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


Instrument Air Piping Testing

Instrument Air Piping Testing

Instrument Air Piping Testing

Hi. We are building a new tank farm and we are having now some problems with our Instrument Air lines testing. They were designed to be threaded and pneumatic tested. The issue is that the construction company is having problems with the criteria of zero leaks during the test (1.1 the Design Pressure, which is 1300kPa). They want to reduce the test to 1.1 the Operation Test (800kPa to 1000kPa). Is there any kind of an acceptance criteria for leakage during testing? Thanks.

RE: Instrument Air Piping Testing

What is your code of construction? If B31.3, why not just use a service test at normal operating pressure?

"Thinking is the hardest work there is, which is probably the reason why so few engage in it."

-Henry Ford

RE: Instrument Air Piping Testing

Dear Tuga29,

The code is B31.3? The material is GI coated CS pipes? Can be tested with DM water at the test pressure. Requires complete drainage of the water after hydro-test followed by nitrogen purging.




RE: Instrument Air Piping Testing

Yes, the Code is B31.3.

That service test at normal operation pressure possibility is only for Category D fluid service right? Our Design Pressure is 1300kPa.

I don't trust these construction guys to use water testing in these instrument lines.

RE: Instrument Air Piping Testing

Ah, I missed the 1300kPa part. My mistake.

"Thinking is the hardest work there is, which is probably the reason why so few engage in it."

-Henry Ford

RE: Instrument Air Piping Testing


(1) Instrument Air Systems (IAS)/Lines are one of the most critical systems in a Process Plant. The IAS system should be installed with special care, by carefully trained workers and kept as dry and as clean as possible.
Introducing water into an IAS system will start corrosion and contamination throughout the whole system that can never be removed.

(2) Are you real sure about the "1300kPa"? It sounds like it's way higher than the normal Instrument Air system would normally have. Remember, Pneumatic instruments are very fragile.

Sometimes its possible to do all the right things and still get bad results

RE: Instrument Air Piping Testing

If these are threaded then you expect some minor leakage.
After all the thread sealant is the only thing actually sealing them.
I have seen such systems where there is an allowed (but very low) leak rate.
Getting zero in this system may be a dream.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Instrument Air Piping Testing

The issue is the criteria of "Zero" leaks.

How is "Zero" being measured? With soapy bubbles? so one bubble every ?? seconds is a failure?

There has been much discussion previously on threaded joints and seal welding / welded joints and the comment often made that sealing threaded joints over a large plant is effectively impossible if the criteria is so stringent as "zero"

Amend the criteria.

This is Instrument Air. Someone needs to get real. I would aim for class IV valve leakage rates (metal to metal seal)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: Instrument Air Piping Testing

IMO, the Instrument Air piping can be service tested with the normal operation pressure if meet the B31.3 Cat. D fluid definition. Of course, it's subject to the Owner's approval.
I thought the experienced Contractor may tight up the screwed joint with the proper joint sealant, the screwed piping can be free of leaking.

RE: Instrument Air Piping Testing

The big issue that we are dealing is that the manufacturer is having leaks. They ddin't tell us yet the gravity, but it doesn't seam good. That's why I would like to know and asked about an acceptance criteria. They asked us if they could only do a service test... it seams they don't have leaks at that pressure. But according to the code we are above the Design Pressure to consider Category D fluid.
Testing at 1.1x1300kpa... what could be an acceptable pressure drop?

RE: Instrument Air Piping Testing

As stated that the design pressure of the instrument air piping is 1300kPa, what's the maximum "operation" pressure of the system?

I don't know any acceptable leak rate for the thread piping testing, because it could be leaking from the multiple screw joints.

RE: Instrument Air Piping Testing

The maximum operation pressure, given by the Design team, is 1000kPa.

RE: Instrument Air Piping Testing

A bit lost following this post.
What pressure are the fittings rated for ?
If they are rated in excess of 1300 kpa x 1.1 then there should be zero leaks (if the installer knows what they are doing).
If less than then there is obviously a problem.
Unfortunately there is no alternative acceptance criteria - either instruct your contractor to provide a code-compliant system or take the responsibility yourself and accept it as non code compliant system.
As noted previously - it is only instrument air.

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