Upgrades improve Diablo Canyon`s waste gas sampling system
With increased reliability the redesigned waste gas
sampling system has reduced the equipment?s maintenance
costs and enhanced plant safety
By Abid Noman, Keller & Gannon, and Edward Dubost, Pacific Gas & Electric Co.
Hydrogen is added to the reactor coolant in pressurized water reactors (PWRs) to scavenge oxygen. The dissolved hydrogen in the reactor coolant evolves from the solution during storage and processing. This gas collects in tanks and vessels used for storage and processing of reactor coolant. In order to preclude the intrusion of oxygen into these vessels, which could lead to formation of an explosive mixture, an inert gas blanket is maintained at a slightly positive pressure.
Gas monitoring recommended
The Nuclear Regulatory Commission?s Standard Review Plan (SRP) for nuclear power plants makes several recommendations to ensure safe power plant operation. In order to prevent conditions that can lead to the formation of explosive mixtures, it recommends continuous monitoring of gases in the various tanks and vessels.
In keeping with the SRP, most PWR plants are equipped with dual oxygen analyzers which analyze the total oxygen concentration in the gases being transported to the waste gas storage tanks. These analyzers audibly announce or enunciate the presence of high oxygen concentration at the selected set-point, usually 2 percent oxygen by volume. If the oxygen concentration continues to rise, the analyzers trip the compressors transporting the waste gases to the storage tanks at the high-high set-point, usually 4 percent oxygen by volume, thus curtailing the processing of waste gases until the oxygen level can be reduced to an acceptable level.
Oxygen analyzers are typically required by the plant?s license (technical specifications) and must be calibrated and tested periodically. Although the analyzers are usually reliable, they measure the total oxygen concentration only in the ultimate waste gas stream. Unfortunately, they are not capable of identifying which of the several tanks is contributing to the high oxygen level. Identification of the tank with the high oxygen level must be performed manually.
However, sampling systems are available that can automatically monitor the tanks and vessels for explosive mixtures. Automatic systems continuously store and trend the data from individual tanks and issue an alarm when hazardous conditions develop. Today, most PWR plants utilize continuous explosive mixture sampling systems. Unfortunately, due to design problems, these systems are maintenance-intensive and have low availability. As a consequence, some plants have abandoned continuous monitoring and are relying on labor-intensive grab sampling. Some power plants selectively sample only a few vessels.
Sampling system design
A typical explosive mixture sampling system consists of several remote vessels from which tubing is routed to a central sample panel. Figure 1 shows a simplified configuration of a typical sampling system. The following points are usually sampled:
Y Volume control tank
Y Pressurizer relief tank
Y Reactor coolant drain tank
YEGas decay tanks
Y Liquid holdup tanks
Y Boric acid evaporator vent condenser
Y Waste gas compressor surge tank
Y Waste evaporator vent condenser
Y Liquid radwaste storage tanks
The flow to the sample panel is established by opening a set of interlocking solenoid valves, one remote, located close to the source, and the other located inside the sample panel. Because most of the tanks are at a lower pressure than the waste gas system a sample pump is required. This pump transports the sample to the sample panel and then back into the waste gas system. The sample pump is typically a diaphragm-type compact, motor-driven machine.
Samples of the gases are analyzed for oxygen and hydrogen content by separate oxygen and hydrogen analyzers, both of which have local analog readouts. A transmitter sends a signal to a recorder and to the enunciators which audibly warn operators of any upset conditions.
Because the tanks being sampled contain warm reactor coolant, the samples contain condensed moisture. To protect the pumps and instruments the sample?s moisture is separated by means of coalescing filters. Most of the waste gas sampling systems for PWRs were designed a number of years ago and are controlled by an electro-mechanical type of timing device. These timing devices actuate various solenoid valves for the tanks to be sampled. They also activate the filter drain.
Typical system problems
Many of the explosive mixture sampling systems do not operate satisfactorily. In general, these systems are maintenance intensive and have poor reliability. Some plants have opted to perform manual sampling instead of correcting the chronic problems associated with explosive-mixture sampling systems.
The electro-mechanical timing device control system is quite inflexible, in that both the sampling sequence and the time required for obtaining a true sample (purge time) are pre-set. Any changes in the sampling sequence, skipping a point for example, require cumbersome hardware changes. Similarly, the purge time is set initially via the hardware. This purge time can only be altered by tuning the hardware, which is inconvenient. Another problem is the continuous opening and closing of contacts which causes the contacts to frequently burn out.
The data logging for the system is done with a traditional pen/strip chart recording device which is not conducive to data trending and is not easily readable.
Typically, vessels are located several hundred feet from the sample panel and the connecting tubing goes through numerous changes in elevation. In addition, the remote vessels contain warm gases which are at, or very near, the saturation point of the gas.
As the gases move through the cool tubing water vapor the gases tends to condense. Due to the changes in elevation, there are several loop seals or moisture traps. As a consequence, the sampling pump tends to be undersized and, in some cases, the flow is blocked completely.
Another problem is that the sample pump operates at vacuum on the suction side which makes it difficult to separate moisture from the flow stream. Any moisture carryover to the pump and analyzers reduces their useful life.
Other mechanical problems include:
Y The setpoints for various regulators in the flowstream, especially the ones downstream of the sample pump, are not optimized to allow discharge back to the pressurized waste gas system.
Y Moisture removal and sample conditioning are not adequate and condensation occurs downstream of the pump.
Y Inadequate instrumentation prevents troubleshooting and identifying of system problems.
Sampling system modifications
at Diablo Canyon
Figure 2 shows the simplified configuration of the modified waste gas (explosive gas) sampling system at Diablo Canyon power plant. Modifications to correct the operating problems entailed replacement of the solid state control system and recorder, replacement of the sample pump, improved filtration and conditioning of sample streams, and enhanced instrumentation for setting system flows and troubleshooting.
The inflexible and maintenance-intensive electro-mechanical control system and recorder were replaced with an industrial grade personal computer-based controller. Sequence of sampling and the sample points selected for sampling can be easily manipulated by the menu
Other parameters can also be modified through the software. One such parameter is the purge time for each source, which is dependent on the line volume. The wiring from the components controlled by the sampling system interfaces with the computer via a control board. Until now, the control board microswitches have shown no wear from the continuous cyclical service of the sampling system.
A color monitor is used to monitor and display the data and alarms. In addition, the data and alarms are recorded on a hard disk and can be recalled, trended and manipulated when required.
The original pump was a diaphragm type, single-head pump with a 1/6-hp motor. From the operating experience it was obvious that the original pump was undersized and not capable of supplying flow to the analyzers and discharging it to the pressurized waste gas system. As a result, it was replaced with a four-head pump driven by a 1/2-hp motor. Two of the heads were connected in parallel and connected to the other two heads in series. This ensures adequate flow, as well as pressure, for a diverse range of operating conditions in the different tanks and vessels being sampled.
The original sampling system filter was a small bowl filter located upstream of the pump. Actuating a solenoid valve intermittently drained the filter. Because no sample conditioning was provided in the original system, it was susceptible to moisture related problems.
In order to eliminate these problems, two modifications were made. First, two large-bowl coalescing filters were installed upstream of the pump. These filters have a very high filtration efficiency, even at high vacuum conditions, with only one filter in service at a time and the other on standby. The filters are automatically switched by the computer software. After a filter is switched to standby, it is purged and drained under positive pressure for a short duration. The positive pressure for efficient drainage is supplied by the pump discharge while the sample tubing is being purged.
A refrigerated dryer was added downstream of the sample pump. The dryer condenses moisture from the flow stream and maintains a low dew point, thus eliminating condensation in the discharge tubing. The condensate from the dryer is purged continuously via a small flow stream obtained from the pump discharge (Figure 2).
Additional pressure gauges and flowmeters were installed to ensure proper flow settings and for subsequent troubleshooting. Pressure readings are now available for pump suction and discharge and for setting the pressure regulators upstream and downstream of the analyzers. Several flowmeters are used for setting the initial system flow rates. Because moisture can block the narrow openings in the meter during normal operation, flowmeters for the filters and dryer drains are bypassed.
PG&E was able to modify the sampling system at Diablo Canyon by adding a minimum number of active components to achieve the design objective. The end result is a system that has improved reliability and maintainability. As a result, the plant has overcome the flow and control system related problems, and the system can now perform its intended design function, thus enhancing the plant?s safe operation. END
Abid Noman is a project engineer with Keller & Gannon responsible for the design and analysis of power plant systems and processes. He hold a bachelor?s degree in mechanical engineering from the University of Oklahoma and a master?s degree in mechanical engineering from the University of California Berkeley.
Edward Dubost is a senior engineer in PG&E?s nuclear engineering department in San Francisco. He holds a bachelor?s degree in chemistry from the University of Santa Clara, Calif. He is a registered professional engineer.