Looking for feedback on a drawdown tank serving flush valve toilets

[nuuvox000]

I have a project for a commercial office building where they want to add a bunch of flush valve toilets and other plumbing fixtures without upsizing the pipe outside of the building. I have concluded that the 2" size will have issues with the added fixtures due to high velocities. Due to the circumstances, upsizing the service line to the building would be very cost-prohibitive.

My plan is to add a "drawdown" hydropneumatic expansion tank that will serve as a pressure and volume reservoir that can fill up slowly with the small building pipe and then provide volume and pressure to run the new flush valve toilets with a larger outlet pipe.

Just some numbers for context:
2" line serves the building
Adding 12 flush valve water closets (1.1 gpf)
Adding 12 lavatories (0.5 gpm)
Adding 4 kitchen sinks (1.5 gpm)

Planned expansion tank is 211 gallons in the penthouse of the 5-story building. I've confirmed that there will be about 55 psi of pressure at the tank. I've sized it so that we can get 60 gallons out of it before the pressure drops to 35 psi (code minimum).

Does anyone have any other thoughts or tips about this? I've wondered about adding a pressure independent control valve to make sure it doesn't pull too much water from the main building pipe but I know that those are typically made for hydronic applications and I'm unsure if it would actually be needed. Let me know what you think; thanks!

Edit: I'm having a hard time finding the right forum for plumbing questions so let me know if there's a better place for this.

 

[LittleInch]

Sounds like a decent plan. You don't need to worry about the main building pipe as the common pressure node will just even things out.

Some of those flow look quite low, but another aspect would be to restrict filling flow to those fixtures or max flow.

I assume your tank volume of 211 gallons is the external size in which case about 40-50% usable volume sounds about right.

There used to be a plumbing forum, but it was rarely used so it was incorporated into one of the other forums.

A schematic sketch of your system would have been useful....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[PEDARRIN2]

Use of a hydropneumatic tank is acceptable. I would assume the building is served by a backflow preventer.

As stated, there used to be a plumbing forum, though I question the "rarely used" when compared to some other forums on the site. But, plumbing has always been considered the red headed step child of engineering - until the toilets stop working. Then we are highly desired.

 

[nuuvox000]

Thanks to you both.
LittleInch: I'm actually working on a schematic today.
PEDARRIN2: There is a backflow preventer for the building. Plumbing has proven to be more difficult for me to master when compared to HVAC.

 

[nuuvox000]

Here's a simple detail if anyone's interested. My next question is if it matters how close the building connection is to the expansion tank. I don't believe it makes much difference, if any.

 

[PEDARRIN2]

Where you have it located is one option. Making it more remote is another option. There is a debate as to which is best. Generally, I put it where I have space for the tank and where I don't have to run a bunch of extra pipe to connect the tank to the system.