Flow transmitter for CW service

[zwsa06]

I am looking at replacing a 12 year old flow mag meter (Manufacturer : FlowTec) with a simple RO and taps. The reason is we have had several plant trips due to faulty reading on the mag meters, which have been calibrated and tuned and whatever else to filter out errors. The magmeters measure cooling water supply flow to burner column, which cools the products stream in a carbon block. The problem arises when we make adjustments in the pH of our cooling water tower basin, which sends these magmeters into a spin. The spikes which occur sometimes can indicate a low low flow in cooling water to the burner, causing a trip, which trips our entire plant at high loads. The original design (back in 1989) had an RO installed in it, but it was taken out as we couldn't get a reliable flow reading. Flow mag meters were installed and everything was running well until we installed our new cooling tower unit. Regeneration effluent from our mixed bed water deminerlisers run into the CW basin, since the effluent quantities were increased. Two CW pumps supply CW at 20-25deg C, 400-420 kPaG to two portions of the burner block, which are independently measured by mag flowmeters. The cooling water line size 10", reduced to 8" at the flowmeters, and the flowrate on each stream is 120 m3/hr.

Are current mag flow meters more reliable in filtering and giving consistent flow readings ? What is the down side in replacing these magmeters with dP taps and RO ?

Thanks.

 

[quark]

As far as magmeters for water are concerned I don't see any substantial change except for part flow detection.As far as DP meters are concerned they are good for water application cheaper than magflow meters but their turndown ratio is lower.

I still couldn't understand how pH value affects the performance of magmeters.

 

[hacksaw]

Check your electrodes. Check for leakage and chemical attack. Platinum electrodes have the least problem when the pH and chemical treatment are all over the map.

The magmeters designed for CW generally have a larger surface area and have provision for on-line cleaning.

By RO I presume you mean an orifice meter. You generally do not want an RO design for flow measurement.

There are a number of insertion flow meters for large pipe and of course there is always the clamp-on ultrasonic meters.

 

[RGCook]

It sounds very much like something is coating the electrode and it is loosing contact, causing a false low. Platinum is generally the best to minimize this kind of thing but there are other metals available that you might want to try. It could be as simple as scale build-up.

You can also try changing the meter's frequency to see if you are suffering a weird harmonic related to your particular system. I know from experience as I have had to do that with Foxboro meters before. I don't know if other meters can do this. In any event, you seem to have a classic case of electrode coating up.

 

[mur]

Installing a RO into the system will increase loss's & Pump H.P. duty, Have you considered Annubars.

 

[TD2K]

Actually, installing an RO in an 'existing' will increase the total resistance through the CW loop, moving the pump back up on its curve and will likely decrease pump Hp slightly.

However, that savings is also at the loss of 'some' CW flow though I suspect the numbers would be neglible if you crunched them.