit; adjusting its resistance to maintain a constant output pressure.

The flow through the pipe is the same from one end to the other, again like a series circuit.

If you remove the pressure reducing valve, you would then somehow reduce the pump output to get the same flow/output pressure at the destination.

-Jon

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a picture is worth a 1000 words lol

a pump curve
heavy black line = pump performance curve
as head increases flow decreases (as does hp at bottom)

thin blue line is the system curve, basically friction loss vs flow
derived from pipe length, friction, fittings, etc
as flow increases so does friction loss

the point they intersect is the operating point
100 ft hd, 1000 gpm, ~30 hp, ~84% eff
(100 1000 8.34)/(60 550 0.84) = 30.1 hp

valve influence
close valve system curve shifts left, open shifts right, always origin = 0

vfd influence
shifts the pump curve up/down
if speed = 1/2 flow drops 1/2

green line = eff
Last edited by Ingenieur; 07-04-18 at 06:37 PM.

4. There is going to be some energy lost as heat in the valve/orifice correct? I don't know how significant it will be.

Energy needed to pump a specific flow rate should remain the same.

We used to heat a tank of lecithin (gets too thick to pump around room temp) by recirculating water through the heating jacket of the tank. Just the friction of moving the water through the system was enough to work, if we needed a quick boost in heat they would stick the steam hose in the balance tank and give it a shot of heat.

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tried to show the change from valve to vfd
removed 50 ft hd from system curve
lowered (slowed) pump curve to achieve same 1000 gpm
est new eff ~ 0.65

hp = (25 x 1000 x 8.34)/(33000 x 0.65) = 10 hp
pump speed ~ 700/1000 (based on original system curve) 70% or 42 Hz

6. Cow
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Originally Posted by ptonsparky
Customer with a 100 HP motor pumping from a large tank to two pressure regulating valves. Input pressure to the valves is 96 PSI. Output is 53 PSI on the load side of one of the valves. Assuming for the moment that they are both set the same.

The only restriction on the inlet side is the short pipe connecting the centrifugal pump to the tank.

The VFD savings calculator I found indicated payback was in less than 2 months by just changing the speed(s) for x amount of time for each. That would be a quick sell.

Seems to me that I would also need to know how much water is actually moving at that 53 PSI in the 6"? pipes leaving the PR valves. What other values do I need to make a better SEWAssumptiveG in cost savings for a customer?
Do you need variable speed to maintain a strict pressure setpoint?

Or would it be simpler to drop the motor/pump sizes and keep it on a starter if maintaining a precise pressure setpoint isn't that important?

Nema starters are reliable and considerably cheaper to replace versus a VFD.

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Originally Posted by Ingenieur
a picture is worth a 1000 words lol

a pump curve
heavy black line = pump performance curve
as head increases flow decreases (as does hp at bottom)

thin blue line is the system curve, basically friction loss vs flow
derived from pipe length, friction, fittings, etc
as flow increases so does friction loss

the point they intersect is the operating point
100 ft hd, 1000 gpm, ~30 hp, ~84% eff
(100 1000 8.34)/(60 550 0.84) = 30.1 hp

valve influence
close valve system curve shifts left, open shifts right, always origin = 0

vfd influence
shifts the pump curve up/down
if speed = 1/2 flow drops 1/2

green line = eff
Ingenieur;

there is is a difference between a positive displacement pump and a centrifugal Pump. As I’m sure you know.

a centrifugal Pump designed for 60 hz, will barely develop any pressure or flow at 30 hz. However a positive displacement pump will still deliver full pressure, but only half the flow at half speed. A centrifugal Pump usually need to be run above 45 hz to develop any meaningful flow or pressure

the Pump affinity laws state
whrn you double the speed of the impeller;
you double the flow,
you square the pressure,
you cube the horsepower

https://www.engineeringtoolbox.com/a...aws-d_408.html

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Ptonsparky

since you mentioned that this was in a 3,000 head dairy. I have to assume it is for the effluent system. The potable water system is probably only a few hundred gallons per minute.

For our non-agricultural members, Many larger CAFO’s (Confined Agricultural Feeding Operations)have a flushing type manure system. Only they have to flush the floor of the entire building.it can take well over a thousand gallons per minute for this system. Much of the water is recirculated. It is not a one pass system.

it sounds like a much larger than needed Pump was installed, a 40-60 horsepower Pump would be capable of what the existing 100 hp pump is doing, if they never need any more pressure. Yes the best solution would be to replace the 100 hp pump with the correct size Pump. Yes there will be some energy savings with a VFD, but not as much as you might imagine. Why? With the throttling valve you are moving the Pump curve to the left and decreasing the amps the motor is using.

It seems counterintuitive to most people, but as you close a discharge valve on a centrifugal Pump, the amps go DOWN, not up. The amps can drop almost 50% if you fully close the valve. Now as you throttle a pump the amps go down, but the amps per gallon pump go up as you move away from the BEP (best efficiency point). That is why the most efficient way is to pick a pump that is running at its BEP.

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it all depends on the pump curve and system curve
the pump may operate at 30 Hz but has lost so much pump eff and motor eff it is not beneficial

typical curve 30-60 Hz

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Originally Posted by Dzboyce
Ptonsparky

since you mentioned that this was in a 3,000 head dairy. I have to assume it is for the effluent system. The potable water system is probably only a few hundred gallons per minute.

For our non-agricultural members, Many larger CAFO’s (Confined Agricultural Feeding Operations)have a flushing type manure system. Only they have to flush the floor of the entire building.it can take well over a thousand gallons per minute for this system. Much of the water is recirculated. It is not a one pass system.

it sounds like a much larger than needed Pump was installed, a 40-60 horsepower Pump would be capable of what the existing 100 hp pump is doing, if they never need any more pressure. Yes the best solution would be to replace the 100 hp pump with the correct size Pump. Yes there will be some energy savings with a VFD, but not as much as you might imagine. Why? With the throttling valve you are moving the Pump curve to the left and decreasing the amps the motor is using.

It seems counterintuitive to most people, but as you close a discharge valve on a centrifugal Pump, the amps go DOWN, not up. The amps can drop almost 50% if you fully close the valve. Now as you throttle a pump the amps go down, but the amps per gallon pump go up as you move away from the BEP (best efficiency point). That is why the most efficient way is to pick a pump that is running at its BEP.
It is for their potable water.

The flush system uses very little additional water.

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