Boost Pump Issue

[IsaacS]

I work on water purification systems for dialysis clinics and I'm having an issue with the city water boost pump. It's a Grundfos CRNE5-5 pump fed by a city water break tank. The pump set for constant pressure control mode maxes out at 60 PSI and constantly runs, so by the time it reaches RO machine after passing across 5 filtration elements through a 1" piping header (sediment cartridge filter, 16x65" softener filter, 16x65" organic scavenger, two 24x72" carbon filters) the RO feed pressure is about 20 PSI which is close to minimum feed pressure for the RO machine. Also the automatic backwash control valves for carbon tanks are Fleck 2850 with 25 GPM flow restrictor and ideally need 25 psi+ during backwash. I tried installing a smaller pump, a Grundfos SCALA2, on the inlet side of the main boost pump and this allowed me to boost main pressure to about 82 psi when RO machine is running (about 13GPM flow), but I'm only getting about 10psi feeding carbon tank during backwash (about 18 GPM flow).

 

[MicEd69]

You need a larger primary pump, not two pumps in series. What is your maximum flow rate through your system?
The Grundfos CRNE 5-5 pump is no longer available.

 

[breplum]

We always use a plumbing engineer when we have special installs like yours, so we are just the “installers” and I love not getting blame for having guessed wrong. Grundfos tech support might be able to help if they still have that.

 

[IsaacS]

We always use a plumbing engineer when we have special installs like yours, so we are just the “installers” and I love not getting blame for having guessed wrong. Grundfos tech support might be able to help if they still have that.

I'm in the same boat, just an installer, but the problem in dialysis is systems get modified over the years--filters are added and subtracted--so even if they were engineered properly to begin with (which is a major if), we're stuck trying to figure out a solution.

 

[IsaacS]

You need a larger primary pump, not two pumps in series. What is your maximum flow rate through your system?
The Grundfos CRNE 5-5 pump is no longer available.

Max flow rate would be 25GPM for carbon tank backwash. RO only demands about 13 GPM. It's weird because this pump is commonly installed for these sized systems but normally not off a city water break tank. I'm tired of these Grundfos variable speed pumps I'd much prefer the old pressure switch setup.

 

[GReynolds929]

@Valveman can advise better when he logs on next, VFD's are designed to be short lived. I would go with a standard jet pump and pressure switch, with a cycle stop valve all sized to meet your needs. Will be much more reliable.

 

[MicEd69]

@Valveman can advise better when he logs on next, VFD's are designed to be short lived. I would go with a standard jet pump and pressure switch, with a cycle stop valve all sized to meet your needs. Will be much more reliable.

I sent a PM to him asking him to respond to this.

 

[Valveman]

I'm tired of these Grundfos variable speed pumps I'd much prefer the old pressure switch setup.

I can help you out with that. The Cycle Stop Valve was designed to replace variable speed pumps or VFD's and uses a regular pressure tank/pressure switch to turn the pump on and off. Once the pump comes on the completely mechanical Cycle Stop Valve can be adjusted to hold from15 to 150 PSI constant as needed. In your case, that pump can only do 24 GPM at 60 PSI. You need a larger pump as you know you are losing 40 PSI through the filters and only getting 10 PSI out. If you need 20 PSI more out, you will need at least 20 PSI more coming in. I am not certain of your application, but it sounds like you need a pump that can do 37 GPM at 80 PSI. 37 GPM is 25 for the backwash and 13 for the RO if they run at the same time.

Setting a Cycle Stop Valve model CSV3A2T at 80 PSI constant will vary the flow rate from as little as 3 GPM to as much as the pump can produce while maintaining the 80 PSI constant at the CSV. A standard 70/90 pressure switch with about an 86 gallon size pressure tank will work with the CSV3A2T to automatically turn the pump on or off when needed, and the CSV will hold 80 PSI constant while water is being used. The CR series pump has a very good drop in horsepower when the pump is running at full speed and simply being restricted by a CSV. The drop in HP is very similar to the drop in HP when the speed is reduced using a VFD. But nobody wants you to know that, or people would not be interested in a VFD to start with.

I spent some time on the Grundfos web page this morning and cannot find a curve for a pump without the "integrated variable speed". I am sure there is such a pump, in maybe the CR10-7 size or so. But they want you to use a VFD so bad they "integrate" them onto the motor so you don't have any choice. A VFD makes pumps much more expensive and short lived, which is the main reason pump companies push them so hard. But many people wrongly believe a VFD saves energy, which makes it easy to sell an expensive and short lived pump.

Here is a little drawing of how to install a CSV on an end suction centrifugal, and another with 4 vertical pumps on a pump station.

 

[Valveman]

I sent a PM to him asking him to respond to this.

Thanks guys. Got pump curves piled up on my desk this morning and trying to catch up.

 

[MicEd69]

they "integrate" them onto the motor so you don't have any choice.

WOW!

 

[IsaacS]

I can help you out with that. The Cycle Stop Valve was designed to replace variable speed pumps or VFD's and uses a regular pressure tank/pressure switch to turn the pump on and off. Once the pump comes on the completely mechanical Cycle Stop Valve can be adjusted to hold from15 to 150 PSI constant as needed. In your case, that pump can only do 24 GPM at 60 PSI. You need a larger pump as you know you are losing 40 PSI through the filters and only getting 10 PSI out. If you need 20 PSI more out, you will need at least 20 PSI more coming in. I am not certain of your application, but it sounds like you need a pump that can do 37 GPM at 80 PSI. 37 GPM is 25 for the backwash and 13 for the RO if they run at the same time.

Setting a Cycle Stop Valve model CSV3A2T at 80 PSI constant will vary the flow rate from as little as 3 GPM to as much as the pump can produce while maintaining the 80 PSI constant at the CSV. A standard 70/90 pressure switch with about an 86 gallon size pressure tank will work with the CSV3A2T to automatically turn the pump on or off when needed, and the CSV will hold 80 PSI constant while water is being used. The CR series pump has a very good drop in horsepower when the pump is running at full speed and simply being restricted by a CSV. The drop in HP is very similar to the drop in HP when the speed is reduced using a VFD. But nobody wants you to know that, or people would not be interested in a VFD to start with.

I spent some time on the Grundfos web page this morning and cannot find a curve for a pump without the "integrated variable speed". I am sure there is such a pump, in maybe the CR10-7 size or so. But they want you to use a VFD so bad they "integrate" them onto the motor so you don't have any choice. A VFD makes pumps much more expensive and short lived, which is the main reason pump companies push them so hard. But many people wrongly believe a VFD saves energy, which makes it easy to sell an expensive and short lived pump.

Here is a little drawing of how to install a CSV on an end suction centrifugal, and another with 4 vertical pumps on a pump station.

View attachment 44802
View attachment 44803

This is great information, thanks a bunch. I hadn’t heard of control stop valves before this but sounds like the way to go. Would I be able to get away with smaller than 86 gal bladder tank due to space constraints?

 

[IsaacS]

I sent a PM to him asking him to respond to this.

Thank you for the referral!

 

[IsaacS]

This is great information, thanks a bunch. I hadn’t heard of cycle stop valves before this but sounds like the way to go. Would I be able to get away with smaller than 86 gal bladder tank due to space constraints?

 

[Valveman]

This is great information, thanks a bunch. I hadn’t heard of control stop valves before this but sounds like the way to go. Would I be able to get away with smaller than 86 gal bladder tank due to space constraints?

Yes you can. But with a 70/90 pressure switch setting, a 44 gallon size tank is probably as small as you can go. However, all 40 gallons do not have to be in one spot. You could use as small as a 4.5 gallon size tank close to the CSV and pressure switch, then an additional 40 gallon size tank or combinations there of could be installed anywhere else in the system you can connect to the pipe.

It is also possible to use a single 20 gallon size pressure tank. At 70/90 pressure a 20 gallon size tank will only hold about 3 gallons of water. But with the CSV3A valve the tank is really only used when/if the demand is zero or less than 3 GPM. If you have no intermittent uses of water like hand washing or toilets (RO system is either full on or off, backwash is full on or off), then a small tank is all that is needed.

Another possibility is using a pressure relief valve or 3 GPM bypass. With 3 GPM continuously bypassing back to a storage tank or somewhere, the CSV3A will keep the pump running 24/7 at at constant 80 PSI as needed, and a pressure tank would not be necessary, just the CSV.

 

[IsaacS]

Perfect, thanks. The only demand would be from backwash/ RO operation. The other thing is the RO operation and backwash never happen together, so peak demand would be 25gpm @25 psi

Yes you can. But with a 70/90 pressure switch setting, a 44 gallon size tank is probably as small as you can go. However, all 40 gallons do not have to be in one spot. You could use as small as a 4.5 gallon size tank close to the CSV and pressure switch, then an additional 40 gallon size tank or combinations there of could be installed anywhere else in the system you can connect to the pipe.

It is also possible to use a single 20 gallon size pressure tank. At 70/90 pressure a 20 gallon size tank will only hold about 3 gallons of water. But with the CSV3A valve the tank is really only used when/if the demand is zero or less than 3 GPM. If you have no intermittent uses of water like hand washing or toilets (RO system is either full on or off, backwash is full on or off), then a small tank is all that is needed.

Another possibility is using a pressure relief valve or 3 GPM bypass. With 3 GPM continuously bypassing back to a storage tank or somewhere, the CSV3A will keep the pump running 24/7 at at constant 80 PSI as needed, and a pressure tank would not be necessary, just the CSV.

 

[Valveman]

Perfect, thanks. The only demand would be from backwash/ RO operation. The other thing is the RO operation and backwash never happen together, so peak demand would be 25gpm @25 psi

Then you only need a pump that can do 25 GPM at 80 PSI. Probably like a CR5-7 or something. There web site wants to pick a pump for me instead of letting me see the curves. Wants to pick a pump with an "integrated variable speed". Ugg! If you can get a curve on a constant speed pump I would be glad to look at it.