7 replies to this topic

Hi all,

Is there a generic value to design an equipment's PSHH as a percentage of its design pressure?
I've heard it's about 90%. Any recommended references on this?

Thanks!
BC

[pleckner]

The short answer is no. You have the liberty to make it anything you want and it is totally dependent on your particular system.

[djack77494]

Phil is absolutely correct in stating that, "you have the liberty to make it anything you want". Nonetheless, I would recommend the 90% value. You would like the alarm to sound before you have a problem and not to just tell you that it's too late to do anything to avoid a problem. The problem occus when the PSV discharges, but actually can start a bit sooner than the setpoint when the valve starts "simmering". 90% of design pressure or MAWP provides a reasonable allowance, and is the value I would recommend.
Doug

So I guess 90% percent is just like a 'rule of thumb' value.

Ok then, thanks for your responses.
BC

[pleckner]

Sorry but I'm going to have to agree to disagree that 90% be considered a "rule of thumb". It depends on your particular system design. You asked for a PSHH. This implies there is a PSH. What are you setting this value at? The PSHH should be a complete shutdown or a move to a fail safe position depending on the constraints of your system. One of these constraints is the system dynamics. If your system can reach higher than normal pressures very quickly and before your valving, etc. can react (run-away reactions?), then even 90% may be too high. On the other hand, if the dynamics are slow, then you might even get away with 95%. You must also consider the equipment in the system and this may have nothing to do with vessel design pressures but some pressure that can damage internal components way before we are talking about vessel integrity.

Now, in your particular case, if you are only looking to stay away from relief, I would agree with Doug but only after you look at your dynamics.

My point is that we can't just use 90% as a "rule of thumb". Your case might fit this 90% limit very well but I don't want other people to think that they should be using 90% as a limit without thinking about their particular design. And, I don't want you to think you can use this 90% limit throughout your plant.

[djack77494]

I was not adequately descriptive in my suggestion that 90% (should have said sometimes) is a good choice. Phil has provided the needed clarification. I have worked on quite a few systems where a relief event was quite undesirable - hence the suggestion that the PSHH would initiate a "last ditch" effort to avoid popping or even simmerring the PSV. It's worth noting that although the action of an HH or LL switch may be drastic (e.g. shutdown) it is not always so. In my mind, having an HH or LL switch implies that an automated action will be taken to mitigate the situation. It also implies that there is an H or L switch which typically just activates an alarm. I can't say it enough, but every situation is different and there are no "one size fits all" solutions in our business.
HTH,
Doug

I was not adequately descriptive in my suggestion that 90% (should have said sometimes) is a good choice. Phil has provided the needed clarification. I have worked on quite a few systems where a relief event was quite undesirable - hence the suggestion that the PSHH would initiate a "last ditch" effort to avoid popping or even simmerring the PSV. It's worth noting that although the action of an HH or LL switch may be drastic (e.g. shutdown) it is not always so. In my mind, having an HH or LL switch implies that an automated action will be taken to mitigate the situation. It also implies that there is an H or L switch which typically just activates an alarm. I can't say it enough, but every situation is different and there are no "one size fits all" solutions in our business.
HTH,
Doug

I was not adequately descriptive in my suggestion that 90% (should have said sometimes) is a good choice. Phil has provided the needed clarification. I have worked on quite a few systems where a relief event was quite undesirable - hence the suggestion that the PSHH would initiate a "last ditch" effort to avoid popping or even simmerring the PSV. It's worth noting that although the action of an HH or LL switch may be drastic (e.g. shutdown) it is not always so. In my mind, having an HH or LL switch implies that an automated action will be taken to mitigate the situation. It also implies that there is an H or L switch which typically just activates an alarm. I can't say it enough, but every situation is different and there are no "one size fits all" solutions in our business.
HTH,
Doug

I just want to add that I agree very much with this, especially the statement that there are no one size fits all solutions in the business of chemical engineering. The use of "rule of thumb" procedures are dangerous and really need a lot of experienced judgement to apply properly. That is distinct from more general rules, which can be regarded as "guidelines" or "good philosophy". When we are talking about relief valves and other pressure safety devices, one guideline or good philosophy for good practice is to set them at the lowest setting that would be consistent with minimizing the risk of premature or inadvertent opening. This could be 50% of the vessel MAWP, (even more), if that fits the operation of the unit. If it is an RV it will usually be at least 25 to 30 psi above the normal maximum pressure expected in the process. If is is an RD it will usually be set with a little higher differential, maybe 50 psi. The more critical design question is what to set as the design MAWP for the vessel or system. This is partly an economic issue, since higher MAWP usually means higher cost, however depending on the size of the equipment, construction materials, etc. higher MAWP design may be a very small premium for the added durability of the equipment. In such cases, the multiple between max operating pressure for the process, and design MAWP for the equipment can be a factor of 2 or 3 or more. When that is the case it does not make sense at all to relate RV and RD setting as a function of the equipment MAWP. Emergency relief activation usually is a very costly event in the plant, and sometimes of itself causes other dangerous events. Relief at high pressure further exacerbates that and should be avoided where a lower pressure relief is possible.