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Wixlandia · 04-10-2020, 18:59

We are replacing a (just above) water seacock for the toilet seacock.

It's a 1 1/2 inch bronze ball valve, BSPP female female threaded.

The hose barb attachment can be either 'marelon' plastic (BSPP male to barb) or stainless 316 (BSPP male to barb).

Most of the seacocks onboard are bronze, with marelon plastic fittings, but I cannot tell what sealant has been used in the join.

I usually use pink (thicker high quality) teflon tape for my pressure plumbing joints, and have never had a leak - though this is much larger (1 1/2 inch) than usual plumbing joints which are mostly 15mm here in Australia.

I know that female plastic and male metal fittings will lead to cracking, but this is a male plastic fitting to a female brass fitting, so this shouldn't apply.

But, what is the correct sealant to use between the brass and plastic? Or, I can buy a stainless hose barb, but it is 6 times more expensive, and will presumably have a galvanic reaction with the seacock.

peghall · 04-10-2020, 19:39

I know that female plastic and male metal fittings will lead to cracking, but this is a male plastic fitting to a female brass fitting, so this shouldn't apply.

But, what is the correct sealant to use between the brass and plastic? Or, I can buy a stainless hose barb, but it is 6 times more expensive, and will presumably have a galvanic reaction with the seacock

You did mean to say bronze, not brass???

--Peggie

Wixlandia · 04-10-2020, 19:54

Quote:

Originally Posted by peghall

[I]

You did mean to say bronze, not brass???

--Peggie

I did indeed. Might I say that the reason for all these changes is part of the 'refresh' of the head, which I planned after recently reading your excellent book (for those who are interested - it's available on kindle, which made it super easy).

We found the hoses were all permeated using your hot water cloth test, one thing leads to another, and we're refreshing all the plumbing and replacing the head.

For the threads, I am happy with PTFE tape or PTFE cord, I also have loctite pipe dope (but find that it is way too runny usually, and leaves a residue). Traditionally I've always used teflon tape, but now feel teflon cord is probably just as good and much easier. Just not sure what to use with plastic/bronze

Compass790 · 04-10-2020, 20:00

You can just use the PTFE tape as well, just gotta be careful not to overtighten.

Used plenty on larger than 38mm screwed pipe. Or do you mean you want to use a sealant as well as tape?
Loctite 5331 is ok for plastic, bit pricey for a single joint though

Wixlandia · 04-10-2020, 20:44

Quote:

Originally Posted by Compass790

You can just use the PTFE tape as well, just gotta be careful not to overtighten.

Used plenty on larger than 38mm screwed pipe. Or do you mean you want to use a sealant as well as tape?
Loctite 5331 is ok for plastic, bit pricey for a single joint though

Great, thanks. Is there a consensus about whether PTFE tape or PTFE cord/thread (e.g. loctite 55) is better for yachts?

garyfdl · 05-10-2020, 09:50

Any sealant compatible with the plastic pipe should work. I would stay away from any that harden, as it will be an issue if you have to break the joint some time in the future.

In my experience, the biggest problem with metal male to plastic female connections is that people over-tighten the connection. As NPT connections are tapered (not sure about BSP) this stresses the female connector and causes it to split.

Cheechako · 05-10-2020, 10:00

I'd just use teflon tape on plastic to avoid any possible interaction. Be careful tightening plastic fittings, you can often tighten them readily until they crack or distort . They dont really bind up like metal telling you you're tight enough.

Rough Magic · 05-10-2020, 10:14

I have used Rector Seal T plus 2 for years, I think it as good as it gets. Works on most every type of pipe/tubing.

https://rectorseal.com/product/rectorseal-t-plus-2/

billgewater · 05-10-2020, 16:25

Use the teflon. Don't over tighten. Check it out after a few days. If it it leaks at all there will probably be some green on the bronze. Tighten a wee bit more. This is not a hot or high pressure joint so sealing will not be severely tested. If it does go belly up, shut off the valve and try again. That's what the valve is for.
I had two similar arrangements and got away with a bit of petroleum jelly. No leaking after ten years.
I am not sure that the plastic hose tail needs to be an expensive one. The thread is under compression as it's screwing into a female thread so it is "safer". Not under any real load unless the hose is moved around too much.

I would be more concerned with ensuring that the hole cut out for the valve skin fitting is absolutely water proofed particularly if the hull is composite. Don't spare the epoxy.
Use a proper marine sealant under the skin fitting and under the inside backing plate. Not silicon.
Re brass or bronze. Brass is for garden plumbing and fresh water. Bronze is for boats. (The exception being LPG gas or refrigeration, where there is no (hopefully) sea water.) Use bronze only. Do not mix brass with bronze as the zinc in the brass will "leave" in very short time causing all sorts of problems. Bronze skin fitting, bronze valve.
When using the loo, it is important that you can sit back and admire your plumbing.(sic) That one may not get past the moderator.