PRVs on same discharge line (steam) 2

[Thulohot]

Hi,

I am trying to confirm whether a PRV can be discharged into an existing PRV discharge vent. We are adding a steam heat recovery vessel which has a MAWP of 150 PSIG, with a PRV set at 30 PSIG. The existing PRV serves a steam boiler with MAWP of 200 PSIG, and PRV set at 150 PSIG. We had originaly planned on discharging the new PRV into the same discharge line as another heat recovery vessel (30 PSIG), but the client asked us to discharge it into the boiler PRV discharge line. The existing discharge line is 4 inch, our heat recovery vessel has a maximum discharge capacity of 600 lb/hr and the boiler's maximum discharge capacity is 10 350 lb/hr.

Both PRVs could open simultaneously. The existing discharge vent is to atmosphere. Couple questions :
1. Isn't the resulting backpressure from the boiler's PRV opening too much for the 10% backpressure requirement for the new PRV?
2. Setting the new PRV at 150 PSIG still wouldn't help with the backpressure requirement as 10% of 150 = 15 < 150 PSIG.

Is there a way to make it acceptable for the new PRV to be discharged into the existing boiler PRV discharge line.

Projecet is in Canada.

ASME section VIII doesn't seem to give many pointers for this type of installation.

Thanks

 

[Latexman]

Will boiler follow Section I and the pressure vessel follow Section VIII?

How long is the 4” header from each PSV to atmosphere?

Good Luck,
Latexman

 

[The Obturator]

Couple of questions/comments that may help;-
1. Is the existing, and will the new, safety valve have open bonnet construction? Open bonnet designs will not be so much affected by built-up backpressure.
2. What is the size of the safety valves, particularly the outlet size?



*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Thulohot]

@Latexman

Yes, boiler is compliant with Section 1 and pressure vessel will be compliant with section VIII.

The pressure vessel will be about 30 feet from the 4 inch header and the atmosphere is around 40 feet (total pipe length) from the boiler PRV (and header). So the pressure vessel PRV will be 70 feet from atmosphere, while the existing boiler PRVs are 40 feet from atmosphere.

@The Obturator

1. The existing safety valves are Kunkle 6021 series (closed bonnet) while the one we had planned was a Spirax 72109-030 (closed bonnet).
2. For the boiler, it has 2 PRVs (redundancy, each sized for 100%) with a 2.5 inch outlet (1 inch inlet), the planned PRV for the pressure vessel will be 1 inch outlet.

 

[The Obturator]

I don't have the experience to calculate the resultant backpressures that would be generated should both valves discharge at the same time. If Spirax are to supply the new PRV, ask them for advise against some of the calculation methods they have. Why does your client not want to discharge to the more suitable other recovery vessel with similar pressure?

One thing which I would advise caution on, is your comment that there are 2 existing boiler safety valves each sized for 100% capacity. Are both of the safety valves 'live' and inlet lines open? or are they on a changeover device (1 redundant/closed, 1 open)? If it is the case that both are open to discharge 100% capacity each, then you will have a serious problem should an overpressure event occur. Basically, you have at least 100% oversizing that will result in valve chatter etc., and ultimate failure. Do you at least have staggered set pressures on these?


*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

 

[Thulohot]

@ The Obturator

I appreciate the feedback and will reach out to Spirax. As for the existing boilers, they are outside the scope of our project and were installed at least 5-10 years ago (not by us). As they were sold as a package by the boiler manufacturer, I assume the design is sound.

I will say though, without having gone into detail about the existing equipment and piping, there are 2 other boilers, each with 2 PRVs identical to the boiler I mentioned in my post which discharge into a common 6 inch header. As the PRVs are closed bonnet, I did see that as odd given the backpressure when one PRV opens. Again, as this is outside the scope, not really pertinent to my original question.

 

[shvet]

@Thulohot
Why do not you want to provide a hydraulic study of discharge piping? This is a rather simple and reliable calculations. So many details around so simple issue. Why are you asking in an anonym forum someone to conduct that instead of you?

 

[Latexman]

shvet, I was also wondering about that.

Thulohot, let's think out loud with rough numbers on this scenario where both PSVs are discharging. The boiler PSV is the sledgehammer, and will carry the day on creating backpressure. (You never really answered The Obturator on what the boiler's two PSVs set pressures are? The same? Staggered?) IIRC, on multiple reliefs on Section I the accumulation limit is 6%. 200 x 6% = 12 psi. Yes, I know the set pressure is 150 psi, but these are rough conservative numbers. 12 psi backpressure on the 30 psi PSV will probably require a pilot operated PSV, but if you raise the 30 psi PSV set pressure to 150 psi, then a conventional PSV can be used. 150 x 10% = 15 psi. I agree with shvet, a proper hydraulic study is needed to sharpen the pencil! It's not difficult. In fact, I ran a quick one to guide my advice here. It took < 5 minutes, but you have to have the right tool(s).

Good Luck,
Latexman

 

[FacEngrPE]

As soon as the client requested using the boiler relief vent, your scope changed to understanding the impact of the existing relief valve system (including the boiler relief valves) on the proposed installation.

As to your new heat recovery vessel relief valve setting, the vessel might be OK with the 30 psig setting + back pressure from the boiler relief, but the process might not be.