PSV Inlet & Outlet Philosophy 1

[Hasham BUKHARI]

I wanted to know the philosophy
between inlet and outlet orifice size relationship with each other in Safety Valves.

 

[RVAmeche]

Again, please elaborate what you mean.

 

[Hasham BUKHARI]

How does outlet size affect the overall discharge capacity a safety valve.

 

[The Obturator]

The criteria affecting the capacity of a safety/relief valve are;-
The Nozzle area and the flow coefficient of the valve.
Dependant on valve application and design, the valve outlet should always be equal or greater than the inlet size.
Other factors affect the flow capacity such as back pressures.
The nozzle area is usually smaller (as in API 526) than the inlet nominal bore.
There can be as much as a 90% pressure drop from set point at nozzle, to outlet flange opening (in case of atmospheric relief). This is generally why outlet sizes are larger and lower pressure rated rated eg, In x Nozzle x Out = 2" x H x 3" with 600' x 150# in x out flanging.
API 526 Established these size combinations with major manufacturers many many years ago.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Hasham BUKHARI]

Thanks Sir;

Supposed we are using PSV ANSI 300# Set Pressure: 30Bar & Mass Flow: 11250kg/h Size 1 1/2"x3"
Orifice Letter: H

Need to further understand, what will be the overall effect if we change the size
of inlet connection to 2"x3" 300#, keeping our process parameters same.

 

[The Obturator]

Hasham,

The flow capacity will be he same. The nozzle area is the same from which the flow is controlled by.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[MortenA]

I tend to disagree - at least with the certaincy. The actual orifice of a PSV may differ from that calculated by API. The manufactor will "adjust" to a similar API area based on his knowledge of his design (from inlet to outlet) . Im not say its different just that it could be. Check with the vendor.

Also, its very common to have a reducer upstream the inlet flange. Why do you want to avoid this? Is the inlet dP too high? Some valves have adjustable blowdown. I think this may be an alternative (although it also have its issues - again check with vendor).

Best regards, Morten

 

[The Obturator]

Morten, my comment re same nozzle. relates to the API effective area/coefficient principle. Of course using ASME Actual areas/coefficient is different between mnufacturers. API will at least cover minimum flow.

The ACTUAL CAPACITY of the SRV has, and always will be, determined by that SRV's ACTUAL nozzle area and corresponding ACTUAL COEFFICIENT of discharge, when calculating acc to ASME VIII. This SRV criteria can be found in the National Board Book NB-18 by SRV design/mnufacturer as you know.

Totally agree with reducer on inlet. Never exceed maximum 3% inlet pressure loss to SRV.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[MortenA]

@avalveman, if your PSV has adjustable blowdown imo you _can_ waiver your 3% - but you need to check. Especially wrt you normal operating pressure. You want to be sure that the PSV will reseat.

Best regards, Morten