When it's been on for a while (maybe half an hour) the pump starts screaming. The plumber returned and bled the pump, then replaced the non-return valve with an ordinary cut off valve but the pump stills screams.
Any ideas?
Thanks
Mary
Possibly cavitation, which is nothing to do with it sucking air in.
It has much to do with the temperature of the water and the pressure available at the pump inlet.
There are energy losses due to friction in the pipes and in every fitting; the energy lost is provided by a pressure loss as the fluid progresses along the circuit. There are high eddy velocities in the pump inlet and these also cause a reduction in fluid pressure.
If the pressure drops low enough and the water temperature is high enough, then the water will boil in the pump inlet. Bubbles of vapour form and these implode as they progress into the pump's discharge.
There is a distinctive sound, like gravel. It will tear lumps of bronze out of the pump's impeller.
The cure is to reduce temperature, increase static pressure and/or reduce frictional pressure losses on the inlet pipework. It more likely if there's little head between pump and the water level in the cold feed storage cistern (assuming it's a vented system).
The pump's usually fitted in the return because the water's cooler. You can also get bubbles of dissolved oxygen coming out of solution, due to the low pressures in the pump inlet, as described by someone above; this isn't cavitation, but it can cause the pump to airlock. A shower pump, for example, will take the water flow from an Essex/Warix/Surrey or similar flange, which is designed so that it doesn't collect the air bubbles from the cylinder.
Ask the plumber whether he thinks it may be cavitation; watch for the blank expression as you do so.