Replacing ball shutoff valve

[b5m]

I have a ball type shutoff valve which has broken and needs to be replaced. The valve will not open or close reliably. The pipes leading into the valve are threaded on both sides and galvanised.
The problem is that there are five valves in parallel and I dont know how to remove only the one valve. If I turn the valve with a spanner it will loosen one side and tighten the other. There is a threaded tube above the valve going into a t-piece which will also loosen one side and tighten the other if I turn it.
Do I need to dismantle all five pipes and valves to replace the one? How do I seal everything up again?

Thanks.

 

[kevplumb]

Do I need to dismantle all five pipes and valves to replace the one? How do I seal everything up again?

yes and fit a union

 

[ChrisR]

Horrible problem. If it helps you can saw/grind through a pipe, unscrew it from both ends and fit screw in ("Male Iron") to copper adaptors for copper pipe. You can also get fittings which go from copper to clamp onto the outside of the galv pipe, but they're bulky.
To fit a union( rotatable flange in the middle) you have to be able to put your own threads on pipes, which is a bit of a pain without good kit.

You can buy "standard length " bits of iron with threads on the end, by the way. Go to bes.ltd.uk and search on "BSP Taper Extended Nipples".