Page 119 - Compression Machinery for Oil and Gas

106 SECTION II Types of Equipment

Several methods have been developed for dealing with fouling situations.
The surest method remains taking the compressor offline and cleaning out
the foulant to restore efficiency. This can be done during an overhaul by
mechanical means or done by performing an offline wash. However, this
requires shutting down the unit and losing production. Less costly methods that
focus on extending the time between cleanings have been developed such as
online washing and nonstick coatings.
An online wash method involves the use of a solvent spray at the inlet and
interstage locations to remove or prevent the accumulation of deposits in the
impeller and gas passages. The composition, amount, and frequency of the
spraying depend on the nature and rate of buildup of deposits. Any spray should
be thoroughly atomized to prevent solid streams of liquids from entering the
unit. The introduction of liquids into a compressor has the potential to create
problems of drainage and erosion. Drain provisions must be made to prevent
the buildup of liquid level inside of the casing as this may result in the collected
liquid passing through the unit in slugs. A compressor with these provisions is
shown in Fig. 3.71. Erosion is a function of the liquid particle size, the quantity
of liquid injected and time. Erosion can be managed by limiting the spray quan-
tity and frequency of spraying to a minimum consistent with the cleaning
requirements. Finding the right solvent for online spraying can be tricky. Steam
has been successfully used as a solvent for some situations such as coke oven
gas. Naphtha has also been used successfully in cracked gas service and other
refinery applications as a solvent.
Coating the rotor and diaphragms with very slick coatings creates a surface
that foulants have difficulty adhering to (see Fig. 3.72). Two common types of
coatings are organic coatings such as polytetrafluoroethylene (PTFE) and elec-
troless nickel. The organic coatings tend to have superior corrosion resistance

FIG. 3.71 Compressor with nozzles for online wash. (Photo courtesy of Elliot Group.)