Q & A > Question Details
During precommissioning of our DHDT unit we want to pressurise the system with nitrogen by makeup gas compressor up to 50kg/cm2g. Can we increase pressure without achieving the minimum pressurisation temperature?
24/02/2021 A: vikaram rawat, IOCL, rawatv@indianoil.in
First of all, it sounds quite impractical that you can pressurise system with N2 upto 50 kg because at those system pressures (even much below 50 Kg) your MUG will trip on high discharge temperature. Secondly, you can pressurise (if your MUG allows) the system upto 50 kg below MPT provided that your HP section design pressure is more than150 kg/cm2 which will hardly be the case. It is for this reason that a nitrogen PSV(with low pop-up pressure setting) is generally provided in a HP separator during the commisioning phase.
13/10/2020 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Pressurizing a rated'pressure vessel to above the recommended limit when the metal is not completely above the minimum pressurization temperature is very risky. Brittle fracture happens without warning and is, for all practical purposes, instantaneous and catastrophic. Doing this violates several codes, good practice strictures, OSHA rules, and most insurance policy strictures. Criminal negligence indictment and conviction are likely if injury or death happens.

It does not matter what the pressurizing media is--bursting stress is the same. Using a gas to overpressure is much more destructive if a rupture happens since there is much more energy contained in a compressed gas than a liquid at the same pressure.
11/09/2020 A: Jake Gotham, InSite Technical Services, jake.gotham@insitetechnical.com
Some companies develop a curve which describes allowable pressure vs temperature. This allows a controlled increase in pressure below minimum pressurisation temperature (MPT), but still only reaching design conditions once MPT is achieved.

The only sites I know of that have tested at design pressure below MPT only did so because they weren’t aware of the risks of brittle fracture. They were lucky enough to not have any failures. I am not aware of any site considering the risk of brittle fracture but continuing to test at design pressure below MPT – but I’d be interested to hear other experience and how this was justified. There have been several failures with ambient temperature hydrotesting. In these events, the equipment damage would delay start-up by many months, but it is unlikely anybody would get hurt. With ambient temperature pneumatic testing, the stored energy is much greater, and there is a real safety risk associated with the activity as well as the risk to the equipment.

A service test at the nitrogen header pressure (i.e. less than the allowable pressure below MPT) is sufficient to find the most common problems (e.g. vents & drains open). Modern gasket and bolting technology gives much better protection against smaller leaks than was the case historically. A high-pressure ambient test may give the impression the system has been fully tested, but this isn’t the case because the temperature changes during start-up cause movement.

Your question refers to pre-commissioning, so I assume you are referring to a new process unit under construction. Some licensors recommend a hot, high-pressure integrity test before catalyst loading. The intention is to start the recycle compressor and furnace, heat the unit up and increase to operating pressure. This activity achieves multiple objectives:
• Function test of various pieces of high-pressure rotating equipment (make-up and recycle compressors. Feed, amine and wash water pumps can also be tested).
• Integrity test.
• Displaces construction debris into the empty reactor. This can be recovered before catalyst loading.
If this test is not performed, the first time the unit reaches operating temperature and pressure is during sulphiding when there is 1-2% H2S in the system. A leak at that point is dangerous, and there is also a risk to the catalyst if there is a crash shut-down during sulphiding. The hot high pressure integrity test takes a few days, but it is invariably worthwhile.

I hope you find this useful. Please get in touch if you would like further assistance.