A bypass takes a safety instrumented function (SIF), or one element of it, temporarily out of service so a trip cannot reach the final element. Sites also call it an inhibit, override, or force. It is what decides how a proof test gets run: shut the process down and exercise the function end to end, or keep running and bypass the function so the test does not trip the unit.
While a function is bypassed, the amount of risk reduction will be impacted. And how much depends on the design of the SIF. Here are some examples.
- A 1oo1 function has no fallback, so protection falls to a person under a documented alternate protection plan, and that only works if they can act within the process safety time.
- A 2oo3 could be degraded to a 2oo2, or be just simply totally unavailable in its entirety depending on the design.
Bypasses can be done via a digital step (for example in the safety PLC) or it can be a mechanical step, such as closing an isolation valve for a pressure transmitter while a calibration is performed or it could be both.
Key Points
- A bypass puts a SIF out of service, so its risk reduction has to be replaced for as long as the bypass is in place. Meaning it will impact PFDavg
- A redundant architecture may only degrade, such as a 2oo3 becoming a 2oo2, but it can also be lost entirely depending on the design.
- The bypass still costs PFDavg and the alternate protection plan does not buy that back. The plan is a separate temporary layer, and it only protects anything if the operator can act within the process safety time.
Example
A 1oo1 high level trip on a surge drum is proof tested with the unit running. The trip output is bypassed in the logic solver so the logic registers a trip without closing the feed valve. The technician closes the transmitter isolation valve, vents the impulse line, and applies known pressures with a hand pump, checking reported level against each applied value and confirming the trip at setpoint. With no redundancy to fall back on, the alternate protection plan puts an operator on an alternate level indication with authority to shut the drum down. Bypass time is logged at 90 minutes.
See Also: proof test, proof test interval, process safety time, compensating measures
Cited Sources