1 reply to this topic

[shinchan]

Hi! Good day to all of you!

I am a junior engineer currently working on a revamp project involving two reciprocating compressors. One of the compressors is motor driven while the other one is steam turbine driven. Both have PRV's located at their discharge. The motor driven one is usually operating while the other one is the spare. However, I have observed that the set pressure for the PRV for the steam turbine driven compressor is slightly lower than the set pressure for the motor driven one even though the design pressures for both compressors are the same. I would like to ask if anyone knows the possible reason for this?

Thanks.

[Art Montemayor]

shinchan:

First of all, I don't believe the reason for the difference in the PSV set pressures is due to the difference in type of driver. It probably is due to something else.

You haven't stated that the service is common to both, that both machines are identical (except for drivers), and that the over-pressure scenarios on the discharge port are identical to both. Consequently, I have to assume that the machines are servicing different process needs. The reason for the set pressure difference then becomes a possibility of the following:

1. Different gases; different service or different discharge conditions;

2. Different discharge piping configuration; you could have an element (like a filter, scrubber, or cooler) in the discharge of one of the machines that reduces the maximum allowable pressure on that system;

3. Even if the machines are identical, the scenarios for each machine may have been developed by different engineers and one of them simply may have made a conservative calculation (I hope) on one of the machines.

Allow me to caution you about the "design pressure" of the compressors. You are supposed to consider the Maximum Allowable Working Pressure when debating the set PSV pressure - not the "design" pressure. Usually the design value is more conservative than the MAWP, so usage of the design value is safe. The important point here is to know the different and distinct elements involved in making calculations and judgements regarding PSVs.

I applaud your interest in noting the difference in set pressures on the machines. But, more importantly, I believe that young engineers should make it a daily practice to mentally (& sometimes by calculations) review the related over-pressure scenarios that affect the critical machines and vessels in their plants. This is one of the most important aspects that affect a plant engineer's key learnings and his/her ability to make positive, safe, and efficient contributions to the safe operations of a plant. It affects your safety and protection as well as those around you. Sooner or later, you will be called to participate in a HazOp and the experience and practice of critically visualizing and analyzing these situations will be of inestimable worth to you.

Although you haven't supplied enought basic data to evaluate the situation, what is more important is that YOU fully understand and appreciate the process of putting together all the possible over-pressure scenarios related to these positive displacement machines and, subsequently, picking the correct worst scenario and evaluating whether or not you have the correct PSV to protect you and your fellow co-workers.

I hope this helps you out. Don't rest until you find the correct answer to your query. Good luck.

Art Montemayor