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I am in a debate with my friends. My point is that in bubbler level measurement, we only need a pressure regulator, a needle valve and a pressure gauge. Through pressure regulator we set the value a little above the head pressure of measuring liquid in an open tank. Needle valve, which is installed just after pressure regulator, is used to set the optimum rate of bubbles. Once the desired bubble rate is achieved through calibration, there is no need to change the opening of needle valve.

My friend says that we have to have a rotameter which should maintain the flow rate of bubbles through some sort of built-in control valve. Can anyone decide through equations who is right?

You need a differential regulator which can produce a constant gas flow rate under varying head pressures. The common everyday regulator is not a constant flow regulator but a constant pressure regulator.

The needle valve on a rotameter will perform the same flow regulating function as a needle valve with no rotameter. The rotometer provides only visual indication of the gas flow.

David

David,

That's what I was taught also, but in reality if you connect the bubble tube to the transmitter and the gas supply to the bubble tube it makes no difference if you supply .01 lpm or 10 lpm of gas, the level reading will stay the same. If the tubing between bubble tube and transmitter is reasonably short, it's convenient to inject the bubble supply right at the transmitter that is purging any moisture or corrosive gas.

We normally use a 0-1 l/m rotameter supplied at line pressure (typically 80# N2) once the flow is set the rate hardly changes for empty or full tank. The change in bacpressure is insignificant compared to the drop across the needle valve.

Regards
Roy

No, you don't need a rotameter, in fact you don't even need a regulator or needle valve. You just need some method of establishing a small flow. I have used the gauge snubber fittings that have a sintered metal flow restrictor. Merium make a nice device, it has a needle valve and a glass bowl. You fill the glass bowl with light oil or glycol and set the needle valve to give you a bubble every second or so, the flow seems to stay constant for months on end. If your transmitter is far away from the bubble tube it's usual to introduce the bubble gas near or at the bubbler to avoid pressure drop due to changes in flow.

I saw an installation once where they had a gauge and selector valve in the control room connected to bubblers in several tanks about 100 meters away. To measure the level the operator selected the tank and then puffed up the line with a squeeze pump similar to the old fashion blood pressure monitor. once the gauge reached max the operator allowed it to stabilize then read the level.

Bubble tubes are very low tech, but the firm I work for still uses them for level, interface and density measurement for a very nasty application in glass lined vessels (Tantalum bubble tubes). They are also still quite common for flotation level in mining.

Your friend is referring to a constant flow regulator, some people spec them but not really necessary.

Regards,
Roy