N E W S R E L E A S E
‘SHURSHUT’ is a Method and Apparatus to Test a Shutdown
Device while a Process Continues to Operate
Patent No: 6631882
‘Shurshut’
is a method and apparatus for testing a shutdown device whose purpose is to stop
the flow of raw material to a process, or of energy to a process, without
stopping the process or upsetting the process.
The shutdown device is the final control element in a Safety Shutdown
System.
The
purpose of a Safety Shutdown Systems is to safely shutdown a process that for
some reason has exceeded the normal operating envelope for that process.
If the process were allowed to continue to exceed the normal operating
envelope, personnel may be harmed and facilities may be damaged.
The process is normally maintained within its safe operating envelope
with the primary control system. If
the primary control system should fail, the process may exceed the safe
operating envelope where upon the Safety Shutdown Systems would shut off raw
materials to the process. The final control element of the Safety Shutdown
Systems is the shutdown device. Normally
this shutdown device can only be tested when a process is intentionally being
shutdown or after a process has been shutdown.
Since the shutdown device is infrequently tested, there is not assurance
that the shutdown device will function to shutdown the process when the process
has exceeded its safe operating envelope.
‘Shurshut’
is a method and apparatus for testing a shutdown device that will assure that
the device will shut down the process when commanded to do so.
The method and apparatus will cause the shutdown device to be stroked 50%
at a frequent interval selected by the user, thus assuring that the device will
shut on demand.
The
apparatus consist of a valve or damper that has a specialty calibrated
positioner that causes the valve or damper to be stroked from a position of 25%
before fully opened to a position of 25% after fully opened.
The valve or damper is dedicated to the shutdown function, and therefore
cannot be used as the primary control device for the process.
The apparatus further requires the use of a programmable logic
controller, the function of which is to control the valve or damper and to
verify that the valve or damper has moved to the position to which it was
directed. The ladder logic required to perform this function is
included with ‘Shurshut’. If
the user does not have a programmable logic controller, one can be included as
part of ‘Shurshut’ as well.
The
method is both the special
calibration of the positioner on the valve or damper and the use of a valve or
damper that is larger than the valve or damper that would normally be used to
control the process. By so doing
the valve or damper can be stroked 50% without affecting the process.
The method includes four features that make an on-line test of the
shutdown device possible and unique:
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This null affect on flow of raw material to the process is attained by using a shutdown device that is larger than the valve or damper that would normally be used to control the raw material flow. For example the valve coefficient of flow (CV) for a 1.25 inch valve is 46. By utilizing a 2.0 inch valve that has a CV of 120, the 2.0 inch valve can be 25 % closed before its CV is equal to the CV of a fully opened 1.25 inch value.
-
The method utilizes an analog signal to cause the shutdown device to move from 25% before the fully opened position to a pre-defined 25% after fully opened position. The result is a 50% change in analog signal to the shutdown device and a 50% change in the shutdown device.
-
The method utilizes a feedback analog signal to confirm that the shutdown device has in-fact moved 50%. A failure of the shutdown device to move to the pre-defined position will sound an alarm and display the failure condition. Immediate corrections can be made to the shutdown device to return it to fully functional state. Without an on-line test a shutdown device that failed to operate correctly can only be corrected after the process has shutdown.
-
The system can be tested as frequently as desired, thus increasing the reliability of the shutdown device. i.e. A continuous process that shuts down once a year can only allow a test of the shutdown device once a year, whereas an on-line test of the shutdown device twice a day results in 720 test per year. There is a greater assurance that the shutdown device will function correctly when it is tested 720 times a year rather than once.
By having tested the shutdown device to 50% of stroke the following has been accomplished:
-
The analog output signal directing the valve to stroke has been tested.
-
The shutdown device hardware (valve or damper) has been tested.
-
The analog input signal indicating shutdown device position has been tested.
Having
tested the shutdown device to 50% of stroke, the device can be expected to
operate to 100% of stroke on demand.
For More Information Contact:
Industrial Control Specialists, Inc.
PO Box 6471, Lake Charles LA 70606-6471
Tel: 337-474-3163
FAX: 337-474-7222
Internet: sales@industrialcontrolspec.com