Process calculations can separate you from your competition, giving you insight into your process and an extra edge. Unfortunately, many companies dont successfully implement process calculations because of poor selection of the platform or fear of the costs associated with development and maintenance of the new software.
Today, process data are readily available at many levels, from instrumentation to higher-level data historians and OPC servers. While each may offer the capability to create process calculations, there are advantages and disadvantages that should be considered. Choosing the right platform can be key to successfully implementing robust and maintainable process calculations at your company.
What are process calculations?
Process calculations are a set of user-developed rules and logic, a blend of real-time and historical process data providing real-time or historical summary values. These distilled statistics furnish vital insight into equipment performance, process economics, environmental or regulatory compliance, quality or any other factors affecting the bottom line.
Examples of process calculations include fermentation rate and efficiency, analyzer performance, equipment utilization, stack emission rates, and optimizations. The purpose of each calculation is to generate information that isnt directly available from the raw measurement values. Ideally, the calculation results are written back to the underlying system where they are available to users along with the raw measurements. Existing tools typically applied to analyze and monitor raw measurement can then be used to analyze the calculation results.
With the proliferation of networks and networked devices in industry, process data are available on many different levels and platforms. Instrumentation is generally tied to a programmable logic controller (PLC) or distributed control system (DCS). A supervisory control data acquisition (SCADA) systems or process historian may provide bi-directional communication to the PLC or DCS. More recently, object linking and embedding (OLE) process control (OPC) servers may provide bi-directional communication to the PLC, DCS, SCADA system or historian. Each process data platform is a potential process calculation platform (Figure 1).
Figure 1. Potential candidates for a calculation platform are shown in a common hierarchy of a control system.
Choosing the correct process calculation platform involves careful analysis of a range of factors, including stability, functionality, maintenance, available tools and life-cycle costs.
When analyzing the various platform choices, you may want to ask the following questions:
- Can those with the process knowledge easily implement the calculations? Many solutions require someone with a configuration access to write the logic. Often, the programmer isnt the technical expert on the process, so there will be a knowledge transfer: from the process engineer to the programmer. These platforms will require significant development effort involving many, expensive hours of writing, testing and debugging.
- How easy is maintenance and editing? There is more to think about beyond the initial implementation costs. Software, like hardware, must be maintained. After a successful roll-out, you may find that the calculations are commonly used across the company at many different levels. The ability for multiple people to edit and maintain the calculations may become increasingly important as jobs duties and roles change. Ease-of-implementation will result in reduced development time and a significant savings to overall cost.
- How open is the solution? Am I tied to a specific vendor or piece of hardware? An open solution implies many things. At its best, the software will be vendor-neutral, utilizing data from a variety of systems and vendors (process data, relational databases) for both inputs and outputs. The platform should also have access to real-time and historical data, and support the output of results to multiple systems.
- How flexible is the solution? Flexibility becomes important as the number and complexity of your calculations increase. You may need to be ability to run calculations at different intervals, add new equations without affecting current equations, or control the order of execution between multiple equations. You may have data in a variety of sources including proprietary databases and files. A black-box product may provide quick-and-easy setup but have limits in handling complex calculations. Therefore, it isnt a good choice for long-term calculation solution choice. Make sure your solution is adaptive to meet your potential future requirements.
A PLC or DCS provides stable and speedy access to the instrumentation measurements in the process. It offers the ability to add programmatic input and output logic (e.g., ladder logic). The problem is that the main purpose of this logic is for control and not complex or multiple-system calculations.
The main advantage of the PLC/DCS platform is reliability and speed. The results of any logic calculations are immediately available within the control system.
Yet, there are many reasons that the PLC/DCS is not an ideal platform for process calculations. The hardware is fairly expensive and provides limited access to other systems and data. A PLC/DCS has only limited or no historical data. Typically, you can access control data from other networked controllers. However, a PLC/DCS platform may be dependent on a specific vendor or depend on a specific control network protocol; this limits the scope of calculations. From the standpoint of commissioning and maintenance, it is costly to develop and maintain complex process calculations in a PLC/DCS. The development tools require special skills and application knowledge, often requiring a PLC programmer.