Is it necessary to install an emergency shut-off valve at the outlet of the expansion end of the expansion compressor?
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Dear seniors, I’m currently facing a problem that’s quite confusing, and I hope you can offer some guidance: 1. According to the specifications, a quick shut-off valve is required only at the inlet of the expansion stage for expansion compressors, whereas no such valve is required at the outlet of the expansion stage. For the project I’m working on currently, the FEED documents specified the installation of shut-off valves at both the inlet and outlet. I believe this is incorrect; in my opinion, shut-off valves should not be installed at the outlet of the expansion stage when the expansion compressor is shut down, as this could lead to runaway operation. I’m not sure if my understanding is correct During the process review, I also expressed my understanding, but the original design insisted that the shut-off valve had to be retained for the outlet, so it was kept in place. 2. Later, during the HAZOP analysis of the expansion compressor process, it was mentioned that if the filter at the inlet of the expansion section became clogged, it could result in low inlet pressure. Therefore, it was recommended in the conclusions to add interlocks for low inlet pressure at the expansion section, as well as interlocks for high inlet pressure at the pressurization section. My understanding is that a low-pressure low-level interlock at the expansion end inlet is not necessary; if the pressure at the expansion end inlet drops, the volume of air entering the expander will decrease, and its speed will gradually fall until it stops automatically. Therefore, there is no need to set up a separate interlock. Regarding the above issues, I would appreciate it if the more experienced colleagues could point out any mistakes in my understanding. (1) Can an emergency shut-off valve be installed at the outlet of the expansion end? Would that cause any problems? (2) At present, the interlock for low-low pressure at the inlet of the expansion side and the interlock for high-high pressure at the inlet of the pressurization side are still retained in the process, as they have been approved by the process owner and modifying them would be rather complicated. Setting values are required during the integration of the DCS system; therefore, I would like to ask how I should set these two values PS: Inlet and outlet pressures at the expansion end: 1025 psig (7.07 MPa) ------- 435 psig (3.0 MPa). Inlet and outlet pressures at the compression end: 410 psig (2.8 MPa) ---------- 520 psig (3.59 MPa)
First question: I’ve hardly ever seen an emergency shut-off valve installed at the expansion machine’s outlet, but personally I think it’s a good idea – it doesn’t cost much~~ Hehe~~ As for the issue of runaway speed you mentioned, it’s mainly caused by the pressure-boosting side; when the emergency shut-off valve activates, the inlet and outlet of the expansion machine are closed, so there’s no airflow and thus no rotation speed. Remember, the expansion side is the active side~~ Second question: It’s fine to install a pressure differential interlock on the inlet filter – it’s useful in case of accidents such as ice or sand blockages. As for whether to disable this interlock during normal operation, that depends on the owner~~ As for the high pressure on the pressure-boosting side, what was the purpose of installing that interlock in the first place? What kind of process have you designed?~~
Thank you for the advice. No differential pressure interlock has been set for the inlet filter; instead, a low-low pressure interlock is installed at the inlet of the expansion section. I don’t quite understand: if the filter is clogged, then the inlet pressure decreases, which means that the amount of air flowing into the inlet side of the expander reduces. Wouldn’t the speed of the expander automatically decrease as a result? It would stop operating automatically once it reaches a certain level. So why is it still necessary? I’m not sure how to interpret the usefulness in the accident you mentioned; please give me more guidance. Thank you! Furthermore, I don’t think it’s necessary to install a high-pressure interlock at the inlet of the pressurization side; a high-pressure alarm would be sufficient
This must be the post-pressurization process for natural gas; when the filter gets clogged, the pressure at the inlet of the expander decreases, and its speed also drops. The automatic shutdown you mentioned is taking things to an extreme. Normally, it’s not allowed to get clogged to that extent. Generally, the differential pressure of the filter is controlled; when it becomes clogged to a certain extent, maintenance is required to prevent further blockage, as this would have a negative impact on the production efficiency and economic benefits of the facility. This pressure difference is necessary, unless the cleanliness of your process gas is guaranteed; in that case, it is also possible to remove the filter screen from an engineering perspective. You are using a post-pressurization process, with a high-high interlock set at the inlet of the pressurization section; I’m not sure what the purpose of this is. If a high-high interlock is set at the outlet to prevent excessive system pressure from affecting subsequent processes, that is understandable :lol
Thank you for your reply. As you said, it is the post-boost process. I understand what you mean – the filter needs to be replaced when the pressure difference across it reaches a certain value. I have also set up pressure difference alarms before and after the filter, as well as an overpressure alarm upstream of the filter. What I’m not sure about now is what value should be set for the low-low threshold. Since I am not very familiar with expanders, in my opinion it should be possible for the unit to have its own shut-off protection, just like other compressors – the unit should shut down when the inlet pressure drops to a certain level. In my system, the set value for this low-inlet-pressure interlock only needs to be slightly higher than the unit’s own set value. I’m not sure if my understanding is correct; I would appreciate it if everyone could give me some guidance. Thank you! I think it’s a mistake to have a high-high interlock at the inlet of the pressurization side; I plan to make changes by removing this interlock and installing only a high-pressure alarm.
If a low-low interlock is installed at the inlet of the expander, it is likely to be for the purpose of ensuring the safety of the expander. For example, if moisture or silica sand gets into the plate sections or the expansion inlet, although inlet filters can capture some of this material, ice crystals can still strike the expander’s impellers, leading to vibrations and increased shaft temperature. As for installing a high-high interlock for the pressure at the pressure-boosting end, this is not common; usually, a high-pressure alarm along with a constant-pressure valve is sufficient. There is no need to implement an interlock that shuts down the equipment in such cases. I suspect that the design here is intended to prevent deviations from normal operating conditions that could lead to accidents like runaway operation~~
Thank you for your reply. This morning I spoke with technicians from a domestic expander manufacturer; they believe that it’s not necessary to install a low-pressure interlock at the inlet of the expansion section – they usually don’t use such a feature. However, they think that this interlock might be installed out of concern that the oil film may not cover the bearings evenly when the speed drops, and it’s possible to stop the machine when the speed falls to that level as well.
It is indeed not advisable for the expander to operate at its operating speed for long periods of time. However, this concern should be addressed through interlocks related to the speed – for example, if the speed drops below 5000 for more than 3 minutes, the expander should be shut down automatically~
I agree with you. I understand that if the inlet filter becomes clogged, the speed of the expander should decrease as the inlet pressure and flow rate drop. Once it falls below a certain set speed – that is, when, as I mentioned earlier, a uniform oil film cannot be formed to cover the bearings – after a period of time, such as 3 or 5 minutes, the unit will be shut down via interlock. But I’m not sure whether foreign manufacturers have set this value; it’s difficult to communicate with them, and they’re reluctant to reply to our emails. It seems that domestic manufacturers haven’t considered such things as much. .
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