Questions on Dip Tube Type Level System

A liquid storage vessel holding a very corrosive liquid has its level measured by a bubbler system, whereby a transmitter measures the backpressure of air inside a “dip tube” inserted into the vessel:

Dip Tube Type Level System

Explain how this level measurement system works, and how it protects the DP cell from the corrosive effects of the process liquid.

Also, complete a calibration table for the differential pressure transmitter in this level measurement scenario, with a calibration tolerance of ± 0.5%.

Assume that the lower range-value of the process (0% level) is exactly the same height as the bottom of the dip tube:

More Questions:

1. The dip tube (or “bubbler”) system does indeed isolate the transmitter from the corrosive process liquid, but how can the dip tube itself survive? And, if the dip tube is able to survive in the corrosive liquid (just like the vessel), why can’t we find a DP cell that can handle it directly without the isolation of a dip tube system?

2. Demonstrate how to estimate numerical answers for this problem without using a calculator.

3. What is your recommendation for setting the position of the needle valve in this bubble tube system? Exactly how far open is “open enough,” and more importantly for what reason?

4. What would happen if the end of the dip tube were to become plugged with debris, blocking all flow?

5. What would happen if the needle valve were to become plugged with debris, blocking all flow?

6. What would happen if the pressure regulator were to become plugged with debris, blocking all flow?

7. Add one or more hand valves to the tubing in this system to provide operators with a simple means of unblocking a plugged dip tube using high-pressure compressed air.

8. Given a 100% full vessel, calculate the amount of pressure that would be registered by a pressure gauge connected to the bottom of the vessel.

9. Given a 100% full vessel, calculate the amount of pressure that would be registered by a pressure gauge connected to the bottom of the dip tube.

10. Given a 100% full vessel, calculate the amount of pressure that would be registered by a pressure gauge connected to the transmitter’s “H” port.

11. Given a 100% full vessel, calculate the amount of pressure that would be registered by a pressure gauge connected to the downstream side of the needle valve.

12. Given a 100% full vessel, calculate the amount of pressure that would be registered by a pressure gauge connected to the upstream side of the needle valve.

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Credits: Tony R. Kuphaldt