A system control philosophy is a document that is used to describe general principles of operation. Variables such as alarm set points or shutdown values are usually described in the Control Narrative. A control philosophy solidifies basic definitions, principles and processes during the design and implementation of a process. It is the cornerstone of how the system will operate. Once approved, it will be one of the documents used to assist in the development of the control narrative and will help facilitate the HazOp or PHA. The level of detail in a control philosophy compared to a control narrative is analogous to the detail in a Process Flow Diagram (PFD) and a Piping & Instrumentation Diagram (P&ID).
In order to begin a control philosophy, the PFD should be well developed while the P&ID is still in development. This document is non-technical in nature and the reader should be able to generally describe how the system operates.
Items in a control philosophy
Besides information that is included in all technical documents, general topics that could be covered in a control philosophy are as follows. Note that this isn't intended to be an all inclusive list.
Operating Philosophy Definition - Describe how the system ties into the rest of the plant. Notate how the system is to operate. Is it fully automated? If not, what systems are automated and which must be performed manually. It should outline how the plant or system operates during normal, stable operating conditions. It should also include how the system will react in process upsets.
System Description - Describe what the total process is doing. For example:
Waste water is collected into T-100 and is pumped out using 3-50% vertical turbine pumps (P-200) which sends the water for injection. Each pump is sized to move 50% of the total flow rate for injection with one pump idle. Pumps P-200A, B & C are controlled using an adjustable speed drive (ASD) and are controlled based on maintaining a low level in T-100. If T-100 liquid level activates a high level alarm, the third idle pump comes online to assist in the disposal of water.
Data Collection & SCADA Systems - How is data collected and, in general, what data is displayed for operators to control the plant? If the system uses a distributed control system (DCS), describe the different systems and what they are controlling. If the system uses a single PLC and no remote input/ output points, this should be notated.
Alarm Philosophy - What types alarms will be employed? How are the alarms being prioritized? E.g. which need immediate operator interaction and which are in place to be recorded but not necessarily annunciated. Include the purpose of each alarm. For example, a high level alarm might alert the operator and a high-high level alarm might shut down equipment to protect it or the overall process.