INLET DESIGN

Inlet design is one of the most neglected aspects of overall separator design - and consequently a regular cause of poor performance. Mist eliminators of any type are designed on the assumption that the inlet gas and liquid load are uniformly distributed. This can be achieved by leaving sufficient disengagement distance between the mist eliminator and the gas inlet and outlet connections. This is an inefficient means of dealing with the problem and is expensive in terms of the vessel size.

A better solution is the installation of an appropriate inlet distribution device. KnitMesh offers several designs ranging from simple inlet baffles to sophisticated vane devices (see Figure 8). The distributor shown in Figure 8 was first used to reduce synthetic oil carryover in a badly designed compressor knock-out pot. Oil and gas was distributed on a simple baffle - this had the effect of shearing the liquid phase into a very fine mist which was carried through into the mist eliminator.

By installing the distributor and a high efficiency mesh mist eliminator, previously significant oil carryover losses were reduced to virtually zero. The distributor effectively disengaged the bulk of the oil, reducing the overall load on the mist eliminator. These distributors of this type are normally effective up to dynamic pressures of about 10,000 Pa where dynamic pressure is calculated as:

Where:
	= gas/liquid mixture density (kg/m3)
	= gas/liquid mixture velocity in pipe (m/s)

At dynamic pressures of under 1,800 Pa, simpler devices such as baffles or cut away pipes can be considered.

Care must also be taken to consider the equipment and pipe configuration immediately upstream of the vessel, which may also have a significant influence on the feed condition. For example, very fine mists are likely to be created as a result of sudden pressure drop through equipment such as choke valves.

Feed conditions likely to result in slug flow may well require a robust design of separator with a heavy duty inlet design; plenty of residence volume for the liquid slugs; and careful location of the mist elimination device to avoid excessive liquid load and physical damage.