Setup of Equipment Units in Accord Builder

Designing Process Automation using Equipment Units

Process Automation covers a wide variety of processes in many industrial sectors but almost all contain common types of equipment items such as Valves, Motors and Instruments. Grouping these equipment items in Units assists both in understanding the process to be automated and in providing a good method for implementing the automation.

There are many good examples of process automaton using units, and the ISA S88 model uses equipment units as a core concept.

In a simple example of a Reception Tank, a Transfer Line and a Storage Vessel,

the Receiver Tank Unit may consist of the Intake Valve, the Tank Level Transmitter and the Tank Emptying Valve, and the Storage Vessel may consist of a Filling Valve and Vessel High Level Switch

and the Transfer Line would consist of the Reception Tank Emptying Valve, a Transfer Pump and the Storage Vessel Filling Valve.

When considered as Units the use of common bits for ‘Unit – Device in Error’ and ‘Unit – Device in Maintenance’ become easier

if any of the Reception Tank Emptying Valve, Transfer Pump or Storage Vessel Filling Valve are in Alarm or Fault then the Transfer Line is in Alarm. Then any program that is using this line would be ‘not working properly’ or In Error. So one Unit state can be used instead of the 3 individual device states. If there are many Reception Tanks feeding the Transfer Line to many Storage Vessels then the advantage of being able to use a Unit state increases as the number of Devices increases.

A Unit usually has States for

  • Reserved: ‘Unit Selected’ – a Program is operating on the Unit
  • Error/Alarm/Fault: ‘Unit – Device in Error’ – a Device in the Unit is in Feedback Fault, or Wirebreak or Tripped…
  • Maintenance: ‘Unit – Device in Maintenance’ – a Device in the Unit is in Maintenance

The Unit may also have other states, sometimes called Modes,

  • Clean – The Unit has been cleaned (has had a CIP Program operate on it)
  • Sterile  – The Unit has been sterilised
  • Full –  The Unit is at High Level, cannot be filled any more
  • Empty – The Unit is at Low Level,

and more states or modes are possible, depending on the process requirements.

So, when a program is operating on a piece of equipment then the program should reserve the equipment so that no other program can start on the same equipment at the same time. The Reception Tank cannot be part of a cleaning program and part of a Filling program at the same time. This is done by writing the Unit as Reserved, or Selected, while the program is Active. If any device in the Unit is in Error then the State ‘Unit – Device in Error’ should be true. Similarly if any device in the Unit is in Maintenance then the State ‘Unit – Device in Maintenance’ should be true.

These ideas are used for allocating devices into Units; the P&ID should be sectioned into Units such that a Program reserving a Unit can do so safely, to avoid product mixing or contamination, and that Devices going into Error can cause a program to go into Error/Alarm correctly.

Allocating Devices in Equipment Units in Accord

Equipment is contained in Units, to reflect process drawing, which are comprised of:

  • Analog Devices           (with PID Loops)
  • Analog Inputs
  • Digital Outputs
  • Digital Devices          (Valves and Motors)
  • Digital Inputs

In configuring a process plant a Vessel Unit contains equipment which is connected to the Vessel and a Line Unit is a usually section of line between block valves or a block valve and an end-of-line drain valve. A Line Unit is a usually a section of plant which is used for transfer.

Items associated with a vessel or section of line are placed in a Unit. There are two types of units in Accord: Vessel Units and Line Units. This is to assist understanding, but the implementation of these types of Units is identical.

In Accord Builder units are grouped for presentation purposes and are both Vessel and Line Units may contain equipment Devices and Instruments. Examine the plant drawing and process description to decide on the use of Units.

A continuous operation system, in which there is one main program, may be configured using only 1 Unit. A sequential system, in which there is more than one program, and the different programs will use different parts of the equipment would be configured using Units.

Unit Common Functionality

A Unit is a logical section of plant, and use of equipment units allows safe operation:

  1. If a device (analog or digital) in a unit is in Maintenance then devices in that unit may not be activated
  2. If a device (analog or digital) in a unit is in Error and the device is enabled to cause a Program Alarm then the program which is using or attempting to use the unit is placed in Alarm.
  3. If a Unit is in use by a program then another program which attempts to select the Unit may be placed in Alarm.

When a unit is configured it may be placed in a program in the Unit Routes component holder in the program. The program will then be able to use, or access, devices and instruments in the unit.

Insertion of Units

Units are inserted into the Model by

  1. Dragging from the toolbox into desired group
  2. Right click on location and adding the device type.

There is a prompt for the object name to be entered. This may be based on the vessel identification in plant drawings (P&ID’s).


In this example the name CIP Tank is given to the vessel unit. A description of the unit should be entered in the vessel properties description tab. Entered information will also be presented in documents.

Add Items to Units

Equipment may be added into the unit corresponding to process drawings. Devices that are required to be linked by maintenance or error status should be added to the same unit.

Equipment items may be added by

  1. Dragging in the appropriate icon from the equipment toolbox
  2. Right clicking on the vessel to select appropriate item type

 The example shows the Devices in the Unit.

Copy and Paste Units

In addition to insertion of devices, right clicking on the vessel gives the options to Rename, Delete, or Copy the Unit.

A vessel or line unit may be copied and pasted within the object unit vessel group or line group. Copying a vessel creates an exact copy of all the information for the vessel, including all components. The copied unit and all the components must be renamed, and re-configured with correct settings.

Copy function in Right-Click for Unit

A copied unit may be pasted within the group, as an exact copy with new components with temporary names. (The names of the copied items are automatically assigned as copies (1)). After the paste function the component devices may be renamed and their short descriptions may be changed. The appropriate I/O addresses must be changed. Other aspects of the copied items may be changed as required.


Linking of Units


Removing a Valve to remove a link between two Units.

This is achieved by selecting the Valve in one unit and right clicking to Delete the Valve. A popup will ask if the Valve is to be removed completely of just to be removed from one Unit. Selecting that the valve is to be removed from one unit only will remove the link between the Units.