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# Deformation of polymer under pressure

## Deformation of polymer under pressure

(OP)
Hello,

I want to put a disk of a certain polymer (let's say polypropylene) so that it can move along an axis inside a steel pressure tank, like a piston or syringe (see image attached). Above the disk there will be air and below it there will be water. When the system comes into balance (the tank will be connected to a pipeline with a certain manometric pressure of the pumps), the pressure in the air and water obviously will be the same. For a specific project the pressure of the water will be 30 bar, and therefore that of the air when the balance is reached (see image attached).

My question then is what's going to be the deformation of the disk because of this pressure from both sides inside the tank under normal operation (compression stress). I am afraid it could reduce its thickness and make it longer, "touching" the walls inside the tank (normally, the disk should not touch the walls so as to make the movement along the axis easier).

I have seen that the elasticity modulus might play a role, but would like confirmation of the formula I must use. The disk, for calculus assumptions, should be 50 mm thick.

Thanks!

Felipe

### RE: Deformation of polymer under pressure

Without seals the disk will sink or float. So what is the purpose of the disk?
30 bar of hydrostatic pressure will have no measurable affect on the disk.

### RE: Deformation of polymer under pressure

(OP)
The only purpose of the disk is to separate air and water so that the air leaks in the water as slow as possible. It is for water-hammer protection in pipelines, i.e, when the pressure of the water goes down, the air will expand and the disk will then push the water down, and the opposite process when the pressure in the water goes up. The disk will float on the water of course.

### RE: Deformation of polymer under pressure

as long as it doesn't seal there will be no pressure across the disc, and hence no force to deform it.
That is presuming that you use a HDPE, if is a plastic with lots of air in it then over time it will deform, perhaps distort, and become denser.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Deformation of polymer under pressure

You can buy a hydraulic accumulator off the shelf to meet your needs.

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