Overpressure Scenarios for Pressure Regulating/Let-Down Package - Double Jeopardy? 2

[Sirius P.Eng.]

We are currently designing a pressure regulating/let-down package for a natural gas pressure regulating and metering station (RMS) that consists of an active regulator, a monitor regulator and a safety shut-off valve (SSV) connected in series - in the direction of flow the SSV comes first, then comes the "Monitor Regulator" and then the "Active Regulator". The Active regulator is to be fitted with an electric actuator. Both the Monitor Regulator and the SSV are self-regulating.
In normal operation, the active regulator regulates gas pressure from 85 barg to 40 barg; while the monitor regulator and SSV are fully open. When the active regulator fails (in a fully open position), the monitor regulator, set at 42 barg, takes over the pressure regulation duty, and when that fails the SSV shuts (at 44 barg) to isolate and protect the low-pressure side from overpressure.

My colleagues and I have discussed at length these issues but I am not very certain and so would like to hear from the experienced and highly distinguished Engineers in this forum.

So here goes, in sizing the PSVs (to be placed downstream of the pressure regulators):

1. Is it double jeopardy to consider that both the active regulator and the monitor regulator fail in a scenario.
2. Is it double jeopardy to consider that both the monitor regulator and the SSV fail.
3. If (1) and (2) can be considered as double jeopardy, then what is/can be the justification for the provision of a PSV downstream of the regulators.
4. If a jet fire scenario is then found to be the governing overpressure scenario, what other means of effective overpressure protection may be provided against overpressure; bearing in mind that during exposure to a localized jet fire, failure can often occur due to localized overheating for which a PSV is ineffective (API 521). Other scenarios considered include blocked outlet and regulator failure - which was deemed not credible.
5. We found that in a similar project, the EPCC contractor did not take credit for the SSV and also did not consider the failure of the Active and Monitor Regulators as double jeopardy. Please share your ideas/experiences/concerns/comments/suggestions, etc.

 

[georgeverghese]

Would suspect the downstream system is a pipeline built to B31.8 code? Have seen several such systems in pipeline pressure reduction service. Am no expert in the interpretation of B31.8 re MAWP(pipeline engineers would know better), but its interpretation is surely different from that for B31.3 (inplant piping). The regulators used are also not the ones familiar to plant engineers ( Fisher and the like); they are much bigger, and may even be claimed to be good for HIPPS service ( or so the manufacturers claim - one example is Mokveld). From memory I recall the pressure transmitters for overpressure detection are configured 2oo3 voting.

The PSV downstream is only for minor leaks across the SSV when closed -less than 5% of rated flow of the regulator, I recall - it is not meant for full flow relief. I dont remember what provisions are made for reset of the trip loop once pressure downstream has been reduced to below PSHH. The systems I have seen are usually 2x75% or 3x50% runs or something similar.

The treatment of B31.8 pipeline overpressure is also seen to be different to B31.3 when you look at the corresponding safety trip device configurations for pipelines in API RP 14C - you can see there is no full flow PSV downstream in 14C. Jet fires are not a concern for B31.8 pipelines I think. There is a throttle bypass across one of the regulators for low flow on startup pressurisation ?

 

[don1980]

In the US, monitor regulators are use used as an alternatives to relief devices on interstate pipelines. Their high integrity (equivalent to a PSV) is maintained by governmental regulations which require inspection/testing every 6 months. Of course, this may vary in other countries where governmental regulations are different. So, I'll answering your questions from a US perspective and leave it to you to determine whether this is applicable in your country.

1) One would not consider failure of the process control regulator and the monitor regulator at the same time. The question is equivalent to asking whether one would consider failure of a process control regulator (or control valve) and failure of the PSV. Of course, that answer is no.
2) One would not consider failure of a monitor regulator and a PSV at the same time. The question is equivalent to asking whether one would consider the failure of two independent PSVs at the same time. Note that even when one PSV is installed, we don't consider the possibility that it will fail to operate. That is, we don't install a second PSV due to concern that the first one may fail.
3) If the monitor regulator is regularly tested, as is required, then there is no justification for also installing a PSV.
4) Jet fires are not considered when sizing relief devices. Relief devices are based on pool fires. When a vessel in exposed to a jet fire, a PSV is incapable of providing effective protection. The localized high intensity heating caused by a jet fire will cause the vessel to fail, regardless of the size of the PSV.
5) No comment - I can't say what the EPC engineers was thinking because I don't have any info about this application, or knowledge of the regulations in your country.