The impact of purging conditions on equipment design
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Recently, when setting conditions, the purge condition was noted in the equipment specification sheet; the equipment team said that if a purge condition is specified, the design temperature and pressure should also be increased. After reviewing some literature, it seems that the equipment needs to be calibrated for purging conditions. But exactly which parts of the equipment are affected, and how is the verification carried out? For example: the designed temperature and pressure of the equipment are 60 degrees Celsius and 0.35 Mpa(g), respectively, but steam at 1.0 Mpa(g) is used for purging. So how should the equipment verify this purging condition? Could some expert tell me that, in my opinion, it is unreasonable to raise the design temperature and pressure of equipment due to professional requirements. Thank you!
Personal understanding: The purging condition refers to the most severe scenario when using 1.0 Mpa saturated steam for purging – at this point, the equipment pressure reaches 1.0 Mpa and the temperature reaches that of the saturated steam. From a technical perspective, it is understood that since such operating conditions are specified, the design pressure and design temperature should be increased to match these conditions; otherwise, the equipment will not be able to meet the purging requirements, unless other process methods are employed to ensure that the purging conditions satisfy the design specifications. The requirements for the equipment specialty are entirely reasonable.
Short-term loads may increase the allowable stress
This is a reasonable request; otherwise, there would be no need to indicate it. There are many such cases on site: when the cooler is shut down, steam purging is used to remove the process fluid (not water) from within the tube side/shell side, which results in leakage problems in the cooler.
That’s exactly how we proceed: when there is a purging condition, we compare it with the original design pressure and temperature, and we use the higher one.
This issue should be taken seriously; in some cases, the equipment can get damaged during purging
What the equipment specialists say makes sense...
The last edit to this post was made by xyfeng007 on 2016-11-17 at 15:07. When using steam for purging, it is necessary to consider not only the increase in positive pressure but also the potential negative pressure that may arise due to the cooling of the steam after purging; therefore, negative pressure checks should be carried out or other measures taken to protect the system against negative pressure；
This depends on the purging time and frequency. A short-term overpressure event can be ignored. But it seems like the purging time for new projects is quite long; it’s better to upgrade instead.
I’m not an expert in equipment, so I’m not sure, but there are a few points that I think should be taken into consideration: 1. During the equipment purging process, the ends of the piping system are under reduced pressure; the equipment is not in a sealed state, which means it isn’t fully subjected to the steam pressure during purging. 2. Purging is used only for starting up and shutting down the facility, and the total operating time per year does not exceed 100 hours. It is necessary to increase the temperature and pressure of the equipment as a whole in order to achieve these 100 hours. I remember that one of the standards for pipelines specifies that when the operating hours for a certain condition throughout the year are below 200 hours, it is permissible for the allowable stress of the pipeline to exceed the normal value by 20%.