Valve failure modes

Table 5.1-3 Motor-Operated Valve Failure Modes...

This study is a good reference for the construction of fault/event trees of systems that are affected by valve performance. The valve failure modes are identified, the associated mechanisms are described in detail, and preventive measures are offered. 

Component Name AIR-OPERATED VALVE Failure Mode SPURIOUS OPERATION... 

Table 11-2. Remote Actuated Valve Failure Modes and Diagnostic Techniques...

Although there are limited common control valve failure modes, the dominant problems are usually related to leakage, speed of operation, or complete valve failure. Table 9-1 lists the more common causes of these failures. 

Additional and new failure modes are introduced with the use of valves that could impact plant performance and operation. Table 9-27 provides an outline of the most probable valve failure modes and potential causes for the SNPP application. Some of these items are discussed in more detail below. These have been accounted for in Section 7 reliability evaluation. 

Failure of the electro-mechanical operator would render the isolation valve inoperable. Depending on the valve design and its position at time of the failure, this could cause the valve to stick shut and fail to open, stick open and fail to close, or stick somewhere in the mid-position. The consequences of these valve failure modes are discussed in 9.5.5.3 and 9.5.5.4. 

In.strument air failure. The consequences of the loss of instrument air should be evaluated in coujuuc tiou with the failure mode of the control valve ac tuators. It should not be assumed that the correct air failure response will occur on these control valves, as some valves may stick in their last operating position. 

An important application of a rupture disc device is at the inlet of a pressure relief valve. The sizing of the pressure relief valve or rupture disc device combination requires that the pressure relief valve first be sized to meet the required relieving capacity. The normal size of the rupture disc device installed at the inlet of the pressure relief valve must be equal to or greater than the nominal size of the inlet connection of the valve to permit sufficient flow capacity and valve performance. The failure modes of rupture discs are [40] ... 

Remember that the failure position of a valve refers to its failure mode if there is a utility failure. A valve can mechanically fail in any position it is possible for a fail closed valve to get stuck in the open position. When doing a process hazard analysis it is important to consider all possible failure positions of a valve, and not only the failure position resulting from utility failure. 

Failure of power or controls to the valve (generally related to the seismic capacity of the cable trays, control room, and emergency power). These failure modes are analyzed as failures of separate systems linked to the equipment since they are not related to the specific piece of equipment (i.e., a motor-operated valve) and are common to all active equipment. 

Table 15.3.4-1 Failure Modes Effects Analysis for Valve A in Figure 3.4.4-6...

Another example is a safety valve in standby service. If demands occur very infrequently, time-related stresses such as external corrosion may have a significant influence. Repeated demands in very dirty service could easily lead to faster degradation and failure, whereas repeated demands in lubricated service might actually enhance performance if the failure mode of interest is failure to open. Failure data based on time or demands can also be skewed if the relief valve is initially damaged or installed incorrectly. 

The data base contains failure rate data plus some failure mode information for process equipment - pumps, compressors, gas turbines, valves, vessels, heat exchangers etc. 

Three reports have been issued containing IPRDS failure data. Information on pumps, valves, and major components in NPP electrical distribution systems has been encoded and analyzed. All three reports provide introductions to the IPRDS, explain failure data collections, discuss the type of failure data in the data base, and summarize the findings. They all contain comprehensive breakdowns of failure rates by failure modes with the results compared with WASH-1400 and the corresponding LER summaries. Statistical tables and plant-specific data are found in the appendixes. Because the data base was developed from only four nuclear power stations, caution should be used for other than generic application. 

Opening segments of the IP2 PRA data analysis section describe the definitions of terms and concepts employed, the assumptions made, and limitations recognized during the data base construction. A set of 39 plant-specific component failure mode summaries established the basis for component service hour determinations, the number of failures, and the test data source for each failure mode given for each component. Generic data from WASH-1400, IEEE Std 500, and the LER data summaries on valves, pumps, and diesels were combined with plant-specific failure data to produce "updated" failure information. All the IP2 specialized component hardware failure data, both generic and updated, are contained in Table 1.5.1-4 (IP3 1.6.1-4). This table contains (by system, component, and failure mode) plant-specific data on the number of failures and service hours or demands. For some components, it was determined that specifications of the system was warranted because of its impact on the data values. 

There are a number of vendors that sell solenoid actuated Teflon valves, which are easily interfaced to a computer. Care must be taken to chose a design where the internal volume at the valve outlet can be flushed easily between steps, however [110], Rotary selection valves have been used as well, but given the number of rotations needed for a 200 cycle deposit, various failure modes revealed themselves. 

The failure mode of ESDVs for gas processing areas should always fail in the closed position, since this is the only mechanism to resolve gas fed fires or prevent explosive vapor buildups. The valves should be provided with an automatic fail close device such as an actuator with spring return specification. 

An FMEA table contains a series of columns for the equipment reference number, the name of the piece of equipment, a description of the equipment type, configuration, service characteristics, etc, which may impact the failure modes and/or effects, and a list of the failure modes. Table 2 provides a list of representative failure modes for valves, pumps, and heat exchangers. The last column of the FMEA table is reserved for a description of the immediate and ultimate effects of each of the failure modes on other equipment and the system. 

Nakamura K, Shiomi M, Takahashi K, Tsubota M. Failure modes of valve-regulated lead/acid batteries. J Power Sources 1996 59 153-157. 

Another, perhaps the most important, controller parameter is the control action, which is set as either direct or reverse. By convention, if the valve position is to increase as the measurement increases, then the controller is considered direct acting. The actual output signal from the controller will further depend on the specified failure mode of the valve. For example, a fail-closed valve will require an increase-to-open signal, whereas a fail-open valve will require an increase-to-close signal. In order to minimize confusion, rather than displaying actual output, most controllers display an implied valve position, which indicates the desired position of the valve. 

---