Full nozzle or semi nozzle - pressure safety/relief valve 1

[EngCutie]

I think I know the mechanical difference between the two: ‘In a full nozzle valve, only the nozzle and disc are exposed to the fluid media when the valve is closed. In a semi nozzle valve, the nozzle, disc, and part of the valve body are exposed to the inlet fluid when the valve is closed’

But when come to choose the valves, which one should I use? Is it purely based on pressure? I remember read somewhere ‘if the pressure is high, full nozzle should be used’, is it true? How high is high? I’m working on a project with design pressure of 15Barg, so I decided to use semi nozzle, is it ok?

Thanks


Cutie

 

[JAlton]

You are correct regarding Full Nozzle and Semi-nozzle. Full Nozzle incorporates the inlet connection (Raised face of Flange or Inlet threads in to nozzle). A semi-nozzle is installed into a valve body/base which has an independent inlet connection. Most semi-nozzle valves, in my experience, are ASME Sec. I, Power Boiler Safety Valves. Such valves are sometimes used in other applications. On the other hand, most ASME Sec. VIII, Div. 1, Pressure Relief Valves are full nozzle designs. ASME Sec. I, Safety Valves are capable of meeting the operational requirements of ASME Sec. VIII-1. However, they are not typically designed for use in a closed system, i.e. backpressure scenario. If you have a Sec. VIII-1, Pressure Vessel to protect, I would suggest you go with an ASME Sec. VIII, "UV" Stamped, Full Nozzle design, Safety-Relief Valve.

JAC

 

[EngCutie]

Thank you, do have a pressure vessel to protect, but the pressure is less than 5 bar, I thought a semi-nozzle would do the job, but luckily, vender quoted full nozzle, it looks like I will never need to use a semi-nozzle relief valve.

Cutie

 

[olcrazy1]

Jalton, not always true that full nozzle valves are Sec.8 and Semi is Sec.1. Most Section 8 pilot valves are semi nozzle, because the pressure pick-up line with some manufactures is at th einlet so they use a semi nozzle on a pilot valve in ASME sec.8 service. Also not always true that Sec.1 valves are semi nozzle, Farris has a Sec.1 valve that is a full nozzle and they promote this as a selling feature since consolidated does use the semi nozzle. So what you said is true in some cases but not always true. Best thing about a full nozzle is like states above when the valve is closed the only pressure retaining parts are the nozzle and disc.

 

[JAlton]

You are correct. My statement was directed toward Direct Spring Valves. Pilot Valves are very nearly all semi-nozzle, but I am an old timer with lots of Sec. I background and not so much Pilot Op. Farris has never been a big player in Sec. I, so I did not even consider their valves in my comments. Farris has a good Sec VIII product in the 2600, but Sec I has never been their forte. My apologies if I misguided anyone. I tried to be clear that I was speaking from personal experience, not making a Statement of Policy for the PRV Industry.

JAC