Thermal Relief - Pipe Protection
Thermal Relief - Pipe Protection
(OP)
I am evaluating a series of relief valves in liquid ammonia service. Pipe only, thermal relief is the only case. I am looking at replacing a bunch of non-certified Rego valves, but before I proceed I would like some input on whether certified valves are really required.
Per API 520 Section 3.8 “Section VIII of the ASME Code requires that capacity certification be obtained for pressure relief valves designed for liquid service”. Section 9 states “Before the ASME Code incorporated requirements for capacity certification, valves were generally sized for liquid service … This sizing method may be used where capacity certification is not required or unknown”.
Interpretation of the last sentence is of most interest. By definition, it would appear that ‘capacity certification is required’, as confirmed by API Section 3.8. However, I wonder if the confusion in the wording of API means that it is not easy to easily interpret ASME. The piping code, B31.3, does refer back to ASME with respect to relief protection.
Anyone else run into this issue?
Per API 520 Section 3.8 “Section VIII of the ASME Code requires that capacity certification be obtained for pressure relief valves designed for liquid service”. Section 9 states “Before the ASME Code incorporated requirements for capacity certification, valves were generally sized for liquid service … This sizing method may be used where capacity certification is not required or unknown”.
Interpretation of the last sentence is of most interest. By definition, it would appear that ‘capacity certification is required’, as confirmed by API Section 3.8. However, I wonder if the confusion in the wording of API means that it is not easy to easily interpret ASME. The piping code, B31.3, does refer back to ASME with respect to relief protection.
Anyone else run into this issue?





RE: Thermal Relief - Pipe Protection
That doesn't answer your question though. It will be interesting to see the rest of the replies.
RE: Thermal Relief - Pipe Protection
The term “capacity certification” simply means that the valve has undergone capacity testing, using the prescribed ASME testing protocol, according to National Board requirements. In other words, the valve has a certified liquid flow coefficient, and it's published in the National Board “Red Book”. To fully understand the text that you cite, you need to understand a little history. Prior to ~1985 there was no such thing as a capacity certified (“ASME code certified”) liquid PSV. Up until then, ASME capacity certification was limited to just vapor PSVs. That is, there was no certified testing protocol for liquid PSVs. In 1985, ASME established the certification requirements for liquid service PSVs, and they were added to the red book. One of the ASME certification requirements stated that the valve’s certified capacity was to be determined at 10% overpressure. Prior to that, almost all liquid valves didn’t reach their capacity until 25% overpressure. So, manufacturers went back and designed liquid trim valves which reach capacity at 10% overpressure.
The first sentence you cite is simply saying that ASME now requires liquid service PSVs to be capacity certified. In other words, if you want your design to be compliant with ASME, then you have to use a capacity certified (“code certified”) PSV – one that has a flow coefficient listed in the red book. The second sentence you cite is effectively saying, “Before 1985, liquid PSVs were sized using the following equations....” And, “This is the sizing method you can use if: (1) your PSV doesn’t have an ASME liquid flow coefficient (isn’t listed in the red book), or (2) your installation doesn’t have to comply with ASME.”
The bottom line answer to your question is this..... If this piping needs a PSV, and you want it to comply with ASME B31.3, then use an ASME certified liquid PSV. Check the vendor catalog to determine whether or not the valve is ASME certified.
RE: Thermal Relief - Pipe Protection
Ref: API 521: Section 5.14 Hydraulic expansion
5.14.2 Sizing and set pressure
" The required relieving rate is not easy to determine. Since every application is for a relieving liquid, the
required relieving rate is small; specifying an oversized device is, therefore, reasonable. A DN 20 × DN 25
(NPS ¾ × NPS 1) relief valve is commonly used. If there is reason to believe that this size is not adequate, the
procedure in 5.14.3 can be applied. If the liquid being relieved is expected to flash or form solids while it
passes through the relieving device, the procedure in 5.21.2 is recommended."
E.g. dont let a detailed engineering contract engineer start to create a calculation sheet for the sizing case.. it is typically not required..
RE: Thermal Relief - Pipe Protection
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9757
"1910.111(b)(9)(ix) A hydrostatic relief valve shall be installed between each pair of valves in the liquid ammonia piping or hose where liquid may be trapped so as to relieve into the atmosphere at a safe location."
don't read too much into the above. it does NOT say "An ASME certified hydrostatic relief valve shall be installed..."
RE: Thermal Relief - Pipe Protection
RE: Thermal Relief - Pipe Protection
In fact, during the initial review of the plant, the NH3 storage standard was dismissed since this facility is an ammonia producer.
My interpretation, while conservative, is to apply the prinicples as outlined by Don. I does seem a bit strange to replace valves where a fractional gpm relief rate is involved, but I can find no basis for maintaining these Rego (non-certified) valves on the basis of saying that the valves are so oversized that they certainly ok.
I hate spending wasted $! Thanks again.
RE: Thermal Relief - Pipe Protection
You need to IMMEDIATLY buy CGA [Compressed Gas Association] G-2.1 -- it has superceded the ANSI K61.1 spec that OSHA cites. Those requirements, through OSHA, have 'force of law' in the USA. Violate them, and somebody may end up in jail.
Further, if handling liquid ammonia, be advised that you have an Auto-Refridgeration potential -- NH3 boils at -29°F. Standard piping materials, SA-53 & SA-106 pipe, and SA-105 flanges, are only certified down to -20°F. To be Code compliant, you will have to use SA-333 Gr. 1 or 6 piping -- it is certified to -50°F; or stainless steel - good to << -250°F.
RE: Thermal Relief - Pipe Protection
Done.
RE: Thermal Relief - Pipe Protection
OSHA 1910.111 specifically exempts ammonia producers, which the OP said they were.
Interestingly, the *main* intent of OSHA 1910.111 was to make it difficult for illegal drug dealers/makers to steal ammonia from farmers, co-ops, low tech distribution facilities, etc. As such, it also addresses security, pin-hole orifices under pressure guages and transmitters, detectors, alarm systems, etc. OSHA figured an ammonia producer would have good security anyway, thus the exemption. The ammonia producer lobbyists saw to that.
Good luck,
Latexman
RE: Thermal Relief - Pipe Protection
Non-Code is Non-Code: gotta have an ASME [or API, if you can find one} acceptance stamp on all primary PSV's [presafety devices; fusible plugs, PSV's, rupture disks, etc.] to have a Code-compliant system.
Ask your Insurance carrier what the financial risk is, if you continue forward with your Non-Code PSV's. It will be in the millions if anyone gets hurt. Makes a few off-the-shelf PRV's or R/D's look pretty cheap, risk/benefit vs. price.
They can be used as a back-up to a Code PRV, or set to pop at a lower pressure than your Code PRV.
RE: Thermal Relief - Pipe Protection
RE: Thermal Relief - Pipe Protection
i worked at an ammonia producer (1500 TPD nh3 plant and typical downstream urea, hno3, nh4no3, etc.) and 60,000 tons of atmospheric storage and we routinely used the small Rego hydrostatic PSV's in liquid piping systems. our insurance carrier and OSHA inspectors (VPP site) never said "boo" about our PSV selections.
if an event happens, regardless of if it was a code or non-code valve, you are pretty much toast either way. that is, whenever anyone gets hurt it is because there was some type of non-compliance.
regarding the auto-refrigeration properties of ammonia, you are fine with A-106 and do not need A-333 or stainless piping for piping that is normally at ambient temperatures, etc.
when an ammonia line is depressured, it will cool but then there is essentially no pressure on the system. a quick look at B31.3 and coincident temperatures and pressures and you will see that ammonia at essentially no pressure (e.g., <4 psig) is acceptable.
that stated, i prefer SCH 10 stainless depending on the situation because too many contract pipefiters will think they are "replacing in kind" carbon steel for carbon steel and you can lose control of piping systems that were originally installed as A333. also, try and find a 3" A333 elbow on short notice vs. 304 SS for repairs/modifications.
RE: Thermal Relief - Pipe Protection
Good luck,
Latexman
RE: Thermal Relief - Pipe Protection
you might want to talk to the enviromental folks and see if the opening of a larger certified "D" PSV results in an ammonia cloud off-site impacting the public.
sometimes the thermal expansion case is because of a hot day. and if you are in nebraska, etc., in the summer time, it is not unlikely that piping in the direct sun can exceed 100°F. and a PSV popping with a long line of saturated 100°F ammonia liquid behind it might stay open longer than was originally anticipated.
RE: Thermal Relief - Pipe Protection
All "code valve" means is that it has undergone capacity certification according to the ASME testing protocol.
As Ben states, one ought not automatically select a D orifice valve for liquid thermal expansion due to ambient heating. In the vast majority of such cases, a much smaller valve is better and safer. A small liquid service valve is a far better selection for those applications rather than a API 526 pop valve, and plenty of those smaller valves are listed in the Red Book (code certified).
RE: Thermal Relief - Pipe Protection
Try and have the set pressure equal to the MAWP of the pipe you are protecting.
Once these valves relieve they can be a nusiance (don't reseat easily).
We usually route the discharge into some type of separator pot. Recently more of the pots have the vapor line connected to a flare header.
OSHA / EPA is getting more strict on NH3 venting etc.
Good luck.
RE: Thermal Relief - Pipe Protection
Bill
RE: Thermal Relief - Pipe Protection
The liquid didn't expand much, but the spring rate of the pipe is so 'fast' that a tiny expansion gives a huge increase in pressure. When your PRV 'burps' out a teaspoon 5ml or so, the thermal expansion has been accomodated and the pipe relaxes back to a MUCH lower pressure. Should be the vapor pressure of NH3 at the skin temperature of the pipe, after the liquid and pipe have cooled some, due to a little autorefrigeration - latent heat of vaporization of 5ml of NH3.
And if all the flanges were a prudent Class 300#, giving a room-temperature MAWP of 740 psig, and the PRV set at 350-650 psig, that PRV will slam shut after after one 'burp'.
Locked-in Liquid Expansion [in a rigid container] is waaay different than relieving an overpressured gas. One 'burp', and you are done.