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In some quick research online, I'm often seeing configurations where a liquid pump is used in tandem with a solenoid valve on the same liquid path.

So, let's say I have a very simple scenario: a standard pump transporting liquid from one container to another container (at a higher position).

If I wish to electronically control the flow/no-flow of liquid, I can simply switch on & off the power supplied to the pump, perhaps using a relay.

So then, what does adding a solenoid valve to that system achieve -- isn't it redundant?

EDIT: Example of a drawing I saw (although I suppose the valve here is a manual one for easier access): enter image description here

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  • \$\begingroup\$ Some pumps may run in reverse when not powered if the output is of higher pressure than the input. \$\endgroup\$ – Dan D. Oct 7 '14 at 3:19
  • \$\begingroup\$ @DanD.: For that, wouldn't a passive element like a "check valve" do the job -- why a solenoid valve? (Just trying to clearly understand components used in fluid control; I'm not very good at this!) \$\endgroup\$ – boardbite Oct 7 '14 at 3:25
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    \$\begingroup\$ A check valve could work - but they often have a greater pressure drop than a solenoid valve that can fully open the fluid path through them. So maybe there is in some way an energy usage tradeoff here. The pump must exert a bit of extra work (energy consumption) to over come the check valve versus the energy consumed to hold the solenoid valve open. \$\endgroup\$ – Michael Karas Oct 7 '14 at 5:30
  • \$\begingroup\$ There's also the consideration that electrical power may be cheap, while more mechanical power means a bigger pump, which is relatively expensive. Plus, check valves can be pretty unreliable. \$\endgroup\$ – Phil Frost Oct 7 '14 at 14:05
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If there were no valve in the line, then when the pump is off, liquid may siphon from the higher container to the lower container until the liquid level is equal in each container.

Some pumps might prevent this by offering no free path for liquid to flow and preventing their internal parts from turning backwards by friction. For example, a screw pump. However, a centrifugal pump is probably more common, and in these, there is nothing preventing the flow of liquid in either direction when the impeller is not turning. Even when the impeller is turning, liquid can flow backwards if the external pressure is enough to overcome the pressure difference generated by the pump.

A solenoid valve solves this problem, and unlike a pump, is designed to block the flow of liquid at some pressure.

A check valve is another solution. However, to open a check valve requires some mechanical work, reducing the efficiency of the pump. Even when open, a check valve introduces more fluid friction than a solenoid valve.

There is an electrical analogy to this dilemma: active rectification. This is when an active device like a MOSFET is used in place of a diode. Whereas a diode has a forward voltage of typically 0.65V, a properly selected MOSFET can have a forward voltage of some mV. This in turn reduces losses in the rectifier. The tradeoff is increased parts cost and complexity.

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  • \$\begingroup\$ The analogy helped me understand active rectifiers better too :-) Regarding the back flow issue, a peristaltic pump wouldn't be vulnerable to this, and thus wouldn't need a valve (at least for this purpose), am I correct? \$\endgroup\$ – boardbite Oct 7 '14 at 15:48
  • \$\begingroup\$ @boardbite Probably. There's always the possibility that water pressure will be enough to turn the pump backwards, or that the pump will be a little bit leaky, but if it's good enough for your application and within the pump's specified operating conditions, there shouldn't be a problem. You should also consider what happens if the pump should fail -- will it siphon 200 gallons onto your living room floor? In a case like that, a check valve or solenoid valve might be a prudent backup measure anyway. \$\endgroup\$ – Phil Frost Oct 7 '14 at 15:54
  • \$\begingroup\$ @boardbite Another possibility may be to have the outlet end of the hose not submerged in the outlet container, or to introduce an air gap so a siphon is not possible. \$\endgroup\$ – Phil Frost Oct 7 '14 at 15:56
  • \$\begingroup\$ Thank you for adding the 2nd comment in particular! This gives me an idea to solve the issue elegantly and inexpensively. \$\endgroup\$ – boardbite Oct 7 '14 at 23:57

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