Understanding Risk Reduction and Safety Control Systems at Industrial Plant Operations

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Written by Dirk Muyskens, Consulting Engineer for Industrialinfo.com (Industrial Information Resources, Incorporated; Houston, Texas). It is important in industrial operations to reduce risk of system failures, while maintaining operational reliability. The guidelines for safe operations of industrial operations are becoming more severe by international safety standards. These standards are particularly relevant for all major industries including oil refining, petrochemical, pulp and paper and electric utilities. Reduction of these risks are critical to the safety of employees and the public, minimizing environmental damage, reducing potential capital losses, shortening business interruptions, and limiting legal and regulatory exposure.

Some of most severe risks due to equipment failure are: explosion, fire, loss of product purity, injury or death to employees, lawsuits and destruction of capital equipment.

The operating companies are aware that a reliable Safety Instrument System (SIS) is of great value, not only because of legal and insurance liability; it serves to provide protection for the previously mentioned hazards.

The instrument safety systems are PLCs (program logic controllers). They receive input impulses from the device that is being controlled, which is fed to a diagnostic protection circuit that decides if the device should be shut down. Then a signal is sent to an output circuit for either a shutdown, or to keep operating. Input signals can be flow-rates, temperature, pressure, other factors, or a combination of these.

There are two considerations in installing safety systems. Safety shutdown is the reason for installing the system, but availability is also important. The short definition of availability is, "will the device work when required."

A numerical system has been assigned to the quality of availability data. This is SIL - or Safety Integrity Level. A control system with an SIL 3 rating is 99.3 percent; SIL 2 is 99 percent, and SIL 1 is 90 percent reliable. The assignment of these SIL levels is dependent on the control instrument, as well as judgment factors on equipment reliability. Not all process equipment needs to have SIL 3 or 2 reliability.

If failure of piece of equipment would not be important in the possibility of fire, personal injury, environmental damage, or other hazards, then a SIL 1 rating is sometimes acceptable. An example would be failure of an air conditioner.

To receive an SIL 3 or SIL 2 rating, it is necessary to install dual safety PLCs, or Triple safety PLCs (Referred to as TMR systems). These computers contain self-diagnosis to double check or triple check whether a shutdown condition exists.

It should be understood that a safety shutdown system is completely independent of a control system (distributed control system or program logic control system). A regulation on this subject has been issued (American National Standards Institute/Instrument Society of America S 84.01).

Installation of safety shutdown systems is a voluntary activity by manufacturing plants. There is no government mandate that these should be installed, and plants do it for self-interest. Plants keep records indicating that they meet standards in case of accidents. Also, the Occupational and Health Administration (OSHA) has authority to investigate safety systems (OSHA regulation 1911).

Steps in assigning the proper instrumentation to a safety system are shown below.

1. Hazard and Risk analysis. Determine the hazards of the process and associated equipment, then make any process modifications associated with these risks. Then determine if any of the safety functions are safety-instrumented functions.

2. Determine for each safety instrument function, the associated SIL.

3. Specify the requirements for the safety instrumended functions.

4. Design the multiple safety integrated systems (SIS) to provide the safety instrument functions and meet the specified safety integrated levels (SILs).

5. To install the SIS according to specifications and drawings and to commission the SIS so that it is ready to final system validation.

6. To operate and maintain the system to ensure that the required SIL of each safety instrumented function is maintained during operation and maintenance.

Industrialinfo.com identifies control systems for over 2,000 units in the U.S. chemical processing, petroleum refining, and power industries. Check out the Industrial Control Systems Database. Users of the database can quickly identify prospects based on the age and type of system currently being used at a plant site.