Fluid handling – Processes – Involving pressure control
Reexamination Certificate
1998-10-15
2001-01-23
Chambers, A. Michael (Department: 3753)
Fluid handling
Processes
Involving pressure control
C137S552000, C137S554000, C137S557000
Reexamination Certificate
active
06176247
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the maintenance of safety devices. Particularly, the invention pertains to the verification of the correct performance of actuator-controlled safety devices comprising movable parts, especially so-called emergency shutdown valves and their control devices, in such a manner that neither production continuity nor the standby state of the devices is impaired.
BACKGROUND OF THE INVENTION
In the various industries, particularly in the petrochemical process industry, systems are used wherein process equipment likely to cause hazards upon failure is provided with valves and other mechanical means designed to bring the process quickly into a safe state if predetermined process parameter limits are exceeded. For example, such a system usually includes valves with single-action actuators, the opening or closing of which releases a built-up overpressure, diverts a hazardous process stream into a holding tank, or the like. Hereinafter, such valves are referred to as shutdown valves (being emergency valves having a closing or an opening function). These valves normally maintain one position, like other corresponding mechanical safety devices, and are consequently at risk for getting stuck if situations forcing a shutdown are not, as is hopefully the case, very frequent. The general safety of shutdown valves is not considered satisfactory when prior art equipment and methods are used. The greatest disadvantage in present systems is, that an existing failure for example, a mechanical component getting stuck—is not necessarily observed when the system is in a standby state, and in an emergency situation the system may be impaired or inoperative.
To verify performance, it is common to test e.g. shutdown valves in a manner simulating a real emergency situation. This practice may in fact cause huge risks, as the workability of the shutdown valve system is temporarily blocked, and if the device is not properly activated, the simulated emergency may become an actual emergency.
One method of testing a shutdown valve is to mechanically limit its travel, thus preventing it from having any significant effect on the process, and check the mobility of the valve within certain limits. This procedure requires the use of, for example, a physical key, and activation of the emergency system is prevented during the testing procedure, at least as regards the unit being tested.
The test is carried out at predetermined intervals, for example twice a year, but the test only proves that the devices are workable at the moment of testing. A fault may develop shortly after the test and remain until the next test. This manner of testing is not a reliable way of verifying operability of the system.
Valve diagnostics using present sensor and digital technology is a fast developing field. For example, in Finnish patent application 96 2406 and European patent application 95 306546, methods are disclosed for surveillance of the condition of control valves using sensors in the actuator and control means of the valve and the analysis of signals from said sensors in a microprocessor to analyze the workability of the valve unit as a whole.
As described above, the surveillance of the condition of a valve or other mechanical device which is part of a safety system involves certain requirements.
DISCLOSURE OF THE INVENTION
General description
A method according to the present invention, which provides for the verification of error-free operation of a mechanical safety device, for example a shutdown valve, while the standby state of the safety system is constantly maintained without impairment. In said method, the safety function and the surveillance or diagnostics function are combined in real time, the safety function nevertheless having a higher priority than the diagnostics function. In a method according to the invention, a component fitted directly to the safety device for activating the safety function is controlled by the plant's high-level safety system which is responsible for safety operations, or alternatively by a logic unit integrated into the device according to the invention, a diagnostics unit being adapted to said activating component. The diagnostics unit is allowed to perform diagnostics not disturbing the process functions whenever the process is in a normal state. In an emergency situation, the diagnostics unit is bypassed, whereby for example neither a fault or disturbance in the diagnostics testing nor an ongoing routine test can impair the safety function. The scope and periodicity of the diagnostics can be programmed into the system in order to achieve a desired level of reliability.
According to one embodiment of the present invention, a device is provided enabling the verification of undisturbed operation of a mechanical safety device while the standby state of the safety system is fully maintained.
The field unit of a safety system according to the invention comprises a microprocessor providing for predetermined self-testing and diagnostics procedures. The field unit further includes a control unit providing for the control of an actuator. By means of appropriate communications means said control unit is connected to, on the one hand, the high-level safety system or the logic unit integrated into the field unit, and on the other hand to the supply of energy to the safety device, e.g. a compressed air supply. The high level safety system or the integrated logic unit activates the safety function directly, bypassing the diagnostics function.
In addition to said field unit, another main component of the safety system according to the present invention is a monitoring unit. The communication link from the high level safety system is routed through the monitoring unit. Preferably, the task of the monitoring unit is to monitor the status of the field unit on-line, and provide the means for presenting status information to the user.
REFERENCES:
patent: 3901273 (1975-08-01), Brouwer et al.
patent: 4428223 (1984-01-01), Trevisan
patent: 4517154 (1985-05-01), Dennis
patent: 4557136 (1985-12-01), Greenhalf
patent: 5000040 (1991-03-01), Charbonneau
patent: 5329465 (1994-07-01), Arcella et al.
patent: 5425270 (1995-06-01), McDonald et al.
patent: 5469737 (1995-11-01), Smith et al.
patent: 232964 (1986-02-01), None
patent: 708389 (1996-04-01), None
patent: 152314 (1985-05-01), None
patent: WO 97/48026 (1997-12-01), None
Högström Karl-Kristian
Winchcomb John
Chambers A. Michael
Neles Field Controls OY
Pillsbury Madison & Sutro LLP
LandOfFree
Method and device for verifying the workability of a safety... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and device for verifying the workability of a safety..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for verifying the workability of a safety... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2505747