In uk.d-i-y Jeff Lowe wrote:
"Ian Stirling" wrote in message
...
In uk.d-i-y Jeff Lowe wrote:

"Tim Downie" wrote in message
...
Gale Pearce wrote:
You can restrict the outflow side of the pump, but not the intake or
you will burn out your pump prematurely - I use a "ball valve" to
reduce the waterflow on my submersible pump on the line to my filter
Gale :~)

I've already got a restrictor and T-piece on the pump outlet so I can
control the output at present. Somehow it doesn't feel "right" to
reduce
the flow this way. I can't help feeling that it's putting extra load
on
the
pump. As I have a reasonably hefty lamp dimmer kicking around, I was
just
wondering if using a device like this would be more mechanically
"sympathetic" IYKWIM. (Probably complete nonsense I know).

Since these are just turbine pumps, there is no difference as far as the
pump is concerned as to the location of the restriction on either the
input
or output. I would not use a lamp dimmer since these are typically just

Cavitation is a real concern if you put it on the input.
This does cause RAPID wear on the impeller.

Since we were talking about lamp dimmers I guess I assumed we were talking
fractional horsepower. I didn't think these capable of enough suction get
below water's vapor pressure at pond temperature. I mean that's 29" Hg. Of
course some ponds and pumps are bigger than others ;-)

That's not quite how it works.
The large amount of turbulence in the pump means that the pressure
varies quite a lot.
It's quite easy to get pressure locally to fall down below the vapour
pressure.
Then the bubbles collapse, and the rebound is VERY powerful.
This happens even to speedboat propellers ofr example.