Photo by American Heritage Chocolate on Unsplash
You can’t bake a cake without a recipe, and you can’t (or you shouldn’t) build a house without a set of blueprints. Every project starts with some guiding design for it to meet successful completion.
Control projects are no different, in fact it is even more critical that the outcomes are understood. The next few articles give you the crucial design foundation for an automation project, starting with the process description.
Note: the examples, specifically of parameters and abnormal conditions, are for illustrative purposes only, they are not meant for industrial use.
Start with What It Does
A control engineer might get other parts of the design first. They might get a drawing, or they might get a list of tags for the system, or they might get both. But neither of these things, although important and covered later, completely tell you what the equipment is meant to do.
Learn the Process, Master the Platform
A control engineer who works with a company that supports one type of industry might be more of an expert on the process. We are going to assume that you, as the control engineer, are not an expert on the process. For this reason, you will need a process description. It could start out as simple as:
A vessel is used to remove as much sediment from dirty oil as possible, both by settling, and by adding solvent and mixing with heat.
That statement alone defines one large sequence and one smaller relationship between two outputs. This is a great start and helps put a lot of the pieces together.
Four Elements of a Successful Process Description
The process description should have the following items:
- A business statement, explaining why it is worth having, as an example:
Oil needs to be pretreated to remove as many solids as possible to eliminate fouling or excessive cost during the next clarification process.
- A description of normal operation, the process that takes place to produce the item of value, example:
Oil is added to the vessel until the tank is about 2/3 full, at which point tank mixing begins. With mixing initiated, solvent is added to the tank to aid soluble solids to come out of solution. The tank is heated while mixing to decrease viscosity. Once it is warm and well mixed, heat is removed, and the tank is allowed to settle. When enough sediment has settled out of solution, the top oil is transferred to the next processing step. The bottom oil is transferred for collection and possible reprocessing.
- Key parameters tell you what the most important values to be monitored or controlled. In this case, examples of key parameters are:
- Tank temperature
- Settling time
- The abnormal conditions below are what the control systems can detect that allow the control systems to interrupt or divert normal operations. Control systems can only react to information that is sent to it. It is not an exhaustive list of everything that can go wrong. Examples of abnormal conditions are:
- Low tank level
- High tank level
- Low Pump Pressure
With a good process description in hand, let’s explore the next source of truth on our list, the window to the process: the Process Diagram.
Disclosure: The views expressed here are my own and do not reflect my employer. This content is for educational and illustrative purposes only; it is not a guide for building or operating industrial equipment. Consult a professional for practical implementation.