Two pumps connected in parallel runs continuously

[John.g]

Yes its strange alright to have a 50 hz motor and as well as that it can run on low voltages between 220 to 240 and high voltages of 380 to 415. I would certainly ask Grundfos if this was installed by mistake as it doesn't make much sense. Can you post a photo of the 60 hz motor nameplate.

I have put together a simple spreadsheet of the VSD which shows the effect of changing the speed which you can input yourself, you may find it interesting.

 

[Julian]

Yes its strange alright to have a 50 hz motor and as well as that it can run on low voltages between 220 to 240 and high voltages of 380 to 415. I would certainly ask Grundfos if this was installed by mistake as it doesn't make much sense. Can you post a photo of the 60 hz motor nameplate.

I have put together a simple spreadsheet of the VSD which shows the effect of changing the speed which you can input yourself, you may find it interesting.

Wow, interesting indeed, I am getting a total hands on education, another proof that formal education needs total overhaul. Our children need not waste too much time in school studying mostly nonsense.
Ok back to topic
First I wanna make sure that you did not mixed up constant pump speed and variable pump speed. Looking at the spreadsheet, briefly ( I need to go to work) I got the hunch that I should set the system at desired speed, corresponding to the building height, and then put it on "on/ off mode" so that it stops when no one is using the tap and ran on that sweet spot rpm whenever it does needs to ran.

 

[John.g]

Wow, interesting indeed, I am getting a total hands on education, another proof that formal education needs total overhaul. Our children need not waste too much time in school studying mostly nonsense.
Ok back to topic
First I wanna make sure that you did not mixed up constant pump speed and variable pump speed. Looking at the spreadsheet, briefly ( I need to go to work) I got the hunch that I should set the system at desired speed, corresponding to the building height, and then put it on "on/ off mode" so that it stops when no one is using the tap and ran on that sweet spot rpm whenever it does needs to ran.

No, I didn,t mix up the two, my spreadsheet (and graph) is dynamic and changes as long as you replicate (by inputting the speed) that the VSD will need to output at the CHANGING flow requirements for any given setpoint (SP) head.
If you set the VSD speed to give you just the desired static head (height) then the top floor will have zero flow, if the building height is say 40 ft or 17 psi you must then add on the required pressure required by the resident say 30 psi so the pump head must be 47 psi, say 50, so the top floor resident will have 30 psi available at his taps/shower and the bottom floor resident will have 50 psi available, pipe friction losses must also be taken into account so IMO the correct way to run this set up is to run on pressure setpoint and maybe also use some logic to start/stop the VSD when demand is zero but VSD is primarily designed for a constant varying demand. however, yes, you can pick and set a sweet point speed based on the "average" flow demand and start/stop the pump using the pressure transducer or simply installing a pressure switch, all very very interesting.
I have also modified the spreadsheet to give a little more info, it graphically now shows the pump+motor efficiencies, just use the flow (x axis) as the reference, the efficiency will be directly above it, on the red trend.

 

[Julian]

No, I didn,t mix up the two, my spreadsheet (and graph) is dynamic and changes as long as you replicate (by inputting the speed) that the VSD will need to output at the CHANGING flow requirements for any given setpoint (SP) head.
If you set the VSD speed to give you just the desired static head (height) then the top floor will have zero flow, if the building height is say 40 ft or 17 psi you must then add on the required pressure required by the resident say 30 psi so the pump head must be 47 psi, say 50, so the top floor resident will have 30 psi available at his taps/shower and the bottom floor resident will have 50 psi available, pipe friction losses must also be taken into account so IMO the correct way to run this set up is to run on pressure set point and maybe also use some logic to start/stop the VSD when demand is zero but VSD is primarily designed for a constant varying demand. however, yes, you can pick and set a sweet point speed based on the "average" flow demand and start/stop the pump using the pressure transducer or simply installing a pressure switch, all very very interesting.
I have also modified the spreadsheet to give a little more info, it graphically now shows the pump+motor efficiencies, just use the flow (x axis) as the reference, the efficiency will be directly above it, on the red trend.

I think you are correct to run the system on pressure set point rather than speed set point, because of the wide varying demand. With this I think the VSD won't be intelligent enough to control or stop via pressure transducer, because it will be running on constant pressure. Unless minimum RPM can deliver a pressure higher than set cut out pressure. All in all I think it would be simpler for me to install a pressure switch in between pump and VSD. And install a bigger pressure tank to minimize start stop cycle. Or if I choose to go further I can also combine a flow switch. As long as water is flowing in the pipe, pump will continue running even if cut out pressure is already attained.
If I may ask how do you compute efficiency based on the given data.

 

[John.g]

There are probably tried and trusted ways of starting/stopping the VSD as there are tens of thousands of these systems about I would think and eventually we will find the answer.

Re efficiencies. Ref Grundfos link in post #66. If you click anywhere in the graph you will all the see the numbers that I inputted in the spreadsheet in columns B to F "under constant pump speed" I clicked on flowrates 0 to 15 ms/hr in 1 m3/hr increments to get all the actual pump conditions in these 16 flowrates.
I then did the calculations in the 16 column table under "variable pump speed" and you can see the formula's used in each of these columns, B to E, and are based on the pump affinity laws (well known) where head is proportional to speed squared, flow is proportional to speed and power is proportional to speed cubed. The efficiencies will remain the same in each row as all the other conditions even though changing will give the same efficiency for each equivalent row as in the full speed table and that's why there is no calculation done in column F.

 

[Julian]

There are probably tried and trusted ways of starting/stopping the VSD as there are tens of thousands of these systems about I would think and eventually we will find the answer.

Re efficiencies. Ref Grundfos link in post #66. If you click anywhere in the graph you will all the see the numbers that I inputted in the spreadsheet in columns B to F "under constant pump speed" I clicked on flowrates 0 to 15 ms/hr in 1 m3/hr increments to get all the actual pump conditions in these 16 flowrates.
I then did the calculations in the 16 column table under "variable pump speed" and you can see the formula's used in each of these columns, B to E, and are based on the pump affinity laws (well known) where head is proportional to speed squared, flow is proportional to speed and power is proportional to speed cubed. The efficiencies will remain the same in each row as all the other conditions even though changing will give the same efficiency for each equivalent row as in the full speed table and that's why there is no calculation done in column F.

Tried the spreed sheet. this is fun I am gonna be late for work again hahaha. It certainly is magic to an unlearned person like me. How did you do it. Well anyway anything that you / I know is simple the opposite is true as well.

 

[quality]

what does 1440 refer to?

rpm .

 

[John.g]

Thanks, the pump will deliver ~ zero flow at 1440 RPM to the top floor if its ~ 31 feet (13.6 psi) from the ground.

 

[Julian]

I was looking at the spreadsheet of grundfos 60hz and I notice how efficiency and flow rate increase/ decrease with just a slight change in pressure. I think I know already what to do.
I was there today tinkering with the VSD but before I did that I switch the emergency switch on so that pump runs continuously if I f*** up. Then suddenly the pump stopped. Luckily I had the presence of mind to push the run button. Is that supposed to do that? is not the emergency switch meant to bypass the VSD? I also flip the emergency switch for the standby to check if its running and nothing happened. So i wired it to bypass the VSD and when I switch on the motor run. Did the previous technician F*** up the installation or am I missing something?