Threaded joints in steam piping

Can anyone tell me if the rules for use of threaded joints in non-seamless pipe (e.g. A53B ERW) on steam service under B31.1 have changed. In the 1995 code, section 104.1.2 only requires seamless pipe for threaded joints in steam pipe above 250 psig. Is the latest edition different, or has there been some addition or change?

 

[MJCronin]

Neil

B31.1 -1998 (104.1.2(C.1)) states that in addition to seamless construction, the pipe must have an ultimate tensile of 48,000psi or better and be of schedule 80 or better. This means A53 Grade A or B are acceptable

Hope that this helps !!!


MJC

 

[TBP]

One thing that you have to watch in Table A-1 Carbon Steel in B31.1 is that there are TWO spec.no/grades of A53A & B pipe listed. One is seamless, the other is ERW. This is can be really confusing.

I have never actually seen a piece of A53B seamless, and don't believe it's normally commercially available. A106B seamless is a readily available grade with the same ratings as A53B seamless.

 

[MikeONeill]

Seamless pipe is currently produced as triple certified. A106B/A53B/API5LX42 SEAMLESS.

Mike

 

When it comes to steam piping, I've found its best to go with welded fittings, socket weld or butt welded. This will increase the capital cost up front, but will pay dividends in the long run by eliminating or minimizing steam and condensate losses that naturally occur over time due to thread loosening or corrosion. The saved energy costs will more than offset the initial cost, not too mention, the R&M costs incurred to repair leaking joints.

 

[MJCronin]

N.E.Hatfield is right...

Socket welded steam piping is the most commonly installed configuration. Socket welded piping, insatlled per ASME B31.1, is stronger than threaded and less prone to any vibration or condensate/startup hammering failures.

Threaded piping on steam systems are also commonly "seal welded" to ensure against leakage. I have never understood this practice... If you go to all that trouble, you might as well have socket welded joints anyway !!

MJC

 

[BREEN]

Hello,

I think that most experienced piping designers will agree that the best pipe joining method to use to assure long life in steam piping is properly fabricated B16.9 welding fittings. There is a temptation to use threaded joints or socket weld joint with small diameter piping which are generally “field routed” and require no drawings. But consider the “baggage” that these bring.

The designer should consider several issues involving threaded connections. Each thread is an incipient crack in the pipe wall. If there is any cyclic bending (and maybe internal pressure) loading involved you would not expect a long service life. When a seal weld is made over the threads (especially if it does not cover all the threads), the inspector MUST be very careful to look for weld "undercut". With small bore piping, the (field) welder usually changed his electrode angle several times as he makes a full circumferential girth weld. Each time he stops the weld and restarts it he has another opportunity to undercut the weld. I would suggest that you peruse B31.1, Paras 127.4.4 (Fillet Welds) and 127.4.5 (Seal Welds) and especially Figure 127.4.4(A)(d). Remember there is a reason why B31.1 specifies a larger stress intensification factor for certain fillet welds (and threaded joints) in Appendix "D".

Seal welds on threaded joints are specifically disallowed by some corporations as a matter of policy, but not by the Codes. In the case of seal welds, it is understandable that there have been bad experiences because many of them are done badly - not good workmanship. Bad welds CAN do more harm than good sometimes. It is important to remember that a seal weld is, first of all a weld, and that fact brings with it certain fabrication and inspection responsibilities (see references below). The B31 Piping Codes, however do not disallow seal welds - but have tried to point out some of their limitations and pitfalls.

Seal welds are defined in ANSI/ASME B31.3, Process Piping, paragraph 300.2 as follows: "Seal Weld - a weld intended primarily to provide joint tightness against leakage in metallic piping." The definition given by ANSI/ASME B31.1, Power Piping, paragraph 100.2 is as follows: "Seal weld: a weld used on a pipe joint primarily to obtain fluid tightness as opposed to mechanical strength".

Further, B31.3, paragraph 311.2.6, states: "Seal welds (para. 328.5.3) may be used only to prevent leakage of threaded joints and shall not be considered as contributing any strength to the joint". B31.1, paragraph 111.5, states: "Seal welding of connections, including threaded joints, may be used to avoid joint leakage but the welding shall not be considered as contributing any strength to the joint". It can be seen that seal welds, per se, are only used in the B31 piping Codes with threaded joints.

Paragraph 328.5.3 addresses fabrication and it adds:
"Seal welds shall be done by a qualified welder. Seal welds shall cover all exposed threads". Welder qualification is addressed by paragraph 328.2. If seal welding is done, paragraph 331.1.3 should be reviewed to determine if heat treating is required. Paragraph 335.3.3 mandates that seal welds shall be made with no thread compound. Of course, there are limitations on where (what fluid services) threaded joints (with or without seal welds) can be used. B31.1 includes similar requirements and limitations. So, no matter that the weld is not a structural weld, its use carries with it all the associated requirements of any other weld.

Obviously, the implication in these paragraphs is that the seal weld should not be considered a structural weld, that is it does not fulfill the requirements for structural integrity. The term "full penetration weld" is not defined by B31.3. All welds that are required by the structural design of a B31.3 piping system shall conform with the requirements of paragraphs 328.1 through 328.6. The purpose here is to develop the full strength of the base material.

At a risk of adding complication to the issue, I will say we have seen the term "cover welds" used when referring to the fillet placed over a groove weld in the fabrication of some pressure vessels. A properly made fillet, covering any other properly made weld will contribute to the strength developed by the completed weld. These welds are not to be confused with "seal welds".

Socket weld fittings are manufactured to the requirements of ANSI B16.11 (and B16.5 for flanges). Others are made to MSS SP-119. The fittings are made in small bore sizes NPS 1/8 to NPS 4 and in pressure classes 2000, 3000, 4000, and 6000. The ID's of the fittings are typically made the same as Sched. 40, 80 or 160 pipe (depending upon their pressure class).

Socket weld fittings have certain advantages over butt welding fittings:

The pipe does not need to be beveled for weld preparation
No tack welding is needed for alignment as the fitting is self aligning
Weld metal will not extend into the bore of the pipe
They can be used in place of threaded fittings thereby reducing the likelihood of leaks
They provide a less expensive method of construction than other welding methods
(the contractor used random lengths of pipe and a bucket of fittings and Afield routes@ the system)

The disadvantages of socket weld fittings include:

The welder MUST be certain to assure the 1/16 inch recess dimension in the fitting (important) (see ASME B31.1, Para. 127.3(E) and , Figure 127.4.4(B) and Figure 127.4.4(C))
The recess (or crevice) between the OD of the pipe and the ID of the fitting promotes corrosion (in systems where crevice corrosion mar be a design consideration)
Their use in certain services is limited in ASME B31.1 and ASME B31.3
The fittings are not as flexible as butt welding fittings (this must be considered in stress analysis)
Socket weld, small bore, field routed systems are seldom documented with "as-built" drawings

As with seal welded threaded connections, the welding inspector MUST be very careful to look for weld "undercut". I would suggest that you peruse B31.1, Paras 127.4.4 (Fillet Welds) and especially Figure 127.4.4(A)(d).

Make sure that when you do your stress analyses, you include the larger SIF that the Code recommends for the fillet welds where the pipe meets the B16.11 components (reference B31.1, Table D-1, note 11). Remember, by far the most common mode of failure that we see in the field with socket welds is fatigue at the toe of the fillet welds. If this is a system that would be expected to see vibration ( or if it is B31.3 "cyclic service" (also see severe cyclic conditions, also see ASME B31.1, Para111.3.1), do not use them. Specify a weld like the illustration in B31.1 Figure 127.4.4(A)(d). Specify the examination of all finished fillet welds with repair for any undercut. Make sure that the welder provides the 1/16-inch gap shown in B31.1 Figure 127.4.4(B)(c).

See ASME B31.3, para.'s 311.2,4,311.2.5, 328.5.2 (and Fig. 328.5.2), 331.1.3 (and Table 341.3.2), 341.4.3, M311.1, and K311.2.3

Good luck with your projects.

Best regards, John.