Because of the critical function of main steam isolation valves (MSIVs) in nuclear applications, keeping these valves running at optimum performance levels is vital. This article covers what a good service team will do to ensure that happens.
MSIVs are designed to close quickly and seal tightly in high-pressure steam service within power plants. In nuclear power, MSIVs are installed in the main steam line that runs from the reactor vessel in a boiling water reactor and from the main steam line of the steam generator in a pressurized water reactor. These valves are designed for fail-safe operating reliability during a loss of coolant accident necessitating closure within three to five seconds. They also must have design simplicity, tight shut-off, low-pressure drop and the ability to open against differential pressure.
Because providing continued optimum performance is so important, MSIV manufacturers have developed technical bulletins that outline recommended maintenance frequency and extensive procedures on how to perform work during a full service outage. For each such outage, a procedure is developed based on the specific plant’s maintenance protocols and the OEM’s valve drawing and specifications. Also addressed are the important safety factors involved based on the power plant’s tools and operational setup.
Whether a full service outage occurs or a valve modification project to install upgrades is underway, the field service team interfaces on a daily basis with the valve OEM. The team works through every detail of the manufacturer’s and the utility’s safety procedures to ensure that everyone involved stays safe while the service work is performed and also to ensure the customer’s expectations are understood and met. This preparation work is conducted prior to a service job and can take weeks. A pre-outage checklist is developed to cover every detail required to perform a safe and efficient service job in the field.
This checklist development process confirms the job’s scope and schedule, reviews the training requirements and rigging qualifications of the crew, verifies the use of special equipment such as respirators and scaffolds and outlines which personnel will hold the tags to lock-out or tag-out equipment.
The service team then sends a list of the minimum required rigging and tooling needed and identifies the proper personal protective equipment needed. To accomplish this, crew members could be asked to complete a personal history questionnaire and respirator training. The service team reviews the safety procedures of the valve OEM and a job safety analysis with the crew. They also review all plant safety procedures and have the crew members review the plant MSIV procedures and work orders. Job permits for rigging plans and confined spaces must be obtained when applicable. The team then reviews the plant’s specific MSIV operating experience with the crew.
Important safety concerns when working on a MSIV include factors such as knowing the exact weight of each component before trying to lift or remove that component from the valve. This may seem obvious to a service person, but when safety is at stake, no guessing should be involved. The weights need to be verified by reviewing a drawing or by consultation with the manufacturer.
The focus of the process also goes beyond the need for safety. Service technicians need to be vigilant about how they handle the different valve components because of the expense of what’s involved. If a part is damaged during disassembly or reassembly, thousands of dollars are involved for replacement costs and schedule delays can occur, which also means an extension of anticipated plant downtime. That’s why a service team is diligent with pre-outage checklists and extremely methodical in how a job is planned and executed.
Parts are labeled and match-marked during disassembly to assure proper reassembly as well as conformance to design requirements. Once the valve is apart, it’s meticulously vacuumed, wiped down, cleaned and the interior of the valve is inspected to ensure foreign material exclusion requirements are maintained. The entire job must be “white glove perfect” before the valve poppet assembly is reinstalled. Sometimes it can take four hours just to clean a valve body.
The orientation of the Y-globe MSIV body cover bore requires that the valve poppet assembly be removed and reinstalled at an exact 45-degree angle. A magnetized torpedo level enables the 45-degree rigging angle of the valve poppet to be maintained during removal and installation. If the 45-degree rigging angle is not precisely maintained, it could cause the poppet to jam in the body bore, which would damage the valve interior that a team may have spent those four hours cleaning. It also could introduce foreign material into the valve assembly.
As the valve is disassembled, spring stops are installed to ensure personnel safety. The air actuator and hydraulic cylinder are removed, followed by the spring hold-down bolts that unload the large springs for removal. Once the bonnet nuts are removed, the valve stem and poppet assembly can be removed, disassembled and inspected. The stem and poppet assembly is then reassembled and installed in the valve (after the inspection and cleaning of the valve body is completed). The bonnet and new packing are installed, followed by the upper structure, springs, hydraulic cylinder and air cylinder.
When the valve assembly is completed, the valve is opened and closed to verify proper operation and stem travel. Once the service crew is finished with all this, the plant’s diagnostic crew checks the valve operation and makes any adjustments before the job is considered completed.
These are just a few highlights of the multitude of details involved in a full MSIV service job. (This column is intended to provide a general overview and should not be construed as a full list of safety concerns or a complete description of all work performed.)
Every plant involved with maintenance considerations for MSIVs should contact the OEM for a full evaluation of the valves in their specific plant as well as that plant’s specific maintenance requirements.
Taking the types of steps outlined here, working directly with OEMs and understanding the specific challenges of a plant ensures nuclear plant personnel will be kept safe and service jobs can be completed to everyone’s satisfaction.
The following is a list of steps that should be taken for MSIVs during every refuel outage. It’s based on years of experience derived from technical phone support, utility feedback, engineering design knowledge and onsite field representation:
Also, standard replacement parts should be on hand including cover gaskets, packing sets, locking tabs or plates, and stem and poppet assembly. Maintenance personnel must be well versed and trained in the MSIV’s details to ensure safety for the crew and plant staff.