maintanance and inspection of screw compressor

The installation should be inspected regularly at intervals that will be determined by the severity of operating conditions. For convenience, suggested maintenance routine may be segregated into two separate but interdependent classifications as follows:

I] OPERATION INSPECTIONS
An accurate operational inspection system is the best means of detecting the need for maintenance work.

a) Continuous log or graph of all pressure and temperature readings should be kept. For the first few weeks of operational break-in, 4 readings per 8 hour shift are recommended. After that, the readings could be reduced to a suggested minimum of two per shift. Any rapid changes in consecutive readings would indicate possible malfunction and should be investigated immediately. Any gradual but consistent change not related to normal ambient or process variations should also be investigated.
b) Any change in the characteristic should or increase in vibration of the unit should be investigated.
c) Oil and water leaks should be repaired when first observed.
d) The water flow from jacket, after cooler and oil cooler should be observed when each reading is taken.
e) Check lube oil level in reservoir.
f) Make all checks recommended for the main driver.

II] PERIODIC INSPECTION
a) Every 500 – 700 hours: Check operation of alarms and controls.
b) Every 1000 – 1500 hours: Drain oil sample from reservoir. Check oil for oxidation, contamination and water. Frequency of oil change depends upon this analysis.
c) Every 7500 – 8500 hours (Annual):

• Inspect rotors through inlet and discharge nozzles. Some polish areas on the rotor lobes are to be expected, due to float and relative shift of the rotors during start up and shutdown. Excessive rubbing between rotors or wear on radial and end sealing strips will require readjustment or possible replacement of the bearings.
• Remove discharge end cover and inspect timing gears. There should be even wear pattern on gear teeth.
• Measure thrust bearing end clearance with a dial indicator. Measure dial bearing clearance with a dial indicator mounted through thrust bearing housing drain opening.
• Loosen inlet end cover and visually inspect radial bearings.
• Remove compressor jacket cover plates and inspect for sediment or calcium deposits. Clean if necessary.
• Remove coupling guards and inspect couplings for broken shims.
• Recheck alignment at all couplings.
• Check and reset, if necessary, all temperature and pressure gauges and pickups.
• Inspect inside of silencers for deterioration of foreign materials.
• Inspect water side of after cooler and oil cooler for foreign material or calcium deposits and clean if necessary.

d) Every 15000 hours (2 years):

• Inspect per paragraph 2c.
• Remove coupling hub at inlet end cover. Measure inlet radial bearing clearances with a dial indicator mounted adjacent to the bearing.

e) Every 30000 hours (4 years):

• Inspect per paragraph 2d.
• Remove and inspect timing gears, thrust bearings, radial bearings and seals.
• With radial bearings in place, indicate shaft runout at all diameters of each shaft with a dial indicator.
• If condition dictate, remove the rotors from the housing, clean and inspect the rotors and housing bores.

shut down procedure of screw compressor

When unit is to be taken off the stream and shutdown, the following procedure should be followed:
1. Gradually open bypass to reduce discharge pressure to ½ normal pressure. Operate unit at this condition for approx. 20 minutes or until the gas discharge temperature levels off.
2. Stop unit. If unit starts turning backwards, immediately close the block valve.
3. Allow lube oil to circulate for 10 minutes after the unit is stopped to properly cool the rotors.
4. If unit is to be shutdown for an extended period in a freezing environment, drain all water from oil coolers, compressor jackets and piping.
5. If unit is shutdown for an extended period of time, following maintenance program should be followed to keep unit ready for service:

Open compressor inlet casing drains to allow condensate to drain out Circulate lube oil for a minimum of one hour once a week and rotate unit several revolutions.For extended down periods, remove silencers and piping from compressor inlet and discharge and spray a light film of lubricating oil on rotor surfaces and machines internal housing surfaces once every two weeks to prevent rusting If the unit shuts down instantly under full load (power failure, emergency shutdown), following procedure is to be followed if unit is to be started:

1. Let the cooling water flow through the oil cooler and compressor
2. Circulate the lube oil for one hour and rotate the compressor drive shaft ¾ turn by hand every 10 minutes
3. Turn until over by hand three revolutions. The unit is ready to restart when it turns normal with no tight areas.
4. Restart unit in normal manner. Operate at approx. ½ normal pressure for 15 min before applying rated pressure

screw compressor-normal operation

Some conditions are subject to slight variations as listed below:

1. Process gas pressures and temperatures - changes in process requirements may cause a variation in operating temperatures and pressures. System temperatures will vary directly with incoming gas temperatures. Temperature differential between inlet and discharge nozzles should remain fairly constant. Pressures will be dependent upon and affected by incoming gas and barometric conditions.

2. Bearing temperatures - bearing temperatures will vary slightly with ambient temperature and with lube oil temperature. Higher lube oil temperatures will produce proportionately higher bearing temperatures. A variation of 6 C to 11 C in bearing o o temperatures is of no consequence.
A running plot of all temperatures and pressures is advisable. Log all gauge pressure and temperature readings at initial start after unit has reached normal operating conditions. Use these initial readings as a guide and check frequently as recommended in “operational inspection”

Any rapid or gradual change, not due to process conditions, may indicate a possible malfunction.

screw compressor-normal start procedure

After all the precautionary measures have been taken, the unit can be started as follows:

1. Turn on the seal buffering gas (if applicable)
2. Start the main lube oil pump for prefabrication. Prelube compressor approx. One hour at initial start and for 10 minutes after each shutdown and at each subsequent start. Start pressure gauges and sight glasses for oil flow.
3. Turn on the cooling water so that a steady continuous supply flows into the unit
4. Check all valves external to the machine for proper adjustment
5. Bar unit over using strap wrench.
6. Make sure process shut off valve and bypass valve is open. Start the driver in accordance with the manufacturer’s instructions and bring unit up to speed. Gradually choke down on bypass valve and bring unit up to pressure over a period of approx. 15 min.
7. Check all lube oil pressure gauges and sight flow indicators to see that the machine is being properly lubricated. Also observe temp. Gauges.
8. Observe the action of the machine to make sure there is no undue vibration or noise.
9. Check auxiliary equipment. Be sure the unit is not operating beyond the rating stamped on the nameplate
10. In the event of an emergency shutdown while unit is operating, do not attempt to restart compressor until it bars over freely.

screw compressor-initial start procedure

1. Run driver up to speed without load as follows:

• disconnect coupling between driver and gear
• pump driver and check rotation
• run driver up to speed, checking for undue noise and vibration

2. Run driver and gear.

• disconnect coupling between gear and compressor
• check alignment between driver and gear
• connect coupling between driver and gear
• start lube oil pump and check pressures and flows to gear
• pump driver and check gear rotation
• run driver and gear up to speed, checking for undue noise and vibration

3. Run in compressor on gas as follows:

• Install temporary wire screens at the joint between the compressor and inlet silencer to catch any weld berries, slag or dirt remaining in the pipe. Screen information is found at the back of this section. Remove screen before putting unit into service.
• Bar compressor over by hand to check for freeness. If any metal- rubbing-on metal sounds are detected in the compressor, determine the cause and correct the same before starting. Use strap wrench to bar compressor.
• Check alignment between the gear and the compressor. Couple compressor to gear.
• Turn on buffering gas and/or other sealing fluid to compressor.
• Start main oil pump and check flows and pressures to bearings. Oil temperature should be 16 C to 27 C before starting unit.
• Make sure process shutoff valve and bypass valve are open.
• Run driver for a period just long enough to bring the unit up to approx. ¼ speed, trip off driver and observe the unit while it coasts to a stop to make sure there is no undue vibration or noise. Bar unit over again to check for freeness.
• Start driver and bring unit up to speed. Do not operate for more than two minutes with zero discharge pressure.
• Gradually close down on the bypass valve and bring unit up to 50% of discharge temperature over a period of 15 minutes. (if applicable) Run for 30 minutes at above condition and observe operation of unit and auxiliary equipment. Check for any unusual vibration or noise.
• Stop driver and let unit coast to a stop.
• Bar unit over to check for freeness.
• Check inlet screen and clean if necessary
• Continue to operate unit for 30 minute interval until inlet screen is no longer picking up dirt
• Always turn injection water off immediately before shutting off main driver. (if applicable)

screw compressor-pre start procedure

Before starting the unit for the first time, and always after a major overhaul or repair,check the following:
1. Be sure the intake, all oil and air piping and internal parts of the unit are free from dirt and moisture.
2. Remove paint and preservative coating, if used, from all moving parts. (See instructions for removing preservatives)
3. Extreme care must be taken to prevent foreign material such as nuts, bolts, tools etc from remaining in or dropping into any of the piping or compressors.
4. See that all bolts and nuts are secure. Some may have loosened in shipment.
5. Check alignment
6. Lubrication- in preparation for start up, flush the lube oil system and adjust the lube oil pressure switches. Check all switch settings in accordance with the lube / seal oil console section.
7. See that all valve internal to the unit are properly adjusted.
8. Check all other auxiliary equipment including safety alarm system to see that it is in proper order
9. Check compressor side inlet drain for condensation at initial start and after a shutdown period.