5 replies to this topic

[h.eghbali]

Hi everyone.

I have a question.

Our Coldbox vendor provided two variables for natural gas stream enter the coldbox:

 

1- Calculated pressure drop: 0.3 bar

2- Allowable pressure drop: 0.4 bar

 

Don't you think the gap between two variables and also the second variable value is very low?! According to these vendor's data, whenever pressure drop increase from 0.3 bar (normal DP) to 0.4 bar, we have to inject methanol to remove hydrates formed inside the coldbox (Assuming hydrate is the reason of DP). But the DP of coldbox is already near 0.4 bar (from initial start-up) and fluctuation of this variable is in the range of +- 0.1 bar.

Thanks in advance.

[latexman]

From an energy perspective, what % of capacity is the normal situation?

[Pilesar]

Are the cryogenic exchanger designer/manufacturer and the coldbox vendor the same? If so, then they have knowledgeable support personnel who can help assess the situation and address operating issues. This equipment is customized for a particular service. Some possible reasons I see for the situation you describe: hydrates, trash in the system, equipment damage, off-design operating conditions, unreasonable vendor operating parameters.

  Troubleshooting cryogenic systems can be tricky. Evaluate the whole system and not just the natural gas path. The variable pressure drop may be caused by variations in the cold streams. The process control system may be out of tune. There may be control valve problems. If you truly have hydrates build up, then fix the dryers.

  Small problems in cryogenic systems can lead to permanent equipment damage if they are not resolved. Good luck.

[PingPong]

Like any heat exchanger, a cold box is not an off-the-shelf item. It is designed and constructed by the vendor according to the specifications of the client.

 

The process engineer of the client has specified all the streams that enter and exit the cold box with temperatures, pressures, physical properties, duties, et cetera, and for each of those streams also an allowable pressure drop is specified. That is the pressure drop that the vendor should not exceed when designing the cold box.

 

In this case the vendor was apparently given a maximum allowable pressure drop of 0.4 bar, and the vendor's actual design is calculated to take only 0.3 bar (in clean condition) so that's OK.

 

It does not mean that the cold box will explode, or not work properly anymore if the actual pressure drop rises above 0.4 bar due to fouling or whatever.

 

When to clean any heat exchanger is an economical decision. Fouling costs energy and/or plant capacity.

[h.eghbali]

Dear Ping Pong.

Thank you. 

 

"When to clean any heat exchanger is an economical decision. Fouling costs energy and/or plant capacity."

 

My question is on this statement. When to clean BAHX? How economical aspects can affect this decision?

[PingPong]

Severe exchanger fouling can cause increased power consumption as well as production loss.

 

To clean a non-spared heat exchanger the plant has to be shut down, which is production loss.

 

It's up to the plant manager to decide whether a shut down is more economical or not.

There is no general world wide guideline for that.