SL/SB-400 Pump Down Procedures

Found another great tech-tip for tech/engineers servicing SL/SB 400 unit, which uses the S391 screw compressor with loading valves. The pump down procedures must be used to minimize the risk of compressor damage.

Compressor Pump Down Procedure

The compressor pump down procedure is used prior to performing service to the compressor itself.

NOTE: Never run the unit if the compressor oil is not visible in the compressor sight glass. The compressor will not function properly if the compressor oil level is low.

1. Install a gauge manifold on the compressor. Attach the low side gauge to the service port on the main suction service valve. Attach the high side gauge to the service port on the discharge service valve. Optionally, attach an additional low side gauge to the service port on the economizer suction service valve.

2. Use Service Test Mode [HS.C] to run the unit in High Speed Cool for about 10 minutes to stabilize the

system. Use Service Test Mode [LS.C] to run the unit for an additional 5 minutes in Low Speed Cool before proceeding to the next step.

NOTE: The unit must be locked in Low Speed Cool Mode using Service Test Mode to assure that both loading valves will be energized for the proceeding steps.

3. Close the economizer suction service valve while the unit is running (in Service Test Mode [LS.C]).

4. Close the main suction service valve while the unit is running. After closing the main suction service valve, note the time and watch the low side gauge to monitor the suction pressure.

NOTE: The unit must be stopped less than 2 minutes after closing the main suction service valve even if the suction pressure does not reach 25 in. Hg vacuum (-85 kPa).

5. Stop the unit when the suction pressure reaches 25 in. Hg vacuum (-85 kPa).

6. After stopping the unit, the pressures in the compressor will start to equalize.

NOTE: Never restart the unit and compressor until 5 minutes after stopping the unit during a pump down. After a pump down, there are unusual temperature gradients in the internal discharge area of the compressor. It is best to let the temperatures in this area stabilize before the next start. It is sometimes common to restart the TK reciprocating compressors after a pump down to get more refrigerant out of the oil in the oil sump. Since the oil sump on the screw compressor is located on the discharge side, an “extra pump down” would not reduce the refrigerant in the oil.

7. To isolate the compressor:

a. Close the discharge service valve.
WARNING: Never close (front seat) the discharge service valve while the unit is running.
b. Recover the refrigerant in the compressor.
c. Compressor service may now be performed. (This includes repair or replacement of the compressor).

Low Side Pump Down Procedure

Low side pump down is used prior to performing service to the low side of the system.

NOTE: Never run the unit if the compressor oil is not visible in the compressor sight glass.

The compressor will not function properly if the compressor oil level is low.

1. Install a gauge manifold on the compressor. Attach the low side gauge to the service port on the main suction service valve. Attach the high side gauge to the service port on the discharge service valve. Attach an additional low side gauge to the suction access port, which is located on the suction tube near the main suction service valve.

2. Use Service Test Mode [HS.C] to run the unit in High Speed Cool for about 10 minutes to stabilize the

system. Use Service Test Mode [LS.C] to run the unit for an additional 5 minutes in Low Speed Cool before proceeding to the next step.

NOTE: The unit must be locked in Low Speed Cool Mode using Service Test Mode to assure that both loading valves will be energized for the proceeding steps.
3. Close the receiver tank outlet valve while the unit is running (in Service Test Mode [LS.C]). After closing the receiver tank outlet valve, note the time and watch the low side gauge to monitor the suction pressure.

NOTE: The unit must be stopped less than 2 minutes after closing the receiver tank outlet valve even if the suction pressure does not reach 25 in. Hg vacuum (-85 kPa).

4. When the suction pressure reaches 25 in. Hg vacuum (-85 kPa), close the economizer service valve while the unit is running.

5. Then, close the main suction service valve while the unit is running.

6. Stop the unit.

7. After stopping the unit, the pressures in the compressor will start to equalize.

NOTE: Never restart the unit and compressor until 5 minutes after stopping the unit during a pump down. After a pump down, there are unusual temperature gradients in the internal discharge area of the compressor. It is best to let the temperatures in this area stabilize before the next start. It is sometimes common to restart the TK reciprocating compressors after a pump down to get more refrigerant out of the oil in the oil sump. Since the oil sump on the screw compressor is located on the discharge side, an “extra pump down” would not reduce the refrigerant in the oil.

8. Perform service functions on low side components using proper procedures.

Hope this helps anyone servicing units with Screw compressors.

Regards A-Z