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Discuss Two pumps connected in parallel runs continuously in the Plumbing Forum | Plumbing Advice area at PlumbersForums.net

Hello everyone, I'm a Plumber/ pump man from the Philippines. I need advice regarding pump problem. I dont know if this the right forum, but here it is. Two pumps connected in parallel runs continuously. What puzzled me is that whenever I partially closed main discharge valve or any of the discharge valve of both pump the pressure drops. Now this my first encounter with such a pump. Please refer to attached files. Is it perhaps a variable speed/ discharge pump that reacts to the pressure transmitter installed at the discharge. Pump is servicing a four story building. pressure is 70 psi drops to 50 to 60 whenever I partially close any discharge valve. Looking forward to your advice.
 

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rpm

Esteemed
Plumber
Grundfos CR10`s 12m head multi-stage booster pumps there. One speed non variable 3kw motors @ 3500rpm and no pressure transmitter on them. Don`t understand why you would want to throttle back the discharge or are you getting cavitation.
 
Grundfos CR10`s 12m head multi-stage booster pumps there. One speed non variable 3kw motors @ 3500rpm and no pressure transmitter on them. Don`t understand why you would want to throttle back the discharge or are you getting cavitation.
I'm just testing if the pump can still deliver more pressure. There is a pressure transmitter installed next to the pressure gauge.
 

Danb

Gas Engineer
Is that the boosted cold
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Does it feed a tank onntop of the four storys?
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Its trying to maintain a constant pressure.
When you close it do you only lose pressure on one side
 
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rpm

Esteemed
Plumber
What is this pressure transmitter, can you post photos of it please?
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I'm just testing if the pump can still deliver more pressure. There is a pressure transmitter installed next to the pressure gauge.
Is that the boosted cold
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Does it feed a tank onntop of the four storys?
Looks like it is in a workshop at the moment.
 
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Is that the boosted cold
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Does it feed a tank onntop of the four storys?
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Its trying to maintain a constant pressure.
When you close it do you only lose pressure on one side
Yes it is boosted cold
No it does not feed a tank on the top floor it has a pressure tank next to it.
Its connected in parallel so both lose pressure. Yes I think its trying to maintain a constant pressure
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What is this pressure transmitter, can you post photos of it please?
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Looks like it is in a workshop at the moment.
The transmitter has a wire that runs to the control board. That is why I was thinking that maybe " the control board sends command to the motor based on the transmitter reading" That is why I was thinking the motor is variable speed. I dont see an auto bypass valve either.
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there is only one pressure gauge on the discharge side of both pumps that sits next to the pressure transmitter.
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The pressure transmitter is connected to the pressure gauge with a tee and has wire that goes to the control board
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I think the reason it runs continuously is because its trying to maintain a constant pressure that does not go up to the set cut out pressure.
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I am sorry I think I had said something wrong. Please refer to the layout drawing I made as I try to explain. Actually the discharge valve1 is currently 20% or so open so like 80% close. Whenever I try to open it pressure drops. When I close main discharge valve pressure also drops. So yes I think its trying to maintain a constant pressure. But if it does, why does the pressure dont recover. I have watch there for like 2 min and the pressure never recover. How long does a pressure of such a system recovers? Whenever I close main valve or open valve1 pressure drops within a minute.
 

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rpm

Esteemed
Plumber
It won`t recover pressure, are you getting flow & pressure mixed up. A quick look at Grundfos site say`s outlet should be DN40 which I don`t think you`re showing in the photographs. You won`t get more out than the manufacture states by adjusting inlet/ outlet valves, if anything you will kill it.
 

Danb

Gas Engineer
Looks to me the the pressure transducer notices pressure drop and kicks selected pump in. When its satisfied turns them off.
If there no non returns with the head off water it pass back through pump 1 when you open valve 1.
Also whats the min and max pressure settings set at?
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The pressure tank next to it that you mentioned,
Should think this is more likely the cold feed for the pump to make sure it has volume behind it .
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Agree with rpm these are on off pump sets.
 
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rpm

Esteemed
Plumber
You mentioned increasing the pressure , you can buy a service kit for these if they are down on duty. I've recon these on site lots of times.
 
Looks to me the the pressure transducer notices pressure drop and kicks selected pump in. When its satisfied turns them off.
If there no non returns with the head off water it pass back through pump 1 when you open valve 1.
Also whats the min and max pressure settings set at?
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The pressure tank next to it that you mentioned,
Should think this is more likely the cold feed for the pump to make sure it has volume behind it .
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Agree with rpm these are on off pump sets.
two pumps run continuously 24/7. But you're right there is no non returns on the head off. I think you're correct with the pressure tank because its just a small one. Not the size to service an entire building. that solves one of my concerns. Excuse my ignorance but what is an "on off pump set"
 

snowhead

Plumber
Excuse my ignorance but what is an "on off pump set"
When the pressure drops below the cut in set point the pump(s) switch ON and run at their maximum speed.
When the pressure in the system reaches the cut off set point, the pump(s) switch OFF.

Modern pump systems are variable speed, so the pump(s) reduce speed to keep the pressure constant.

Have you checked that there is no air trapped in the pumps?
 

oz-plumber

Esteemed
Plumber
Quite a lot of variables with this one and it could be a situation where you start with the basics and go on from there.

1/ non-return valves on outlets of pumps
2/ pressure switches - faulty, not calibrated, not connected.
They are there, but not switching off the pumps - obviously not working as intended.
3/ Pressure vessel - shouldn't have much to do with the operation of the pumps or pressure switches on the pumps. It's only there to stop a lag in water pressure when the pumps are stopped and the pressure switch hasn't activated ( very basic description of what it does )
 

rpm

Esteemed
Plumber
Unless I`m looking at this all wrong I would expect to see two pressure gauges on the discharge pipes or one on the pipe after they merge together if they do this.
Also wondering if a photo of the control board might help.
 
There seems to be two different pumps fitted, one is 12.1 m3/hr, 47.2 M head @ 3550 RPM, the other nameplate shows a 50HZ pump, 2990 RPM, can't see the flow/head so if fitted in the philippines will run at 60 HZ so maybe thats fitted in position 1 and throttled in to prevent o/load trip out (pump affinity laws). If so, it seems a very strange match but whatever the set up, there should most certainly, as pointed out above, be NR valves fitted because not only will you get reverse flow through the pump, you can/will also get reverse rotation of the pump which will result in motor burnout on auto restart or if restarted without manually shutting the discharge valve.
 
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There seems to be two different pumps fitted, one is 12.1 m3/hr, 47.2 M head @ 3550 RPM, the other nameplate shows a 50HZ pump, 2990 RPM, can't see the flow/head so if fitted in the philippines will run at 60 HZ so maybe thats fitted in position 1 and throttled in to prevent o/load trip out (pump affinity laws). If so, it seems a very strange match but whatever the set up, there should most certainly, as pointed out above, be NR valves fitted because not only will you get reverse flow through the pump, you can/will also get reverse rotation of the pump which will result in motor burnout on auto restart or if restarted without manually shutting the discharge valve.
Edit: Maybe both 60hz pumps have 50 hz motors fitted and the installation is in the UK.
 
When the pressure drops below the cut in set point the pump(s) switch ON and run at their maximum speed.
When the pressure in the system reaches the cut off set point, the pump(s) switch OFF.

Modern pump systems are variable speed, so the pump(s) reduce speed to keep the pressure constant.

Have you checked that there is no air trapped in the pumps?
Ok so this is not a variable speed pump and I should stop looking from that angle.
I will try to air purge the pumps.
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Quite a lot of variables with this one and it could be a situation where you start with the basics and go on from there.

1/ non-return valves on outlets of pumps
2/ pressure switches - faulty, not calibrated, not connected.
They are there, but not switching off the pumps - obviously not working as intended.
3/ Pressure vessel - shouldn't have much to do with the operation of the pumps or pressure switches on the pumps. It's only there to stop a lag in water pressure when the pumps are stopped and the pressure switch hasn't activated ( very basic description of what it does )
You could be correct about all of these. But how does this correlate to the dropping of pressure whenever I make adjustment on valves that is suppose to increase pressure. If pump maintains speed and power closing outlet valve should increase pressure in between valve and pump where the pressure gauge (and transducer) is located, no exception. also opening valve one full (which is currently about 20% open) should increase pressure, but the opposite happens. What other explanation could there be, if there is any. other than the pump is "trying to maintain a constant pressure". You see I don't want to touch anything until I am able to connect all the dots. This is a hotel and I don't want to trouble the guests as that won't be good for the owners.
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Unless I`m looking at this all wrong I would expect to see two pressure gauges on the discharge pipes or one on the pipe after they merge together if they do this.
Also wondering if a photo of the control board might help.
I will take photo of the control board and more photo of the system when I go visit again. I am not hurrying with this one. I told the client that I wont touch anything unless I am absolutely sure of what to do and that they can hire someone if they want to.
 
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Danb

Gas Engineer
Without seeing the set up is hard to say for certain but It looks like to me that there for constant cold pressure.
As mentioned earlier.
Fit nrvs for a start.
Check min and max pressure settings, mechanical or digital control.
Make sure pump supply tank.is large enough to cope with demand,
As mentioned earlier you may get air in system if the tank runs dry And get pump issues.
 
Assuming both pumps running then yes shutting in valve 1 should increase the pressure reading on this pump but the pressure read by pump 2 should decrease as this is effectively reading system pressure.
If you are running at 60 HZ then a fully closed valve should give ~ 85 psi, if running on 50 HZ then the closed valve pressure should be 58.5 psi assuming full speed in both cases, as you mentioned 70 psi above then it looks like a 60 hz frequency or else pressure gauges are faulty.
 

Danb

Gas Engineer
The pumps may take breaks as well once pressure is stable.
Soon as a tap or showers run pumps kicks in on pressure drop.
Hotels are great bet there's leaks and cisterns overflowing into pans all over the place. Pumps I should think will end up running all the time
 

rpm

Esteemed
Plumber
Bleed valves are just under the motor & pump shafts coupling. These rarely suffer from trapped air due to the mulitstage design.
Very wise to be cautious untill you fully understand the pumps and the installation, if in doubt then walk away.
 
Thanks everyone for helping out. This one really puzzle me. I'm a guy who messes with everything and I can normally find my answers on my own. This is one of the few cases where I have to ask help so thank you.
There seems to be two different pumps fitted, one is 12.1 m3/hr, 47.2 M head @ 3550 RPM, the other nameplate shows a 50HZ pump, 2990 RPM, can't see the flow/head so if fitted in the philippines will run at 60 HZ so maybe thats fitted in position 1 and throttled in to prevent o/load trip out (pump affinity laws). If so, it seems a very strange match but whatever the set up, there should most certainly, as pointed out above, be NR valves fitted because not only will you get reverse flow through the pump, you can/will also get reverse rotation of the pump which will result in motor burnout on auto restart or if restarted without manually shutting the discharge valve.
Edit: Maybe both 60hz pumps have 50 hz motors fitted and the installation is in the UK.
Thanks Jhon, there are NR's on the suction side of the pumps, so I dont think reverse rotation can/will occur. The nameplates I publish is one for the pump and one for the motor. Motor and pumps are of the same model. I mean pump 1 and pump and motor 1 and motor2 is the same.
 
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Without seeing the set up is hard to say for certain but It looks like to me that there for constant cold pressure.
As mentioned earlier.
Fit nrvs for a start.
Check min and max pressure settings, mechanical or digital control.
Make sure pump supply tank.is large enough to cope with demand,
As mentioned earlier you may get air in system if the tank runs dry And get pump issues.
Another thing that does not look right is that the pressure tank is very small for the system. 30 gal or so. This are are digitally control. Did not see any mechanical PS. I will take more pictures.
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Forgive my insistence, But I wanted to correlate every suggestion to the phenomena of pressure decrease whenever I close outlet valve. Unless we can explain this phenomena its more of a hit and miss. I will go and gather more info and take more photo. Thanks again. But before I go. Does anyone have a suggestion on what to check? Also I wanted to make sure that this is not a variable speed system. How do I do that?
 
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snowhead

Plumber
Also I wanted to make sure that this is not a variable speed system. How do I do that?
Post the model number of the whole pump set, not the numbers of the pump or motor that you have posted already.
Post a picture of the front of the control box.
If it was variable, you would hear the speed of the motors increasing and decreasing as the system pressure varied and the pumps tried to match it.
There would either be an inverter box on the side of each motor of within the control box.
There would most likley be a display on the front of the box showing the speed.
 

rpm

Esteemed
Plumber
Stab in the dark - Is motor rotation correct and inlets & outlets of the pump the right way round?

Why put NRV on inlet side?
 

Danb

Gas Engineer
Check your nrv before pumps seen then fail before.
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Trying to up load pics of a system I see today says there to big.
Had transducers variable pumps and a large feed tank
 
Thanks everyone for helping out. This one really puzzle me. I'm a guy who messes with everything and I can normally find my answers on my own. This is one of the few cases where I have to ask help so thank you.

Thanks Jhon, there are NR's on the suction side of the pumps, so I dont think reverse rotation can/will occur. The nameplates I publish is one for the pump and one for the motor. Motor and pumps are of the same model. I mean pump 1 and pump and motor 1 and motor2 is the same.
Thanks but the motor nameplate(s) suggest 50 hz and the pump rating is based on 60 hz, can you say where these pumps are installed and the max running speed/pump curves can then be established.
Have to considered shutting the main discharge valve fully, very briefly, and the indicated pressure will then tell alot. You could also, again very briefly, shut each pump discharge valve fully and note the pressures in both cases.
The 30 gall. pressure (expansion vessel?) will probably act as a buffer/accumulator to give a steady pressure if pumps are variable speed controlled and if stop/start will reduce the frequency.
 

Danb

Gas Engineer
I imagine the nrv will be the type with a simple flap design.
I've seen the paddle flap damaged and corroded
 
What puzzled me is that whenever I partially closed main discharge valve or any of the discharge valve of both pump the pressure drops.
I might be missing something, but assuming the pump(s) are on either their P1 or P2 curves that's how they are intended to operate. They reduce their outlet pressure when the flow is reduced. This is usually an energy saving measure. The spec for the pump is here:

 
Excellent observation Chuck, smart pumps with proportional pressure (PP) control, so in maybe answering Julian's original question, the highest pressure is obtained by throttling one pump until the other reaches its max speed but the most economical way should be to run with both pumps unthrottled but this will give the lowest pressure and may not satisfy all the hotel guests but will also give the greatest redundancy in the event of any pump failure, the guy who set it up probably knew what he was doing.
 
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Post the model number of the whole pump set, not the numbers of the pump or motor that you have posted already.
Post a picture of the front of the control box.
If it was variable, you would hear the speed of the motors increasing and decreasing as the system pressure varied and the pumps tried to match it.
There would either be an inverter box on the side of each motor of within the control box.
There would most likley be a display on the front of the box showing the speed.
Thanks, I will take more photos and do I you said.
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Stab in the dark - Is motor rotation correct and inlets & outlets of the pump the right way round?

Why put NRV on inlet side?
Your stab is interesting. I'm sure at least one pump (pump 2) is working well because pressure is 70 psi. There could be something wrong with pump 1. I will also check for this.
The pump is drawing water from below. installing NRV on the discharge side will cause draining of water from the suction side when pump is on standby. Oh yeah, I remember there should be a foot valve on the cistern. added to check list.
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I might be missing something, but assuming the pump(s) are on either their P1 or P2 curves that's how they are intended to operate. They reduce their outlet pressure when the flow is reduced. This is usually an energy saving measure. The spec for the pump is here:

Thanks Chuck. If this is the case there should be a flow sensor. I did not see one. But I will check again.
 
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"What puzzled me is that whenever I partially closed main discharge valve or any of the discharge valve of both pump the pressure drops".
Assuming both pumps running then yes shutting in valve 1 should increase the pressure reading on this pump but the pressure read by pump 2 should decrease as this is effectively reading system pressure.
If you are running at 60 HZ then a fully closed valve should give ~ 85 psi, if running on 50 HZ then the closed valve pressure should be 58.5 psi assuming full speed in both cases, as you mentioned 70 psi above then it looks like a 60 hz frequency or else pressure gauges are faulty.
I'd like to stress that there is only one pressure gauge for the whole system. Please refer to the drawing I made.
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The pumps may take breaks as well once pressure is stable.
Soon as a tap or showers run pumps kicks in on pressure drop.
Hotels are great bet there's leaks and cisterns overflowing into pans all over the place. Pumps I should think will end up running all the time
Yep, leaks is one thing I am considering also. And if its high up will cause siphoning.
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Bleed valves are just under the motor & pump shafts coupling. These rarely suffer from trapped air due to the mulitstage design.
Very wise to be cautious untill you fully understand the pumps and the installation, if in doubt then walk away.
"if in doubt then walk away." yep that I will do. But hey this is the first time I encountered such a system with such a problem. An opportunity to learn and of course the excitement. I am not very good at control systems. But I was able to design and built a control system for a remote controlled gate I have constructed. I design and built the whole thing with a friend. Still running after two years. But looking at the control board of this pump system... I will need help with this also, i'm sure.
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Thanks but the motor nameplate(s) suggest 50 hz and the pump rating is based on 60 hz, can you say where these pumps are installed and the max running speed/pump curves can then be established.
Have to considered shutting the main discharge valve fully, very briefly, and the indicated pressure will then tell alot. You could also, again very briefly, shut each pump discharge valve fully and note the pressures in both cases.
The 30 gall. pressure (expansion vessel?) will probably act as a buffer/accumulator to give a steady pressure if pumps are variable speed controlled and if stop/start will reduce the frequency.
These are multi stage pumps capable of high pressure. do you think I can close the main discharge valve fully with out damaging anything.
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I might be missing something, but assuming the pump(s) are on either their P1 or P2 curves that's how they are intended to operate. They reduce their outlet pressure when the flow is reduced. This is usually an energy saving measure. The spec for the pump is here:

I'd like to stress my correction to the wrong info I provided. I did made the correction already but it appears that Chuck haven't seen it. Actually when I open valve 1 pressure decrease. And since pump 1 is also running. pressure must increase but the opposite happened.
 
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Danb

Gas Engineer
Sounds like you just lose your head when pump are off or valve closed. Imagine a vertical pipe pump push water to the to keeping pressure up. When pump of of valve closes allows water to settle under it's own pressure and drops abit.
Just a thought
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Might explain the pressure drop,
Because you don't lose all pressure do you just partial.
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Might explain the pressure drop,
Because you don't lose all pressure do you just partial.
 
I'd like to stress my correction to the wrong info I provided. I did made the correction already but it appears that Chuck haven't seen it. Actually when I open valve 1 pressure decrease. And since pump 1 is also running. pressure must increase but the opposite happened.
I haven't read the entire thread but that is what should happen if the pumps are in PP mode. When there is one pump running you have flow Vdot and head deltaP across the pump. Put a second pump in parallel then the flow per pump is halved so deltaP is also reduced. Depending on what's drawing the flow, that may also drop causing the pumps to reduce the pressure even further.

A smart pump doesn't need an external flow meter to operate, the flow is proportional to the pump RPM, which the motor controller knows. Also, don't forget that the 'pressure' referred to on pump curves is the pressure difference (hence deltaP) across the pump, not the outlet pressure.
 
Just thinking there again re these pumps, P1 and P2 refer to pump efficiency and pump+motor efficiency and grundfos give them on all sort of fixed speed pumps and even though pump pressures vs valve throttling can only logically be explained by some form of speed control I really wonder are these smart pumps like their much smaller domestic circulator cousins?.
 
Sounds like you just lose your head when pump are off or valve closed. Imagine a vertical pipe pump push water to the to keeping pressure up. When pump of of valve closes allows water to settle under it's own pressure and drops abit.
Just a thought
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Might explain the pressure drop,
Because you don't lose all pressure do you just partial.
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Might explain the pressure drop,
Because you don't lose all pressure do you just partial.
I'm pretty sure I still have my head in 1 piece. hahaha. Kidding aside I think the first step is to make sure this is a or not a variable speed intelligent pump system.
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I haven't read the entire thread but that is what should happen if the pumps are in PP mode. When there is one pump running you have flow Vdot and head deltaP across the pump. Put a second pump in parallel then the flow per pump is halved so deltaP is also reduced. Depending on what's drawing the flow, that may also drop causing the pumps to reduce the pressure even further.

A smart pump doesn't need an external flow meter to operate, the flow is proportional to the pump RPM, which the motor controller knows. Also, don't forget that the 'pressure' referred to on pump curves is the pressure difference (hence deltaP) across the pump, not the outlet pressure.
Thanks again, very nice tutorial on intelligent pumps but still does not explain the phenomena I have observed. Logically an intelligent pump system should minimize bill while giving the same pressures on the taps as the dumb pumps. That said an IPS must maintain same pressure on the taps at all times. I am imagining when all taps are close both pumps must be on standby. When one tap is opened primary pump kicks in at minimum speed and then increase speed as more taps is opened. When more taps are in service that primary pump cannot cope up, back up pump kicks in. If IPS lose pressure when closing discharge valve then its a dumb pump or in other words there is something wrong with it. The only thing I can think of is the control system starting from the transducer to the receiver/ interpreter. But I Will go and do as Snowhead suggested and perform same test as before to make sure. Now I am seriously considering that I indeed lose my head momentarily hahaha.
 
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Just thinking there again re these pumps, P1 and P2 refer to pump efficiency and pump+motor efficiency and grundfos give them on all sort of fixed speed pumps and even though pump pressures vs valve throttling can only logically be explained by some form of speed control I really wonder are these smart pumps like their much smaller domestic circulator cousins?.
Most probably yes because I still don't see any explanation to losing pressure when main discharged valve is closed other than the pump is trying to maintain a constant pressure.
 

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