Using system-medium-operated gate valves at Oskarshamn 123 December 1999
The replacement main steam isolation valves at Oskarshamn are more reliable and easier to maintain than conventionally operated versions.
When Oskarshamn 1 (a 460 MWe BWR) started up in 1974, it was provided with four main steam isolation valves (MSIVs). These were motor-operated gate valves – there were two in series on each of two steam lines, inside and outside of the containment. In a BWR system like Oskarshamn these must be capable of bottling up the radioactive steam within the reactor in the event of an emergency – handling 2 500 000 lb/hr (320 kg/s) at a pressure of 1015 psig (70 bar g).
In 1998 the first unit at OKG’s Oskarshamn plant underwent a comprehensive modernisation programme, in which, among other upgrades, the main steam line piping was renewed between the steam generators and the turbine. Along with this renewal, it was determined that plant safety at Oskarshamn 1 could be significantly enhanced by replacing existing motor-operated MSIVs with system-medium-operated gate valves. They are thought to be more reliable than motor operated valves and are fully capable of meeting current loss of coolant accident (LOCA) requirements. As a result, the four original MSIVs have now been replaced with system-medium-operated gate valves.
System-medium-operated valves have been well known for many years, but this was the first time that system-medium operation had been used for gate valves.
A photograph of one of the nominal 20 in (500 mm) valves installed at Oskarshamn is shown below left. It measures 6 ft 7 in (approximately 2 m) from the valve centreline to the top of the position indicator.
The reason for using system-medium-operated gate valves in applications such as providing main-steam-isolation service in nuclear power plants is to achieve the highest reliability possible in this safety-related emergency dead shut-off service. The levels of reliability required by regulators in such applications are constantly being upgraded. A system-medium-operated valve can meet reliability criteria because it requires no large outside power source to function.
In the case of gate valves, the operating forces required can vary widely and unexpectedly due to gate/seat ring friction. As a consequence, wherever this high level of reliability is required, external actuators can become extremely large and heavy – essentially out of proportion.
Typically, the height of a conventional gate-valve that must be pneumatically-closed and hydraulically-opened, is much greater than that of a system-medium-operated gate valve (see diagram far right).
The diagram above right compares the details of angle valves and gate valves of a system-medium-operator design. Angle valves of this design have been around for several years, but pressure losses through an angle valve can easily be ten times those through a similarly sized gate valve.
In existing installations where conventionally actuated gate valves have experienced difficulties, it is possible to retrofit existing valve bodies with new valve internals and system-medium-operated actuators. This is especially advantageous, from a cost and down-time point of view, where existing valve bodies are welded into the piping system.
System-medium-operation has been an accepted and proven valve actuation technology for well over 20 years. Operation can be based on either a “pressurise-to-close” or a “depressurise-to-close” principle. In the valve design shown in the diagram above, the upper and lower piston chambers are unpressurised in the “open” position. To close the valve, a pilot valve is operated to pressurise the upper piston chamber. To reopen it, the pilot valve is switched to depressurise the upper piston chamber. If required, the pilot control system can be made multi-redundant – solenoid, electro-mechanical, hydraulic or pneumatic pilot valves can be used.
One of the many contributors to the necessity for large, heavy, overdesign in many gate-valve actuators is that, because of the sliding friction between seat rings and disks, the force needed to open and close the valves is very uncertain. As well as creating uncertainty, this heavy sliding friction itself can be a source of damage.
This is not so for system-medium-operated gate valves. During closing, a control element incorporated into the piston ensures that the intermediate space between the disks remains pressureless throughout the entire stroke. There is no contact between the disks and seat rings. Once the closed position has been reached, the intermediate space between the disks is pressurised by closing the venting line of the lower piston chamber, and the disk on the downstream side seals tightly under the full pressure differential. In the closed position, the gate valve inner space and both of the piston chambers are under system pressure.
To open the system-medium-operated gate valve, the pilot valve is switched to depressurise the upper piston chamber. As the pressure there drops, the intermediate space between the disks is simultaneously depressurised, and the disks move away from the seat rings before the opening stroke begins.
Thus there is no friction – no sliding wear – during either opening or closing operations, and the “sliding” and “sealing” functions are entirely separated.
Control Components’ system-medium-operated gate valves underwent many hundreds of stroke tests at the company’s Winterton facility. CCI does not have the capacity to carry out full flow ”blowdown” testing of the valve and simulation of pipe breaks. Testing of this type was carried out by Siemens at its facility in Karlstein, Germany. To meet the requirements of the Oskarshamn plant these tests were carried out independently by Siemens, and witnessed by TÜV.
Once in place, regulations covering MSIVs at Oskarshamn require periodic full-stroke testing of the valves. To accomplish this, the reactor is run down to approximately 50% load so that the total steam production can be passed through one steam line without producing a scram signal. Then one of the MSIVs is fully closed slowly to avoid any rapid transients. This is done by isolating the steam feed through the closing pilot valve and redirecting it through a small orifice, sized to produce the required slow closing.
Since the disks do not touch the seat rings during opening or closing operations, the seating materials are not subject to sliding wear. The danger of damage to the seats is therefore minimal.
The valve inner parts, such as the guide rails and disks, are simple to dismantle. The guide rails are merely push-fit located, and when the valve is opened, they can be lifted out of the valve body without further valve dismantling. The disks themselves are provided with a bayonet coupling and can therefore be removed very quickly. This is most important when the gate valve is located in high radiation fields where all service work means exposure of personnel to radiation dosage.
There are no pressure-boundary penetrations. Thus there are no operational seals such as gland bushings subject to wear or leakage. Consequently all service work associated with gland bushings such as tightness testing, controlled pre-forming of packing, and stem-friction testing is eliminated. The elimination of an external actuator obviates expenditures on spare parts, service, maintenance and testing.
The new valves were inspected, after some six months in operation, when Oskarshamn underwent its most recent planned outage in the second quarter of 1999. The valves had experienced no problems in operation.
Following this first successful installation, CCI has won two further contracts to supply system-medium-operated valves. Eight valves were supplied to Novovoronesh 4 and installed in early 1999. This Russian-designed VVER plant uses six valves in the main steam lines. More recently, CCI won an order to supply system-medium-operated valves for the Forsmark 1 and 2 plants in Sweden. Two system-medium-operated valves will be used at each plant, where each feedwater isolation will use three valves: one electrically operated; one system medium operated; and one manually operated.