Alarm trips: the ups and downs – Part 1
Thursday, 12 November, 2009
Something happens - a signal peaks or falls - and you need to know. A limit alarm trip can trigger the response needed to maintain normal, and safe, operations.
A limit alarm trip monitors a process signal (such as one representing temperature, pressure, level or flow) and compares it against a preset limit. If the process signal moves to an undesirable high or low condition, the alarm activates a relay output to warn of trouble, provide on/off control or trigger an emergency shutdown.
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While limit alarm trips are best known as a sure way to activate a warning light, siren or bell when a process problem occurs, they are also called upon to do much more. In fact, today’s highly flexible and versatile alarm trips can be found working in a wide range of applications, under an impressive list of pseudonyms. See the sidebar on the next page for a list of some of them. Hard versus soft alarms Because they are hard wired into the process and provide relay outputs, independent limit alarm trips are often referred to as ‘hard’ alarms. This term differentiates a hard alarm trip from the software implemented alarm (a ‘soft’ alarm) which is found within a distributed control system (DCS) or a programmable logic controller (PLC). Why use hard alarms? Every plant performs alarm functions using soft alarms within their DCS or PLC. As such, some might argue that hard alarms are not necessary. However, hard alarm trips complement DCS and PLC systems by providing redundancy, simple control and critical safeguarding. Because of the potential consequences to plant and personnel, hard alarm trips continue to be the accepted industry standard for a wide range of primary alarming functions, as well as for backup of DCS and PLC strategies in critical emergency shutdown (ESD) and safety-related systems (SRS). Soft alarms can be susceptible to common-mode failures (such as failure of a computer-based system’s power supply, hardware or software) that could disable all of the soft alarms in the entire system. Soft alarms may therefore be inappropriate for providing the degree of protection demanded for some critical applications, such as those found in ESD systems or safety instrumented systems (SIS). |
Other names for alarm trips
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Hard alarms are not exposed to the adverse effects of a common-mode failure because they maintain complete independence from the DCS or PLC (Figure 1). Hard alarm trips distributed throughout a facility can be used to provide warnings and safety backup measures in the event of a common-mode failure. That’s why in critical and safety-related applications, the use of hard alarms is a requirement of many insurance companies.
Another good reason why hard alarms should be considered in place of, or to back up, soft alarms is that rather than intermittent scanning of individual points as is accomplished by a DCS or PLC, each hard alarm provides continuous supervision of an individual process signal. In some fast-changing applications, the computer’s scanning speed or network throughput time may be inadequate. In addition, hard alarms are typically easier to set up, which eliminates potential programming errors. They are also less prone to failure, inadvertent changes and tampering.
Basic limit alarm trip functions
Anything from simple annunciation to shutdown of an entire process can be handled by a limit alarm trip. An alarm trip accepts an input signal from a monitoring or control instrument, such as a signal transmitter or sensor. When the monitored variable falls outside of a user-set ‘trip’ point (also called a ‘set’ point), the alarm trip activates one or more of its relay outputs. The relay(s) are typically used to control a warning light, annunciator, bell, pump, motor or a shutdown system.
In most units, once an alarm trips, it remains in an alarm condition until the process signal re-crosses the trip point and passes out of the deadband. An adjustable deadband makes it possible to increase or decrease this range, thus affecting what point the relay returns to its normal, non-alarm state.
Using this relatively simple cause-and-effect action, limit alarm trips can be economically used in a wide variety of basic and complex applications:
- To warn of trouble by providing a hard alarm output when a process signal exceeds a high or low limit.
- To create an independent ESD system to avert undesirable situations in the event of a central power failure or DCS shutdown.
- To provide redundant warning or shutdown capabilities to back up and compensate for failure of DCS or PLC soft alarms.
- For simple applications, to replace over-complicated PLCs with alarm trips that are easier to set up and use.
- To reliably and cost-effectively provide on/off control of pumps and motors in batching and similar applications.
- To sense dangerous conditions and shut down control equipment before it is damaged.
- To monitor an input for a change in value, and trip an alarm when the input rate-of-change exceeds a selected rate, over a selected time period.
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High and low limit alarms A high or low limit alarm is triggered when the value of the variable being measured exceeds a preset high or low alarm trip point (Figure 2). This type of alarm trip monitors temperature, pressure, level, flow, position or status variables, and is typically used to warn of unwanted process conditions or to provide emergency shutdown. Alarm trips with multiple relay outputs A limit alarm trip can have one, two or even four relay outputs. Typically, each relay output can be set to respond to a different trip point. This would include any combination of high or low alarm trips, with different trip point settings for each. Some alarm trips also offer the option of setting the relay to trip if there is an input fault (such as a broken sensor), or to alert that there is a problem with the alarm trip itself (Figure 3). |
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The following examples describe how alarm trip points might commonly be set for a dual output limit alarm trip. Of course, if the alarm trip had four relay outputs, any combination of these same trip options could be applied to the remaining two relays.
- High alarm: A status change (alarm condition) of a single high alarm occurs when the input rises above the trip point. The status will return to a non-alarm condition when the input falls below the deadband.
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- High/low alarm: A dual high/low alarm accepts one input and has two relays, each with a separate trip point (Figure 2).
- On/off control: A limit alarm trip can also be used as a simple on/off controller such as those required in level applications (pump/valve control) when filling or emptying a container or tank (Figure 5).
In Part 2
In Part 2 of this article, we will look at some other uses of alarm trips, describe fail-safe versus non-fail-safe operation and discuss alarm trip configuration options.
Moore Industries Pacific Inc
www.miinet.com
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