Explosion in a gasometer

A major vapour cloud explosion (VCE) occurred at a gas storage tank (a gasometer) of a coking unit.
Several days before the incident, several gas detection alarms had frequently sounded. Also, the detectors of the tank oil sealing system had started to issue low-level alarms.
3 days before the incident, an inspection confirmed leaks in more than one corner of the tank wall. Despite all these signals, the management did not take measures and arranged to resume the operation of the gas tank.
On the day of the incident, all gas detection alarms were in over-range and as well as the monitoring points of the sealing oil level.
Later in the day, the gas tank exploded suddenly, causing the destruction of the gas tank body and damaging in the nearby benzene washing tower and the removing tower. Crude benzene leaked from the reflux tank in the crude benzene section on the north side of the tank and the waste lubricating oil in the trench on the north side of the company's power plant, causing a fire.

The INITIATING CAUSE was the deterioration of the gas sealing of the gasometer.

This was due to two factors: a decreased viscosity of the sealing oil and an inclination of the piston exceeding the process requirements. This caused the loss of the sealing oil and the dropping of the static pressure below the gas pressure in the gas tank.
This allowed the formation of a large amount of gas mixed with air, in the upper part of the gasometer, which was a semi-confined space.
IGNITION was possibly caused by non-explosion-proof electrical system installed on top of the aviation obstacle light or the video monitoring camera. An alternative ignition source could have been the guide wheel of the gas tank jammed and rubbed against the guide rail due to the inclination of the
Piston.

The hazard detection systems in place worked as designed, and the onsite personnel had duly notified the management of the abnormal situation, during the days before the explosion. There would have been enough time to inspect and take the required safety measures to avoid escalation.

gasometer

During several days before the explosion, severa gasalarms and low-level oil sealing alarms had sounded. An inspection had been performed, after which the oeprations had restarted as usual.

The detailed timeline of the facts before the accident:
15 days before the incident, the liquid level of the piston sealing oil in the gas tank had started to decrease and the 10 gas detection alarms in the gas tank frequently sounded.
8 days before, the sealing oil level had dropped far below the nominal level (200mm instead of the standard value of 280 ± 40 mm).
The abnormal situation had been reported to the company's management, who however did not take the required safety measures. The safety department issued a notice requiring a check on the cause of the flammable gas alarms.
3 days before the incident, an inspection of the facility found leaks in more than one corner of the tank wall. Nevertheless, the management did not take measures and arranged to resume the operation of the gas tank. One hour later, a full-scale alarm sounded on 3 monitoring points. In the following two days, also the carbon monoxide gas detection alarms continued to sound.
On the morning of the day of the incident, all 10 detection alarms in the gas tank were in the over-range and there was a 0-level alarm at 2 monitoring points of the sealing oil level.
Later in the day, the gas tank exploded suddenly, causing the destruction of the gas tank body and damaging in the nearby benzene washing tower and the removing tower. Crude benzene leaked from the reflux tank in the crude benzene section on the north side of the tank and the waste lubricating oil in the trench on the north side of the company's power plant, causing a fire.

DESCRIPTION OF THE GASOMETER (also called GASHOLDER)
The gas tank was a thin, oil-sealed, dry gas tank with a nominal volume of the gas tank was 50,000 m3(effective volume 48,791.16 m3), at a design pressure of 3500 Pa. Its later wall was a 20-sided steel structure.
The vertical movement of the roof was part of a piston moving up and down. The movement of the piston was actuated by the pressure inside the tank. The pressure was determined by the injection or removal of gas, so that the piston had the role of stabilisation of the pressure. The sealing between the vertical side of the piston and the tank wall was guaranteed by a oil sealing mechanism.

The SAFETY SYSTEMS of the GASOMETER consisted in a piston guide wheel, an anti-rotation device and a oil level alarm system. Accessories to this were an oil pump station, a mechanical tank capacity indicator, a piston inclination measuring device, automatic control and monitoring alarm facilities, an emission and emptying device, aviation obstruction beacons, a spotlight.

COKE OVEN GAS is highly flammable, toxic, colorless, and has a sulfurous odor. It has a lower explosive limit of about 4.4 percent and is less dense than air. U.S. Steel identified coke oven gas as an extremely hazardous substance. Exposure to high concentrations can cause asphyxiation due to the displacement of oxygen.
The composition o f the gas stored in the gashonder had a composition of Hydrogen (55.0–60.0%), Methane (23–27%), Carbon monoxide (5.0–8.0%), Nitrogen (3.0–7.0%), Ethane (2.0–4.0%), Carbon dioxide (1.5–3.0%), Oxygen (0.3–0.8%).

The incident caused 10 fatalities among workers,33 injuries, and a direct economic loss of 32 million yuan.

The scientific article used as a source calculated a TNT mas equivalent of 7323 kg, an equivalent death radius of 42m and an equivalent radius of serious injury of 70 m.

This is a case where the detections measures in place to identify a deviation from safety operative conditions worked as planned.

The heigh of the roof was lowering, and both the safety gas detectors and the sealing oil detectors sent several levels of alarm, able to inform the onsite workers of the existence of an abnormal situation. According to the source, also the information system aiming to involve the management of the plant worked. All this information was clearly indicating that a considerable amount of gas was leaking through the sealing and diffusing I the upper part of the tank. On top of that, an inspection had confirmed the occurrence of more leaks.

The management did not act on this evidence, failing in their leadership responsibilities. According to the source of this incident, there was a complete lack of safety awareness at all company’s levels, safety trainings being only a formality. Therefore, personnel and managers could not realise the consequences of the leak and cumulation of gases in the semi-confined space above the roof. The main responsibility, however, is with the company managers:
(1) They did not take the appropriate measures during the emergency, disregarding the clear signals and not understanding the consequences of this negligence
(2) They failed in designing and enforcing effective emergency procedures which could have guided the personnel actions in case of abnormal operations.
An additional failure, only hinted at by the source, was a failure in managing changes. Apparently, the type of sealing oil had been changed before the incident, and in particularly the oil viscosity had decreased.

The following technical corrective measures should be taken (it is however not known how the company re-designed the facility):
(1) The validity of the sealing system of a gas tank should be guaranteed in every moment. The quality of the sealing oil should be checked regularly, and oil not meeting the specifications should be replaced immediately to prevent leakage.
(2) Prompt and effective actions shall follow alarms. Procedures must be designed and enforced, to guide personnel and supervisors during emergencies.
(3) Effective risk assessment should be based on worst-case scenario, explosive zone identified, and the explosion-proof electrical equipment should be designed and installed there. The possibility of mechanical sparks should be eliminated, for example on the piston guide rail (lubrication control)/
(4) The piston movements should be better controlled and regular inspected, to prevent worn-down and loss of sealing due to deformation.