Dear friends,

For a revamp of methanol plant, while checking adequacy of PSVs for blocked outllet and emergency power failure cases, will it be possible to use the redundant PSV(Keeping both the PSVs on line for revamp condition). (Earlier design was 1 + 1 for 100 % redundancy).

What is the code requirement?

GKKannan.

Yes, you can use your 1+1 PSVs as two operating without the redundancy. Code allows you to stagger the set pressures, but you don't have to. If you do, then the valves must be removed and sent to an authorized facility to change the set pressures and re-test and certify them.

One problem you can run into though is chatter. If the two combined are grossly oversized for the worse case scenario, than during any relief you can expect chatter; not a good thing.

It is possible to use both PSVs as working. However, you may need to shut down the unit or section when you have maintenance requirements.

I came across situations where there is only one PSV on vessel (no stand by). But in such case, there was a whole standby vessel itself with another PSV on it. So, you don't have to shut down process if you need any maintenance on PSV.

I have not seen any standard or code for stand by philosophy. It is mostly based on the overpressure protection philosophy of the owner's criteria.

However, a code does say that when you have multiple PSVs, there should be a difference of 5% in the set pressure. This is to ensure gradual increase of flow rate through PSVs in case of emergency.

In your case, I would suggest following.

Is it possible to change the "orifice" of the PSV? It is quite possible to increase the capacity of PSV by changing higher size of orifice in same PSV. This must be verified by PSV vendor. You can ask vendor if the same PSV can be used by just changing the orifice inside it. In such case, you won't have to use stand by PSV to meet
the required relieving capacity.
What you will have to ensure is the adequacy of line size of inlet and outlet piping which you will also need in case if you use both PSVs. The pressure drop in inlet line should not be more than 3% of set pressure as per API codes. The back pressure in outlet of PSV should also be checked as per type of PSV and the flare piping network.

Have you checked the required area for new capacity of governing case? How much it is larger than the present orifice area?

Regards,

I have to take exception to something Nirav said in his post. As I said in my previous post, you can stagger the set pressures but you do NOT have to. Now, I don't really know what the European codes say but as far as ASME Section VIII, Div 1 goes:


"When the required capacity is provided in more than one
pressure relief device, only one pressure relief device
need be set at or below the maximum allowable working
pressure, and the additional pressure relief devices MAY
be set to open at higher pressures but in no case at a
pressure higher than 105% of the maximum allowable
working pressure, except as provided in ( below."

Note the use of the word May. If this was mandatory, then ASME would have used the word SHALL. So you can have the set pressure of one of the PSVs at 105% of the MAWP (or design) but again, you do NOT have to.

I like his suggestion about seeing if the PSV vendor can change the "orifice" of the existing PSV but you will have to have the valve re-tested for certification. That is something the PSV vendor should do anyways if the valve is modified in any way. Note that a PSV really doesn't have an orifice in the true sense of an orifice, it has a nozzle.

Further to Nirav's statements. Yes, you must check the lines for the new capacity. I'm very concerned about both the inlet pressure drop exceeding the 3% rule significanly and increasing the built up backpressure against the PSVs. The former creating a chatter problem, which I mentioned in my previous post and the latter causing a loss of PSV capacity. NOTE, the 3% Rule is NOT a code requirement. First, API is not code; it is a recommended practice, note the title API RP520/RP521. The "RP" stands for Recommended Practice. This is not code by any means. It can be considered "Good Engineering Practice", which is almost as good as code in my book but is not code (or law). Second, the 3% rule is actually stated in ASME Section VIII but it is included in the Nonmandatory Appendix M. Again, not code (law) but should be considered "Good Engineering Practice" nevertheless.

A final note to all or our readers and particpants. It's not that I want to be anal about the terminology and such. We must all recognize what is required by law(s) and what are standards, guides and just good engineering practices. We don't want to be doing things that will increase cost and really don't give us any safety benefit. Even the 3% Rule can be violated without consequence. I've written about this in articles published on this web site.

Thanks for your attention.

Hello Leckner,

I fully agree what you say about the difference between "code" and "Recommended Practice" or "good engineering practice".

I would still say that "recommended practices" should be followed to the extent possible to avoid any unwanted or unforseen situations. However, if it is not feasible either from economics or practical point of view (mostly in revamp cases), we can deviate from practices, but deviation should be based on experience and of lower magnitude.
Further, we must not and can not deviate in any case from code(s) or law(s).

Again, thank you very much for more elaborate explanations on this topic.

Warm regards,

And please be carfeful about interlocked valves. 1+1 normally implies the inlet valves are interlocked - one LO and the other LC. This may not always be the case, but in most circumstances will be. In the new scenario both the valves be LO or removed.

Regards