6 replies to this topic

[SawsanAli311]

I have been going through the various details related to PSV chattering due to exceeding the inlet pressure loss criteria for vapor services in comparison to the PSV blowdown set by the manufacturer. 

However, I was recently discussing with another Engineer the statement in API 520 PII- section 7.3.4  : that keeping the pressure loss below 3% becomes progressively difficult for low pressure and / or as the orifice PRV increase'' . I believed that this difficulty in meeting the criteria for such scenarios are related to low relieving pressure mandating larger PSV area (as calculated by API 520 PIi.e. effective area). Accordingly, the rate area would also be large leading to fast pressure drop as soon as the PSV opens specially when the operating pressure is low and is not widely different from the PSV set point. However, the Engineer disagreed with any relation between the difficulty of meeting the 3 % and the size of the PSV or the fact of low pressure applications. I need your views on this. 

 

Additionally, I was reading several opinions that in case 3% inlet pressure loss can't be met, one of the solutions is either to use a pilot operated valve with remote sensing, increase the blowdown of the PSV if possible OR install staggered PSVs. I was challenged  by the same engineer that staggering smaller PSVs will not help in resolving the issue of exceeding the 3% stating that eventually both PSVs will have their inlet lines experiencing the same flow. I am aware that symmetrical piping also should play a role in resolving the issue, YET the Engineer insists that having a staggered installation of PSVs has no solution with regards to the inlet pressure loss. I also appreciate your views on this. 

 

Thanks alot

[fallah]

 

Sawsan,

 

Firstly, a lower set pressure will lead to higher ratio of inlet pressure loss to the set pressure where meeting the 3% rule becomes so difficult especially when would be accompany with higher PSV orifice size indicated higher relief load.

 

Secondly, in general, using staggered PSV's instead single PSV if there is common inlet line with the same size and length as the single PSV; cannot resolve the problem to meet the 3% rule in full relief load and it might having improvement just in relieving through the first PSV in the scenario with the lowest relieving rate. 

[SawsanAli311]

Thank you very much Fallah for your answer, 

 

Accordingly, we agree on the first point about the difficulty in meeting the inlet pressure losses. 

 

However, for the second point, the staggering would only happen if we also replace the common piping to the multiple PSVs otherwise problem would not be resolved. Yet, can you please clarify why the low set PSV will have improvement in relieving?: ''and it might having improvement just in relieving through the first PSV in the scenario with the lowest relieving rate. '' 

I was under the impression that the lower set PSV (at MAWP) would be the one affected more by potential exceeding the 3 %. Can you kindly please clarify why there would be improvement in the first PSV. 

 

thanks 

[fallah]

 

Sawsan,

 

Because with the same inlet line (partly common and partly dedicated symmetrical inlet line) and almost the same set pressure (just 5% lower regarding the last PSV set pressure) you have lower (and might so much lower) relieving rate hence lower pressure drop (for the scenario with lowest relief load) along the relevant inlet line.

[SawsanAli311]

Thanks Fallah, 

 

I was going through one of the design engineering guidelines and I saw that staggering PSVs (one 1.05 *DP and the other at DP)  for DP/1st PSV SP less  than a specific value of around 10 barg, would make staggering for preventing PSV chattering impractical  due to the close difference between the inlet PSV tolerance of 3% of the DP and the higher set of 5% of the DP''

 

can we conclude from this recommendation that we tend to stagger when find that the benefit of having a 2nd PSV set at 5% above the 1st PSV set pressure would make the application of 3 % inlet pressure loss requirement valid.In other words, if we see that having a 2nd PSV at 1.05 of the DP would still lead to having the 3% pressure tolerance exceeded due to the proximity of the inlet pressure to the set pressure, then we should consider other means of mitigating 3% inlet pressure loss such as the use of pilot operated non-flowing PSVs?

 

Regards, 

[astro]

You'd do well to refer to the (now a little dated) paper by Smith, Burgess and Power, "Relief Device Inlet Piping: Beyond the 3 Percent Rule" available at https://www.smithbur...blications.html (be sure to take note of the errata). Well worth a read.

 

As reviewed in the paper noted, valve stability one cause of valve instability is due to an over-sized valve that's required to relieve at excessive turn-down. This is a valid case for justifying a multi-valve installation with staggered set points. Another alternative to address a wide relief range is a modulating pilot. The former option is generally preferred because conventional valves are typically applicable.

 

As far as I'm aware, the API STDs refer to this point in API STD 521 at 5.8.2.2 Vapor Emission (cf. 6th Ed). However, the point is easily missed if you don't know of its existence. If the point is located somewhere else, please feel free to point it out for me.

[SawsanAli311]

Astro:

 

Hi, 

 

Thank you very much for your valuable input.

 

In fact, oversized PSVs are one of the examples of potential PSV cycling below the natural frequency and as far I have known, API 521 section 5.8.2.2 is where it relates the PSV stable performance to having a capacity above 25% of its rated capacity. Yet, also API 520 P2 section  also discusses that oversizing is unavoidable specially due to the difference between the effective and rated PSV areas as well as the wide rangeability of required relieving capacities for the various relief contingency scenarios. Therefore, as a way to overcome the oversized PSV cycling issues is to stagger the PSVs.

 

I was discussing at the beginning the statement of API 520 PII section 7.3.4 for the difficulty in meeting the 3%  inlet pressure loss criteria for large area PSVs and /or low pressure applications. Oversized PSVs and the difficulty in meeting the inlet pressure loss criteria are somehow related due to the a=fact that with large area, the upstream pressure would not be sustained to create kinetic forces which can overcome the spring force. Specially in full bore design PSVs ( as in API 520 PII 7.3.7.3), the difficulty in meeting the 3% inlet pressure loss is difficult to be met specially that it would need to be estimated based on the valve's rated capacity in case of conventional or balanced bellow PSVs therefore API recommends the use of modulating type pilot operated PSV where the required relieving rate would be used in assessing the 3% inlet pressure loss.

 

For the last point which I added regarding the staggering at low pressures, actually I realized that the statement in that guideline was comparing the  ASME section VIII PSV set pressure tolerance of 3% to the 1.05* 1st PSV set pressure, therefore, the guideline was stating that under low pressures, staggering may be impractical due to the very close limit to the PSV set pressure tolerance.