Issues regarding welding materials for steel-in-steel directly-buried steam pipes
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Our project supplies steam to external users using prefabricated directly-buried steam pipes (steel-clad steel); the inner pipe is made of 20#, while the outer pipe is a spiral-welded Q235B pipe. The design institute provided us with J427 as the welding material for our welding designs. One of our technicians suggested using J507, saying that its price is similar to that of J427, but it performs better, results in a higher success rate in inspections, and is also easier to obtain. I’m not sure if this statement is correct Dear sea friends, is there anyone who is familiar with welding materials? Or are there any relevant guidelines suggesting which welding materials should be used for welding which types of materials? Also, for our inner tube, we plan to use TIG welding for the root pass and MIG welding for the cover pass. So what material should be used for TIG welding? So, there’s no need for TIG welding as a backing layer for the outer tube; just beveling it and then welding it is sufficient, right?
This post was last edited by nj1952 on 2017-8-8 at 19:00. (1) The design adopts equal-strength welding, in which the strength of the deposited metal is the same as that of the base material, satisfying the requirements of the specifications. (2) Welding shall be carried out in accordance with NB/T 47015-2011 \"Code for Welding of Pressure Vessels\". (3) 20 steel, Q235B welding materials: Shielded metal arc welding: J427 electrode ； TIG welding: H08A wire.
How to use TIG welding for the root pass in linear engineering? You can’t just carry around argon cylinders everywhere.
Buried pipelines should undergo pressure testing in segments, with water pressure applied to the inner pipe and air pressure to the outer pipe.
From what I understand, for one of the projects here, only the outer tube is subjected to a gas-tightness test, using 0.4 MPa of compressed air at a pressure that is maintained for 30 minutes. The inner tube was tested using 100% RT, the outer tube using 100% UT; no hydrostatic testing was performed on the inner tube.
The inner tube is subjected to RT testing, but not hydrostatic testing. Performing RT testing only ensures the quality of the welds; it does not guarantee that there are no issues with the inner tube. After all, the steel-clad pipe products purchased are only responsible for the quality of their respective segmented parts, and not for the inner tube.
Each section of the steel-in-steel pipe is in good condition, and the weld quality is also good; there should be no problems. Unless they damage the originally good pipe sections during construction, such as accidentally cutting through the pipe during gas cutting. Of course, using water pressure is better, but to do that you need a water source and a pump, and those conditions aren’t available on site. Can the inner tube also be tested for airtightness?
The inner tube is not airtight. The outer tube is a pipeline at atmospheric pressure, and the airtightness test is conducted to prevent leaks in the outer tube, which could lead to a reduction in the insulation efficiency. Generally, an airtightness of 0.1 Mpa is sufficient for the outer tube. The inner tube is a pressure pipeline that needs to be pressurized to the design pressure; obviously, airtightness alone cannot achieve this design pressure.
It’s hard to find a water source with sufficient water pressure. The conditions are difficult to meet.
Pressure testing is carried out in segments; even if there are a few fewer pipe sections, pressure testing is still performed on them. Once buried pipes are underground, it is extremely difficult to repair them once problems arise.