'Finished water' pressure.

[itsmoked]

I'm doing the controller for a Manganese and Iron filter system for, essentially, and apartment. The last job I used pressure transducers to monitor the well pressure into the filter and the finished water output pressure. I also monitored the backwash supply pressure so I could suspend backwashing if someone dropped the backwash pressure below spec using it for irrigation or fire sprinklers.

We want to be able to initiate backwash by the filter pressure delta.

I'm told to save on the finished water transducer by dropping it. The argument is since the finished water is piped to the top of the storage tank. its pressure will always be the same - simply the head pressure to the top of the tank.

After scratching my head for a while I can't see much argument against this.

Got any cautions or arguments?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[3DDave]

That would be the minimum pressure - any excess pressure would add to the velocity of the water entering the tank.

 

[bimr]

The pressure in the pipe will vary. I assume that you have a valve at the tank to shut off the flow when the (gravity?) tank is full.

For a pressure piping system, the pressure will be constant at all points in the system. When the valve at the tank shuts off the piping pressure should increase. If the pipe has an open end, the pressure should be lower.

I would go with a differential pressure switch at the tank. There will be times when the system is running and that is when you want to measure the differential pressure. When the system is not running, there will be no differential pressure. A pressure switch is also inexpensive and will assist if you need to do any troubleshooting of the filter.

 

[itsmoked]

Hi bimr;

The pressure in the pipe will vary. I assume that you have a valve at the tank to shut off the flow when the (gravity?) tank is full

Negatory. On these small systems you switch off the well pump and that stops the flow to the storage tank.

Essentially you watch the tank with a large hysteresis float.

Water drops a ~foot command the well ON.

After X hours you do a backwash. (This can be problematic if your water changes quality seasonally.)

Alternatively you watch the delta pressure across the filter when it reaches Y you do a backwash. (You back this up with an 'X hours' alternative to keep the filter system exercised.)

So, the differential is only watched during well-running operations. The flow is virtually constant with the exception of the slowly blinding-off filter.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LittleInch]

If all you want to do is look at the dp across the filter then in that system then you can dispense with the final pressure transmitter.

But I would still add a small guage for commissioning and trouble shooting.

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

 

[itsmoked]

Thanks Little. Yes, there definitely will be gauges. In fact commissioning will include "read the gauge and enter the pressure into the controller for calculation of filter differential".

Keith Cress
kcress -

http://www.flaminsystems.com