Triple modular redundancy provides safe shutdown

The Hidrojen Peroksit hydrogen peroxide plant at Bandirma in Turkey uses GE Fanuc Genius modular redundancy to provide control of the emergency shutdown system (ESD).

The Hidrojen Peroksit hydrogen peroxide plant at Bandirma in Turkey uses GE Fanuc Genius modular redundancy to provide control of the emergency shutdown system (ESD).

Such safety critical systems are vital in chemical plants where fire is a major hazard.

The current plant was built with safety as the key factor in its design.

It replaced a previous factory on another site that burned down during commissioning after ten years of erection work.

The great danger with hydrogen peroxide is the low ignition temperature of the working solution.

It can fall so low as to be self-combusting at ambient temperatures.

As well as building in ultra-critical Emergency Shutdown (ESD) systems new processes developed have enabled the ignition temperature of the working solutions to be significantly raised - in some cases from 37C up to around 70C.

Hydrogen peroxide is well established as an environmentally friendly deodorising and bleaching agent.

It is used in organic and inorganic processes, for textile and pulp bleaching, metal treatment, in cosmetic applications, as a catalyst in polymerisation reactions, in industrial waste treatment and in the control of bulking in waste water treatment.

Current world demand is around 1.8 million tons.

This is provided by three major companies.

The installation at Bandirma is currently capable of producing around 20,000 tons per annum.

There are two main steps involved in the manufacturing process before the final product is ready for storage and despatch.

First comes the organic section.

Here a working solution is made from three different chemicals.

Hydrogen is produced from natural gas and passes through the solution.

Air is then supplied to the solution so that oxygen is taken up by the hydrogen in solution to form hydrogen peroxide, H2O2.

The H2O2 is in solution in water and at this stage the concentration of H2O2 is about 30-40%.

Most industrial processes require solutions with concentrations of between 35 and 50%.

So the second stage of the process involves a distillation column.

At Hidrojen Peroksit the process produces an H2O2 concentration of around 70%, which is ready for storage.

The distillation process itself is potentially hazardous as the low concentration solution is heated on entering the column and eventually boils.

The vapour rises through trays where the distillates are drawn off at various stages.

The complexity and the high risk levels involved in the production process demanded high control system availability when the new production facility was designed.

Any extended interruptions in system availability have significant effects on the chain of production of the hydrogen peroxide.

Management at Hidrojen Peroksit selected the highest-level control systems available, to provide safety throughout the process.

Their control system selection criteria included: a requirement that any changes to the plant should not increase the risk, in fact they should reduce it; and that single system faults should not cause plant production interruptions.

The whole plant is controlled by a distributed control system (DCS) which handles 925 I/Os, and the GE Fanuc triple modular redundancy emergency shutdown system which handles 138 I/Os.

Critical points are shared by both systems via hardwired cables and duplicated each time.

The emergency shutdown system uses three GE Fanuc 90-70 PLCs whose CPUs are running identical software.

Each CPU is connected via a Genius bus controller to the same I/O system via a Genius bus.

this triple modular redundancy arrangement is an option from GE Fanuc's Genius modular reduncy (GMR) systems which offer a wide range of redundancy configurations.

The GMR system mainly controls the hydrogenation, oxidation and filtration units, which are the core parts of the organic section and the distillation unit.

Light and sound signals warn the operator of any malfunctions.

Emergency trip buttons installed on workstations are hardwired directly to the GMR system.

However, the control system itself takes responsibility of the plant in the case of any DCS failure, or trip condition.

For example, if the pressure or temperature rises above the normal operating range in the distillation column, the system will automatically close the inlet control valve.

When emergency shutdown conditions occur, the GMR system sets all the related equipment into "failsafe" mode and acts on the interlock conditions.

Thus the whole system shuts down in a safe and controlled manner.

According to the plant management, although the fault tolerant technology is inherently more expensive than traditional simplex controllers, the ongoing cost of ownership is greatly reduced.

There are fewer plant shutdowns, lower maintenance costs, and, should a major fault occur, then the system will shutdown in a controlled manner without the danger of a catastrophic fire as occurred at the previous plant.

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