Steam Safety Valve Sizing

[rugbymale]

I am a recent Mechancial Engineering graduate and have been asked to look into the addition of some steam safety valves. I am going over some calculations for safety valves that have been done by others in the past. The difficulty that I am having is with the steam capacity of the safety valve. It looks to me like in the past they have taken the rating of the PRV (lbs/hr) and multiplied by 1.25 in some cases and 1.1 in others. I am wondering what the proper way to calculate the steam capacity of the safety valve is.

Thanks,

Mike.

 

[rugbymale]

I forgot some information. The PSV would be down stream of a PRV on a 200 PSI steam line. The down stream pressure would be 150 PSI.

 

[TBP]

Look at the upstream safety valve setting. A line that OPERATES at 200 PSIG may very well have a safety valve set for 250 PSIG. For example, it could be fed from a boiler, with those safety valves set for 250. How much more will your PRV pass if something goes wrong back at the boiler, and the valve now has an inlet pressure of 250, rather than 200?

 

[zdas04]

Get a copy of API RP 521. The 1.1 factor (times set pressure) is allowable overpressure in all be fire case. The 1.25 probably should be 1.22 which is the allowable overpressure for fire case.

David

 

[MortenA]

That will be API 520 - and remember that theres a new revision out in 2008.

Best regards

Morten

 

[zdas04]

API RP 520 title is Sizing, Selection and Installation of Pressure Relieving Devices in Refineries, Part I and II

API RP 521 title is Pressure-Relieving and Depressurizing Systems, Fifth Edition.

What did he say that makes you think he's working in a refinery?

David

 

[rugbymale]

Actually this is a steam distribution line at a university campus. We are looking at upping the pressure in the main line, and one of the distribution lines will need this new PRV station.

 

[skearse]

To size them according using API methods you'd really need both. And they're both standards now, rather than recommenedd practices (RP), but to the OPs question:

What is it you are trying to protect? Is it an ASME Code rated vessel, or just the piping downstream of the pressure reducing valve? In any case, the MAWP of whatever you are trying to protect is going to determine the set pressure of the safety valve. The sizing is going to be determined using methods from API Stds 520 & 521 and/or the ASME Codes as applicable. Depending on what you are protectign will help determine what the capacity of the valve is-if you have to go through a 'rigiourous' analysis because it is a vessel you are protecting then you will need to consider failures that can cause an overpressure in the protected equipment, such as an external fire, if that is possible, or failure of the reducing valve. If the downstream equipment is already protected, then you need to concentrate on ensuring that the pressure in the line will not exceed the allowable working pressure in the pipe.

 

[davefitz]

You will first need to obtain a copy of the asme design code for which the pipeline is designed for. Likely to be B31.1 or asme sect I ( if it is the boiler superheater outlet safety valve). Each code has explicit rules related to safety relief valves. The nest step is to read the relevant section in that code.

It may be that if multiple (N) relief valves are provided , then only 1 of N valves needs to lift at or below code design pressure.The others may lift at higher , staggered set pressure, but the set pressures must insure that the max permitted overpressure is not exceeded, inlcuding the effects of (set presssure error) and ( accumulation).

Your plant's re-rating of the relief valves for 25% or 10% over mfr's stamped rating might be permitted provided the initial rated lift pressure was lower than the max overpressure permitted by the design code, minus accumulation minus set error. If true, you may want to confirm with the authorized inspector or insurance agency that the plant is permitted to re-rate the relief valve capacity.

But you must recognize that all components attached to the piping must be suitable for that new max overpressure ; and if something fails, they will view such are-rating very critically.

 

[MJCronin]

Rugby...

Get a copy of ASME B31.1. It contains special flow and ratings requirements for PRVs at pressure reducing stations on steam systems.

Alternately, that section of B31.1 states that you can design the low pressure portion of the system to accept the higher upstream pressure and not have to install a PRV. This may be an advantage to you in this system.

Tell us more about the system....
pressures... temperatures....how old is it ... what size pipes are you using ? do you have enough traps ? are all the traps working ???

These systems can be killers....especially during fall startups...

-MJC

 

[MJCronin]

Rugby,

Its a little off topic, but are you aware of this website ?

http://www.kirsner.org/index.html

The articles written by Kirsner are excellent both in thier imporance and in the clarity of writting style.

-MJC

 

[UtilityLouie]

I think that TBP has the right approach...

Find where the reliefs are set on the system at the upstream of the PRV. This is your worst case inlet pressure to the PRV. Talk to the valve manufacturer and get the max choked flow through that valve at the worst case inlet pressure. This is the most flow that can go through your valve no matter what. Figure out what the delta p across the valve is that gets you to choked flow and see how that fits in with your system. I'd still use choked flow as the max flow, but you'll feel better about the end solution.

I would work with relief valve manufacturers on the final sizing. I've picked valves myself, but the vendor always seems to come up with a valve that is more appropriate. They deal with these valves every day. Most engineers in system design don't size relief valves every day.

One note: If your Maximum Allowed Working Pressure (MAWP) of the system down stream is too close to the operating pressure standard spring relief valves could chatter or fail to seat. Select a pilot operated valve in this case. And by close I mean if the operating pressure is within 10-15% of the MAWP use a pilot operated valve.

Of course there are a lot of other questions... If the system was designed around the upstream MAWP, do you even need a PRV down stream? If there is nothing feeding into the system other than high pressure steam side and there is nothing fired on the low pressure side... You may not even have to install one...