High-liquid-level trips

Compressors are also served by high-liquid-level trips in their upstream knockout drums. These high-liquid-level trips work in the same way as the low-level boiler trips discussed above, except that the mercuroid switch is activated by a rising, rather than a falling, liquid level so as to protect the compressor from a slug of liquid. 

In automatic systems the filler sets the quantity to be filled on a meter, which closes a valve when this required quantity has been delivered. Overfilling has occurred because the wrong quantity was set on the meter, because there was already some liquid in the tank (left over from the previous load), and because the filling equipment failed. For these reasons many companies now fit their tank trucks with high-level trips, which automatically close a valve in the filling line [8]. 

The portion above the high-level trip This portion performs vapor-liquid sepsu-ation uid is sized using a knockout drum-sizing criterion (e.g., 237, 365). If all product is liquid, this portion is not needed. 

The rationale behind the definitions of iow demand mode and high demand or continuous mode in lEC 61508 is based on the failure behaviour of a safety-related system due to random hardware faults. Underlying much of the reasoning is the distinction between safety-functions that only operate on demand and those that operate continuously . A safety function that operates on demand has no influence until a demand arises, at which time the safety function acts to transfer the associated equipment into a safe state. A simple example of such a safety function is a high level trip on a liquid storage tank. The level of liquid in the tank is controlled in normal operation by a separate control system, but is monitored by the safety-related system. If a fault develops in the level control system that causes the level to exceed a pre-determined value, then the safety-related system closes the feed valve. With such a safety function, a hazardous event (in this case, overspill) will only occur if the safety function is in a failed state at the time a demand (resulting from a failure of the associated equipment or equipment control system) occurs. A failure of the safety function will not, of itself, lead to a hazardous event. This model is illustrated in Figure 4. 

US BWRs have level 8 trips to protect the reactor from getting water in the steamlines, into turbine-powered high-pressure coolant injection (HPCI) turbine, reactor feedwater pump turbines and the main turbine. The SRV s are not designed to relieve liquid as presently configured. 

There was also an independent safety switch on the tank, which provided the operator with a visual and audible alarm in the control room when the level of liquid in the tank reached its specified maximum level (the ultimate high level). This alarm also initiated a trip function to close valves on relevant incoming pipelines. 

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