Instrument Air System

Another incident occurred on a plant where the pressure in the instrument air system was maintained with nitrogen when the instrument air compressor failed. Two operators who were required to wear air masks attached them to the instrument air system. Unknown to them, the compressor had broken down, and the system was full of nitrogen. They both died [16]. 

The degree of drying desired will vary with the pneumatic equipment and application involved. The aim is to eliminate further condensation in the airlines and pneumatic tools or devices. Prevailing atmospheric conditions also have an influence on the approach that is most effective. In many 100-psig installations, a dew point at line pressure of from 500°F to 350°F is adequate. Other applications, such as instrument air systems, will require dew points of minus 500°F. 

Problems witbin tbe polisher unit caused operators to respond by attempting to unblock a cboked condition using instrument air. The air was at a lower pressure than the condensate and this caused water to enter the air system. This was not a standard procedure and the commercially supplied polisher unit was not built to standards consistent with the plant. Water in the instrument air system caused several instruments to fail and ultimately initiated a turbine trip. This interrupted heat removal from the radioactive core. The heat generation within the reactor was halted automatically within a few minutes by dropping metal rods to absorb neutrons within the core. 

An instrument air system normally consists of conventional oil-lubricated compressors and is used for operating instruments and machinery where no contact with the product or product environment exists. 

The task of purging pipelines for maintenance is almost second-nature to well-experienced operators in this unit. Typically, pipeline clearing is routine and is uneventful. This time, however, the utility dry air system was also being utilized as a source for instrument air in the operating area. Hence, this corrosive material was able to backflow throughout the instrument air system into monitoring and control systems. The backflow created expensive instrument damages. 

Damage to analyzers and instrumentation from the intrusion of acid gases into the instrument air system was extensive. Repair costs were about 300,000. 

The release resulted from backflow of a corrosive into a utility air header because of human error and from purges, bleeds, and leaks on the instrument air system that was being supplied from the utility air system. 

Clearing any system with instrument air as a pressure source is a bad idea. Other pneumatic sources, such as utility air, plant air, or nitrogen, should be used instead. However, the Three Mile Island instrument air was at a lower pressure than the water stream on the resin polisher system. Despite the presence of check valves, there was a reverse flow water entered the instrument air system causing several instruments to fail, and the turbine tripped. Through a series of other errors, the water covering of the radioactive core was uncovered, allowing an escape of a small amount of radioactivity. Due to the widespread negative public reaction, the U.S. nuclear industry received a setback. If it were not for that improper hose connection or the erroneous one-minute modification,... 

Other support facilities such as utility and instrument air systems, steam generation, flare relief systems, maintenance, firefighting, warehouse and administration buildings. 

The actuator system for a process control system in the CPI is typically composed of the control valve, the valve actuator, the I/P transmitter, and the instrument air system. The actuator system is known industrially as the final control element. In addition, a variety of optional equipment is designed to enhance the performance of the actuator system, such as valve stem positioners and instrument air boosters. 

Booster relays are designed to provide extra flow capacity for the instrument air system, which decreases the dynamic response time of the control valve (i.e., the time for most of a change to occur). Booster relays are used on valve actuators for large valves that require a large volume of instrument air to move the valve stem. Booster relays use the pneumatic signal as input and adjust the pressure of a high flow rate capacity instrument air system that provides pressure directly to the diaphragm of the valve actuator. 

The final control element consists of the instrument air system, the I/P converter, and the control valve (the valve and the valve actuator). The fastest way to identify gross problems with the final control element is to plot both the manipulated variable and the controller output. If the manipulated variable does not follow the controller output, there is probably a problem with the final control element. 

The analysis of utility systems such as steam headers and instrument air systems can be difficult because it is not always clear where the nodal boundaries are located. A discussion that starts in one area can become very far-reaching and include almost the entire facility. 

As described abeady in Chap. 6, instrument air and compressed air for operating purposes are normally supplied by the ah compressor in the air separation unit A receiver in the instrument air system ensures a continuous supply during malfunctions for a period of 30 minutes. A diesel-powered auxiliary compressor is usually installed to start up the plant and ensure compressed air supply during downtimes. 

Nielsen D, Platz O, Kongs0 HE (1977) Reliability analysis of a proposed instrument air system, Ris0 -M-1903. Roskilde/Danemark... 

The Disassembly Basin level indication and alarm systems, associated instrument air system and associated electrical power... 

Instrument air systems provide motive power for many components in a power reactor. The system is classified as a non-safety related system for many reactors, with safety related components assumed to fail in the safe position (on loss of air), or provided with safety related accumulators. Operating experience has revealed that the performance of air-operated safely related components may not be in accordance with their intended safety function because of inadequacies in the design, installation and maintenance of the instrument air system. Further, system recovery procedures and operator training may not be adequate to cope with loss of instrument air conditions. 

Degradation of instrument air systems can lead to failures in safety related systems, including possible common-mode failures. Failures have occurred in several systems, including auxiliary feedwater, residual heat removal, main steam isolation, BWR scram, service water, emergency diesel generators, containment isolation, and fuel pool seals. 

Instrumentation air systems are provided with redundancy and independence, and are also connected to the emergency power supplies. Meanwhile, air-operated valves for safety related systems are structured to keep the safety functions by moving into the fail-safe position, at the loss of the air supply, and are tested to ensure their functions periodically. Emergency procedures on loss of air are prepared at the nuclear power plants. 

Reliability of instrument air systems was tracked by the USNRC as Generic Issue 43 (in NUREG-0933). This issue was initiated in 1981. 

USNRC issued a number of Information Notices to alert its licensees to problems with instrument air systems. A report, NUREG-1275, Volume 2, "Operating Experience Feedback Report-Air Systems Problems was issued in 1987, covering 29 events at several plants. 

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