Issues regarding flange connections for small-diameter seamless steel pipes
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This post was last edited by pigletxp on 2020-5-26 at 10:46. I would like to ask: in systems that are not under high pressure or high temperature, such as ordinary heating hot water, chilled water, cooling water, compressed air, etc., where the temperature is below 100°C and the pressure is below 1.0 MPA, can seamless straight pipes be used for small pipe diameters of DN50 and below via threaded connections? Are the corresponding fittings (elbows, tees, reducers, etc.) available? I’m currently working on a project that requires seamless steel pipes for the pipelines, in specifications such as D57X3.5, D48X3, D38X3, D32X3, etc. The construction team has informed me that only Marlowe threaded pipe fittings are available, but these fittings are designed to be used with welded steel pipes and do not match the specifications of the seamless pipes. Does that mean that for seamless steel pipes with small diameters, welding connections are also the best option, while flange connections should be used for valves? If all the pipes are connected by welding and only the valves are connected by threading, can matching valves be purchased?
1. Both valves and pipe fittings are available with threads; you can refer to municipal hot water pipelines for guidance. 2. There is no issue with the welding used for pipe fittings – the problem might lie in the specifications, such as using American-standard steel pipes with Chinese-standard fittings. 3. Your pipes are of Chinese standard. 4. Welding is still the better option, as it reduces maintenance issues in the future
The construction contractor reported that threaded fittings made of malleable iron are currently available, but their specifications are designed to match welded steel pipes. Since there are differences in outer diameter and wall thickness between seamless steel pipes and welded steel pipes of corresponding specifications, they do not fit well together. The specifications state that forged steel threaded fittings GB/T14626 should be used for the threaded connections of seamless steel pipes, but the construction party says they are unavailable.
It’s because the construction company didn’t contact me: lol
GB12459 has corresponding fittings, with the smallest size being DN15. Note that there are two series for the outer diameter, and it is necessary to ensure they match; otherwise, misalignment will occur
The last edit to this post was made by Zhang Long201314 on 2020-5-29 at 09:17. 1. Based on your description, it seems to be industrial pipes; the outer diameter of the pipes you purchased belongs to the II series. The fittings specified in the GB14383 standard are of the I series (weldable or threaded), and they do not match the pipes you bought, so they cannot be used. You can purchase fittings of the II series in accordance with the GB12459 standard for welding purposes； Therefore, small-diameter pipes need to be welded, while valves can use flanges. 2. However, the threaded valve connections are all of the I series, so in principle threaded connection is not feasible. 3. The pipes for instrument air can be connected using threads; they are all galvanized pipes with an outer diameter belonging to the I series. Valves can also be connected by threading, with on-site threading machines being used by the construction team for this purpose.
Your question mainly involves clarifying two aspects: whether the pipes used are of Series A or Series B； Types of two-pipe threads ； Fittings with different pipe diameters and pipe threads correspond to various fitting standards, and there are no fittings that cannot be purchased (as long as they meet the standards). Additionally, non-toxic flammable media at 100 degrees and below 1 Mpa can fully use threaded fittings.
The construction company is trying to trick you; how is it possible not to be able to buy it? Tin plumbing fittings definitely won’t work; they burst under even slight pressure. To buy forged pipe fittings (GBT 14383-2008 forged socket-welded and threaded pipe fittings), the price is many times higher.
Under the operating conditions provided by the poster, a threaded connection method can be fully utilized. It is estimated that only some pipe fittings lack threaded accessories, such as reducers. Therefore, a combination of welding and threaded connections seems to be the more reasonable approach.
It can be purchased, but it uses steel pipes from the Series 2 (small diameter) type. No problem