Choosing the right Process Calculation Solution by
Dane Overfield
What are Process Calculations?
Process Calculations are a set of user-developed rules that use real-time and historical process data, along with other data such as lab values, manual inputs, etc., to provide real-time or historical information about performance, economics, environmental, regulatory compliance, quality, or other calculation-based operations parameters. Ideally, the calculation results are stored and available to others along with the inputs.
Why use Process Calculations?
Process Calculations can add valuable insight into how a plant, process, or unit is functioning. This can lead to higher efficiency, reduced waste, more effective maintenance, higher quality, reduced emissions, quicker turnarounds, and reduced energy usage.
Some typical Process Calculations
Process calculations can be used wherever the desired information is not directly available from the raw measurement values. This includes items such as
Cure rate
Flow compensation
Fermentation rate and efficiency
Process analyzer performance
Mass balance
Equipment utilization
Stack emission rate
Optimizations
Advanced abnormal condition detection
Process Calculation Rules
The calculation rules can often be more complex than the process calculations themselves. The rules include items such as:
Determining if or when a calculation should be made
Starting and ending new calculation periods (new batch)
Dealing with missing or invalid inputs
Choosing a Process Calculation Platform
Where can Process Calculations be performed
Due to the availability of process data in multiple systems and platforms, process calculations can be created at multiple levels (PLC, DCS, SCADA, Process Historian, OPC Server, Spreadsheets)
Problems with many Process Calculation solutions
The platform and solution decision can have a great impact on the success of implementing and maintaining the calculations, as well as the ability to make the calculation results available to others.
Typical Process Calculation solution problems:
They are often developed by someone other than those with the process and calculation knowledge
They may depend on specific hardware or vendor software versions
Calculation documentation usually separated from calculation rules
No standards for implementation resulting in multiple solutions from multiple users
Process Calculation Solution Goals
The Process Calculation solution may be chosen because the platform already exists, not necessarily because it is the best solution. When deciding on a solution, you should make sure it meets the following goals:
High performance and wide functionality
Calculations can be easily created by those with process knowledge
Rules are easily maintained, edited, and documented
It has the ability to read from and write to multiple process data sources
Examining Various Process Calculation Solutions
There are advantages and disadvantages to each Process Calculation platform solution.
DCS (distributed control system) or PLC (programmable logic controller) Advantages:
High reliability
High speed
Results available within the control system/PLC
Disadvantages
Expensive real estate
Complex calculations are not easily implemented
Limited visibility of equations and logic
Not easily maintained
Limited data availability
Limited or no historical data access
May require special skills to implement
Limited test and debug environment
Difficult to change/edit
Limited access to other systems (LIMS, Enterprise, RDBMS)
SCADA (Supervisory Control and Data Acquisition) or Process Historian Advantages:
Access to real-time and possibly historical data
Advanced implementation tools
Easier test and debug environments
Relatively good maintainability
Relatively inexpensive computing resources (PC)
Disadvantages
Only process data within the system is available
May require special skills to implement
Often highly dependent on SCADA or Process Historian vendor software versions
Spreadsheets (with process data-retrieval add-ins) Advantages:
Access to real-time and possibly historical data
Easy to implement
Relatively good maintainability
Relatively inexpensive computing resources (PC)
Good for creation of reports
Users often familiar with the software Disadvantages
Results typically available within the spreadsheet
Limited real-time computing component to write results back to the system to make results available to decision-makers
Cannot be scheduled (calculations typically executed in an ad-hoc manner)
What is an OPC Calculation Engine?
An OPC Calculation Engine is a PC based OPC client software package that provides a framework for easily and efficiently creating calculations that use process values (tags/ items) as inputs and outputs, utilizing the OPC standards for reading and writing data.
OPC provides a standard set of specifications for reading and writing current and historical values to the underlying process data store (DCS, PLC, SCADA, Historian ). An OPC client can read and write process data in a vendor-neutral manner, provided that an OPC Server exists for the data store.
Use of an OPC Calculation engine allows a user to create sets of calculations that may be high in number, complexity or both. OPC Calculation Engines typically consist of intuitive development tools (rules editor) and run-time tools to schedule and control the calculation execution.
OPC Calculation Engine Advantages & Disadvantages Advantages:
Data from a wide variety of sources is available using the same data access standards
No knowledge of subsystem internals is required
Advanced tools for implementation of calculations
Advanced test and debug environment
Provides a structure to keep calculations organized
High maintainability
Allows those with process knowledge to implement calculations and logic without requiring special skills
Rich set of calculation and data manipulation functions available
Plentiful and inexpensive computing resources
Can easily integrate with special purpose third party calculation modules such as Steam Tables, API Methods, Web Services and relational databases.
Results can be directed to a variety of destinations
Calculations are system independent and transportable
Scheduling can be time or event-based. Individual equations may be execute or not based on the result of other equations.
Calculations may be used to easily move data between systems
Disadvantages
Depends on quality OPC servers for connection to data sources
Not typically suitable for sub-second calculation rates
Not as reliable as PLC or DCS
Example OPC Calculation Engines
OPC Calculation Engines such as EXELEs OPCcalc or Honeywells Advanced Formula Manager (AFM) use Microsofts Visual Basic technology as the underlying computing environment. They provide the user with a robust and user-friendly environment for the creation of OPC-based process calculations.
