VFD Savings

[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?

 

[Besoeker]

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?

Yes, you need head and flow.
That said, two months would be unusual in my experience.

 

[winnie]

Does the system run continuously?

Do you actually know the power being drawn by the motor?

You need to somehow figure the power being drawn to estimate the savings. Since hydraulic power is flow times pressure, you are correct that one approach would be to know the flow in the pipe.

But the electrical input to the motor is another approach. The motor is rated 100hp but is probably not being used at full capacity.

As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

So if the system currently uses 65 hp, then with the vfd it would use 53/96 * 65 hp.

This estimate ignores things like pump efficiency and pipe losses, and just focuses on the losses in the valves.

-Jon

 

[ptonsparky]

Does the system run continuously?

Do you actually know the power being drawn by the motor?

You need to somehow figure the power being drawn to estimate the savings. Since hydraulic power is flow times pressure, you are correct that one approach would be to know the flow in the pipe.

But the electrical input to the motor is another approach. The motor is rated 100hp but is probably not being used at full capacity.

As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

So if the system currently uses 65 hp, then with the vfd it would use 53/96 * 65 hp.

This estimate ignores things like pump efficiency and pipe losses, and just focuses on the losses in the valves.

-Jon

I should be able to handle that with my old FLuke 43B.

Renting a clamp on flow meter was my other option or measuring the tank and determining how many gallons the pump removes in x amount of time.

 

[don_resqcapt19]

Does the system run continuously?

Do you actually know the power being drawn by the motor?

You need to somehow figure the power being drawn to estimate the savings. Since hydraulic power is flow times pressure, you are correct that one approach would be to know the flow in the pipe.

But the electrical input to the motor is another approach. The motor is rated 100hp but is probably not being used at full capacity.

As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

So if the system currently uses 65 hp, then with the vfd it would use 53/96 * 65 hp.

This estimate ignores things like pump efficiency and pipe losses, and just focuses on the losses in the valves.

-Jon

Would that ever be high enough to give a reasonable ROI on the installation of a VFD?

 

[kwired]

Does the thing ever have circumstances where the load is closer to 100 HP? If not best investment would have been a smaller motor up front. If it is determined you do see max load of ~65 hp then that probably means you at least need a 75 HP motor.

Does cost of 100 HP drive justify that route vs changing to a 75 HP motor?

This is presuming a centrifugal pump also. I have a client with a positive displacement high pressure pump application (50 hp motor on this one) where the load does vary to some degree depending on what product they are putting through it, but their main concern was maintenance costs on some pump components, slowing the pump down yet being able to main needed pressure and flow made VFD attractive on this application. I think they run it between 40 and 50 Hz most of the time, but has lessened the replacement parts needed since we put the drive on it. When running at 60 Hz the pump parts are moving at faster speed and wearing out faster, to regulate pressure and flow they simply adjust bypass valves in the system, which they still do running on the vfd but it is a balancing act of getting the right pressure and flow but at lowest input speed as possible without overloading the motor.

This is pump that injects liquid product into a dryer, getting the right spray pattern into the dryer effects what kind of powder consistency comes out of the dryer, so pressure and flow rate through the nozzle is critical to final product, but not every product has same solids content plus temp, pressure and humidity of ambient has some impact and needs a certain amount of fine tuning every time they run it.

 

[winnie]

As a rough estimate the change in power consumption will just be the pressure ratio across the valves.

Would that ever be high enough to give a reasonable ROI on the installation of a VFD?

Let's imagine best case for the OP; system runs at a full 100 hp 24x7.

100 hp is about 75 kW. 1800 kWh per day.

Assume a 100% efficient pump; 75 kW of hydraulic power (pressure * volume) exits the pump.

Pressure is being dropped from 96 to 53 PSI After the pressure reducing valves, there is only 41 kW of hydraulic power

If we instead run the system at lower speed to get only 41 kW of hydraulic power out of the pump then we only use 984kWh per day, saving 816 kWh per day.

This is a best case back of the envelope, and it would give a pretty rapid payback.

As kwired noted, if the load isn't changing, it might make lots more sense to reduce how hard the pumps are being pushed, so that the pump output pressure is lower and there is less loss at the pressure reduction valves.

-Jon

 

[ptonsparky]

Does it run continuously? I’d have to say, yes. Especially now. 3000 Head Dairy.

 

[Ingenieur]

if running at constant flow and all that is required is a press drop resizing the pump is the best option

you can get a rough approximation without knowing flow since it will cancel when looking at a ratio of the power vfd/valve

since the dp is so high 43 psi ~ 100' a vfd may pay off

assuming piping losses ~ 50% of the valve or 50' head
we can calc an ~ flow assuming motor at 90%/hp, pump eff 60%
hp = q x 150 x 8.34 /(60 x 550 x 0.6) = 90
q = 1400 gpm

hp loss due to valve ~ 100 x 1400 x 8.34/33000 = 35 hp or $25k/yr ($0.1/kwh, continuous)

the best (only) way is to get the pump curve
plot the system curve with valve
apply affinity laws to get pump head to 53 psi/100' hd
compare the hp for both

 

[don_resqcapt19]

Let's imagine best case for the OP; system runs at a full 100 hp 24x7.

100 hp is about 75 kW. 1800 kWh per day.

Assume a 100% efficient pump; 75 kW of hydraulic power (pressure * volume) exits the pump.

Pressure is being dropped from 96 to 53 PSI After the pressure reducing valves, there is only 41 kW of hydraulic power

If we instead run the system at lower speed to get only 41 kW of hydraulic power out of the pump then we only use 984kWh per day, saving 816 kWh per day.

This is a best case back of the envelope, and it would give a pretty rapid payback.

As kwired noted, if the load isn't changing, it might make lots more sense to reduce how hard the pumps are being pushed, so that the pump output pressure is lower and there is less loss at the pressure reduction valves.

-Jon

Ok, I assumed that the reduction of pressure on the discharge valve also resulted in a reduction in flow.

 

[Ingenieur]

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

 

[winnie]

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

 

[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

 

[kwired]

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.

 

[Ingenieur]

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

 

[Cow]

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.

 

[Dzboyce]

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/affinity-laws-d_408.html

 

[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.

 

[Ingenieur]

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

 

[ptonsparky]

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.