Screen or Perf?

Both screen and perforated sheet are used in PMDs. They both can prevent gas penetration while allowing liquid flow.

Perforated sheet's main advantage is that it is more easily welded and handled in manufacturing. Unfortunately it is often the poorer performing porous element.

Screen is more difficult to weld and handle but offers four potential advantages over perforated sheet:

1) Screen comes in finer pore sizes and thus higher bubble points. If a higher bubble point is required then screen must be used.

2) At the same bubble point, screen flow losses are almost always lower. This does not preclude perf sheet but more area is required.

3) At the same bubble point, screen transient capability is almost always higher. This does not preclude perf sheet but more area is required.

4) Screen wicks and perforated sheet does not. Anywhere where either gas penetration may occur prior to depletion (e.g. a trap entrance) screen is highly desired since it will reseal itself without other PMD components. In addition, anywhere gas may be in contact with both sides of a porous element, screen is required. Perf sheet cannot hold its bubble point with both sides exposed to gas.

Porous elements are used in all PMDs to prevent gas ingestion. Once wet, a porous element will prevent gas penetration using surface tension. For example, the illustration to the left shows the gas-liquid interface at bubble point within a plain Dutch weave screen.

The screen warp wires are shown in blue and the weft wires in red. The gas surface is modeled and is cyan where in contact with the weft wires, green where in contact with the warp wires, and black where in contact with the liquid. As shown, the gas has to penetrate into the triangular opening formed by two adjacent weft wires and a warp wire. Interestingly, this triangular opening is nearly perpendicular to the screen face.

The pressure needed to force gas through this triangular opening is dictated by the liquid's surface tension property and the curvature of the gas-liquid interface in the opening. It is termed the screen's bubble point. The model to the left was constructed using Surface Evolver and predicts the screens bubble point. However, with complex screen geometries it is far easier to experimentally measure the bubble point.

Four porous element properties are key to analyzing PMD performance: the bubble point, the flow losses, the transient volume available, and whether the porous element wicks. These are all experimentally measured.

The bubble point most easily determined experimentally by injecting a column of gas beneath a submerged porous element. The height of the gas column under the element when bubbling first occurs is the bubble point. Bubble point testing is most often conducted with isopropyl alcohol and the results translated into the propellants.

The flow losses are also measured. With water flowing through the element the head loss is measured as a function of approach velocity. The data is easily translated into the propellants.

The transient volume available in a porous element is more difficult to obtain but is measured similarly to the bubble point. By pressurizing one side of a wetted porous element with gas and measuring the pressure differential across the element as a function of volume needed for pressurization one can obtain the transient volume available as a function of pressure.

Finally, the wicking rate is measured by wetting one end of screen and timing the surface front within the screen.