Cause and Effect Matrix

Table of Contents

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Introduction

This unit operation, which is very similar to a spreadsheet, models a Cause and Effect matrix used in designing and operating plant safety systems. It processes a number of causes and a matrix, and determines the resulting effects.

As a simple example, it can be used to open a valve if a vessel level or vessel pressure exceeds a certain value.

Connections Page

Add causes and effects on this page. Click "Add Causes" or "Add Effects" to use the variable navigator to select a number of variables to be added. Click "Switch" to add a cause which is controlled manually, i.e. not linked to a variable. Similarly, click "Add Effect" to add an effect which is connected to an output object. You can also delete causes and effects from this view, and use the arrow buttons to change the order in which a cause or effect appears in the matrix (and are processed).

You can also add variables to a cause and matrix by right mouse clicking on the variable and choosing to send it to the cause and effect matrix. Or hold the right mouse button down over the "DRAG" text in the cause and effect view, and drop it on a spreadsheet cell or digital switch state for example.

Note that effects generally send 0 or 1 signals. Hence it is common to have a spreadsheet cell or digital unit operation as the output. The valve unit operation also has an on off state variable which translates into a 0% or 100% desired valve opening.

Parameters Page

Global Settings

  • Always Update Outputs - Always push an effect output to its associated object, even if the output value has not changed.
  • Use Local Switched for Outputs - Lets you manually set the output for all effect.
  • Use Output Reset - Any new effects added will have the "Reset?" option checked, and this option will also be checked for all existing effects.
  • Bypass All Outputs - Do not send any outputs to the output objects. This can be used during start up or to temporarily disable the matrix.
  • Trace All Alarms - Trace alarms in the trace window when they trigger.
  • Reset All Outputs - Any outputs with "Reset?" which have tripped, will be reset. Otherwise such effects will remain tripped.
  • Hand Pulse Switch Duration - If a cause has a manual switch, you can pulse it on or off for this duration of time.

Cause Variables

  • Description - Enter your own user description here.
  • Tag - Enter your own tag here.
  • Switch - Indicates that this cause is set by a manual switch, its value is not determined by comparing the variable value to the trip value.
  • Override - This cause is effectively ignored in calculations.
  • Invert - During matrix calculations, the on or off state of this cause is reversed.
  • Off Delay - If the variable exceeds the trip, its value must consistently remain above the trip values for this length of time, before it will be treated as having tripped.
  • Value - If the cause has a variable, this shows its current value.
  • Alarm - If a value is entered, it can be used to raise an alarm if the Value exceeds the Alarm.
  • Trip - If a value is entered, the cause will trip if its Value exceeds the Trip value.
  • High - If checked, the Value has to the higher than the Alarm and Trip values to trigger, otherwise, lower.
  • Pulsed behavior - If the switch option is checked, this can be set. Latched mode means that the switch is manually set and remains at the set value. Pulse On and Pulse Off can be select to have the switch go on or off for a certain length of time only.
  • State - Shows the current cause state (Healthy(1) or Tripped(0)).

Effect Variables

  • Description - Enter your own user description here.
  • Tag - Enter your own tag here.
  • Switch - Indicates that this effect is set by a manual switch, and not determined by the matrix outcome.
  • Bypass - This cause is effectively ignored in calculations.
  • Invert - The reverse of the effect state is sent to the output object.
  • Reset? - Indicates that this effect must be reset after it trips, before it can change again.
  • Reset - Click here to reset a tripped effect.
  • First Out - Shows which cause first tripped this effect.
  • Local Switch - Shows the value of the local manual switch, if enabled via the Switch settings.

Matrix Page

The matrix determines which and how certain causes impact an effect. Press the "Legend" button to see the strings. In the screen shot, you can see that for Cause-1 the Value exceeds the Trip value, and hence the input has tripped. The X in the matrix for Cause-1 and Effect-1, indicates that this input will trip Effect-1, as shown. Cause-2 has a green mark in the matrix next to it, indicating that it is healthy. The T5 for it under Effect-1 means that the value of Cause-2 (Vessel Pressure) has to exceed to bound for at least 5 second continuously before it will trip Effect-1.