I have come across this problem myself. An old and very expierienced plumber gave me this advice. Take two medium size hammers and tap the main nuts on opposite flats at the same time. Work your way around each nut a couple of times then try the spanners again. Dont hit too hard you don't want to destroy the system just free the nuts. It worked for me when I had almost given up.
I managed to release one side, but the other was impossible. I had to resort to draining and taking the gate valves off with the pump.
The stubborn one finally came off with the pump in a vice, the gasket fell apart though, which brings me to another question - can you buy just the gaskets for gate valves, are do you generally need to buy a complete assembly?
If you look at the nuts - new or old, see if there's two squares cut out each side, place these over the flange part of the valve, you can normally cut the nuts off old pump valves and replace the nuts without draining or removing the valve.
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