Risk Detection How PLCs Help to Avoid Hazards and Increase Plant Safety

Author / Editor: Deepak Makhijani, Consultant / Dominik Stephan

In the present times, when strategies to ascertain workplace safety and environment conservation have become a necessity, fail-safe PLC systems can work wonders. These systems minimize the risks involved in critical applications where the possibility of hazards is high. The fault-tolerant architecture ensures that the failure of a component does not interrupt the process operation, thus ensuring safety.

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Fail-safe PLC system is used in critical safety applications in refineries, petrochemical/chemical plants
Fail-safe PLC system is used in critical safety applications in refineries, petrochemical/chemical plants
(Picture: ABB)

Today, workplace safety and protection of workers as well as the environment are top priorities while preparing a plant design, and are routinely incorporated into operating and maintenance procedures. In general, design engineers and plant operators can use four different approaches to reduce risks in a chemical process plant, namely:

  • Seek inherently safe designs
  • Devise passive design solutions
  • Install active (engineered) protection
  • Implement procedural or administrative controls

To improve both process operations and plant safety, many operators are turning to advanced analytical tools, diagnostic devices and smarter field instrumentation to monitor critical operating variables, such as the presence and concentration of combustible gases; oxygen concentration in potentially explosive environments; flow rates and temperatures in pipelines and vessels, etc.

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Combined with today’s open digital communication protocols, expert software and advanced control systems, these diagnostic devices can improve the real-time monitoring and control of process operations; reduce the time needed for repair of various equipment components; simplify troubleshooting; minimize unscheduled downtime; and reduce the risk of dangerous conditions such as undetected equipment failures.

What is a Safety Integrated System?

The safety integrated system (SIS) is one of the latest systems being used in refineries and petrochemical sector to mitigate hazards and reduce the level of risk. TUV is a governing body that offers support during the complete lifecycle of a product from concept to development and testing to certification. The IEC 61508 safety standard defines safety as ‘freedom from unacceptable risk’.

Some layers of protection can be used to reduce unacceptable risk to an acceptable level. The amount of risk reduction for each layer is dependent on specific nature of the safety risk and the impact of the layer on the risk. Economic analysis should be used to determine the appropriate combination of layers for mitigating safety risks.

When SIS is required, one of the following should be determined:

  • Level of risk reduction assigned to SIS
  • Safety integrity level (SIL) of the SIS

Typically, a determination is made according to the requirements of the ANSI/ISA S84.01 or IEC 61508 standards during a process hazard analysis (PHA). A process demand is defined as the occurrence of a process deviation that causes an SIS to transition a process to safe state. SIL can be considered as the statistical representation of the availability of an SIS at the time of a process demand. It is the litmus test of acceptable SIS design and includes factors such as device integrity, diagnostics, systematic and common cause failures, testing, operation and maintenance.

Fail-safe PLC

In modern applications, a fail-safe programmable logic controller (PLC) is used as the core of SIS. Safety integrated PLC system is used for fire and gas applications, and improves plant operations by responding in a pre-determined manner.

The fire and gas logic is implemented in a fault-tolerant PLC control architecture that ensures that no single point of failure will cause an inadvertent action or prevent the fire and gas systems from sounding the alarm and taking appropriate actions. All system failure modes are designed so that loss of any component (CPU, I/O module, power supply, communications, etc) will not only ring an alarm but will also allow the system to isolate the failure and continue providing safe and uninterrupted operations.

National and international standards for safety and critical control systems require that companies document their systems as well as design, maintain, inspect, test and operate them in a safe manner. Fail-safe systems need to comply with the standards; procedures must be written for operating, testing and maintaining the system for the specific application. This requires the services of agencies affiliated to TUV that have the thorough process knowledge for documenting the system.

Applications for Fail-safe PLCs

The fail-safe PLC system is used in critical safety applications in refineries, petrochemical/chemical plants and other industrial processes. For example, in reactor and compressor units, plant trip signals – for pressure, product feed rates, expander pressure equalization and temperature – are monitored and shutdown actions taken in the event of an upset condition. Though traditional shutdown systems implemented with mechanical or electronic relays provide shutdown protection, these can cause dangerous, nuisance trips.

The fail-safe PLC system increases integrity, providing automatic detection and verification of field sensor integrity, integrated shutdown and control functionality, and direct connection to the supervisory data highway for continuous monitoring of safety critical functions. Some major applications areas include:

Boiler Flame Safety

Process steam boilers function as a critical component in most refinery and process applications. Protection of the boiler from upset conditions, safety interlock for normal startup and shutdown, and flame safety applications are combined into a single integrated PLC system. In traditional applications, these functions would be provided in individual, non-integrated components.

But with a fault-tolerant, fail-safe controller, the boiler operations staff can use a critical resource more productively while maintaining safety at or above the level of electromechanical systems.

Turbine Control Systems

The control and protection of gas or steam turbines require high integrity as well as safety. Continuous non-stop operation of the fault-tolerant controller provides turbine operator with maximum availability while maintaining equivalent levels of safety. Speed control as well as startup and shutdown sequencing are implemented in a single integrated system. Unscheduled outages are avoided by using hot-spare modules for the I/O. If a fault occurs in a module, a replacement module is automatically activated without operator intervention.

Offshore Fire and Gas Protection

The fire and gas detectors are located in the process area as well as the building blocks, so that in case of any possibility of gas leak or fire, the fire suppression systems act immediately. The application demands highly accurate control systems to take immediate action. These systems should also be failproof, so that breakdown of any component of the control system should not affect the timely action of emergency shutdown systems.

The operator should be intimated of a hazardous condition well in advance, so that operating personnel could vacate the area, which otherwise could harm life and property. The fail-safe PLC system enables this through on-line replacement of faulty modules. Faults in individual modules, field wiring and sensors are managed automatically through built-in diagnostics. Analog fire and gas detectors are connected directly to the controller, eliminating the need for trip amplifiers.

Traditional fire and gas panels can be replaced with a single integrated system, saving space while maintaining high levels of safety and reliability. To sum up, employing fail-safe PLC systems in critical applications is the need of the hour to avert risks to mankind and environment.

* First published in PROCESS India June 2014

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