Interlock testing

Construction, Leaks, Cavitation Interlocks Tests, Bypass Procedures... 

Interlock testing is performed on a periodic basis, usually every 12 months, following detailed written test procedures. The test procedure for each safety interlock defines the actions, including the approval system and the mechanical work required to ensure reliability. These procedures cover the testing activity from the process measurement device to the final element, which is often a control valve. [26]... 

The technical paper described the interlock testing program to consist of more than merely verifying that the interlock works. There is also inspection of the interlock equipment for deterioration. The function test on instrumentation is required following mechanical work on one or more of the safety system elements. In short, all devices or systems that have been disturbed enough to affect their normal function will be function tested prior to being restored to service. [26]... 

Administrative controls are in place to ensure periodic channel functional testing and calibration of Ap Instrumentation, and ventilation system fan sequencing interlock testing. 

Ventilation Hot Exhaust (Zones 1 and 2A combined exhaust) Control of off-site radioactive material releases. 1. Hot exhaust confinement boundary integrity must not be compromised by ventilation system fan mis-operation or malfunction. 2. HCF operators must maintain the required functional performance of confinement controls. 3. HCF operators must verify the functional performance of the hot exhaust HEPA filters. 1. A ventilation system fan sequencing interlock ensures that the backup exhaust fan is automatically started upon loss of the primary operating fan. An aiann alerts the operations staff upon loss of the operating stack exhaust fan. 2. Administrative controls are in place to ensure periodic ventilation system fan sequencing interlock testing. 3. Administrative controls are in place to ensure periodic functional performance and efficiency testing of the HEPA filters. 

The reactor will be shut down twice a year for off-line maintenance and safety interlock testing. All protection layers identified in the LOPA that provide risk reduction must be tested at this same frequency. Note Since this is a batch operation, some SIS components could be tested more frequently (e.g., the vent valves could be tested before each batch is started) if necessary to meet the target PFD. Presently, the SIL verification calculations described in Clause 5.3.4.3 indicate that the higher test frequency is not required. However, if operating experience shows that SIF component failure rates are aotually higher than assumed in the PFD calculations, higher frequency testing of some components could be implemented. 

NOTE — Each interlock test procedure has its own unique safety considerations. The following text must be modified to meet specific application requirements. 

Knitted fabrics are produced from one set of yams by looping and interlocking processes to form a planar stmcture. The pores in knitted fabrics are usually not uniform in size and shape, and again depend largely on yam dimensions and on the numerous variables of the knitting process. Knitted fabrics are normally quite deformable, and again physical properties are strongly dependent on the test direction. 

Pour-Point Depressants. The pour point of alow viscosity paraffinic oil may be lowered by as much as 30—40°C by adding 1.0% or less of polymethacrylates, polymers formed by Eriedel-Crafts condensation of wax with alkylnaphthalene or phenols, or styrene esters (22). As wax crystallizes out of solution from the Hquid oil as it cools below its normal pour point, the additive molecules appear to adsorb on crystal faces so as to prevent growth of an interlocking wax network which would otherwise immobilize the oil. Pour-point depressants become less effective with nonparaffinic and higher viscosity petroleum oils where high viscosity plays a dominant role in immobilizing the oil in a pour-point test. 

Although the traditional point of reference for safety interlock systems is a hard-wired implementation, a programmed implementation is an alternative. The potential for latent defects in software implementation is a definite concern. Another concern is that solid-state components are not guaranteed to fail to the safe state. The former is addressed by extensive testing the latter is addressed by manufacturer-supplied and/or user-supplied diagnostics that are routinely executed by the processor within the safety interlock system. Although issues must be addressed in programmable implementations, the hard-wired implementations are not perfect either. 

As part of the detailed design of each safety interlock, written test procedures must be developed for the following purposes ... 

Assure that the interlock (hardware, software, and I/O) continues to function as designed. The design must also determine the time interval on which this must be done. Often these tests must be done with the plant in full operation. 

Execution of each test must be documented, showing when it was done, by whom, and the results. Failures must be analyzed for possible changes in the design or implementation of the interlock. 

These tests must encompass the complete interlock system, from the measurement devices through the final control elements. Merely simulating inputs and checking the outputs is not sufficient. The tests must duplicate the process conditions and operating environments as closely as possible. The measurement devices and final control elements are exposed to process and ambient conditions and thus are usually the most hkely to fail. Valves that remain in the same position for extended periods of time may stick in that position and not operate when needed. The easiest component to test is the logic however, this is the least hkely to fail. 

The assessor should also find out whether an effective testing program is in place to help ensure the serviceability of process measurement equipment. The successful toller should have an established calibration program to address the accuracy of critical measurement equipment. Safety critical process parameters should be monitored and critical process equipment should automatically interlock when monitoring instrumentation detects safety critical deviations. Interlocks should either facilitate a remedy to the critical deviation or bring the process to the zero energy state. These instruments and interlocking devices should be routinely tested to ensure operational reliability. 

Does the facility have a program for regular inspection and testing of process safety valves and other process safety devices including interlocks ... 

A defeat mechanism to bypass the door interlock may also be necessary for the purpose of testing. 

Test This is the position of the trolley when the power contacts are isolated but the control circuit is still connected, because it is tapped directly from Ihe auxiliary bus. This condition is essential to facilitate testing of control circuits with functional interlocks, without energizing the connected lotid. Isolation This is the position of the trolley when the power and the control circuits are both isolated. Depending upon the site requirements, sometimes the control circuit may be required to be still energized for some test requirements. 

This lest is conducted to establish the satisfactory functioning of mechanical parts, such as switching devices and their interlocks, shutter assembly, draw-out mechanisms and interchangeability between identical draw-out modules, A brief procedure to test these features is as follows. 

The test may be considered successful if the operations of the interrupting mechanism, the interlocks and other mechanical features after the test are found satisfactory. 

A more recent process, the P2 etch [60], which uses ferric sulfate as an oxidizer in place of sodium dichromate avoids the use of toxic chromates, but still provides a similar oxide surface morphology (Fig. 15) allowing a mechanically interlocked interface and strong bonding [9]. The P2 treatment has wide process parameter windows over a broad range of time-temperature-solution concentration conditions and mechanical testing confirms that P2-prepared surfaces are, at a minimum, equivalent to FPL-prepared specimens and only slightly inferior to PAA-prepared surfaces [61]. 

The furnace was fitted with interlocks that should have isolated the fuel supply if the tube wall temperature or the pressure of the heat transfer oil got too high. Neither interlock worked, and neither had been tested or maintained. The set-point of the high tube wall interlock had been raised far above its original set-point, from 433°C to 870°C, a simple way of putting it out of action [15]. Changing the set-point of an interlock is a modification and should be allowed only when the equipment is capable of withstanding the new conditions (see Chapter 2). 

Trips and interlocks should be tested after a major shutdown, especially if any work has been done on them. The following incidents demonstrate the need to test all protective equipment ... 

Limits, interlocks and inspection. Cut-outs, limiting controls and interlocks are tested. 

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