INSTRUMENT Solenoid OPERATED VALVES

By: CURT R. DUPILL; US Nuclear Sales Manager, Automatic Valve Corporation (AVCO)
The SOV (Solenoid Operated Valve) is an original type of valve; in that you can categorize the SOV in different or multiple valve programs due to the dynamic electrical and mechanical construction. Depending on the program protocol the SOV could be under the AOV, I&C, Component, Electrical or a plant system engineers program. The system engineer will usually play a role to monitor and PM (Preventative Maintenance/Changeouts) SOV’s whether installed in the MS (Main Steam), FW (Feed Water), or BOP (Balance of Plant) system. Due to the fact that air service SOV’s have such a demanding role in the operation and control of a power plant, occasionally a designated program is delegated primarily to the SOV component (Typically under the AOV program).

An SOV is an electromechanical valve used in a liquid or gas medium which is controlled by an electric current through a solenoid coil; when energized or de-energized, will change the position of flow path configurations dictating the desired control (Open/Close) and speed regulation of a larger component or mechanical system. The general purpose of an SOV is to control or regulate a mechanical function.

There are many roles of an SOV in a nuclear power plant whether it be a process (liquid media) SOV; an SSPV (Scram Solenoid Pilot Valve) in a BWR installed on HCU’s (Hydraulic Control Units) which control the insertion and speed of the CRD’s (Control Rod Drive); a DCV (Direct Control Valve) SOV on HCU’s; or pneumatic SOV’s installed on actuators. In order to focus on a specific SOV subject for the practical purpose of the large installed base in nuclear plants, we will focus this article on instrument air SOV’s.

In a power plant there are 4 main actuation methods utilized to operate a valve; Motor, Hydraulic, Pneumatic and a combination of Hydraulic/Pneumatic. Again, in this article we are focusing on the actuation aspect of operating the larger valve component (ex. Butterfly, Gate, Globe, etc.) in a power plant. For simplicity reasons let’s categorize two primary pneumatic actuation technologies as the following: “1” Single Acting; and “2” Double acting (Fail Closed or Fail As Is). These SOV’s dictate the mechanical cycling or open & close function of the valve by controlling where and how fast the stored energy in the cylinder is released or re-directed. The 3 and 4 type SOV’s are used in the pneumatic actuation applications. It is estimated that 85% of all applications installed are3 Way SOV’s.

The most logical and common question asked when specifying whether to use a 3W (way), or 4W (way); NC (Normally Closed) or NO (Normally Open) SOV is:


It all depends whether the actuator is a (A) spring return, fail closed (which means 3WNC SOV), (B) spring return, fail open (which means 3WNO SOV), (C) double acting, fail closed (which means 4W single coil solenoid SOV), or (D) double acting, fail as is (which means 4W double coil SOV) (See illustration of 3 & 4 Way SOV’s).
Another determining variable made when selecting an SOV is the time to Open or Close a valve. The Size or Cv (flow) of the SOV depends on the speed you want to stroke the valve. The plant design EQ (Equipment Qualifications) specifications will determine the valves stroke time in which to comply and by default the actuator cylinder size and speed will convert the Cv of the SOV size for each particular valve. An unequivocal factor to selecting the correct SOV is to determine the Cv (Flow) value. To put it in even simpler terms, the required speed for the actuator closing the valve will have picked the Cv of the SOV.

Another factor to consider when selecting an SOV is the DIRECT and PILOT operated feature. DIRECT Acting SOV’s do not need any system air pressure in order for the SOV to change positions/stroke. Cv again is the key variable when choosing a direct or pilot operated SOV. If the Cv is >0.5 the coils charge will usually not be enough to change the state of position and/or leakage will occur through the exhaust port. Therefore a PILOT assist SOV must be used with typical MOP (Minimum Operating Pressure) of 25 to 50 PSIG in order to switch the position state.

Volume (Air) is always a critical topic for an SOV to function correctly. Most SOV’s have high flow (Cv) capacity and it is important that the volume of air supply is not reduced significantly deterring the functionality of an SOV. Therefore, let’s review the most common instances where lack of volume can cause SOV functionality issues. A precision regulator (air set) will deter an SOV to function if installed upstream from the SOV which reduces the volume to fractions of one Cv (less than 0.1 Cv). If there is a regulator installed upstream from the SOV contact the OEM as they should be able to advise if the regulator application will be suitable from the regulator model. Also, there are circumstances where there may be glitches in the plants instrument air systems; whether there is a lack of volume at the air source, accumulator, undersized compressor, or undersized tubing lines to the components. There are also scenario’s where too many components are being operated by an undersized air source and when the control room operates multiple components there is not enough Air pressure to actuate the valves. Also, keep into perspective that a lot of the traditional SOV diaphragm designs have the tendency to leak (exhaust port) which can be a major draw from a systems air source as well as put continuous strain on air compressors.

PNEUMATIC AIRPACKS:
In more critical primary nuclear systems (ex. Main Steam); there are more complicated SOV configurations which includes multiple types of SOV’s working in conjunction with each other; not only to cycle the valve components but to perform multiple functions for safety, and exercising purposes. These more complicated SOV combination packages would be considered control panels or Airpacks (See Picture). Typically these Airpacks are installed on very critical Valves within a power plant system, for example on Main Steam Isolation Valves (MSIV’S) in a nuclear plant. The Air pack consists of a 3 coil solenoid manifold in which 2 of the coils work in redundancy with a 2 way and 4way SOV to cycle the MISV eliminating SPF (single Point Failures). SPF’s have been a growing concern by the NRC for all nuclear systems( ex. Feedwater); and OEM’s have reacted swiftly and offer a single body dual coil design to eliminate any possibility of a component being tripped due to a coil failure(see picture). Let’s get back to the Airpack configuration. Finally, the purpose of the 3rd coil is designed to work together with a 3 way SOV and restrictor valve utilized to exercise (slowly) the stroking of the MSIV.



SOV PM PROGRAMS:
As we mentioned there are thousands upon thousands of Instrument Air SOV’s in multiple systems within a power plant. Because of the large installed base of SOV’s to operate major valve components in critical systems; plants require a very proactive and demanding PM (Preventative Maintenance)/Change out program to be followed. In general summary the program is as follows: A typical nuclear plant PM program might consist of delegating a PM frequency code on an EQ & SQ (Environmental & Seismic Qualification) SOV Component tag #. Each SOV tag # determines that particular SOV’s correlation with location in the plant & environmental condition (temperature, Radiation & Seismic level, etc.). The program owner must clearly categorize each SOV for a PM/Change out by correlating the environmental condition with the SOV models EQ/SQ life, the Coil (H/F class) & elastomer (fluorocarbon/Viton or Buna) MSDS (Material Specification Data Sheet), and finally the plants outage cycle. The elastomer and coil will always dictate the PM frequency as they are traditionally the weakest link to fail in an SOV.
Nuclear Utilities are continuously looking for ways to reduce PM frequencies on SOV’s not only for the economic and labor factor but also for (HP/ALARA:Dose Reduction) and Safety reasons as well. For example if an SOV PM program has the potential to cut a PM frequency down by 1/3rd the PM reduction in turn will reduce more than 30% of the dose which would have been received (Electrical Maintenance technicians) under the traditional program.
Simple PM Frequency evaluations can be performed which can prove Plant life PM reduction ratios to be as effective as 7 to 1 depending what component tag model and location the SOV is associated with. The data can be easily pulled from 2 reports, SOV Usage & Frequency report, from the plants valve database. At that point the Program owner simply evaluates, compartmentalizes and prioritizes the highest frequency and usage, determining the most logical starting point to change to more reliable SOV Designs with a longer qualified life.
In conclusion, collectively Solenoid operated valves compared to other valves and components play an enormous role within a nuclear power plants system. With that said, an SOV will function to the best ability when the correct Cv is chosen for the particular application, the volume is not reduced and most important the SOV PM program is continuously challenged for efficiency.