gate valves - close watertight ?

When they are brand new and shiny, probably. After a while in service, probably not. I have never had one that worked after a year, even with monthly maintenance like opening and closing.

Is that true for domestic gate valves? I thought the main use of a (domestic) gate valve was throttling, since they cannot be relied on for isolating ! Simon.

If used for throttling they wear quicker particularly with high pressure hot due to cavitation. I wouldn't rely on gate valves for anything when there are better (and usually cheaper) alternatives.

Yep, I've never been inclined to use one ! Simon.

They may close off completely when new - but as soon as any crud deposits in the bottom of the gate they don't tend to seal properly.

Their other party trick is the spindle breaking off internally, leaving a knob that turns and nothing actually happens - it stays open or closed

- wherever it was left when it broke.

I an industrial application, you fit them in a position so crud doesn't collect.

Baz

Can't see how you can avoid it when used on fresh water applications in a hard water area. Even if they don't fail to seal they can still break

- especially if not used regularly.