Instrumentation & control
A new age for automation21 January 2011
The development of new reactor designs, along with IEEE specifications that demand higher levels of performance and reliability, are driving suppliers to re-qualify their existing valve automation products. By John Gonzales
Valves are an important part of every nuclear power plant, controlling fluid flow, temperature, level and pressure, and also helping to ensure the safety integrity of the plant. Depending on the application, valves can be either rotary or linear. Rotary valves can be ball, plug, or disc design, and linear valves can be globe or gate. Valves can be operated manually, electrically, or pneumatically to perform their required functions. Manual valves are increasingly being replaced with pneumatic and electric actuators to allow plants to become more automated, reducing the man- hours required to operate valves and increasing operator and plant efficiency.
Actuators are used in a nuclear plant to operate valves on main steam lines; ventilation, service feed water and reactor isolation systems.
Valves fitted with electric actuators are known as motor operated valves (MOV) and valves fitted with pneumatic actuators are known as air operated valves (AOV). Actuators for both MOV and AOV valves are available in linear and rotary designs. Linear actuators operate globe or gate valves by raising and lowering the valve stem to control the flow through the valve. Rotary actuators operate the stem of ball, plug, or disc valves in a clockwise or counter- clockwise motion, in either 90 or 180 degrees of travel to control the flow through the valve.
Ninety-degree rotary pneumatic actuators are used within the nuclear industry for normal operation and as part of the plant’s safety integrity system. They can be used to modulate the valve opening to provide some degree of control, or they can be configured to move the valve from fully open to fully closed or vice versa. The actuators can be set up to fail open, fail closed, or fail in place. Typical pneumatic rotary actuators are operated by a diaphragm or a piston-type cylinder. The cylinder operator is mounted on a housing and yoke mechanism that transfers the linear motion into rotary motion. The opposite side of the housing and yoke mechanism of the actuator can be connected to a spring to provide a static force to drive the actuator in a certain direction.
Pneumatic rotary actuators generally operate in one of two different ways. They can either be spring-return or double-acting. Double-acting actuators have a pneumatic cylinder and housing. They require air power to operate the actuator in both clockwise and counterclockwise directions. Spring return (or single acting) actuators have a pneumatic cylinder, housing, and spring. They require pneumatic power to operate the actuator in one direction, and on loss of the pneumatic power the spring will rotate the actuator in the opposite direction.
When selecting an actuator for any valve application, it is very important to size the devices correctly to ensure that the valve can be operated at all times. The size of the piston or cylinder and the pneumatic supply pressure determines the amount of force available. This force is then transferred through a piston rod to a scotch yoke mechanism (that converts linear motion to rotary motion).
In the USA, the new IEEE and new plant design specifications require that actuators meet both the Electric Power Research Institute (EPRI) sizing criteria and the plant’s sizing requirements. These stringent specifications ensure that the actuators have more then enough torque to operate the valve before, during and after an event.
For each nuclear station there are typically four different variations of actuator required: environmentally-qualified, seismic-qualified, safety-related and commercial (balance of plant). Environmentally-qualified (EQ), seismic, and safety-related actuators must go through rigorous testing and qualification programmes to validate and document their performance when used in the critical containment areas and for safety related functions. The actuators are tested to verify that they will perform their designed safety function before, during and after an event.
The actuators subjected to EQ testing are aged for the service life required by the IEEE and individual plant specifications and are subjected to thermal ageing, radiation ageing, loss of coolant accident (LOCA)/main steam line break (MSLB) testing, seismic testing and other functional tests. The EQ actuators are unique in construction and must be purchased as such from the manufacturer’s factory.
The Nuclear Procurement Issues Committee (NUPIC) and the Nuclear Industry Assessment Committee (NIAC) audit actuator manufacturers’ facilities and sales channels. These are US organizations made up of multiple plants. Actuator manufacturers must have a quality assurance programme for nuclear service that conforms to the specifications of 10 CFR PART 21 and 10 CFR 50 Appendix B requirements.
The final choice of actuator specification must be based on the requirements of its particular application, which can vary depending on the reactor type. This, coupled with new IEEE qualification standards, is driving suppliers to re-qualify their products, which can take approximately 12-18 months. IEEE standards are US standards; there are different standards outside the USA, although some have accepted the IEEE standards.
Historically, pneumatic quarter-turn actuators offered to the nuclear industry were qualified to the ‘old’ IEEE EQ, seismic and safety-related standards listed in Table 1. New qualified actuators will be required to meet or exceed the new IEEE specifications listed in the table.
The main differences between the old and new standards are an increase in service life (from 40 to 60 years), an increase in seismic requirements (depending on the location in the plant), an increase in radiation levels, an increase in pressure ratings, an increase in temperature ramp (which is instantaneous), and an increase in the dwell time for all of the above.
Automation and controls
Historically, in most nuclear applications, the controls applied to pneumatic actuators typically consisted of limit switches, solenoid valves, and filter regulators for on/off valves. The trend is now changing towards more complex control systems that require the automation element to perform different functions including:
- Fail in place—on loss of electrical or pneumatic signal—and then close after a certain amount of time
- Modulating service
- Fast close
- Move to fail function after a MSLB event or LOCA event.
Automated control systems must be designed, tested, and certified to ensure that they meet the required specifications for each application. The test certification becomes part of the final Certificate of Conformance (C of C) for the specific order.
Identifying and understanding any loss of performance in control devices is becoming increasingly important. Development in automation technology has seen the introduction of digital positioners for diagnosing valve condition. Digital positioners increase the response, performance, and repeatability of the automated valve packages.
The nuclear industry requires support for automated packages as per 10 CFR Part 21 and 10 CFR 50 Appendix B programmes with documentation for qualified controls from the individual control manufacturers. Valve controllers that meet the new automation requirements are becoming more readily available as manufacturers are re-qualifying their systems to meet the new specifications.
Maintenance and support
Another area that has seen tougher standards is in the area of device maintenance and support. Previously, nuclear specifications required 40-year service and support for products (valves, actuators, automation). The latest nuclear specifications have increased the required length of service and support to 60 years.
For products offered for new plant designs, manufacturers must identify service intervals and be prepared to provide complete service kits for soft seal replacement. These kits must be proven through the qualification testing process and identified in the operating and maintenance manuals for the qualified actuators.
Most nuclear facilities elect to refurbish the actuators themselves, but manufacturers still need to have a strong support structure should they require technical support. The required service interval of a legacy pneumatic actuator is every five years. Current models should extend this to ten-year service intervals, giving new power plants longer serviceable life and significantly reducing the amount and frequency of maintenance required. The service intervals are determined by the manufacturers’ recommended standards. The IEEE standards require that service intervals are listed per the products’ manufacturer.
Some of the latest actuators on the market, including Emerson’s Bettis NG-series, are designed to facilitate ease of service while the actuator is valve-mounted. Other developments in actuator design include housings that are totally enclosed to protect personnel from moving parts, minimise the possibility of scotch yoke and piston rod misalignment and prevent mechanical damage.
External field-adjustable stops allow accurate valve positioning at both the open and the closed positions. Also corrosion and wear protection extends the actuator life and provides resistance to the effects of harsh environments and ageing.
John Gonzales, nuclear project manager, valve automation, Emerson Process Management
|AUMA expands its range with smart actuator|
German company AUMA Riester GmbH has supplied electric actuators for nuclear power plants in Europe, Russia and Asia over the last 30 years. The company has been selected as the sole manufacturer and supplier of electric actuators for the Olkiluoto 3 EPR under construction in Finland, and in 2009 its products were supplied to the Beloyarsk and Rostov nuclear power plants in Russia.