CASE STUDY ON BOILER ACCIDENT

CHIA BAK KHIANGASST. EXECUTIVE ENGINEER

OSD, MOM

OVERVIEW OF UTILITY BOILERS

The boiler involved in the accident was a water tube unit with attached economiser and superheater. Total heating surface is 2203 sq.m and its design pressure is about 12 000 KPa and it can produce 160,000 kg/hr of steam.

The burner system can burn 8 different types of fuel using various nozzles.

The boiler is protected from overpressure by 2 PSVs at the steam drum and 1 PSV at the superheater.

OVERVIEW OF UTILITY BOILER

OVERVIEW OF BOILER CONTROL SYSTEM

Boiler Control System

Control the operation of valves and actuators

Monitor critical control

functions for safe operation of

boiler

Status of valves during normal light up

INTRODUCTIONOn 9 Dec 2000, at about 2:30am, three personnel were trying to re-start the boiler when an explosion occurred inside the furnace of the boiler.

The three personnel were badly injured with more than 50% 2nd degree burns on their bodies.

Two of them subsequently passed away later in the hospital:

• Deceased 1 - Technician/ Male / 23 yrs old

• Deceased 2 - Technician/ Female / 21 yrs old

PHOTOGRAPHS OF BOILER AFTER EXPLOSION

PHOTOGRAPHS OF BOILER AFTER EXPLOSION

PHOTOGRAPHS OF BOILER AFTER EXPLOSION

PHOTOGRAPHS OF BOILER AFTER EXPLOSION

PHOTOGRAPHS OF BOILER AFTER EXPLOSION

DESCRIPTION OF ACCIDENT

Boiler was on LPG firing. Night Order was given to light up diesel burner in Boiler.

The three personnel attempted to light up the diesel burner at about 12:30am. They made several attempts but were unsuccessful.

At 2:20am, they attempted to light up the diesel burner. However, the boiler experienced a master fuel trip which shut down the boiler totally.

While restarting the boiler on LPG, an explosion occurred.

OBSERVATIONS & FINDINGS

The boilers were in the commissioning stage at the time of the accident. Written operational procedures were available for cold and hot start-up of the boilers

Investigations revealed that the startup team encountered some difficulties in lighting the boiler with LPG some time back. To overcome the problem, they devised a temporary manual bypass method.

This bypass method was not the same as the operational procedures.

OBSERVATIONS & FINDINGSThe bypass method was used by the startup team as a temporary measure and they had stopped using it when a permanent solution was found to overcome the problem.

This method was only to be used by the startup team and no process technicians were instructed to use it.

Investigations revealed that process technicians were present working on this method with the startup team when it was used. This method had been used on several occasions by most of the process technicians

OBSERVATIONS & FINDINGS

Company Internal Safety Management System

Investigations revealed that the S.M.S. was not effectively implemented in the plant prior to the accident

• There was no Management of Change approval put up for management approval to use the temporary bypass method.

• The bypass method required the opening of 2 bypass valves. There was no Control of Defeat. procedures put up to the management for approval to remove the sealed wire on these valves.

OBSERVATIONS & FINDINGS

Company Internal Safety Management System

• Pre-Startup Safety Review (PSSR) was claimed to be carried out on the Boiler. But the PSSR document was not available for our review during the investigation.

• It was found that the bypass valves did not have any sealed wire when the startup team first implemented the bypass method. However, the team did not find out further why there was no sealed wire on these valves.

OBSERVATIONS & FINDINGSTraining & Experience

All technicians were given 8 months of orientation and training programme. This included technical and S.M.S. training.

The 2 deceased were Process Technicians but were not certified boiler attendants. The injured was a Supervisor and a certified 1st Class Steam Boiler Attendant.

The injured claimed that he was unaware of the bypass method and that it was being used on 9 Dec. He also felt that the training provided was insufficient for him to operate the boiler.

SITE FINDINGS

• Site investigations after the accident confirmed that the 2 bypass valves were 50% open. This confirmed that the bypass method was utilised to restart the boiler.

• Data records confirmed that the LPG control valve was about 66% open just before the explosion.

• The block valves before and after the control valve were fully open.

• A direct path was therefore established to allow LPG to enter the firebox, resulting in the explosion of the boiler.

Status of valves after accident

Fuel Flow Line after accident

1st Trip valve

100% closed

1st bypass valve

50% open

2nd bypass valve

50% open 2nd Trip valve

100% close

Control valve

66% open

Block valve

100% openBlock valve

100% open

Block valve

100% open

CAUSE OF ACCIDENT

• Use of temporary bypass method to restart the boiler after it had tripped.

• Two bypass valves of the trip valves were opened without first closing the two block valves, downstream of the LPG control valve

• Non-compliance of the company internal S.M.S.’s safety requirements:

- The use of unauthorised temporary bypass method

- The removal of sealed wire on the bypass valves.

CONCLUSION

LPG

FIRE

TRIANGLE

Air (Oxygen)

Hot Furnace Wall

LESSONS LEARNT

• All personnel who are operating boiler must follow Safe Operating Procedures.

• Authorisation must be obtained before introducing change to the boiler system or procedures.

• Ensure all personnel who are operating boiler received adequate training and supervision.

• Ensure proper documentation.

ACTIONS TAKENThe company had been instructed to carry out a thorough inspection and examination on the remaining Boiler and carry out necessary rectification works to restore the boiler to safe operating condition.

The company had also thoroughly reviewed the BMS and carried out rectification to improve the system.

They had also reviewed and audited their internal S.M.S. to identify weaknesses and to close such gaps.

THANK YOU

Don't Neglect Your Boilers Operation Just Because They Operate

Automatically