For instrument piping connections under high-pressure conditions, should socket welding or butt welding be used?
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For pressure grades of CL1500 and CL2500, should butt welding or socket welding be used for the pipe fittings used to connect the instruments? For example, installing pressure gauges, differential pressure instrument pressure guiding tubes, and the like
With such high pressure, socket welding is still the better choice.........
This pressure poses no problem for fittings, whether they are socket-welded or butt-welded; it depends on the skill of the welder. Some pressure conduits are quite long – how can butt welding be used in such cases? There’s no issue with that. Once the appropriate pipes and welding materials are chosen, it comes down to the welder’s proficiency; after all, this isn’t about welding complex pipelines. In terms of the construction requirements in the chemical industry, the pressure tapping points and sampling points for process pipelines are located at the welding points of the process units, that is, at the first valve and the first weld seam
This post was last edited by feng*aosa on 2015-7-2 at 12:14; for instrument pipelines of PN420 and below, both butt welding and fillet welding are acceptable. However, intermediate joints should be used at the welding points; it is not advisable to weld the pipes directly together.
Welding is preferred, after all it is a groove weld with a high sealing coefficient. You can also check the pipeline grade table or the actual conditions on site; it shows the connection methods for small-sized fittings/valves (1/2, 3/4) used for backwashing/venting, to see whether socket welding is permitted. (Here, butt welding is mostly used for high-pressure applications.) For high-pressure levels, the instrumentation piping should use materials/connection methods consistent with those used in the piping discipline.
For grades 900 and above, butt welding should be employed, while socket welding should be avoided as much as possible. Because socket welding does not allow for the inspection of weld quality, or it is extremely difficult to inspect weld quality. It is recommended that socket welding can be used for classes below Class 600, as the requirements for welding quality are not as stringent as those in high-pressure applications.
In the Instrumentation and Control Installation Manual-2012, both butt welding and socket welding methods are covered. However, in practical field situations, such as in a project we designed for a client, the root valves used were socket-welded globe valves (CL1500/2500). To facilitate work by the welders on site, the manufacturer welded sections of pipe before shipping the valves, so that the connectors, tees, and pressure lead pipes at the site could be installed using butt welding – this is a better approach. It is also important to note that during welding, the wall thickness of the piping used for instrumentation must be consistent, such as 18*4 or 19*5.
There is no problem with grooving and butt-welding high-pressure instrument pipes; most domestic instrument valves are also welded by butt welding. Imported high-pressure instrument valves tend to use socket welding, while transmitter connections are all welded by butt welding. For steam and water instruments in power plants that can withstand 20 MPa, the same procedures are followed.
Which standard is this, expert?