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Many pumps, when turned off, will still pass water. If there's any pressure at all upstream of the pump, the impeller will turn and water will leak through in the forward direction.

To prevent backflow of course, a check valve is used.

What's the term for a pump that forms a pressure seal when off, preventing flow in the pumped direction?

  • Are you talking about pressure causing the pump to flow backwards? That could be resolved with a check valve. If you are talking about pressure in the direction the pump normally moves water, I don't know. I'm not aware of any pump designs with impellers that are perfectly water-tight. – Hank Oct 14 '15 at 3:12
  • I have clarified – Bryce Oct 14 '15 at 4:09
  • As @chris mentions, a pump that forms a pressure seal is a positive displacement type (think pistons, although there are many types; even some that have rotary elements similar to impellers). If you could enlighten us as to the application, you may get more useful answers. – Jimmy Fix-it Oct 14 '15 at 4:27
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I believe the term you are looking for is a positive displacement pump.

However, such a pump is not generally constructed using an impeller and such a pump is only sometimes used for liquids.

Here is a link to a brief discussion of the characteristics of centrifugal pumps versus positive displacement pumps:

http://www.pumpschool.com/intro/pd%20vs%20centrif.pdf

For low pressure water applications a centrifugal pump and check valve in series are usually used to prevent back flow.

  • For solar use, there are available, positive displacement diaphragm pumps. They have inlet and outlet check valves on the diaphragm chambers. Other positive displacement pumps like rotary lobe pumps will attempt to turn into hydraulic motors if they don't have a check-valve in the outflow. – Fiasco Labs Oct 14 '15 at 4:18

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