Porous elements used in propellant management devices are principally woven screen and drilled perforated sheet. Porous elements can prevent gas penetration while allowing liquid flow. This unique characteristic results from the surface tension forces present within the porous element.
Porous elements are found in every PMD. Trap entrance windows are screen covered. Gallery arms use large amounts of porous material to provide inflow windows along the length of the gallery arm. Perforated sheet can be found at the outlet of a vane PMD. If some porous element is exposed to gas but sufficient area submerged, liquid will easily flow though the element while the gas is prevented. At a measurable pressure differential, gas will begin to flow. This is termed the bubble point.
The bubble point of a porous element is dependent upon the surface tension properties of the fluid and also the pore size in the porous element. The smaller the pore size the higher the bubble point. However, small pores mean small transient capability and higher flow losses. Higher bubble points do not necessarily result in more capable PMDs.
Woven screen wicks which means it will self reseal if gas were to penetrate it. Perforated sheet does not wick and should not be used where gas could reside on both sides of the porous element. Thus perforated sheet is almost always the last barrier to gas in a PMD but is almost never used on large gallery arms nor on trap entrances. Screen is more difficult to weld but is generally preferred due to its transient and wicking properties.