Preventing plant draining 1

[Lemoni]

I have a situation where a water treatment plant that operates intermittently drains on shutdown to the clearwell because much of the process equipment and piping is at higher elevation than the discharge point (clearwell manifold). (see attached sketch).
To prevent this, is it best to install an anti-siphon loop with an air-vac valve as shown on the sketch or is it possible/permitted to install a pressure relief valve on the clearwell manifold. The PRV would open on plant start-up and close on shutdown, preventing water to drain through by gravity.

thanks

 

[bimr]

You would probably be better served with a level transmitter installed on the clearwell, a level controller, and a control valve on the feed line into the clearwell. This scheme will allow you to control the level of the clearwell. The control valve will be in place of the proposed prv.

It is not really clear what your systems consists of. Are the filters pressure filters or gravity filters? Is any of the equipment automated?

Do you have a PI&D drawing? Any proposed fix should be incorporated into the overall control scheme.

 

[Lemoni]

Thanks for the quick response bimr.
The intent of the figure I posted was to show the relative elevation of the process equipment and piping. I've attached the P&ID with notes.
The entire plant is automated and is controlled by the liquid level in the clearwell (start-up, shutdown, and flowrate). The problem is that most of the plant drains to the clearwell on shutdown. How can this best be prevented?

thanks

 

[bimr]

You will just need to install a motorized valve prior to the isolation valves (HV-201 & HV-211) that are flowing into the clearwell. The valve should energize to open when the pumps are started and close when the pumps stop. Note that you already have a PRV on the line to protect from overpressure from an inadvertent valve closing.

If the pipe is oversized at that location, consider installing an orifice to maintain backpressure. (An oversize pipe will allow the pipe to drain even when the system is operating, not just when the system is off.) However, this scenario probably does not exist.

You already have a flow control valve FCV 201; another one at the clearwell would be counter productive.

A valve similar to CV-233 should work.

The air vac valve will probably be more expensive and will be a source of contamination.

The pressure reducing valve will be an energy waster as it will increase the pumping cost.