Isolator valve help

[harlequin]

I'm hoping someone here can offer some advice on what I think is very basic plumbing.

I just found a puddle of water under my sink. Tracking the fast dripping back, the water was coming from the hot water pipe to the kitchen tap. The pipe appears to have two isolator valves (?) so I grabbed a screwdriver and tightened the lower one as much as possible. This is the valve:

http://www.cl.cam.ac.uk/~rkh23/valve1.jpg

The dripping stopped. I assumed I'd just shut off my tap, but to my surprise the hot water flows freely from it. I can't tighten the valve any more. Thinking I'd done something stupid, I loosened the valve and eventually water started spraying from the screw piece (where the screwdriver goes). I quickly re-tightened it.

The leaking has stopped, but I'm confused about what this valve does and I worry something is wrong. Tracing the pipe up, I get to these:

http://www.cl.cam.ac.uk/~rkh23/valve2.jpg

(left is hot, right cold) which look to be valves too. But why have two valves on the hot? There's only one on the cold.

Can anyone explain to me why tightening the lower valve appears to have no effect but to stop the leak? Am I being really dumb?

Thanks
Rob.

[plumbbob]

Can anyone explain to me why tightening the lower valve appears to have no effect but to stop the leak? Am I being really dumb?

The valves don't work as you think! They do not screw and unscrew at all. In fact they only turn through 90 degrees from the off/on position. Turn it further and it is just going on/off/on/off etc. If the screwdriver slot is in-line with the pipe the valve is "open". If it is pointing across the pipe then the valve is shut. Simple as that. If they won't continue to turn indefinitely then then the valve is choked with limescale.

The water leak you describe is just the valve leaking. They do this! I have had brand new valves which spurt water when you turn the screw. Normally the flow stops if you wiggle it but if yours continue to weep then they need replacing.

[harlequin]

Ah. Thanks! That makes more sense. Still not sure why there's two valves but that's not a big concern.

There was a small amout of leaking last night which I eventually traced back to the very top connection to the flexible pipe. I could screw this using my fingers so I got a wrench and tightened it. Since then, no leak so finger's crossed!

The other (cold pipe) seems damp, but I'm putting this down to condensation as it is pretty cold to the touch!

Thanks for the help
Rob.

[plumbbob]

Still not sure why there's two valves but that's not a big concern.

One for the hot (left) and the second (right) for the cold supply.

[harlequin]

Ah - sorry - that wasn't what I meant. There are a total of three valves: two on the hot and one on the cold. It's probably just the result of some earlier plumber not bothering to remove one.

Thanks,
Rob.