Partial stroke testing system coupled with fuel control valve

Abstract

A system for on-line testing of an emergency shut-off valve includes a first emergency shut-off valve (first valve) and a flow control valve (second valve), with the system being configured to allow the second valve to serve as a combination flow control and second emergency shut-off valve. A subsystem is also provided for testing the first valve without fully closing the first valve in response to a signal from the control. In this subsystem, a solenoid valve bleeds off pressurized fluid to move the first valve from a fully opened to a partially closed position. A bypass around the second valve allows it to be tested as the second emergency shut-off valve, allowing the second valve to close completely without shutting down the process. The use of the two emergency shut-off valves in series wherein either valve can shut down the process provides a level 3 safety integrity level.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation-in-part of U.S. patent application Ser. No. 12 / 038,830, filed on Feb. 28, 2008.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]This invention relates to a system for improving the Safety Integrity Level (SIL) rating of process heaters (e.g., industrial heating systems) using partial stroke testing of an emergency shut-off valve coupled to a supplementary fuel control valve.[0004]2. Description of the Related Art[0005]In the oil, gas, petroleum and power industries, natural gas or other combustible gas is often used to provide the required heat or combustion motive power for the desired operation, more generally referred to herein as a “process.” Various conditions may occur that necessitate immediate shut down or tripping of the operations, process or plant. In those industries, a majority of the final control elements of a shut-off system are implemented with fast acting shut-off valves....

AI Technical Summary

Benefits of technology

[0016]In addition, the partial stroke testing system in accordance with the present invention will not interfere with a plant trip, i.e., a full shut down due to an emergency condition. Furthermore, if a partial stroke test is being conducted at the time of a plant trip, the partial stroke test will contribute to a more rapid closing of the emergency shut-off valve. Thus, the partial stroking design acts as a backup to the main trip mechanism.

[0017]Accordingly, such systems should reduce the cost of insurance or risk coverage. In essence, the system partially closes the emergency shut-off valve to a predetermined position to test and confirm its ability to function and to ensure its availability on demand in the event of a plant emergency.

[0019]There is also a widespread concern among process operators about the Probability of Failure on Demand (PFD) of final shutdown elements, such as shut-off valves used in safety applications. Partial trip testing of the final shut-off valve coupled with the supplementary emergency shut-off valve according to the present invention will serve to improve the PFD rating of shut-off action in such systems. In Table 2, the first column represents parameters of interest, where MTTF is the mean time to failure, xv is the shut-off valve, cv is the control valve, Ti is the test interval, and PITT is partial instrument trip testing. The second column is the test interval (in years), and the PFD in the third column is the probability of failure on demand corresponding to the test interval in the second column.TABLE 2Utilizing shut-off (XV) with the control valve (CV) in 1oo2 configurationMTTF (xv) = 35 yearsTi (CV)PFDMTTF (cv) = 35 years0.51.09E−4Ti (XV) = 4 years11.14E−4Ti (CV) = 0.5, 1, 2, 3 years21.31E−4Diagnostic Coverage (CV) = 85%31.48E−4Diagnostic Coverage (XV) = 80%T(PITT) = 1 week

[0024]The essential control components of an emergency shut-off valve are the main trip solenoid valve with manual reset, quick exhaust valve and a fluid actuator. The control components of the fuel control valve are a solenoid valve and a fluid signal from the I / P converter to the actuator of the fuel control valve to accomplish the fuel regulating function.

Problems solved by technology

However, any failure in the internals of the shut-off valve may lead to a fuel leakage to the heater or other component being supplied by the combustible gas or fuel passing through the valve(s).

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