thePetroStreet

Dear TPS Experts,

I have a situation at site where 4" CS piping is being fabricated for Liq. Ammonia (lethal service) at ambient temperature. We are planning to carry out PWHT of all butt welded joints as per relevant Piping Specification.

Now my query is related to few socket welds of small drain nozzles (1~1.5") of this piping network. Can we exempt heat treatment considering small throat thickness? What is the code requirement of PWHT on these socket joints?

Regards - Qaisar Abbas

What's the material of construction, its carbon steel (A106 Gr. B) or killed carbon steel (A333 Gr. 6)? For liquid ammonia service I have mostly seen killed carbon steel. Also please mention the design temperature?

I believe the material would be killed carbon steel.
Lethal services are dealt in Category M fluid service as per ASME B31.3. And as per para M331, no requirements less stringent than mentioned in Table 331.1.1 should be followed. And for socket weld joints, for P-1 materials having throat thickness less than 16 mm may be waived off from post weld heat treatment. This is exactly as per piping code ASME B31.3.
Now as per my knowledge and experince, piping owner always defines much more stringent requirements above minimum requirements by code for piping in lethal service like liquid ammonia. For your case, I have seen piping owners going for PWHT of 100% of joints in killed carbon steel which have to deal with liquid ammonia no matter whether they are butt or socket.
Being stringent for lethal service especially having lower temperature is always advisable otherwise, you may face issues after some time during operation.

I want to add a point here. For handling anhydrous liquid ammonia, extensive care must be taken as there is always a chance of SCC of carbon steel piping but if you are handling liquid ammonia with required amount of water also to avoid SCC, you may not consider PWHT after careful analysis.
Check with your process and operations people in order to take the final decision.
PWHT effectively reduces the chances of SCC. See this technical paper to find more details.
http://www.nationalboard.org/Index.aspx?pageID=182

I agree with mechcolor, code does not state explicitly about carrying out PWHT of killed carbon steel in lethal service as it leaves the decision with the owner.
Qaisar, what do the project specifications by owner / designer say about this?

Thank you all for valuable inputs.
Let me specify our circuit for subject discussion: We have A-333 Gr6 (P-No.1) piping with liquid ammonia @ design pressure of 550 psig & design temperature of -45~100 F.

Piping spec available for this project does specify 100% PWHT (without mentioning about socket welds). However, I recently came to know about code references. These are from UW-2 (service restrictions) & UCS-56 (2)(c)(2) (PWHT requirements) of ASME Section VIII Div.1.

"When the thickness at welded joints exceeds 5⁄8 in. (16 mm) for carbon (P-No. 1) steels and for all thicknesses for low alloy steels (other than P-No. 1 steels), postweld heat treatment is required." (UW-2 second last para)

"PWHT is mandatory for welded joints of all thicknesses if required by UW-2, except postweld heat treatment is not mandatory for groove welds not over 1⁄2 in. (13 mm) size and fillet welds with a throat not over 1⁄2 in. (13 mm)". (UCS-56 Table for P-1)

So in conclusion, PWHT of socket welds in lethal service is not required below nominal thickness of 13mm.

Regards - Qaisar

Neither UW-2 nor UCS-56 applies to piping.
And the para you referred from ASME Sec. VIII UW-2(d)(2) applies to pressure vessels or parts subjected to direct firing.

Eventhough code is exempting based on weld throt thickness it is advisable to go for PWHT of socket welds considering lethal service.There could be a possibility of delayed weld crack in service if PWHT not done during construction period and relatively higher amount of stress will be induced in case of socket welding.

Though its officially my 1st entry to Petrostreet and little weird as well...

What about Stainless steel at small bore in Liquid Ammonia applications...?

Stainless Steel is perfectly fine with liquid ammonia services rather you would be over killing the material requirement. Most of the grades are good up to temperature as low as -196 deg C.
A-333 Gr. 6 (killed carbon steel) is normally preferred over SS to save much of cost.