Instrument Air in Nuclear Power Plants
Nuclear power plants produce electricity for people, business and industry. Electricity is produced in a similar fashion as fossil fuel (i.e., coal, oil, etc.) power plants, using steam to drive a turbines which spin an electrical generator, producing the electricity. Energy, in the form of heat, to produce steam is comes from the fission of Uranium235 atoms rather than the burning of fossil fuels. Once energy has been removed from the steam, the remaining energy is either used in preheating condensate (liquid water) that will be used to make more steam or it is removed in a condenser heat exchanger. In the condenser, the steam condenses to liquid as energy is removed by cooling water from a lake, ocean, river, or to the atmosphere via cooling towers. Once the remaining energy is removed and the steam becomes liquid condensate, the liquid condensate is reheated in the steam generator, producing high energy steam for use in the turbine (1)(2) .
|Oconee Nuclear Station at Dawn|
Instrument Air for Measurement and Safety
To produce electricity in the most efficient manner, the flow of steam and condensate, as well as levels in the heat exchangers must be monitored and controlled. Many of the instruments that monitor flow, level, pressures, and temperatures incorporate instrument air quality compressed air (Instrument Air) to transfer information. Flow and level control is accomplished by the throttling operation of air operated valves (AOV’s). The AOV’s require instrument air: clean dry compressed air. Normally, the resulting load on the Instrument Air System is relatively constant. The high quality air prevents the small ports on instruments, controls, and AOV’s from becoming clogged with debris, moisture or oil, which could prevent the proper operation of the equipment supporting the efficient production of electricity.
CANUG (Compressed Air Nuclear Users Group) was formed, in 1988, with the objective being to exchange information between Compressed Air Systems Engineers serving nuclear power plants. Eighty members share the NEED to improve the reliability, availability and reduce the cost of the compressed air systems that serve the valves, instruments, and other components utilizing compressed air in controlling the conversion of nuclear energy to electricity.
As noted, nuclear energy is produced by the fission of Uranium235. The fission process as well as fission products resulting from the fission, produce radiation. The safe and proper operation of a Nuclear Power Plant ensures the nuclear fuel and fission products will be contained in a acceptable configuration so as to not cause harm to plant operators or the general public. At some nuclear power plants, instrument air is required to assist in safely shutting down a nuclear power plant in the event of an emergency ensuring that the barriers to the release of radioactive contamination will be maintained. Yet, in most nuclear power plants, instrument air is not required to perform these functions, as all the components required to perform these functions either do not use compressed air or the air controlled components fail in the position required to prevent release of radioactive contaminants (3).
As at all industrial facilities, compressed air is also used for supplement uses such as operating air driven tools and pumps. This is commonly designated as service air and is separate from the Instrument Air System. The Service Air System may be used as backup for instrument air.
Instrument Air at the Oconee Nuclear Station
Oconee Nuclear Station’s (ONS, three unit station) original Instrument Air System configuration incorporated three Worthington Reciprocating Compressors (HBB 14x13, 489 cfm each) supported by refrigerant dryers. During initial startup and operation, Oconee’s Instrument Air system normally operated with two compressors in run, one in standby, supporting the operation of Unit’s 1 and 2 air operated valves and instruments, as well as tooling and sewage ejectors. ONS realized more compressed air capacity was required. In the mid 70’s, a separate Service Air System was installed, using two oil-flooded Sullair Rotary Screw compressors (150 Series), to support tooling and sewage ejectors.
The Service Air System was tied in to the Instrument Air System as a backup. This proved advantageous as well as problematic. The Worthington’s were high maintenance and Service Air was often used to supplement Instrument Air. Since the Service Air system was supported by oil flooded compressors, the Instrument Air System was frequently contaminated by oil, causing operational problems. The oil problem was eliminated with the installation of Deltech Color Change filters at the Service Air to Instrument Air crossover.
By the mid 80’s, the Nuclear Regulatory Commission (NRC) had gathered quite a bit of data concerning problems experienced at nuclear power plants that resulted from poor Instrument Air quality and reliability. The NRC issued Generic Letter 88-14 (INSTRUMENT AIR SUPPLY SYSTEM PROBLEMS AFFECTING SAFETY-RELATED EQUIPMENT). This report referenced NUREG-1275, Volume 2 "Operating experience Feedback Report-Air Systems Problems" which indicated “that the performance of the air-operated safety-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.”
In addition to Generic Letter 88-14 (4), the Institute of Nuclear Power Operations (INPO) issued Supplemental Operating Experience Report (SOER) 88-1 (Instrument Air System Failures), written to address system failures that had been initiated by poor quality Instrument Air (IA).† It noted at that time, “system failures caused by instrument air failures are occurring at a rate that indicates greater attention to instrument air systems is warranted.”†(5)
Addressing Issues with Instrument Air
ONS, as well as the rest of the Nuclear Industry, addressed these issues. At ONS, an additional air compressor was installed; Sullair 32/25 400, a 2200 cfm oil-flooded tandem rotary screw compressor. Dual two-stage pre-filters and two SAHARA heatless desiccant dryers (each rated at 1400 scfm) were installed in parallel downstream of the new compressor. This configuration, now the primary source of Instrument Air, operates approximately 340 days per year and is taken out of service each quarter for preventive maintenance. The Worthington compressors have become the backup source of air and the supporting refrigerant dryers were replaced with two 750 scfm heatless SAHARA desiccant dryers installed in parallel. This arrangement has worked well, providing a much more reliable source of dry, clean compressed air (6).
Oconee Nuclear Station is unique with its Instrument Air System configuration, using an oil-flooded compressor, as well as having one central source supply for all three nuclear units. A majority of the nuclear industry use oil-free compressors; rotary screw or centrifugals. Most of their systems are configured such that each nuclear unit has its own air source. Though each unit has its own source, most sites have cross connects between the unit’s instrument air supplies. The cross-connects are kept isolated except during emergencies, ensuring that a problem on one air system does not affect the operation of multiple units.
|The CANUG 2009 Conference was held at Disney's Contemporary Resort and featured Compressed Air Challenge training. The trainers were Bill Scales and Tom Taranto.|
Instrument Air Specifications for New Nuclear Power Plants
There are new nuclear power plants on the horizon. These will be built or supplied as modular units. There will only be a few designs to choose from and therefore, the plants will be identical. By designing and building them this way, it will be much more cost effective and easier to have them licensed for power operation. When the concept of similar power plants was first presented, the Compressed Air Nuclear Users Group (CANUG) took it on as a challenge. The challenge was to provide to the power plant designers an ideal instrument air system configuration, one that was not only reliable, but also cost effective and flexible. Each of the present CANUG members has an Instrument Air System which has some shortcomings and this was an opportunity to provide input to the future plant designers, eliminating those shortcomings.
Defense-in-Depth of Three
With respect to the Ideal Instrument Air System, the CANUG requests a defense-in-depth of three. This will allow for one instrument air train to be in Maintenance while the other two are configured in Run and Standby. Each of the three trains should consist of a compressor and associated inter/after cooler(s), wet receiver, pre-filter/mist eliminator, desiccant dryer and after filter. To enhance the flexibility, the component trains ought to have the ability to be cross-connected as well as have connections to the systems which would allow an outside source of air. Dry receivers should be used at the outlet of the after filters and as needed at the far reaches of the Instrument Air System.
The compressors should be sized to carry approximately 75% load to allow for growth or changes in system dynamics. Their suction sources should be in areas not subject to high temperatures, sources of contaminants or highly moisturized air. The compressor controls should be such that a failure on one controller will not affect the capabilities of the other two compressors.
The piping between the compressor and the dryer should be stainless steel so as to minimize the corrosive effects of highly moisturized air. This will also minimize the corrosion brought on by other atmospheric contaminants that could be initiated by the discharge from other industries in the area. Likewise, the wet receiver should be either stainless steel or coated such as to prevent corrosion.
The pre-filter should be a high quality coalescing filter supported by automatic drain traps. A “mist eliminator” should also be used depending upon the type of compressor used and the expected contaminants in the area of the suction of the compressor.
Drain Traps (supporting the aftercooler/intercooler moisture separators, the wet receiver, and the prefilters) should be “Zero-Loss” drain traps with at least 3/8” ports to prevent blockage. Each of the drain traps should have their own individualized drain discharging to a sump. They should also be supported by a bypass line and valve that allows the drain trap to be removed from service for maintenance and yet allow the supported component to remain in service.
The CANUG discussed the type of desiccant air dryer to be used in this system and decided upon a heatless desiccant dryer. The reason was a simpler design and operation than the internally heated or blower purge dryers. We understand that in the long run, this is more costly, but were considering reliability over cost. This may still be up for discussion.
Both wet and dry receivers should be installed. The wet receivers provide a surge volume for dryer purges as well as another opportunity to collect and condense moisture in the compressed air. The dry receivers prevent over rating the dryers on sudden surges in the instrument air header as well as an accumulator volume to support operation of downstream components.
The CANUG also considered installing dual-parallel pre-filters and after-filters to increase the margin of reliability. This allows the removal of a filter from service without taking the whole train out of service.
In addition to the monitors provided with the equipment installed, there needs to be additional monitoring capabilities in order to observe the health of the Instrument Air system. Dewpoint monitors should be installed at the outlets of each air dryer if they do not come as part of the air dryer package. There should also be sample taps throughout the system to allow periodic monitoring of air quality; downstream of each of the pre-filter/mist eliminators (oil and hydrocarbons) and the dryer after-filters (particulate and moisture) as well as strategic locations throughout the system. Moisture can enter the Instrument Air System through leaks and thus periodic moisture monitoring at locations downstream of the after-filters might prove advantageous (Fick’s law). Flow and pressure indications should be available at strategic locations to provide an indication of use (trending) as well as assist in diagnosing problems. Flow indication upstream of the dryers would provide input as to the purge being used by the dryers. It would be good to add the ability to measure the amount of electricity (KW, amps) being drawn by the compressors. This could provide valuable insight concerning the efficiency of the compressors as well as a second check of flow indications.
Since the issuance and response to NRC’s Generic Letter 88-14 and INPO’s SOER 88-1, air quality has not been a great issue other than ensuring we are continuing to meet the commitments we made in the past. But, reliability of the Instrument Air Systems continues to be a challenge. The Institute of Nuclear Power Operations provides an operating experience data base of problems experienced throughout the nuclear industry that is regularly reviewed for applicability. Nuclear Power Plant System Engineers review this data base as part of their System Health Reporting and consider it for potential applications. No formal response is required, as in the case of Generic Letter 88-14 and SOER 88-1, but it is incumbent upon each nuclear facility to seriously consider the applicable events listed in the data base, the causes and resolutions, so that similar events do not happen at their respective sites; “Operating experience; heed it or be it.”
Preventative Maintenance Guide
In the mid-90’s, the nuclear industry noted that there had been challenges to the reliability of equipment essential to the production of electricity using nuclear energy. They commissioned the Electric Power Research Institute (EPRI) to assist in developing a set of Preventive Maintenance standards to support the nuclear industry. Instrument Air System components were selected to be part of the study and development of the EPRI PMIR (Preventative Maintenance Information Repository). A number of nuclear industry personnel as well as compressed air industry representatives were involved in the development of this guide. Out of this effort, plus a previous gathering of information involving Instrument Air System Engineers throughout the nuclear industry, EPRI produced the Compressed Air System Maintenance Guide (TR-1006677). This is an excellent resource for developing a preventive maintenance program and developing an overall understanding the Instrument Air System. The EPRI PMIR templates are used throughout the nuclear industry as a benchmarking tool in the development of preventive maintenance programs.
As can be noted, the nuclear industry is unique: in order to use nuclear energy safely, the nuclear industry needs to work together. Though there may be competition in the realm of selling and distributing electricity, there is tremendous cooperation between the nuclear sites, sharing “Operating experience; problems and resolutions” for the good of the industry.
- Nuclear Power Plant Illustration: By Russell D. Hoffman: http://www.animatedsoftware.com/environm/nukequiz/nukequiz_one/nuke_parts/reactor_parts.swf
- Nuclear Regulatory Commission Animated Power Plant: http://www.nrc.gov/reading-rm/basic-ref/students/animated-pwr.html
- In Reactors, Radiation is Trapped and Contained in Several Ways: http://www.nrc.gov/reading-rm/basic-ref/students/radiation.html
- NRC Generic Letter: http://www.nrc.gov/reading-rm/doc-collections/gen-comm/gen-letters/1988/gl88014.html
- Institute of Nuclear Power Operations: http://www.inpo.info/
- Regulatory Effectiveness Assessment of Generic Issue 43 and Generic Letter 88-14 (NUREG-1837): http://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1837/
- Nuclear Industry Support Groups: http://www.nucleartourist.com/basics/inpo.htm