In the event of a fault with the instrument itself, is it necessary to activate the safety interlock to ensure safety?
Thread Content
According to the requirements of GB50770, the SIS system must ensure fault safety; does this mean that in order to maintain safety, any failure in the instruments involved in the SIS interlocks must also trigger a safety interlock? Is there any regulatory basis for this?
The standards are quite clear on this: interlock mechanisms are only necessary if a subsystem is unable to ensure fault safety for the main system; if there is redundancy, then such interlocks are not required.
We need to consider it, huh? If the diagnostic coverage meets the requirements, is it still necessary?
It is about determining whether the action of that subsystem can completely prevent the further spread of the fault; if it can, then no such setup is necessary, but if it cannot ensure that, then interlocks need to be installed.
For example, if a human operator detects a fault in the instruments, they can manually press the emergency stop button, or manually close or open valves remotely, or shut down the pump, in order to ensure that the equipment remains in a safe state. Automated interlocks are also necessary for this purpose, right?
For example, in the simplest case, when the liquid level in the storage tank rises to the warning level, the emergency shut-off valve activates and shuts off the flow. At that point, it’s impossible to ensure the safety of the entire system, as the feed pump has not yet stopped operating. Therefore, interlocking is required: when the emergency cut-off valve for imports is closed, the interlocked feed pump stops. In general manufacturing, it is rare for a single instrument to be sufficient to ensure the safety of an entire vehicle; multiple instruments need to operate together in sequence to achieve this.
What you’re referring to is manual interlock shutdown; if it’s set to automatic interlock, then when a worker detects a fault with the instrument and manually presses the emergency stop button, the relevant valves, pumps, and other related instruments will be automatically shut down.
What I mean is, if the instrument fails, should the safety interlock be triggered automatically? If there is a fault with the instruments, the control station should be able to detect it, and then the interlock can be activated manually. Does this meet the requirements?
The requirements are met; the specifications only require an automatic interlock, and the specific triggering conditions can be either automatic or manual.
A malfunction in a gauge does not necessarily trigger interlocks; it is necessary to analyze each specific situation. For example, a malfunction in a flow gauge may trigger a low-low interlock, while a malfunction in a pressure gauge that results in low pressure will not trigger a high-high interlock. In other words, each case must be analyzed on its own
If I want to set an interlock that is triggered by a fault in the measuring instrument itself, can I do so freely? For example, normally an interlock is activated to open the vent valve when pressure is high; can I set it so that a fault in the instrument causes the instrument to output a signal at full scale, thereby triggering the interlock and opening the vent valve? As you said, it’s malfunctioning and can only output a low value?