For the SGCH and SGXH 355 compressor models, which are equipped with angular contact ball
bearings, an active regulation of the oil pressure to SB-2 is necessary. A Balance Piston Regulator
(BPR) is necessary because the gas force varies with the position of the slide valve. A low slide valve
position could reduce the load below the minimum necessary for the axial bearings. There are a
number of BPR configurations available and it is important to select the one that works well for the
application.
An example of a balance piston regulator configuration is shown in
, and consists of the
following components:
•
Pressure regulating valve
: The discharge pressure determines the compressor thrust balance.
The standard setting for the pressure-regulating valve is 3.5 bar [50 psi] below discharge
pressure. The pressure setting might be different depending on the operating condition.
•
Solenoid valve
: Energizing, or opening, the solenoid valve pressurises the balance piston with
full oil pressure from the oil manifold, by-passing the pressure regulating valve. De-energising,
or closing, the solenoid valve pressurises the balance piston with oil pressure regulated by the
pressure regulating valve.
Signals from the control panel operate the solenoid valve. For a standard regulator setting, the
solenoid valve should open when the slide valve position is 70% or greater and close when the
slide valve position is 65% or less. Depending on the operating condition of the compressor the
opening and closing slide valve position might be different. You can change this in the Unisab
controller.
•
Orifice
: The orifice ensures oil supply to the inlet end bearings during upset conditions such as
start-up.
Figure 26: Balance piston regulator principle diagram. For SAB 355 H and some SGCs that
require controlled oil pressure to the balance piston
SAB 283-355 A-Frame OHU 6042 and OHU 7650 (including ATEX) Screw Compressor Unit
56
