Pneumatic Testing Pressure for Galvanized threaded pipe 2

[nickypaliwal]

Till what pressure have you experience a successful pneumatic test of threaded CS/galvanized CS piping ?

As per recent experience, we are facing issues with pneumatic testing of galvanized CS above 7barg. We need to actually test it at 16barg.

It starts leaking at threaded joints.

 

[pennpiper]

nickypaliwal,
16 Bar = 268PSI?
I think you need to supply more information such as:
- What is the purpose of this system?
- What is the environment and location of this installation?
- Why are you trying to use threaded, Galvanized Carbon Steel pipe?
- Why are you not using High Pressure Tubing commonly used with Pneumatic systems?


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

 

[nickypaliwal]

16 barg = approx. 246 psig

The service in this case is instrument/utility air and is on-plot piping network in desert region. Galvanized piping is recommended since there are chances of water condensation. we have experience with Galvanized CS in this region and nearby facility so we plan to follow the same in new facility but the test pressure here is higher.

The query is only for the max. pneumatic test pressure for threaded fittings of CS/Galvanized CS.

 

[europipe]

Threaded fitting are in 1500, 3000 and 6000#, so that is not the problem,
it is probably not properly installed, normally there is no problem.

 

[LittleInch]

In theory there is no limit to the pressure for threaded joints.

Galvanised joints can be an issue if the galvanizing has been done after threading or is not done well.

Any threaded joint really needs some sort of sealing paste, compound or tape to get the bubble tight seal.

What thread pattern are you using?
Is it a taper thread?
what joint sealing system are you using?



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

 

[Artisi]

if we are going to be pedantic, 16 bar X 14.50377 = 232.06 psi (according to Bing), but it's fairly prevalent any way as it is probably down to poor assembly technique.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[moltenmetal]

Your sealant system is the problem. We routinely pneumatically test galvanized steel threaded systems to the full limit of 150# class (285 x 1.1 = 313 psig) without leakage, using the cheap malleable cast iron 150# fittings. In fact we've taken that system to 300# class without difficulty using 3000# threaded fittings.

The only fitting we've seen fit to do away with is any union beyond 150# class. The unions at 150# are available with brass seating rings, and that makes the difference on the leakage. Above 150#, those unions are steel on steel and tend to leak- we use Victaulic or flanges instead above 150#.

If your NPT threads are cut and assembled properly and you use high density teflon tape (1.3-1.5 s.g.) top-dressed with a good-quality anaerobic pipethread sealant (Henkel Loctite make several good ones), you will be all set

Skip either the tape or the paste sealant and you WILL have problems, even with a system which is initially leak tight. The combination is needed to ensure both good anti-galling during assembly AND resistance to thermal cycling over time.

This sealant system is from an old Dow spec. We've been using it for 20 years and it works like magic- as long as you stay below 350 F which is the limit of the paste sealant.

 

[blacksmith37]

Traditional pipe threads are a poor choice for a critical pressure system. They rely on pipe dope to seal ( as has been noted). Possibly a better dope like API will help. Also the accuracy of the thread contour define how large a leak path must be sealed by dope. Oil production tubulars use metal to metal seals for high pressure.

 

[moltenmetal]

The service is utility/instrument air, not a "critical pressure system". NPT threads work just fine if you use the right sealant system.

 

[weldstan]

Your problems are not new. They are a result of improper installation and tightening therein. Unfortunately for you once improperly installed, it takes quite some time and effort to correct the problem.

 

[nickypaliwal]

Thanks for all your valuable comments.

The proposal under discussion is to provide seal weld of the threaded joints. DO you gentlemen foresee any issue other than safety (due to Zinc Fumes) and damage on internal surface of welded/HAZ area.

 

[TGS4]

Welding galvanized is all kinds of bad. Please research liquid metal embrittlement. People have died due to this. Please don't let your people be counted among the casualties!

 

[nickypaliwal]

@TGS4: We plan to remove the galvanized layer for some portion of length and carry out the welding so LME will be taken care, if there is chance of any issue. In any case I am not aware if LME is possible for CS pipe. It is generally for SS.

Although we do not prefer to use welding but there does not seem to be an alternate solution since it is about 600m long pipe already installed on site.

Like to add that the threads pattern is tapered as asked earlier.

 

[rconner]

When a book named “Methods of Joining Pipe” by J.E. York was published near seven decades ago in 1949 (in which there is a pretty detailed 22 page chapter of technical issues related to various “Screwed Joints”), I believe such methods were however already well-known and long-standing methods of piping connection. This work explains that while the primary responsibility for the tightness of pipe joints is commonly assigned to the Contractor, it is desirable that the Engineer as well as the Contractor understand the “fundamental requirements” for making screwed joints which are “tight”.
I noticed much of this is prefaced with the advice, “Good clean-cut, uniform threads make tight strong joints, eliminate leaks, extend the life of the installation and result in complete satisfaction with the fabrication and the piping material.
Carelessly cut, wavy, shaved or rough threads result in a weakened piping system, localization of corrosion, high maintenance cost, and condemnation of the workman and the piping material.” [At least I guess condemnation may not be as bad as damnation!!]
Even way back then there were many different types of materials threaded and thread assembled, and this reference made clear that how all this is suitably accomplished from thread cutting to assembly etc. varied with materials. How to accomplish all this right even with standardized threads and all sizes etc. is thus not necessarily as simple as it sounds.
At the end of the passage there was discussion of many thread sealants du jour that were also used at the time for various services, concluding with a mention of “wicking” (different than “wiping”!). I think wicking was basically as it sounds something like wrapping some sort of cord that probably looked not unlike a candle wick, and that may or may not have been soaked in some kind of dope, in the thread valleys and in essence that worked sort of like a gasket or packing when tightened up (while the author mentioned this was used in some parts of the country, I don’t think he in general was necessarily too keen on this practice).

 

[moltenmetal]

Seal welding of galvanized piping is NOT AN OPTION. The pipe and fittings are galvanized on both the interior and exterior, and you would need to completely remove ALL thread sealant before even attempting a weld. When you were done, assuming you completely removed the zinc from both the weld area and HAZ on the interior and exterior, including the crests and roots of the female threads of the fitting (you are VERY unlikely to succeed in doing this!), you will end up with a pipe system which will corrode heavily at every single weld.

You need to go back and re-do the work correctly.

A good thread sealant system is more important than the surface finish of the threads themselves, but improperly cut threads are a challenge to seal.

 

[TGS4]

LME is for all steels. And the effort required to COMPLETELY remove the zinc is going to cost more than replacing the entire system. You've chosen galvanized threaded pipe. You've been given great advice here by several people. Just do it right and you'll be ok.