What does it happen to my expansion control valve?

[hazel]

There are three expansion control valves in the bottom of H2 stripper in the cold box.
The normal process conditions are following:
1. Stripper operates at 18 kg/cm2.

2. The liquid CO on bottom of stripper flows paralleled through three expansion valves, but only one of the temperature of the first expansion valve is warming up in past 2 months, from -186? to -180?.

3. The first expansion valve decreases the pressure from 18 to 0.35 kg/cm2,the second one is from 18 to 2.5 kg/cm2, the third one is from 18 to 6.5 kg/cm2.

4. All the pressures in both sides of valves remain as usual, the same as the openings of the three valves.
We have slightly increased the first control valve opening, but it did not work.

The problem is how do we solve this abnormal situation, even we shut down the cold box unit.

Do we need to disconnect and take out the valve in the cold box? What should we check for the valve?

Is it possible that some substances block the pipe before the control valve in such cryogenic temperature? But why did not happen on the other control valves.

According the Joule-Thomson Expansion, does the control valve’s opening influence the temperature indicated in the expansion side or just reflect on the energy?

 

[25362]

Some thoughts for your consideration.

In all three cases the upstream liquid CO has a slightly negative J-T coefficient. One condition for the J-T effect to take place is that the kinetic energies on both sides of the throttling valve shoudn't differ much.

In the first (lower downstream pressure) expansion valve it appears that vapors/gases forming downstream have a higher kinetic energy per unit mass, measured as V²/2, and may result in friction. V is linear speed. Purity of CO may play a role. I may be wrong.

 

[BCJ]

I am not completely sure I understand your question, but here are a couple thoughts.

1. Many things can and will form hydrates at the temperatures you are describing. A wash with methanol or ethanol can remove the hydrates.

2. Sometimes oils that are frozen at those temperatures can be removed by depressuring that section and letting it warm up and draining it.

BCJ