Bouncing Level Indicator

[Thealanator]

We have several liquid nitrogen tanks equipped with Barton level indicators. One of the tanks is pressurized with nitrogen gas through a regulator and solenoid valve. When the solenoid is opened the level indicator starts bouncing 1-2 times per second. Bouncing stops after the solenoid closes. This same tank used to be pressurized with a pressure-build coil. I never saw the indicator bounce.

Seems like there is something related to heat leak, but I don't understand what is happening. Any insights?

 

[iainuts]

Is the gas being injected into the tank tied into one of the lines for the dP gage by any chance? Or are the lines totally separate going into the tank?

 

[Thealanator]

Separate lines, I am fairly certain. I think the pressurization manifold runs the length of the tank up at the top.

 

[iainuts]

Is the pressurization any faster with the solenoid valve? I've seen gages bounce when liquid is pushed into the lower dP tube and flashes off against some warm part of the tube. At least, that's my theory which I've discussed with tank manufacturers. Note that the dP tubes are cryogenic on one end and warm at the other so any liquid flow into the tube that might boil off and flash will create a small but sudden change in pressure in that line. Note also there is generally a liquid trap in the lower dP line to prevent liquid from going past a certain point. If liquid gets past that point, it generally will flow to a hotter location resulting in flashing.

If the rate of pressurization is the same between the PB coil and new method of pressurization, the above might not provide a satisfying answer.

 

[Thealanator]

That seems to make sense. Do you think the same thing would happen during a tank fill? I can see see how it would be more pronounced in a skinny vertical tank (level increases faster with a given liquid flow rate), as opposed to a horizontal tank.

 

[iainuts]

I would think it is possible any time there's a pressure or liquid level change in the tank. Generally, adding liquid to the tank causes the tank pressure to change.

 

[hotcap]

I have had gauge bouncing almost entirely disappear by using gauge snubbers on the leg lines of bouncing DP gauges. Happens sometimes when filling top fill also.