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We are observing a lot of vacuum pressure fluctuations in our VDU which is operated at 21 mm Hg. During fluctuations, we tried to switch over ejectors with different capacities, however, no effect was observed. Pressure fluctuations are happening on their own and it is becoming steady after a certain time, say one day. How do we solve this problem and what is the possible root cause?

Aug-2021

Answers

Sergey Glistovchenko, Slavyansk ECO, Sergey.Glistovchenko@gmail.com

If your fluctuations start since start-up or main overhaul of VDU make sure you are using an absolute PT at the top of tower. One of our VDU reacted to the weather changes.

Apr-2022
Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com

Much has been written on the subject of ejector troubleshooting (examples below), so I’ll resist the temptation to repeat it all here and limit myself to the following observations:

1. If the system works well most of the time, but fluctuates or deteriorates at other times, the problem is likely to be caused by a process issue rather than a mechanical problem in the ejector/condenser system. Examples include:

Backpressure. Does the final condenser vent to a furnace, flare, atmosphere or somewhere else? Is it possible to route it to a different system to see if there is an improvement? Is something else relieving to flare and generating a backpressure? Are there flame arrestors that need cleaning? Steaming out the pipework can remove ammonium salts that often deposit in this service.

Steam quality. Does the motive steam come from a dry (slightly superheated) system? Are there wet steam sources in the same header that could affect the motive steam quality intermittently? If there is a steam separator vessel, is the drain blocked, or level instrumentation faulty? Is the insulation on the steam pipework in good condition?

Condenser back-up. Do an IR scan of the condenser drain line during normal and upset condition — you may be able to see a change. If the condenser vapour line is hotter than the drain line it can also indicate this problem.

Recycling ejector. If you are operating parallel ejectors, in some situations one ejector can operate in reverse. If one ejector is in reverse the outlet nozzle will be cool and the inlet nozzle will be hot — the opposite of a healthy ejector.

2. Vacuum system troubleshooting cannot be done from the office or control room. It requires time on the ejector platform listening to the system, monitoring temperatures and doing pressure surveys during good and bad operation.

3. Many vacuum systems have insufficient or non-functioning instrumentation. Providing measurement of motive steam temperature & pressure local to the ejectors, pressure on each condenser and temperature on the vapour from each condenser would provide invaluable troubleshooting information.

4. In the absence of the instruments mentioned above, a regular (twice per year) temperature and pressure survey while everything is operating well provides a good baseline to compare measurements with when the unit is misbehaving.

Good luck!

https://www.digitalrefining.com/article/1001321/troubleshooting-vacuum-systems

https://www.digitalrefining.com/article/1001330/operating-vacuum-distillation-ejector-systems
Aug-2021
Sanjay Bhargava, Bharat Petroleum Corporation Limited, sbhargava2160@gmail.com

Check following:

1. Steadiness of steam pressure at ejector.

2. Steam condition - whether moisture coming intermittently in steam.

Aug-2021
Paul Stobbe, Saudi Aramco, stobbepe@yahoo.com

Check the tower sections for flooding.

Aug-2021
Doug Morgan, Searles Valley Minerals, morgan@svminerals.com

Any process that has recycle and for which all of the degrees of freedom are under feedback control will oscillate. Oscillations in highly automated facilities can travel from one unit to another. One of our favourite pastimes in Ammonia/Urea plants is tracking down the source of oscillations across several units. If you are NOT seeing oscillations, your processes is NOT under control and variables are drifting. If you are "closed loop" and oscillations are too high, I would recommend

1. deploying "Delta Tuning" directly on the pressure loop and directly associated loops

2. changing the CV/MV pairings to something less interactive.

3. Looking beyond the VDU for the culprit.

Aug-2021
Roy Bisnett, Brightmark Energy, roy.bisnett@brightmark.com

Aside from what was already stated, it could be an issue with the dip legs. Especially if the water has not been neutralized. Corrosion causes pitting which can usually be found in the water lines that hold the vacuum. Changes in vacuum affect water level and make the pressure indication bounce.

Aug-2021
James Risko, TLV, Risko@TLVengineering.com

There can be multiple reasons, so it can be worthwhile to consider various component aspects. Here are several questions relative to the steam supply & ejectors themselves:
1) Is the steam supply properly drained through steam traps - or are some traps blocked allowing slugs up into the ejectors?
2) Is there a steam trap at the base of the steam supply riser into the ejectors?
3) Is there a separator on the steam supply feeding the ejectors?
4) If the steam supply is superheated, is the desuper heater properly functioning with a steam trap discharging visibly for confirmation?
5) If there is an orifice plate prior to the valve, is there a steam trap immediately before the orifice plate to prevent pooling?
6) Is there a separator after the desuper heater?
7) Are the ejector nozzle and throat within manufacturer tolerance.

These 7 questions may help isolate potential issues on the ejector side. Next, on to condensers.
8) Are the condensers heavily fouled?
9) Is there sufficient cooling water at the proper temperature or does it vary?
9) A key problem can be that the drain traps on the condenser system are not properly selected or installed with proper balance lines. It is useful to check those closely. If you want to see more of this topics, please check the following link - https://www.tlv.com/global/TI/webinars/common-refining-petrochemical-plant-steam-application-problems-pt2.html

Aug-2021