Compressor Maintenance Tips

Below is a long list of maintenance tips for rotary vane compressors. Not all of these tips might be applicable to your specific compressor, but most can be useful for maintaining the performance of most compressor types.

Cylinder

A. Cylinder Bore Allowance
Maximum bores are based on having a maximum blade standout of 60% of the blade width. If the maximum bore is exceeded, the blade deflection will cause high friction and jamming in rotor slat and failure.

B. Radial Grooves in Cylinder
These grooves look like machining grooves completely around the cylinder bore and are caused by foreign materials entering the compressor and being pushed by the blades around the cylinder causing wear grooves.

C. Longitudinal Ridges or Corrugations (Washboarding)
These are lines traveling along the length of the cylinder, parallel to center line of the cylinder and usually occur at the bottom of the cylinder because of chattering or back and forth and is caused by excessive wear in the rotor slot as the blade moves backwards as pressure is released from maximum to minimum at the discharge porting. This is at the bottom of the cylinder. Longitudinal lines are also caused by the bouncing of blades at the change in cylinder to undercut bore on the undercut design when there is excessive wear of the cylinder. It is also known that at low temperature blade lubrication becomes very viscous causing the blade to jump and come away from the cylinder. This occurs at cold spots in the cylinder where there are water cooling cores.

Blade and Rotor

A. Mushroom
Mushrooming or flattening of blade tip is caused by high tip pressure due to buildup of materials in the bottom of the slot. The blade is not allowed to retract into the slot causing high pressure at mechanical deformation of the blade. Mushrooming is shown in trying to insert the blade tip into the rotor slot with the tip downward. The blade cannot be installed because it has excessive thickness at the tip. Foreign particles should be cleaned from the bottom of the slot every six (6) months. These materials are at the lowest pressure in the cylinder assembly where materials can accumulate. Mushroom can also be formed by over heating of blade tip at high pressures and temperatures causing an actual melting of the tip. In this case the blade does not show actual deformation but melting and spreading of the tip.

B. Bell- Mouth
Blade bell-mouthing is a narrowing in thickness of the center portion of the blade where it travels in and out of the slot. It could be caused by sharp objects that wedge between the blade and the slot, but most likely is a result of the rotor slot wear causing excessive back & forth movement of the blade and in the slot back and forth causing it high friction and wear on the sliding faces of the blade. The rotor slots will generally wear wider at the top over a period of time and bell mouthing will occur.

C. Tapered Wear along the Length of the Blade
This will occur because of misalignment of the rotor and the cylinder on rebuilt or worn units. It can also be caused by higher dust loading at one end of the compressor and/or higher heat at that end. Generally, a blade will run with more clearance on one end. If the clearance gets excessive there will be higher temperature at the edge of the sides of the blades causing higher wear. The tapered wear patterns if continued will to lead to failure at the short end of the blade because of the blade will have greater standout at that end. After failure, the end can be inspected to see that there is a taper in that length of the blade. This can be avoided by reducing the discharge temperature, dustloading, and having the compressor realigned. Also, if this is observed before failure, blades can be turned over to even out the tapered wear pattern.

D. Maximum Blade Wear Allowance
The maximum allowance on blade wear is ½” for most sizes. If this width wear allowance is exceeded, the blade has a greater tendency to jam in the slot and fail.

E. Blade Edge Damage
This is caused by insufficient clearance between the fixed and expansion heads, not having the proper fixed and expansion clearances between the rotor and the heads. When there has been rotor face contact with heads due to overheating of compressor, the rotor may be machined smooth and new blades installed without shortening their length to compensate for the shorter rotor. Therefore, blades will not have sufficient clearance. This procedure is not recommended.

F. Oblique Wear Tip Profile
A normal wear tip profile will not be symmetrical but will have an arc shape that is somewhat oblique having one side lower than the other side as shown. In some cases, this oblique wear pattern maybe exaggerated in one direction, which indicates that the blade is actually deflecting due to either a very highly worn blade or rotor slot causing that oblique wear pattern and parts should be reworked at this point. This oblique wear profile is exaggerated under higher pressures as well. Rotor slots should be inspected and re-slotted for next large blade thickness.

G. Blade Delamination
There are several opinions on what delamination is and is not. The blade tip will normally wear in a fashion to produce an oblique tip and at the edges of this tip where the tip meets the face; there will be small splinters of fiber that tend to come loose at this point. This is not delamination and actually is normal wear pattern at this point the blade has no bending stress. Delamination occurs within the laminate due to high bending stresses and sometimes cannot be observed within the composite. The highest stress occurring at about half the width of the blade. Tip delamination also occurs because of high pressures due to dirt built up in the rotor slots. The edges of mushroomed blades can be sanded and blades reversed in its slot using the non-worn toe as the tip. This has been successfully done to extend the life of blades.

H. Moisture and Corrosion
The carbon blade does not absorb moisture and doesn’t swell like other blade materials. It is chemically stable and can be used with many corrosive gases.

I. Blade Melting
At about 500°F the carbon blade will reach its crystalline melting temperature at which a flow of resin will occur and deposit itself in the cylinder. This will occur first at the tip which is the highest pressure and friction point of the blade. It also occurs along the face due to the pressure of blades rubbing in the slot. The normal operating actual blade temperature is about 200° - 250° F for compressor running at 80° inlet and 300° - 350°, 30-40 psig discharge pressure and 0 psig inlet. This is well below the maximum blade temperature. The water in the cooling jacket will tend to keep the cylinder cool which also keeps the blade tip cool as well. Loss of water flow may cause the tip to reach a melting temperature as well as excessively high discharge temperatures. In some cases, vacuum pumps may reach very high internal temperature due to high compression ratios and high adiabatic temperatures following the high compression in proportion to high pressures following an adiabatic curve. Therefore, compressors running at high inlet vacuums can have excessive discharge temperature. In this case, the air is also rarified, which causes localized heat internally in the compressor causing overheating and also some cases there is not enough air flow passing the discharge temperature sensor to indicate high temperature and shutdown the unit.

Compressor Drive

A. For bolt drives measure belt tension occasionally and make adjust mounts to increase bolt HFC. Note that the newer co belt does not require high bolt tension, as shown.
B. For direct drives, check pin couplings for wear and replace. Worn pins will cause excessive vibrations.

CAUSES FOR COMPRESSOR FAILURE

A. Lubrication
Lubrication rates for cylinders and bearings are quite plentiful for the application. There is generally not a concern for insufficient lubrication. Lubrication rates have been reduced successfully using carbon blades in certain applications. Loss of lubrication can occur and the compressor will run for about thirty minutes safely. Those compressors with digital no flow timers will activate an alarm if a full cycle is not completed in four minutes. There are cases when oil will complete its cycle, although the lube lines to compressor are broken or leaking. All lube lines to compressor should be inspected periodically. An alarm is also caused by excessive oil pressure. The blowout disk mounted near the lube pump will fail at 1400 psi and oil will return to reservoir. Check for blocked oil lines or oil filter pressure due to block filter of the main oil line to the distribution blocks. This high pressure may cause blowout of disks. Also, high viscosity oil during cold weather causes higher oil pressures.

B. Dirt or Foreign Particles at the Inlet and Discharge of Compressor
The inlet piping and filtering system should deliver clean air through the compressor inlet. The location of the inlet filter should be in a clean area and also a cool area. Blocked filters will create high dust loading, high velocities, and high vacuums, all having the adverse effects on the compressor operation. High dust loading at the inlet of the compressor, due to entrainment at flange connections and opening in the pipes. Also, light gauged ducting used for compressor inlet piping is not recommended because of possibilities of dust entrainment. Higher inlet vacuums will cause the compressor to operate as both a vacuum pump and a compressor both adding to the internal compression ratio of the compressor causing high temp internal compression temperatures. This can produce very high wear rates of blades.

C. Discharge Pressure
a. The Single Stage Sliding Vane Rotary Compressor operates in some cases to a maximum of 60 PSI, but it’s generally between 20-35 PSI. Higher compression ratios are produced at higher altitudes for the same discharge pressure causing higher temperature operation; this must be taken into account.

b. The compressor may actually pass the foreign particle to the discharge having the inlet being the source of material. Although, the materials may backup into the discharge line due to faulty operating NRV, and backflow to backup into the compressor. The non-return valve mounted in the discharge piping is not designed to prevent the back flowing of materials back into the discharge part of the compressor. The valve is designed to prevent the backflow of air to the compressor discharge causing the compressor to reverse its rotation after shutdown.

D. Moisture in the Air
On very humid days, or 100% relative humidity, air will enter the compressor under saturated conditions. Although the compression will elevate and superheat the moisture above the saturation level, this superheated air at high pressure will be of no consequence in the compressor, other than raising the specific heat of the air. In certain applications liquid moisture may enter the compressor inlet washing out the lubrication and causing a reduction in lubricating properties of the oil. In these cases, a specific lubricant is recommended.

E. Ambient Temperature
It is recommended that the inlet air piping should be located to feed the coolest air possible for compressor inlet. For every degree of increase of air inlet temperature, there will be a corresponding increase of discharge temperature causing the temperature of internal components to operate at higher temperatures and reduced life.

F. Cooling Water
The cooling water delivered to the compressor should be less than 90°F and should remove about 15°F of heat from the compressor. The flow rate for single stage compressor is 10 gpm /100 hp compressor motor. It is recommended that the temperatures and pressure gauges are installed in the cooling water inlet and discharge of compressor. This would give an indication if heat is actually being removed. The cooling water core may be clogged with dirt and mud and cooling water is not effective. In this case, both pressure and temperature gauges will indicate that. Generally cooling core pressure drop for clean core is 2-3 PSI. Concerning the water cooling, for every increase in degree of inlet water cooling above 60°F there will be an increase in 1/2°F of the discharge temperature. For circulating cooling systems, the heat removal may not be sufficient enough and overheating may occur, because of the undersized cooling systems during high ambient temperatures. Also, in some case, too much cooling may occur causing condensation in the compressor or accumulation of oil deposits or build-ups of areas of oil in the cylinder causing blade bounce in the cylinder and wear. It is recommended to shut-off cooling water to compressor when compressor is not operating.

When servicing compressor be sure to inspect head gaskets and replace gaskets if necessary applying correct head bolt torque given in manual. This will assure that no water will leak into cylinder and wash out lubricant.

A water flow sensor is supplied for installing in the cooling water inlet piping. Upon loss of cooling water, the sensor will activate an alarm.

G. Inlet Pressure
The single stage compressor can operate at a slight inlet vacuum of 8” wg maximum. A vacuum indicator installed at compressor inlet flange will indicate excessive inlet vacuum (red signal) at 15” wg. If inlet vacuum is excessive the compressor internal temperature will rise reducing the life of blades.

H. Discharge Temperature
It is recommended that the discharge air temperature probe be cleaned periodically every 6months from buildup of carbon deposits and material. The deposits will insulate probe and will not sense the actual discharge temperature.

The maximum operating temperature should not exceed 350° - 380° F. At higher temperatures, blade life is reduced. There will also be a greater amount of lube oil in vapor phase reducing this effective lubrication of oil. For very high temperatures, a higher viscosity oil is recommended.

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