Hi, I've been looking at some PSV/RV catalogues and I noticed an Orifice Designation Letter, all I know is it is based on API standards and different letter represents different sizes and that manufacturer's also have their own definition.

However just like to know what does this letter represent on the Valve? Is there an orifice plate inside the valve as I thought it operates based on Springs?

PSVs have a opening, like circular. Which can be assumed as an orifice. The opening is kept closed by spring force, until sufficient pressure reaches, above the spring force. When operating pressure reaches this spring force, fluid will be released through this opening. Opening is expressed in orifice area, sq.in or sq.mm.

API 526 (if I remember) mentiones standard orifice areas available. These are expressed with different letters, like D,E,P etc. Each letter represents a particular orifice area, like 0.11 sq.in for D orifice.

Hope you have been answered.

Sastry

When do we use the name Safety Valve, Relief Valve and Safety Relief Valve seeing it has to do with the Design Type.

Also some Design Basis is based on Fire while others are based on BackFlow, Control Valve Failure. Can you guys explain also abit about these?

Please refer to API Std 521 - ISO 23251 - Ed 5 - Jan 2007 - Pressure-relieving and Depressuring Systems section 3 for definition of Safety Valve, Relief valve, etc...



Please refer to API Std 521 - ISO 23251 - Ed 5 - Jan 2007 - Pressure-relieving and Depressuring Systems, section 4.3 for understanding of overpressure scenarios...

Grab technical manual from ANDERSON and GREENWOOD Pressure Relief Technical Manual, CROSBY'S PSV Engineering Handbook, etc (CLICK HERE) for better understanding of Pressure Relieving Devices...

Taking to heart what I've seen Phil Lechner discuss when talking about the thermodynamics of pressure relief, it is more correct to think of the internals of the RV as resembling a nozzle and NOT an orifice. A perfect nozzle will act isentropically, while expansion across an orifice is closer to an adiabatic process.
Doug

Doug,
Good remark...

I believe this should have been known by API long time ago...but API still keeping it as orifice ? I am still struggling myself why API keeping this definition... Any enlighten points ?

Sorry Joe, but I don't have any further enlightenment to provide. Perhaps the concept of an orifice is easier to comprehend than that of an undefined shape called a nozzle??

Doug,
Thanks for your response.

Let see how i struggle with this nozzle and orifice...

I have seen some PSV "nozzle" is convergence type. Is convergence nozzle would behave more like a orifice ?

I also have seen the PSV "nozzle" is rather short (<100mm). Can we consider it is a "thick wall" orifice which behave differently than "thin wall" orifice ?

As per Dr. Fauske studies, equilibrium will not reach if nozzle length less than 100mm. Can this "nozzle" behave like a orifice as the equilibrium has not been reached ?
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I am sorry. I should stop for this. Otherwise it end up a debate on nozzle and orifice.

I like your statement "...A perfect nozzle will act isentropically..." and what Phil's believe and defensed till last drop of.... It give some conservatism from PSV sizing perspective...

Thanks a again for your contribution.

Joe,

If I could continue on your line of thought in the post above, but add a thought or two of my own creation . . .

While a PSV may perform somewhere between the theoretical models of an orifice and a nozzle, it is closest to the nozzle.

From past experience, more flow will usually result from a frozen equilibrium model (flow in PSV nozzle does not reach full equilibrium within the nozzle as it flows along) than a homogeneous equilibrium model (flow in PSV nozzle reaches full equilibrium within the nozzle as it flows along). This is kind of a safety factor for the PSV vendor and Engineer who is sizing it for multi-phase flow. This effect should not get cancelled out by the flow coefficient of the PSV which is usually based on single-phase flow, like water, steam, and air.