Unscrewing limescale-encrusted thread on a potable water system

[Caecilius]

Following on from an earlier post about a pressure reducing valve on a megaflo cylinder, I've decided to remove the cartridge and clean out the filter.

Guess what - it won't unscrew, and seems to be stuck fast with limescale. Here's a pic:

So the question is: what's the best way to tackle this? I assume plus-gas and WD40 are not a good idea because this is connected to the potable water supply. Would it be worth using some sort of descaling cleaner to remove the limescale, or wouldn't that get down into the threads?

I'm using a 12" adjustable spanner, so one option is to use a longer one - like a big set of stilsons, but I'm worried about putting too much stress on the surrounding pipework.

[The Medway Handyman]

Get a blow torch on it if possible, the expansion will crack the limescale.

[stuart noble]

Giving the spanner a sharp tap with a hammer is better than putting pressure on it. It's the impact that shifts things

[Nick]

Would there be any plastic/rubber components within? Heat might damage these. What about cleaning thoroughly with wire brush, and a few taps around the periphery with a toffee hammer. Then a few drops of Plus-Gas or similar at the joint, leave to soak and repeat as seems fit. I can't imagine WD40 having much effect. A few drops isn't going to greatly contaminate the system. Then a well fitting ring spanner and give it a decent thump whilst bracing adjoing pipework. If all else fails, and I hesitate to say this...........................................................angle grinder. Nick.

[Caecilius]

Yes, I suspect there may be heat sensitive stuff inside, so I'm wary of using a blowtorch.

I wish I could use a proper spanner, but it's about 40mm (or one and a half inches) across the flats, so I don't have a spanner that would fit. And most spanners only seem to go up to about one inch anyway. So I need to use an adjustable, which is not the best thing for these circumstances.

No, stilsons come before angle grinder surely.

[Tim+]

Can you soak a rag in descaler (vinegar maybe) and drape it over the valve? Maybe wrap it in cling film to stop it drying out.

Tim

[stuart noble]

grinder.

Do acids dry out? IIRC the stronger ones at least get wetter when exposed to the air

[Caecilius]

The pipes are copper, and the valve body is brass. So I don't think vinegar would be a good idea, as it tends to corrode it.

I'm sure I can remove the surface scale easily enough, but I think it's the stuff in the threads that's causing the problem. So I'd need something that would wick down the threads and dissolve the limescale without corroding the metal.