Page 112 - Air and gas Drilling Field Guide 3rd Edition

5.4 Intermittent (Positive Displacement) Compressors 103

Operation at
Design Pressure
Design Pressure
(Discharge)
Pressure

Volume
Operation Above
Design Pressure

Discharge Pressure
(Design Pressure)
Pressure

Volume
Operation Above
Design Pressure
Design Pressure

Pressure Discharge
Pressure

Volume

FIGURE 5-9. Rotary compressors operating with back pressure at the built-in design pressure
(upper), with back pressure above the built-in design pressure (middle), and with back
pressure below the built-in design pressure (lower)[1].

cannot efficiently expel the gas volume within it. Thus, the fixed volumetric flow
rate (at a given rotation speed) will be reduced by slippage within the compressor
from the volumetric flow rate when the back pressures are equal to or less than the
built-in design output pressure [1]. This slippage can be the result of a seal struc-
tural failure. This is why a safety valve is usually stalled on these compressors so
that the compressor will automatically shut down (or be slowed down) when
the back pressure to the compressor exceeds the design fixed pressure.