Filters & Control valves

[98769876]

Hi (Question regarding sodium chlorate plant pump tank transfer)

We have a small tank (2 m3) that gets intermittent flow to it. From this tank we pump to the storage tanks. First we have the tank then the pumps (2 pumps pumping in parallel), then a control valve (control the 2m3 tank level by adjusting flow) then a cartridge filter and lastly the storage tank. We installed the filters in the system recently and also pressure transmitters before and after the filter. the purpose of these pressure tr. are to measure filter dP- but with the system as is it does not work. dP decrease then increase then decrease- and is very arratic the measurement- we cant use it as an indication to know when to change out the cartriges- the reason for this is is there is a control valve before the filter with a pressure drop of 170-220 kPA (20 % valve opening)- the pump tank incoming liquid is also "like pulsing" the way it comes. The pressure drop of the filter should iNCREASE to 30 kPa over a 12 hour period. We have similar filters sthe somewhere else but the set-up is differ:ent: Big tank (buffer & good head), No control valve the filters and then the next tank to which it transfers to.
We are thinking of moving the position of the control valve to after the filters to eliminate the variation in pressure measuerment - will this help?

Will there be a back pressure from the filters that will affect the dPs? i dont thinks so.
One thing- the two pumps are pumping in parallel but their impellor sizes are different - from what I understand they will fight each other- could this be a cause for the eratic pressure readings?

 

[DRWeig]

It sounds like you are trying to measure DP of a filter while the flow going through the filter is varying according to the level control algorithm. As the flow changes, so will the DP across the filter. In order for filter DP to make sense when deciding it is time to change it, the measurements must be made at the same flow rate.

Upstream or downstream of the control valve will give the same results.

Do you also measure flow in the line? If so, you can measure the filter DP at a given flow rate, and as the flow varies, you can compute what the DP would be at the same flow rate as the initial measurement. For simplicity you might start with your measurement at some flow rate that gives a good mid-scale reading on your DP transmitter. DP varies with the square of the flow. That is, at twice the flow, the DP will be four times as much. At half the flow, DP will be one-fourth as much.

A little experimentation and a little math will give you decent results.

I assume in all the above that your fluid is not compressible, does not change densities or flash at the valve, etc... In other words, it fits Bernoulli's law in the same way that water at reasonable flow rates with reasonable pipe sizes.



Good on ya,

Goober Dave

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