eNatech - Natech Accident

Natech Accident

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Type

Natech Accident

Date

2009/07/23

Time

01:12

Natural Hazard

Lightning in Ibbenbüren, North Rhine-Westphalia, Germany, Germany, 2009

Site

Site 14, Germany

Status

Published

Units Involved

Name
Cell room
Type
Process: Other
Description
The cell room contains the low height (a few tens of cm) and large surface (several tens of m2) cells, inside which the electrolysis takes place.

A 3 mm thick layer of mercury flows on the slightly inclined bottom of the cells and acts as a cathode, on which the brine (NaCl) previously purified (removal of carbonates, sulphates, calcium, manganese ions and metals traces) runs at a speed of 1 m / s.

The anodes made of titanium coated with ruthenium and titanium oxides, are arranged parallel to the surface of mercury at a distance less than 5 mm.

Chlorine gas is directly formed at the anode by redox at high temperature (T = 80°C) and in acid medium (pH = 4) to avoid chlorine disproportionation.
Name
Chlorine processing
Type
Process: Other
Description
The hot and wet chlorine gas produced in the cells is fed into a collecting pipe leading from the cell room to the primary chlorine processing, which comprises wet gas cooling and low-pressure compression. These tanks are directly in front of the cell room.

Further chlorine processing takes place in an adjacent building.
Name
Emergency power generator (Stand by set)
Type
Other: Other
Description
The 110 kV voltage of the external electricity supplier is transformed to 10 kV in a substation near the site and then led to the 10 kV station of the site.

The lower voltages required are produced in a series of step down transformers and are then distributed via several conduction rails.

Vital aggregates for preventing hazards are connected to the emergency conduction rail.

In case of voltage failure the stand by set is automatically started and switched to the emergency conduction rail.

During normal operation the emergency conductor rail is supported by the internal power distribution. If the network monitoring registers a voltage dip and classifies the dip as failure, the emergency power generator is started. On matching the connection conditions, the emergency power generator connects to the emergency conductor rail and supports the units with power.

After return of the power, there is an automatic synchronized changeover to the normal power supply.

Event Sequences

Name

Blockade of the power supply

Unit

3. Emergency power generator (Stand by set)

Description

A voltage dip was registered by the internal power network monitoring, the emergency switch was opened, and the emergency power generator was started.

The voltage dip had a very short duration, 30ms - 200 ms.

The internal network was restored after this short duration, during which interval the emergency power generator failed to connect to the emergency conductor rail.

The opened switch could not be closed, as the contradictory commands caused a blockade.

The whole emergency conductor rail had no power to support the units, while the internal network was available.

Initiating Event	Critical Event	Major Event
Lifeline: Loss of electrical power / short circuit At 01:12 (local time) a lightning in overhead line caused a voltage dip in the internal power network (30 ms - 200 ms) Contributing Factors Measure: Emergency shut off / safety valves: Unclear Due to the voltage dip, the emergency switch was opened and the emergency power generator was started. The internal network was restored after very short duration of the voltage dip (30ms - 200 ms), during which interval the emergency power generator failed to connect to the emergency conductor rail. Equipment: Blockage: The whole emergency conductor rail had no power to support the units, while the internal network was available. This led to contradictory commands that blocked the opened switch, which could not be closed.	-	Event Sequence: Break of the mercury surface covering the steel bottom of the cell The medium-pressure compressor of the chlorine processing unit failed, while the low-pressure compressor remained operating.

Name

Break of the mercury surface covering the steel bottom of the cell

Unit

1. Cell room

Description

Due to the blockade in the power supply, the medium-pressure compressor of the chlorine processing unit failed, while the low-pressure compressor remained operating. Due to this failure, the intensity for the electrolysis was automatically lowered to 6 kA.

The failure of the mercury pumps caused a break of the mercury surface covering the steel bottom of the cell, which is connected to the negative pole of the direct current.

As a result, hydrogen was directly formed in the cell at the bare steel cathode.

Initiating Event	Critical Event	Major Event
Event Sequence: Blockade of the power supply The medium-pressure compressor of the chlorine processing unit failed, while the low-pressure compressor remained operating.	-	Event Sequence: Explosion of the chlorine-hydrogen mix

Name

Explosion of the chlorine-hydrogen mix

Unit

2. Chlorine processing

Description

The hydrogen produced directly in the cell was drawn together with the chlorine gas into the collecting pipe to the chlorine processing unit, where it exploded in the low-pressure system.

Substances Involved

Name
Chlorine
CAS No
7782-50-5
Involved Quantity
0.5 ton
Name
Hydrogen
CAS No
1333-74-0

Initiating Event	Critical Event	Major Event
Event Sequence: Break of the mercury surface covering the steel bottom of the cell	-	Explosion: Other The probable sources of ignition were either an electrical sparking or electrical discharge in the cell or a spontaneous decomposition reaction due to the steam injector in the collecting pipe. Contributing Factors Measure: Instrument / control / monitoring devices: Inadequate The situation was complicated by the fact that some monitoring devices, which were not yet switched to uninterrupted power supply but were still connected to the emergency conductor rail, failed. These were measuring devices for hydrogen in the chlorine absorption unit, and for the chlorine concentration after the absorption unit. In addition, the 6kA circuit was not monitored by the potentiometer. As a consequence, the operator could not get a complete and confirmed overview of the plant status. This resulted in the built-up of a dangerous chlorine/hydrogen mixture in the chlorine processing system for about 30 minutes. Explosion: Deflagration After the first explosion in the collecting pipe, air was drawn in the low-pressure system by the compressor and caused subsequent deflagration and detonation. Dispersion: Substance in air Up to 500 kg of chlorine gas were released in the course of the accident. A small portion was released due to the failure of the chlorine absorption unit after 01:12 (local time), but most of the gas was released after the explosion via the ruptured tanks.

Emergency Response

Response Planning Activities

The major accident prevention policy with regard to power failure is based on the emergency power supply of the units necessary to prevent hazards.

These units are especially the mercury pumps and the chlorine absorption unit. The electrolysers are expected to be out of operation during power failure.

Response Teams and Equipment Involved

On-site systems (e.g. sprinkler, water cannon)
Local fire fighting team

Response to the Natech Event

Chronology of the accident upon alarm activation at 01:30 (local time):
01:38 - Arrival of the on call team “electrical workshop”
01:41 - Loud bang, devastations in the low-pressure system of the chlorine processing
01:45 – 02:00 - Electrical workshop team puts emergency power rail into operation, decouples intact and damaged units (chlorine low-pressure/ medium-pressure)
01:45 - Sprinkler system in operation, arrival of fire brigade
02:00 – 03:00 - Measurements of the fire brigade in the surrounding and at site. As a precaution, construction of a water shield
03:00 – 05.00 - Removal of chlorine from the different parts via absorption unit, shut down of plant

The spread of the gas was hindered by the sprinkler system and the water shield.

Consequences

On-site Injured

Human Health Impacts

One member of the staff inhaled chlorine for a short time.
One member was injured in the leg and hospitalised less than 24 h.

There are a number of people potentially at risk in the vicinity of the site.
The plant is located in an industrial estate which houses among others four chemical plants consuming products of the chlor-alkali-plant. Adjacent to the west there is a commercial area. The estate lies in rural setting alongside a channel. The shortest distance from the chlor-alkali plant to an inhabited area is about 750 m, to a school about 1100 m. A farmhouse is situated at a distance of 520 m.

Environmental Impacts

There were no environmental consequences according to the criteria of the European scale of industrial accidents.

Chlorine concentration measured by the fire brigade ranged between 0.1 and 1.5 ppm.

The AEGL-1 level for 30 minutes had been exceeded at two measuring locations.

The AEGL-2 level for 30 minutes was not exceeded.

On-site Material Losses

0.34 M USD

Down-time of the Unit / Installation

840 hour(s)

Economic Impacts

The whole low-pressure system of the chlorine processing was destroyed.

The property damage resulting from the accident in the plant resulted to approximately 237,000 euro.

Production losses lasted for 5 weeks.

Lessons Learned

Lessons Learned on Equipment

Recommendations of the expert survey report:
- Optimisation of the control technology for the emergency supply in order to exclude blockades.
- Design of the emergency power supply ensuring a safety integrity level (SIL) 2 according to EN IEC 61508/61511*.
- Alternative 1: a redundant stand by set with a monitoring device in SIL 2 quality.
- Alternative 2: abandonment of emergency power supply in the safety policy.
-Visualization of the switching status of the power supply, completion of connecting control devices to uninterrupted power supply.
-Similar accidents (e.g., France 1995) and explosions (Norway, Sweden, The Netherlands and France) highlight the lack of means for detecting hydrogen in the collected and processed chlorine – improvements in such measuring devices are warranted.

According to the expert, there may well be comparable power supply systems in other plants. He reckons the technical expertise concerning the power supply system drawn from this accident of general importance.

*Action taken: The design of the emergency power supply in SIL 2 quality was not realizable. Instead, the redundant stand-by set was implemented (alternative 1) but failed to work due to the very short voltage dip (30ms - 200 ms) that led to contradictory commands in the control system, resulting in a power blockade. Additionally, the lack of emergency power supply is regularly regarded in the risk analysis (alternative 2).

Lessons Learned on Organisational Aspects

- Necessity of training operators for exceptional situations. To achieve this aim, two strategies were discussed:
1) The expert recommended additional instructions.
2) The company emphasizes on the training of detailed understanding with the argument, that no instruction could comprise all possible hazards.

- Implementation of the expert’s recommendations by the competent authority.

Lessons Learned on Mitigation Measures

- Power failure should not be considered a binary event (voltage/ no voltage) in hazard and risk assessment studies.
- The consequences of very short voltage dips (30 ms – 200 ms) have to be considered in hazard and risk assessment studies.
- When hazards due to power failure are possible, the emergency power supply should be designed in a quality ensuring a safety integrity level (SIL) 2 according to EN IEC 61508/61511.
- Alternatively, the option of emergency power supply may be abandoned in the safety policy, requiring the operator to review the risk analysis, which should lead to different risk management solutions and to a review of existing plant design choices.

Lessons Learned on Emergency Response Aspects

-Alarm of the failure of the emergency power supply and the mercury pumps in SIL 2, emergency shutdown in SIL 2.

ID: 19, Created: Bogdan DORNEANU, 2013-08-07 15:30:19 – Last Updated: Kyriaki GKOKTSI, 2023-11-20 14:54:30

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