I am trying to hook-up a bubbler type level measuring system for sump tank with connection to a dP transmitter to DCS. The sump tank is used to collect hydraulic oil, chemicals and liquid disposal from an offshore oil platform.

I am using a 3/8" OD tubing for the instrument air supply and seems that it is not working due to blockage on the tubing. Please recommend what type of tubing is usually required for such liquid measurement. I am proposing to use a 1/2" or 5/8" OD tubing to avoid the blockage. Any suggestion?

 

A 3/8" SS316 tubing should not be a problem, and this is usually the industry tradition. Probably the reason why the tubing is clogging is because the air supply may have not been set properly, which causes the sump, at its max level and head higher than the air setting, to enter the tube internal. Another solution is to install a check valve at the tip of the tubing to prevent back flow of the sump to the tubing.

 

I think that 1/2" OD tubing is good for this application, the most important thing is that this tubing not create a back pressure to the system. I mean that you will have a indication in the level transmitter without liquid present in the sump, therefore you must test the bubbler level system outside the sump connecting the 1/2" OD tubing and observe if there is sometime differential pressure sensed by the transmitter.

If this is not present the tubing diameter is good and you can install it again in the sump and check the supply pressure to the tubing. maybe you will need rise this pressure, how many inch or feet are you measuring with this bubble system? is the sump tank to atmosferic pressure or not?

Regards
J.Pinzon
Instruments Techniccian

 

I've used bubblers many times in wastewater wetwell level measurement. 3/8" tubing is more than adequate for a bubbler, and you can neglect pressure drop because of the low air flow. I would set supply pressure at 2 to 3 times the maximum wetwell depth. A rotameter with control valve and differential pressure regulator will give you constant flow across the wetwell level range. Instead of a check valve at the end of the bubbler tube, I recommend either of two ways to make it more resistant to plugging - cut the end of the bubbler tube at a 45 degree angle or add a short length of 1" to 2" tube/pipe cut square with V-notches.

One thing you seem to lack is a purge circuit. This is a 3-way valve in the line from the bubbler feed to the transmitter that, when actuated, isolates the transmitter and feeds air directly from the regulator to the bubbler for a few seconds, bypassing the flow controller. This high pressure burst clears any accumulated sludge from the end of the bubbler tube. Typically, this is done once a day either manually or automatically.

Mike

 

Hi Pizon, I am measuring around 6ft and sump is to atmospheric.

I can send you a pdf file of the sketch if you can provide me your email.

Thanks.