I work in downstream clinical manufacturing and I’ve always been curious to understand why process fluid will not move through tubing if the pump is post-filter? There was a time where I accidentally placed the pump on the outlet side and the pump started making noises as if it was drawing up air despite the inlet tubing being fully submerged in liquid.
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Post filter the resistance pulling fluid through the filter causes fluid pressure to drop between the filter and pump. That leads to increased cavitation (bubbles forming from water vapor on the pump blades) It’s super unhealthy for most pumps because the bubbles collapsing is like a ton of tiny hammers, in addition to it just not pumping as well. Similar happens if you run a pump too fast, or with other things that drop pressure pre-pump like pulling fluid upwards.
you cannot pull a liquid, so on when the the pump is on the output all it can do is lower the pressure and let the (low) pressure before the filter push the liquid through, if that is not enough the pump will just be spinning making bubbles because the pressure gets so low the liquid boils. It is like pushing a rope
if the pump is on the input of the filter there is no restriction input of the pump and the only limit on pressure on the filter is how much pressure the pump can make. Like pulling a rope
Okay so the pump pulls water in one side and pushes it out the other. It can push out at any pressure- whatever the pump is capable of. But pulling, there’s a limit (not of the pump, but in general), and it’s a bit over 14.6 PSI.
That’s because there is actually no such thing as suction. Now before you tell me I’ve gone off the rails, I will explain. This may be a bit longer than the usual ELI5 explanation.
All around us is air, a mix of mostly nitrogen and oxygen. In our atmosphere, the air is under pressure- the Earth’s gravity pulls the air molecules down, so at sea level they are pressurized to about 14.7 PSI. We don’t feel that because we live our whole lives under that pressure. Our bodies actually require outside pressure to live.
So let’s say you take a plastic drink bottle and start sucking on it. You are removing the air from the inside. The bottle will crinkle, but not because you’re sucking- it crinkles because the air outside the bottle is pushing in at 14.7 PSI, and you’re removing the air on the inside that was previously pushing out at 14.7 PSI.
Same thing is true with your pump. Let’s say your membrane works at 30 PSI and thus you feed it with a 30 PSI pump. If the pump is before the filter, it can create 30 PSI of pressure to push water through the filter. But if you put the pump after the filter, it can pump water out of the pipe, but the only thing pushing water in from the other side (through the filter) is the ambient atmospheric pressure. Thus, your 30 PSI pump only runs the filter at 14.7 PSI. And the pipe between the filter and the pump is even lower than 14.7 PSI.
The noises as if it was drawing air- the ‘bubbles’ are actually steam. Behind the pump’s impeller blades, pressure becomes so low that the liquid boils (even though it’s still at room temperature). This is called *cavitation*.
The pump is designed to have a ready supply of liquid, and it can pressurize that to 30 PSI (in our example). But it can’t pull liquid through the filter any faster than the atmosphere can push fresh liquid back in to replace what the pump removes.
[This video](https://www.youtube.com/watch?v=glLPMXq6yc0) is a helpful explanation for the bubbling. In it, the scientist takes cold water, and reduces the air pressure around it. The water spontaneously boils, even though it’s cold. That’s the same thing that’s happening inside your pump.
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