[Puddlebrook]

I have a 1000 ltr water bowser which i'd like to connect to a water pump and use with a hose. What happens when the pump is running and I turn off the nozzle at the end of the hose.
Is there a pressure relief valve?
does it stop pumping?
does it keep pressurising until something explodes?

Also any recomendations on pump specifications would be nice. Need to water plants/crops during dry spells so would imagine high volume fairly low pressure. Thats just my guess.

[Womble]

Hi Puddlebrook,

That depends on the type of pump.

If it's a centrifugal pump (most likely), the water will just stop flowing. The pump will continue to run though, and will eventually heat up the water inside it which is going round and round but not actually pumping. If you're only doing that for short periods of time, like moving between containers it will be ok, but avoid doing it for long periods.

If it's a 'positive displacement' pump (unlikely), then that type do keep pressurising until something gives. For that reason, that type of pump usually has an internal relief valve which passes water back from the outlet to the inlet if you dead-head it.

Some systems come with a flow or pressure switch which will automatically detect that the hose is off, and then stop the pump for you.
Re specifications, "normal" domestic water pressure is usually a bit over 1 barg (you might also see 1 barg written as '10 metres of water head'), and normal flow out of a tap is something like 10 to 15 litres a minute, IIRC, so I'd use that as a starting point and go from there. If you want a bit more flow and pressure, specify a slightly larger pump, but the supplier should be able to help you select the right pump for your needs.

Hope that helps!

[Puddlebrook]

Womble thanks for the reply

I wonder if I could fit a pressure relief valve that would feed back to the tank. It would only need to be slightly higher than the outlet pressure surely

[Womble]

Yes, you can certainly do that, if you think it's needed. Here's an example pump curve (sorry, it's a bit cluttered!):







The key piece of information is the minimum allowable flow. What you'd do is install the relief valve on the outlet of the pump, and pipe the relief discharge back into your bowser. Probably the best thing to do is to select a relief valve with a nut on top which adjusts the spring pressure, making sure that it is rated for up to the full shut-off head of the pump, and for the minimum allowable flowrate (in your case, this is going to be quite a small relief valve - probably no more than 3/4" diameter):





Then all you do is to prime the pump, then whilst running it against a dead head, unscrew the adjuster nut until the valve lifts. Hopefully, now, when you use the hose, the relief valve will reseat because the pressure produced "moves along the pump curve", e.g. to the point marked "best efficiency point" - can you see that the pressure at that point is less than at the shut-off pressure?

Does that make sense?  I do this stuff for a living, but it's difficult to explain simply!

[Puddlebrook]

Once again Womble, thank you for your reply

I'm an ex aircraft technician so I fully understand the design. The hardest thing will be finding the correct fittings. Water pumps appear to have barbed outlet fittings rather than threads.

[Puddlebrook]

Womble, qiuick question
If I were to buy a 1 inch outlet pump what thread is likely to be on it? I'm guessing 1 inch BSP

[Womble]

It could vary, but if it's a 1" screwed thread, it will be BSP, yes.

[chrismahon]

We've just bought a pump from Lidl. Google PGPS 1100 A1. The inlet and outlet is 1" BSP, but it didn't have any outlet fittings so I bought a male/male 1"BSP fitting and added a 19mm push fit hose connector to it. Also have a 15mm. It is designed for exactly what you want- sucking out from a well or reservoir and pumping at high pressure to a distant point or a spray nozzle. Works a treat, but you must prime the pump before switching on so it is filled with a small watering can via removable plug. It probably pumps at a pressure which is near the limit of hosepipe connectors- 4.5 bar. The manual is available translated so you can get the technical details.