Page 301 - Compression Machinery for Oil and Gas

286 SECTION II Types of Equipment

the downstream system by a check valve and a large unloading valve is placed
in the recycling line. The unloading valve recycles the full compressor flow
from the discharge side to the suction side. During start-up the pressure drop
in the recycle line is max. 50kPa (discharge pressure suction pressure
+50kPa) and therefore the starting torque is less than during normal operation.
Also the mechanical loads on the bearings during start-up are much lower which
prevents metal to metal friction in the bearings at low speeds. After the com-
pressor has reached the desired speed the unloading valve is closed and the dis-
charge pressure begins to rise. Finally, the discharge pressure exceeds the
pressure in the downstream system, the check valve in the discharge line opens
and the compressor delivers gas into the system.
It should be noted that after the unloading valve has been closed gas is
sucked out from the suction volume and the suction pressure may drop unless
enough gas is delivered from the upstream system. The drop of suction pressure
can be reduced by a large suction volume.

Starting of Flooded Screw Compressors
Similar to oil-free compressors, oil-flooded compressors with sleeve bearings
must start unloaded. In most cases, unloaded start can be accomplished by mov-
ing the slide valve to the minimum flow position. Opening the recycle valve can
be helpful but is not always necessary. As with any hydrodynamic bearings, pre-
lubrication is required to prevent bearing damage. This is accomplished with a
separate oil pump driven normally by an electric motor and sometimes by a steam
turbine. The flow of oil required for prelubrication is normally 15%–25% of the
fully flow required. One option is to use a small prelube pump and have the main
oil pump be shaft driven. Another option is to have a separate 100% oil pump that
provides the oil for prelubrication as well as normal operation. To prevent filling
the compressor body completely with oil, automated shut-off valves block the
flow of oil to the main injection ports in the rotor chamber, and the oil only flows
to the bearings and driveshaft seal. Since the pressure requirement for the oil is the
same during prelube as during operation (normally 250–320kPa higher than gas
discharge pressure) but the required flow is much lower, the oil system must be
designed to handle both prelube and normal operation.
Once the compressor has reached minimum speed, the prelube pump (if
used) is turned off. The recycle valve can be closed, and the slide valve can
move to increase the flow capacity of the machine. The suction and discharge
check valves will open and gas is moved downstream.

Stopping of Dry Screw Compressors and Settle-Out Pressure
During shutdown dry screw compressors are also unloaded. Immediately with
the driver stop signal the unloading valve opens and the gas in the discharge line
expands into the suction line. The discharge pressure drops rapidly and the