Costs metrics

Testing for contamination in pneumatic control systems is required when contamination of the system is due to observed changes in instru-ment operation and calibration stability, or when evidence of oil or water is found on instrument filters, at bleed ports, or in system dirt legs.

Testing for Oil and Water Contamination

In a pneumatic system, the first line of defense against oil and

water contamination is a dual unit oil and water separator followed by a particulate filter, rated at 3 microns, which is generally installed be-tween the high pressure storage tank and the dehydrator. Close inspec-tion of the glass bowl of the separator will reveal the presence of oil or water. By blowing down the separator and collecting any contaminants in a clean cloth, the contaminants can be inspected with a magnifier for identification, or sent to a lab for exam.

After oil contamination is detected, it may be necessary to install oil test tubes at several points in the system. This is discussed in more detail in Chapter 8, “Maintaining Pneumatic Control Systems.”

Testing for Particulate Contamination

In a pneumatic system a check for particulate contamination can be made when testing the oil and water separator for liquid contaminants.

The particulate contaminants may include wear particles from bearings and seals, debris from piping installation, and dirt.

Testing for General Contamination

The filter elements installed in many makes of receiver-controllers are used to test for general contamination. When excessive pressure drop is suspected across a line filter, the filter should be replaced and the old filter cut open for inspection. Many filters have instruction sheets for the inspection of used filters and the interpretation of findings.

Interpretations of Test Findings

After the testing has been done, the test findings must be inter-preted. Where contamination is abnormal, a program must be imple-mented to determine what steps must be taken to correct the contamina-tion problem and place the system back in proper operacontamina-tion.

Elimination of Source of Oil

The usual measures which are employed to stop oil contamination include eliminating the source of the oil. Because oil generally originates in oil-lubricated compressors, replacement of oil-lubricated compressors with oil-free compressors is a positive measure to eliminate that source of contamination. Oil left in piping must be removed. Methods for clean-ing pipclean-ing systems are described in Chapter 8.

Elimination of Source of Water

Water generally originates as vapor in atmospheric air and is

con-densed out of air during the compression cycle.

One way to minimize the amount of water in the system is to limit the amount of water vapor introduced into the system. This can be done by piping conditioned supply air from a dehumidified air duct into the controls air compressor intake.

The volume of air pumped by the compressor is so small as to be no factor in the HVAC system load.

Testing for overall control system functions is required annually, and after each addition to the HVAC system building usage.

Testing Operating Controls

Testing operating controls for setting requires a point-by-point re-view of system documentation to determine the desired values and verification of each of those settings, which may include main setpoint, reset setpoint, throttling ranges, proportional band, and ratio or au-thority.

After the control setpoints and settings are verified, they must be justified, which may require simulation of inputs, such as with gradual switch or manual transmitter from main air for pneumatic devices or by use of decade box to input resistance values to simulate electronic sensors or by use of potentiometer to simulate controller inputs on slide wire type bridge circuits in electric controls. The process of jus-tification is that of proving the control outputs to be appropriate to the control inputs.

After the controls have been justified, they must be tested for func-tion. Function is tested by observing for appropriate action, whether the direct acting reset of a direct-acting controller is giving reverse reset of the controller, or whether the chilled water valve is opening on a rise in space temperature.

Testing Controlled Devices

Controlled devices to be tested may include dampers, valves, or relays, either electric or pneumatic.

Dampers and valves must be tested for freedom of motion.

Damper motion may be restricted because of the friction caused by seals and blade linkages, particularly on low-leakage damper designs. Valve motion may be restricted when stem packing is over tightened in stop-ping leaks, or after a stem has become galled due to leakage of the

controlled fluid causing the stem to seize in the packing. In pumping systems without pressure control, dynamic forces in the piping system can overcome the actuator power and prevent movement of the valve in response to the control signal.

Actuators must be tested and observed for direction of travel in order to justify proper control response.

Actuators must also be tested and observed for extent of actuator travel in order to verify that damper and valve linkages are properly adjusted to provide full positioning of controlled device when full actua-tor travel is applied.

Testing of minimum positioning relays and other multi-positioning devices is a necessary part of this work.

Servicing Air Filters

Air filters in a pneumatic system include air intake filters on com-pressor, particulate filters in the main air supply dual element unit, and

“finger” filters at individual control components.

The required frequency of change should be based on exposure to contaminants and pressure drop. The possibility of media blowout due to excess pressure drop is very good reason for changing filters. The required frequency can range from a few months to several years and is determined by experience.

No part of a pneumatic system should be left unfiltered except under emergency conditions and then only for the shortest possible time.

Verifying Tight Connections

Electric, electronic, and pneumatic systems all require periodic at-tention to tightness of connections between control components and their interconnecting piping and wiring systems.

Electric and electronic control system connections may be checked for tightness by using a volt-ohm-ampere multimeter to measure the resistance across the joint. A measurable resistance across a joint is con-sidered excessive and must be corrected.

Pneumatic control system connections may be given a rough check for tightness by listening for the hiss of an air leak. Suspected leaking joints may be checked using the soap bubble test. Where major air leaks are suspected in pneumatic controls system air mains due to loss of air pressure, a sectionalizing exercise must be performed. Using

sectionalizing valves, which may be added, divide the system in half and apply a pressure test. Determine which half of the system the leak is located in, then divide that part of the system in half, and repeat the exercise until the leak is located.

Cleaning Sensors and Remote Bulbs

Many sensors depend on a flow of air over the temperature or humidity sensitive element in order to perform their sensing function.

Buildups of dust or dirt on sensors and remote bulbs will result in de-lays in sensing and inaccurate measurements of sensed medias. Each type of sensor requires a different cleaning method to avoid damage or impairment of sensing function.

Humidity sensors may be cleaned with a soft brush, like a photo-graphic lens brush, or by air jets such as those generated by a hand-held medical syringe. Do not use solvents in cleaning humidity sensors.

Temperature sensors of the wire wound type should be cleaned by the same methods as for humidity controllers, except that temperature sensors may be sprayed with a solvent, such as is used for cleaning TV tuners. Other temperature sensors may be cleaned with a cloth or brush and may be sprayed with a solvent.

COMMONLY FOUND CONDITIONS