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Gerrard

Is there an equal and opposite force from gas movement due to pressure gradient force?

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The balloon ( or rocket ) effect was discussed on the first page ( in January ), when there was confusion about the set-up.
A pump's inlet does reduce the relative local pressure, so it isn't simply outflow.

The pump is causing the flow by reducing the relative local pressure, then accelerating the fluid out.
As such, it is the pump which experiences the reaction forces.

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A hole as little as 1m below the water line of an atmospheric tank will produce a water jet around 4 m/s without being accelerated by anything other than the potential energy of its top surface, whereas suction lines for typical common-or-garden centrifugal pumps are rarely designed to run above 1 m/s, often much less. So the potential energy of the liquid top surface is more than sufficient to flood a pump suction without any assistance from the pump. The fact that some pump suctions are able to run at a partial vacuum is because the pump internals are sealed within an air-tight casing and cannot 'see' atmospheric pressure. The actual operating suction pressure is set by suction side static liquid head less friction losses and is independent of the pump. The pump simply adds its rated differential head to whatever absolute liquid head it's provided with at its suction flange, and that sets its discharge liquid head. 

As I said, there's no such force as suck. 

 

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