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Electric, Hydraulic & Electronic Control Systems
It is usual to have three installation wide control systems on a process plant, and a host of local control systems.
Plant Wide Control Systems
The Process Control System (PCS)
The PCS takes care of running the process and utility systems within normal parameters. Some operators call this system a SCADA system: Supervisory, Control And Data Acquisition.
The Emergency Shut Down System (ESD)
The ESD supervises important /critical parameters in the process and utility systems and will shut down the plant if predetermined values are superseded, e.g. a hi hi pressure level is reached. The ESD system normally has graduated shutdowns with each shutdown level given a name e.g. ESD1, ESD2, ESD3 etc. The levels correspond to severity with an ESD 1 shutting down the complete plant, an ESD 2 perhaps closing down a complete system, etc. See our page on Levels of Shutdown for more details.
The lower levels of the ESD system are known as the Process Shut Down (PSD) system.
The Fire and Gas System (F&G)
The F&G system, unlike a PCS or ESD system, is not process related but is instead facility related. While the first two supervise and control the process, the F&G system supervises and controls hazardous situations i.e. a fire is detected or a gas release, on the facility.
Local Control Systems
Not every plant will have all the systems mentioned below, however some or all of them will be present in most plants.
High Integrity Process Protection System (HIPPS)
A HIPPS system is typically a local and normally small control system. In principle this is an ESD system but it is totally independent from the plant wide ESD to ensure a sufficiently high Safety Integrity Level (SIL) according to IEC 61508: Functional safety of electrical/electronic/programmable electronic safety-related systems and IEC 61511: Functional safety - Safety instrumented systems for the process industry sector.
Local Control System (LCP)
Local control systems are usually housed in a panel close to the piece of machinery that the LCP controls e.g. compressed air unit, water maker unit, HVAC unit. A Local Control System has typically a limited interface to the PCS.
Unit Control System (UCP)
A UCP is very similiar to a local control system though typically has a lot more two way communication with the PSD and ESD. An example of a UCP would be the system that controls a compressor package driven by a gas turbine.
Specialist Control Systems
It is not unusual to find "Specialist Control Systems" that feed into LCPs, UCPs or even the PCS or ESD. For example, both an anti surge system on a compressor, or a vibration monitoring system on a piece of rotating equipment would be considered as specialist control systems.
The following pages on Control and Instrumentation.com give more detail on other topics associated with Electric and Electronic Control Systems:
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