Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


PSV Inlet & Outlet Philosophy

PSV Inlet & Outlet Philosophy

PSV Inlet & Outlet Philosophy

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

RE: PSV Inlet & Outlet Philosophy

Again, please elaborate what you mean.

RE: PSV Inlet & Outlet Philosophy

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

RE: PSV Inlet & Outlet Philosophy

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.

RE: PSV Inlet & Outlet Philosophy

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.

RE: PSV Inlet & Outlet Philosophy


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.

RE: PSV Inlet & Outlet Philosophy

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

RE: PSV Inlet & Outlet Philosophy

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.

RE: PSV Inlet & Outlet Philosophy

@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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


3D Scanning in the Automotive Industry
With over 100 years of production history, the automotive industry has been at the forefront of manufacturing technology since its inception. Whether the transformative technology of the day was the assembly line, the integration of robotics into the manufacturing process, or the switch from steel to aluminum frame chasses, the automotive industry has consistently implemented advanced technology into its manufacturing and production workflow to improve manufacturing and product performance. Today, the same is true. Download Now
Green light on lidar: Developing low cost systems for autonomous vehicles
Lidar has been around for quite some time, but to date, it’s been custom—and expensive. Right now, there isn’t a clear-cut solution that’s suitable for all applications, such as lidar in autonomous vehicles. As they explore options, optical and mechanical engineers are forced to make choices and tradeoffs during the design process. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close