Point alarm

Another serious distraction to a process operator is the multiple alarm event, where a single event within the process results in iTuuti-ple alarms. When the operator must individually acknowledge each alarm, considerable time can be lost in silencing the obnoxious annunciator before the real problem is addressed. Air-handling systems are especially vulnerable to this, where any fluctuation in pressure (for example, resulting from a blower trip) can cause a number of pressure alarms to occur. Point alarms (high alarms, low alarms, status alarms, etc.) are especially vulnerable to the multiple alarm event. This can be addressed in one of two ways ... 

Ganging alarms. Instead of individually issuing the point alarms, all alarms associated with a certain aspect of the process are simply wired to give a single trouble alarm. The responsibility rests entirely with the operator to determine the nature of the problem. 

Control set points, alarm, and interlock switch points (as applicable) Engineering range and signal type/level Operating/calibration tolerances Fail-safe mode... 

Point alarms (high alarms, low alarms, status alarms, etc.) are especially vulnerable to the multiple-alarm event. This can be addressed in one of two ways ... 

Ganging alarms. Instead of individually issuing the point alarms, all alarms associated with a certain aspect of the process... 

Instruments run on last known set-point, alarm conditions are not received by control system Error will not automatically alarm on control system if last received process parameter within acceptable range... 

The theory of Fourier series tells us that we can find all of the different bn needed to match any initial condition F x, 0). At this point alarms... 

Chart 3 Control points and alarms. Control point/Alarm Results Date... 

In the preceding points, alarm life cycle is not mentioned but with introduction of various standards on alarm management, alarm life cycle has become part and parcel of it. Naturally, the same will be dealt in the latter part of the discussions. Before that, let us find out what benefit does alarm management offer ... 

There are three basic requirements that Hquid level control devices are designed to satisfy alarm functions, pump/valve control, and transmitted output signal to track level continuously. Alarm devices provide warning or shutdown functions when process levels pass a predeterrnined point in the vessel pump /valve control devices turn on/off pumps or open/close valves at predeterrnined levels in the vessel and transmitters provide a proportional output signal over a predetermined span to send to a local meter or signal back to a control room. 

Displacer units are tolerant of specific gravity shifts within their range, require no power to operate, are intrinsically safe, and can be easily field cahbrated. A single displacer unit can control up to three alarm set points or up to three pumps with different spans. They can be caHbrated for set points or pump spans from 0.15—30 m from the top of the vessel mounting. 

Heat build-up Replace screen with one correctly sized and/or due to too fine or clean screen blocked outlet, Install pressure indicator downstream and screen. upstream of mill for conveyed systems Measure temperature at strategic points in mill casing to detect and alarm product temperature rise CCPS G-12 CCPS G-23 CCPS G-29... 

First-out annuneiators are highly desirable and should provide suffieient points to aeeommodate the speeified alarm and shutdown funetions. Standard features should inelude aeknowledge/silenee, lamp/test, and both audible and visible deviees for general alarm indieation. A dry auxiliary eontaet from the shutdown eireuit must be provided for expander inlet trip valve aetuation. Additional sets of eontaets are useful for other alarm requirements. 

During testing, all alarm and shutdown funetions must be simulated to verify proper operation of the annuneiator display. The test teehni-eian must plaee his initials next to the tabulated set points as verifi-eation of eompletion. 

The minimum alarms and trips reeommended for eaeh major driver and driven maehine should be a low oil pressure alarm, a low oil pressure trip (at some point lower than the alarm point), a low oil level alarm (reservoir), a high oil filter differential pressure alarm, a high bearing metal temperature alarm, and a metal ehip deteetor. See Table 15-1. 

A differential pressure switch set to alarm when the pressure drop reaches a predetermined point protects against the loss of oil flow. In addition to the differential pressure switch, a two-way, three-port valve with a pressure gauge is piped in parallel with the differential pressure switch for accurate indication of inlet and outlet oil filter pressure. When a single transflow valve is used with a cooler-filter installation, the differential pressure switch and pressure gauge assembly should span the cooler-filter system. 

Onee the data points have been deeided, limits and alarm must be set. This is a long and ehallenging task, as the limits on many points are... 

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