Reflexiones personales

Maintenance of an automatic control system pneumatic control air supply system is planned to assure these four basic needs for control air:

1. Assure reliability of air supply by making the system free from breakdown.

2. Assure pressure of air supply by maintaining the compressor in proper working order with piston rings, shaft seals, valves, and other moving parts within proper tolerances.

3. Assure cleanliness of air supply by filtering makeup air at the compressor intake and by filtering air leaving the tank with a particulate filter in series with an oil filter.

4. Assure dryness of air supply by obtaining makeup air from the driest available source and then dehydrating high pressure air before pressure reduction.

System Reliability

System reliability applies to compressors, tanks, motors, motor starters, control devices, dehydrators, filters, and air distribution system.

The most common problems in system reliability stem from com-pressors and are lubrication related.

A common failure train is found when a system air leak occurs which is large enough to cause continuous compressor operation with no cool-down periods. As heat is generated in compression, the com-pressor builds up heat due to lack of off-time, and the lubricating oil heats up and loses its lubrication ability. Without lubrication, the com-pressor bearings and seals soon fail and cut into the rotating surfaces.

Failures may occur in a single part or may progress to the point that the entire compressor requires repair or replacement after only one episode of overheating.

Bearing Temperatures

Motor and compressor bearing temperatures should be checked with a surface-reading thermometer during normal operation to estab-lish baseline data on bearings before trouble occurs.

Motor Starters

Failure of electric motors due to burnout can be avoided by provid-ing proper overcurrent and phase failure protection in motor starters.

Motor starters may be thermal overload type manual switches on frac-tional hp single-phase motors or magnetic starters for large single-phase and three-phase motors.

The most common electro-mechanical starters are magnetic contac-tors with current sensing overload heater relays. Starters for 3-phase service may have overload heaters on only two phase legs. Should an excessive current flow occur in the unprotected phase leg, the motor can be damaged to the point of failure. An overload heater should be pro-vided for the third phase leg. For 3-phase starters without phase-failure relays, retrofit current monitoring relays should be installed to open the starter contactor holding coil circuit on failure or reversal of any phase leg or under- or over-voltage of the electric service.

Maintenance operations for manual switches are:

1. Verify the proper overload heater rating for the motor size and ambient temperature.

2. Verify the security of wiring connections.

Maintenance operations for magnetic starters are:

1. Verify the proper overload heater ratings for the motor size and ambient temperatures of both the motor and starter.

2. Ensure smooth faces on current-carrying contacts. When rough contacts are found, either dress contacts in place with a burnishing tool or replace badly damaged contacts with a complete new set.

Control Devices

Compressor motor operation is controlled by a pressure switch sensing pressure in the air tank or receiver. Typical setpoints might be:

cut-in at 70 psig to start compressor and cut-out at 80 psig to stop com-pressor. Compressors larger than about 15 hp may use solenoid unloader valves to relieve pressure in compressor to allow the next start to be unloaded.

Maintenance operations for controls include:

1. Verify settings of pressure control, test average off-time, adjust cut-in setpocut-int as needed to provide desired off-time.

2. Observe repeatability of cut-in and cut-out paints. Where control action is not repeatable, replace the pressure controller.

Dehydrators

Most pneumatic control systems installed in the past 25 years must have very dry air for proper operation. The water-cooled shell and coil aftercoolers used in older systems do not provide adequate moisture removal. A refrigerated dehydrator capable of cooling maximum system airflow rate to at least 10°F dew point is required. The most frequently used dehydrators are air-cooled refrigerated packages with an integral water drainer.

Maintenance operations for dehydrators include:

1. Ensure continuous electric service to the dehydrator.

2. Observe discharge from the water drainer to verify that the dehy-drator is refrigerating the air. Take corrective action as required to make the dehydrator operate and provide dehumidification.

3. Assure a clean condensing coil. Provide an air filter cut from 1/2”

thick roll media to cover the air entering side of the air-cooled condensing coil. Change the filter regularly to assure adequate clean airflow.

Filters

Modem control systems require very clean air. Filters for particu-lates and oil vapor should be installed in the high pressure air line be-tween the air tank and the pressure regulating valve, generally upstream of the dehydrator. The particulate air filter will trap particulates in sizes 5 microns and larger and the filter cartridge must be changed when pressure drop exceeds the manufacturer’s recommended value.

Pressure drop testing can be done with pressure test ports on each side of the filter. The oil filter will trap oil in sizes 0.3 micron and larger and the coalescing oil filter must be changed regularly. Both filters may trap water droplets and must be fitted with drain outlets to allow manual draining of oil and water.

Maintenance operations for filters include:

1. Measure pressure drop across particulate air filter. When observed pressure drop exceeds value recommended by manufacturer, re-place the filter element.

2. Measure pressure drop across coalescing oil filter. When observed pressure drop exceeds value recommended by manufacturer, re-place the filter element.

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