High/low alarms

The oil eoolers and filters are eontrolled by a loeal temperature eontrol loop with remote eontrol room indieation and high/low alarm. The eoolers and filters also have an indieating differential pressure alarm. These usually feed into a eommon high alarm to pre-warn a need for switehing and filter element replaeement. 

High/low alarms. A high alarm is generated when the value is greater than or equal to the value specified for the high-alarm limit. A low alarm is generated when the value is less than or equal to the value specified for the low-alarm limit. 

Schematic diagrams of the control system are shown in Figures C-59 through C-67. There are many design features common to all control systems which promote system reliability. The high-low alarm monitors called summing point monitors are used extensively in all control systems.

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 ... 

With respect to their response, the discussion should emphasize why these are important anci why they adjust certain control settings. Among the deviations on which analysts should focus the discussion are the high and low alarm settings. Some alarms will require rapid response. Alarms may give insight into equipment-operation boundaries as well as process constraints. 

Provide high/low temperature alarms to shut off feed... 

Annunciation control (for low pressure alarm, high pressure alarm and operation ot the safety valve etc.)... 

A typical lubrication oil system is shown in Figure 15-1. Oil is stored in a reservoir to feed the pumps and is then cooled, filtered, distributed to the end users, and returned to the reservoir. The reservoir can be heated for startup purposes and is provided with local temperature indication, a high-tempera-ture alarm and high/low level alarm in the control room, a sight glass, and a controlled dry nitrogen purge blanket to minimize moisture intake. 

For a fired reboiler, a pump-around system is used with an FRC to maintain constant flow. There will be a low flow alarm plus fuel shutoff. There will also be a high flow alarm plus fuel shutoff, since a tube rupture would reflect itself in a high flow. 

Consider need for high/low temperature alarms for refrigerated storage these should be inspected and tested regularly. Consider need for mitigatory measures (fire, blast, fragment-resistant barricades/screens), electrical and electrostatic safeguards, personal protection, disposal etc. 

With flammable gases, eliminate all ignition sources (refer to Chapter 6). Possibly provide additional high/low level ventilation background gas detectors to alarm, e.g. at 40% of the LEL. With toxic gases, possibly provide additional local ventilation monitors connected to alarms appropriate air-fed respirators. (The flammable/toxic gas detectors may be linked to automatic shutdown instrumentation.)... 

If HjS is continuously present in the flare gas or if the flare seal drum also functions as a sour water disengaging drum, then the effluent seal water must be routed to a sour water stripper, desalter, or other safe means of disposal. Withdrawal from the drum is by pump in place of the normal loop seal arrangement. Two pumps are provided one motor driven for normal use, and the other having a steam turbine drive with low pressure cut-in. The seal drum level is controlled by LIC with high and low alarm lights plus an independent high level alarm. 

To prevent similar incidents from happening, if heavy oil is being transfen ed into a tank, incoming oil should be kept below 100°C, and a high-temperature alarm should be installed on the oil line. Alternatively, water should be drained from the tank, the tank kept above 100°C. and the tank contents circulated before the movement of oil into the tank starts. In addition, the movement of oil into the tank should start at a low rate. 

If a service is used continuous , it may be connected permanently to process lines. If the service pressure is liable to fall below the normal process pressure, then a low-pressure alarm should be provided on the service line. If the process pressure is liable to rise above the normal service pressure, then a high-pressure alarm should be provided on the process side. 

Consider need for high/low temperature alarms for refrigerated storage these should be inspected and tested regularly. 

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