[PSUBS-MAILIST] Valve part 2

[Sean T. Stevenson via Personal_Submersibles]

First off, can you confirm that both the male and female thread are of the same thread standard (NPT, ISO / BSP, etc.)? These tapered thread forms are not all interchangeable.

To get deeper engagement on a tapered pipe thread, you can run a tap in the female thread, or run a die over the male thread, or both. (Where the tools are available, it's never a bad idea to dress both sides of a tapered pipe thread connection prior to assembly just to avoid problems with fouled or damaged threads). In this case, I'd be inclined to try to dress the male thread with a die first, because it is likely out of true geometry due to the welding. A die that size won't be a cheap tool though if you don't already own it.

Consider also the function of a PTFE (or similar) pipe thread sealant. The sealant actually serves three purposes, the first of which is, as expected, to seal the spiral thread gap. The second purpose is to prevent galling between similar alloys (particularly an issue with AISI Type 316 / 316L stainless and equivalents like CF-8M) during make up. The third, and arguably most important purpose is to act as a thread lubricant which facilitates greater engagement and interference of the tapered thread by reducing the rotating friction between the parts, ultimately resulting in a smaller gap to be sealed, and consequently a more successful seal.

When working with tapered threads, I typically tighten them hand tight plus 1.5 to 3 full turns, but that is specifically with smaller diameter fittings. 2" and larger fittings may have different recommendations for number of turns as someone else already mentioned, and of course often tapered threads are used to make up fittings with a final required orientation, which isn't necessarily ideal for obtaining optimum assembly torque / engagement. Importantly though, "hand tight" applies to a connection which has first been lubricated with pipe thread sealant, and will typically be quite a bit further engaged than when test fit dry.

Sean

-------- Original Message --------
On May 24, 2021, 02:05, James Frankland via Personal_Submersibles wrote:

> Hi all.
> I didn’t get any responses to my valve dilemma. I’ve made another video with the problem right at the start so you don’t have to watch it all. I would really appreciate some advice here, I’m not sure what to do.
>
> https://youtu.be/qJhVeFrUwWI
>
> Thanks James
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