Page 305 - Compression Machinery for Oil and Gas

290 SECTION II Types of Equipment

as injection quantity increases. This is an indication that a certain amount of
heat will always be taken by the gas and the compressor materials themselves.
Oil-flooded screw compressors are designed in such a way that the injected oil
leaves the compressor discharge port along with the gas. The amount of oil to be
injected depends on the load and the rotating speed. Too much oil injection can
cause a “choke” effect at the discharge, increase dynamic losses (reflected by
higher shaft power), and increase vibration. Too little oil injection is generally
reflected by higher discharge temperature. In some cases, as with gases with
water vapor content, it is desirable to keep the discharge temperature as high
as possible to stay above the dew point. In these instances, the amount of oil
injection must be controlled to maintain the desired discharge temperature.
Both compressor types tolerate liquid injection well as long as the liquid
comes in a steady and controlled flow. An uncontrolled inrush of a large amount
of liquid like an overflow of a separator must be avoided by any means because
liquids are not compressible and thus the size reduction of the compression
chamber will cause extreme forces and torques on the rotors. This effect is
called “liquid hammer” and can cause massive damages of rotors, bearings,
seals, and inner casing surfaces. The authors have never experienced a bursting
of the steel or ductile iron casing, and this is not expected because the high
torque in this case causes a failure of the coupling or the driving shaft and
the compressor comes to an immediate standstill.

Recommended Instrumentation and Monitoring
For dry and oil-flooded screw compressors the instrumentation listed in
Tables 6.2–6.5 is recommended. It should be noted that additional monitoring
may be required depending on the application.
Shaft vibration monitoring can give much information when interpreted by
an experienced vibration specialist. It should be noted however that shaft vibra-
tion monitoring is much more expensive than casing vibration monitoring
because 8 x–y probes (4 at each rotor) plus 2 key phasors are required. There-
fore, x–y probes should be used only when the interpretation of vibration data is
required but not if only automatic shutdown is required.

Typical Design Range (Discharge Pressure, Pressure Ratio,
Volume Flow, Driver Power, Molecular Weight)

Design Range for Dry Screws
The maximum discharge pressure for dry screws is limited by the casing design.
Typically, the casings are not designed for the individual application but are
designed for certain flange pressures (e.g., Class 150, Class 300, 1.6, 2.4,
4MPa, etc.) Vertical split casings have higher pressure limits than horizontal
split casings because it is easier to seal the vertical split including a fully