bad valve

I live in a 1998 manufactured home with PVC plumbing. The main shutoff valve has crusted open (I assume because of the hard water around here) and can not be closed. The valve is located in the laundry room behind a small panel door in the wall. Is there any way to repair this (just in case I ever need to turn off the water in the future) and why did it crust open so quickly?
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Turn it off at the street connection, take the stem and handle portion out, clean it all with CLR, put it back together. If it's sideways, you may need to use a baggie with rubber bands to keep the solution in while it works. And when you have it back together, and made sure there aren't any leaks, make sure it is all the way open.

and
small
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These valves are not very expensive. I would just buy a new one and replace the old valve. After you replace the shutoff valve, turn it wide open and then close it about turn. Then every few months, close and open the valve to keep it working and prevent it from getting stuck open again.
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replace
valve
How do I get the old valve off? It looks like it is glued in place.
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Ahha! That's why I suggested using CLR instead of replacing the valve. I suggest a trip to Home Depot (sometimes called The Borg around here) or somewhere else you can get advice (that 'glue' is the solvent used to connect the PVC pipes together. "Lost" is a seriously inexperienced neophyte, and our initial thought to 'replace the valve' isn't such an easy task. Bet he first has to figure out where the water company property service connection happens to be. Actually, it's possibly controlled by the mobile home park management.

and
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Sorry to "Lost", who suggested replacement. I lost track of who posted what. Lot to be said for top-posting.

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If it was a connection to which I could apply a wrench I think I could manage... but while I can think of several things that would eat through the pipe nicely I don't think that's quite the effect I want in this particular situation...

I believe there is a second valve outside... box set in the ground with a green cover, no?
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