VFD Savings

[Dzboyce]

I do the well pumps for a 6,000 head dairy. They have 4 wells all about 600 ft deep. It pumps 500 gpm max. We have a 40 hp submersible on a Yaskawa drive, a 20 hp submersible on a drive, and a 10 hp submersible on a drive. The 40 hp is the primary pump and pretty much runs continuously between 55 and 50 hz. It is set to 60 psi. If it can’t keep up with demand, then the 20 hp sub comes when the system drops to 50 psi and then holds it at 55 psi. The 10 hp sub takes care of the half dozen residences and can be added to the feetlot drinking water system. This water system also provides the makeup water for the sewage system.

The cattle’s water is also routed thru the plate chiller for cooling the milk. It takes about 50 Gpm for the plate coolers. I’ve also got a fourth well at the milking parlor. It was originally drilled to provide the employee drinking water. But it is just a backup well now. The only time it is used is if there is a power outage.the milking parlor had an backup generator to keep the milking process going. Then this well is run off the generator to provide cooling water to the plate coolers. Cows have to be milked regardless of the power working or not.

 

[ptonsparky]

FWIW

The motor is running at 30.3 KW with a PFC installed and corrected to .93. Motor label indicates a PF of .84. The two 6" Clay valves dump into a 12" line. They can use fresh water to flush with if they have to. My SEWAG then would be that a 40 HP would be close to meeting the day to day needs.

I installed a 100 HP VFD last year. There is no room for that size of equipment in this building. No room for the existing PP for that matter.

Thank you to all.

 

[winnie]

If I understand things, you have a 100 Hp motor running at about 40 Hp, pumping with an output pressure of 96 psi, followed by pressure reducing valves that get the pressure down to 53 PSI.

Making the motor smaller might give you a little bit of savings; if you have a same speed motor, than a 50 Hp motor would sill use 40 Hp and pretty much use the same power input, possibly _slightly_ more efficiently.

Changing the pump or pump speed so that the output pressure drops will give you more savings. The pressure reducing valves are converting mechanical power into heat, and if you can reduce the pressure on the input side of the valves you will reduce this heat production. If you have 40 Hp of mechanical power upstream of the valves then you only have about 23Hp of mechanical power downstream of the valves, with 17 Hp going into heat.

-Jon

 

[Ingenieur]

If I understand things, you have a 100 Hp motor running at about 40 Hp, pumping with an output pressure of 96 psi, followed by pressure reducing valves that get the pressure down to 53 PSI.

Making the motor smaller might give you a little bit of savings; if you have a same speed motor, than a 50 Hp motor would sill use 40 Hp and pretty much use the same power input, possibly _slightly_ more efficiently.

Changing the pump or pump speed so that the output pressure drops will give you more savings. The pressure reducing valves are converting mechanical power into heat, and if you can reduce the pressure on the input side of the valves you will reduce this heat production. If you have 40 Hp of mechanical power upstream of the valves then you only have about 23Hp of mechanical power downstream of the valves, with 17 Hp going into heat.

-Jon

the 40 hp includes the pressure drop losses
which are substantial

valve drop 43 psi ~ 100 ft hd
if we assume piping losses at 25 ft hd and assume pump eff at 0.7
40 = Q x 125 x 8.34)/(33000 x 0.7)
Q ~ 900 gpm

if valve drop is gone
P = (900 x 25 x 8.34)/(33000 x 0.7) = <10

so removing the valves (reducing their drop) changes hp from 40 to 10
huge savings 200 kwh/yr

need pump curve
need system curve

pump should be sized for ~ 1000 gpm at system losses and valves used to trim 10 ft hd or so vs the current 100

 

[Besoeker]

FWIW

The motor is running at 30.3 KW with a PFC installed and corrected to .93.

The question is about VFD savings. You can't normally add PFC to the motor side of a VFD.

 

[ptonsparky]

The question is about VFD savings. You can't normally add PFC to the motor side of a VFD.

We added the PFC a few years ago to help with utility billing. That PFC unit will disappear if a VFD is ever installed. This thread is related to another I had about power readings on my Fluke 43B. The trick, as you have suggested in the other, will be to add PFC and make it all work with whaterever harmonics I have or will introduce.

 

[Besoeker]

We added the PFC a few years ago to help with utility billing. That PFC unit will disappear if a VFD is ever installed.

Good man!

This thread is related to another I had about power readings on my Fluke 43B. The trick, as you have suggested in the other, will be to add PFC and make it all work with whaterever harmonics I have or will introduce.

Yes. Detuning is, in my experience, is generally required. To somewhat lower than fifth harmonic.

 

[Ingenieur]

are the valves automatic?
what controls them?

what is the avg gpm?

it looks like the pump can be turned way down
maybe to 10-20 hp
to reduce pressure from 125 to 25 ft hd (same flow)
sqrt(25/125) = new/60
new = 27 Hz, you may have issues that low

need pump and system curves before anything is done
it appears poorly engineered now, you do not want the same result after the changes

 

[Dzboyce]

In post #22, Ptonsparky indicated that the valves were clay valves. I’m going to assume he meant that they are Cla-Val brand automatic control valves.

https://www.cla-val.com/waterworks

I have used Cla-Val, Singer, and Nelson automatic control valves. Depending upon the pilot controls, they can be a simple on-off, pressure reducing, ordsure sustaining, flow control, or a number of other function control valves.

it sounds like in this application, the Cla-Val is being used as a pressure reducing valve, however we Do not know that for a fact. We do know that because the Cla-Val is introducing a restriction in the discharge, the Pump curve is moving to the left, and consuming less horsepower than FLA.

Motor FLA, can be reduced mechanically by restricting the discharge of the Pump, or it can be reduced electrically be changing the speed of the pump. Either method changes where on the Pump curve the pump is operating.

my SWAG is that there might be a maximum amperage savings of 10% by installing a VFD and eliminating the control valves, it would make much more sense in the long run to replace the pump and motor with a more properly sized one.

 

[Ingenieur]

what we don't know is everything
flow or pressure control?
manual or automatic?
pump curve
system curve
flow?
valve position? 0% open 50? 100?

really all the stuff we need

they could be throttling the valve to regulate flow
he says in the op that they are prv's, imo self regulating
the high dp is just a consequence of poor sizing
if the valve is fully open a vfd won't help

 

[Besoeker]

whrn you double the speed of the impeller;
you double the flow,
you square the pressure,
you cube the horsepower

That's why VFDs save money when running at reduced speed on centrifugal pump applications.
Yes, yet those simple points seem to be getting missed in favour of convoluted calculations mostly based on assumed operational parameters.

 

[Ingenieur]

something does not compute

looked at Cv values for a 6" globe (vs angle) clay valve
they use the same body for auto, prv, etc
Cv = 440
from their chart 40 psi dp = 2800 gpm
and the valve is ~70% open

does the pump feed BOTH valves?
that be at least ~ (5600 x 100 x 8.34)/(33000 x 0.7) = 200 HP

 

[Ingenieur]

Yes, yet those simple points seem to be getting missed.

no one is missing it
it's been stated a few times
but if you can only slow the pump a few Hz to deliver the same Q and P it gets you zip

this is a mechanical engineering issue, not electrical, and imo a vfd is a patch, not a solution

it looks like pump is too large and valves too small (pumping at 40 out of 100 hp with 100 ft hd drop on the valve)
2 x 6" valve area <<56 sq in, feeding a 12" line ~ 113 sq in
put a pump in with the same flow at -100 ft hd as the existing
adjust prv's

he's meeting his needs now with 40 hp
a large (most?) portion of it wasted across the valves

let's say he can slow the pumps to reduce pressure from 90-60
and the prv still outputs 50 (drop reduced from 40 to 10)
assume constant system loss of 30

P1/P2 = (H1/H2)^1.5
40/P2/(120/80)^1.5
P2 ~ 22 HP ~ 49 Hz

based on the limited info if the mechanicals were sized properly a 10 HP may do

again
need basic info
pump curve
system curve
control method
layout, 1 pump > 2 valves?

I assume the prv's keep a header pressurized and float valves or something keep the cow troughs full, but demand is flow/qty, not press

 

[Ingenieur]

what we know
40 hp
96 - 54 = 43 psi drop
final head is 54 psi or 125 ft hd
pump does not care if loss is dynamic thru a valve or static lift up grade

40 = (Q x 125 x 8.34) / (33000 x 0.7)
Q ~ 900 gpm which is pretty consistent

this means valve loss
Q ~ 32 HP
which means 10 hp will do it

what does not jive is the drop of 43 psi across the valve
from clay valve this is ~ 2800 gpm EACH

 

[Besoeker]

no one is missing it
it's been stated a few times
but if you can only slow the pump a few Hz to deliver the same Q and P it gets you zip

this is a mechanical engineering issue, not electrical, and imo a vfd is a patch, not a solution

It's about VFD energy savings. That's why they are widely used for pumping applications.
I'm sure you must know that.

 

[Dzboyce]

A pressure reducing valve moves the point on a fixed pump curve left or right.

A Vfd moves the entire Pump curve up or down.

Much depends on the Pump curve for the pump in question.

a few years ago I drilled a replacement well for a frozen french fry processing plant. That plant’s entire production goes to the twin arches Far East restaurants. As I recall, the lineshaft turbine pumps for both the old well and the new well were 300 hp. The plants water needs were 1,000 to 1,700 gpm. The old pump was on a vfd and ran between 55 and 60 hz, depending upon water usage. The Pump in the new well had a pump curve better suited to a VFD. It ran between 48 and 58 hz for the same amount of water useage.

 

[Ingenieur]

It's about VFD energy savings. That's why they are widely used for pumping applications.
I'm sure you must know that.

not necessary if pump and valve sized properly
100 hp pump, 40 hp load (30 of it valve drop, ie, wasted)
control valve with 40 psi drop, should be ~ 5 psi
it's a patch and not the best choice for this application
get a pump with the same flow at ~80 ft less head
same flow, inlet 55 psi, out 50 psi
likely 10 hp

 

[Besoeker]

not necessary if pump and valve sized properly
100 hp pump, 40 hp load
control valve with 40 psi drop, should be ~ 5 psi
it's a patch and not the best choice for this application
get a pump with the same flow at 100 ft less head
likely 10 hp

Your speculation is noted.

 

[Ingenieur]

Your speculation is noted.

yours is dismissed, you are missing the forest thru the trees
mech sizing issue not elec
he gave us
hp
pressures
valve size
flow is moot since the same before/after change

 

[Ingenieur]

prv assembly https://www.cla-val.com/documents/pdf/E-90-01.pdf
valve https://www.cla-val.com/documents/pdf/E-100-01.pdf