In message , Set Square
writes
Most pumps seem to come with dire warnings about not running them dry.

Anyone know why this is? Does a pump rely on water to cool it - albeit using
'coolant' at 80-odd degrees? Or is water needed to lubricate the bearing in
some way?

It would sometimes be useful to test a pump dry - by running it for a minute
or so -before fitting it, and I'm trying to assess the consequences of so
doing.

It uses water both as a coolant and as a bearing for the ceramic shaft.

I did run a new Grundfoss 15/50 dry when I needed it as a load (well, I
only paid a fiver for it) and had no water to hand

It lasted about 20 seconds before it started squealing like buggery and
I had to remove the power

I don't recall if it got hot

I'm not really sure exactly why you might find it useful to run one dry
and what you would hope to achieve, but I would put it in the category
of less than sensible things to do

--
geoff