On the origin of the “Regulations on Flanges for Extremely High, High, and Liquefied Petroleum Gas Media”

After spending a week in Haichuan, I noticed that the level of activity there is much lower than before; there are fewer people around, and even fewer who are willing to talk. Sigh! This phenomenon is not good. A bunch of information related to pipe flanges can be found through searches, but none of it explains their origin; one only knows what they are, but not why. Understanding the origin helps us better comprehend the original intent behind regulatory requirements. I hope everyone will speak up actively. Of course, I expect the technicians to provide plenty of support, and those who participate actively should be rewarded with points (it’s only since I haven’t been here for half a year that I realized how few points there are – they get used up in no time, haha)

It is said to be an explosion of a liquefied gas tank in Jilin

It is said that the explosion was caused by a leak at the flange seal of a liquid ammonia storage tank; it was to serve as a warning that this regulation was established.

This post was last edited by Douwan on 2017-10-30 09:17. You are right. It was an explosion accident caused by a leak in a liquefied gas tank. But it wasn’t Jilin; it happened in Xi’an on March 5, 1998.

This post was last edited by Douwan on 2017-10-30 at 10:08. At around 18:40 on March 5, 1998, a leak and explosion occurred in a gas storage tank at the Liquefied Petroleum Gas Management Office of Xi’an Gas Company; a second explosion took place about 10 minutes later. Two more intense explosions occurred at around 19:12 and 20:01. Flames shot into the air, and the mushroom clouds resulting from these explosions lasted for over 10 seconds, reaching heights of 150–200 meters. In particular, the last explosion was the most violent; the streets in Xi’an’s western suburbs were illuminated as if it were day, and 100,000 residents in the area became panicked and rushed to flee their homes. The explosion killed 11 people (7 firefighters and 4 station staff), and 1 person is missing. 34 people were injured, most of those with burns becoming disabled for life ； The economic losses are enormous. 　　I. Course of the accident: At around 3 p.m. on March 5, the wife of Luo Limin, a temporary worker at Xi’an Gas Company’s liquefied petroleum gas management department, suddenly noticed that there was a leak at the bottom of the large spherical tank No. 1, which was used to store liquefied petroleum gas. Liquefied gas was spraying out from the ball valve with a hissing sound; the white liquid quickly vaporized once it came out. Frightened, the woman dropped her broom and ran to the duty room at the facility to report the incident. The management promptly organized personnel to carry out emergency repairs. The leaking No. 11 large gas tank had a volume of 400 m3; the high pressure caused the liquefied gas inside the tank to burst out through the damaged ball valve. There was no wind at that time, and the concentration of dangerous gases at the scene kept increasing. The staff used over 30 cotton quilts to seal the ball valve, and then sprayed water from fire hoses onto the quilts — the liquefied gas flowed out of the valve in liquid form, quickly vaporized; due to its very low temperature, the water sprayed on froze almost immediately ； The leakage has decreased. But the high pressure would from time to time break through the insulation covering the frozen area, leaving the workers at a loss; they could only use water guns to spray water and dilute the leaked liquefied gas. Since liquefied gas is denser than air, it sinks to the ground after being emitted, forming a mist layer about a foot high that grows thicker and rolls forward; its pungent odor can be detected from great distances. At 16:51, staff at the management office called 119 for help. 6 minutes later, a fire truck from Xi’an Fire Brigade Unit 7 arrived at the scene. Yang Zishu, the deputy squadron leader of Team 7, led 5 people to the scene. By that time, the ball valve at the bottom of Tank No. 11 had already broken. Yang Zishu immediately ordered everyone present to hand over their communication devices, cut off the power supply at the site, eliminate all sources of fire, and prohibited vehicles from passing by the area; water guns were used to disperse the gas. A few minutes later, seeing that the liquefied gas leak was severe and the situation was becoming difficult to control, Yang Zishu requested additional support from the fire department in the form of 5 more vehicles. Five or six minutes later, reinforcements arrived at the scene one after another. Due to the extremely low temperature of the liquefied gas after it was released, when firefighters went down into the tank bottom pool, condensation ice formed on the bottoms of their pants within ten to twenty seconds. Liquefied gas in a liquid state floated and rolled at a height of one or two feet above the ground, and the pool was covered with ice. By then, several firefighters had fallen ill from the poison and were carried out by those who arrived later. Since the members of Squadron 7 who arrived first did not have gas masks, most of them were poisoned and lost their combat capability, so they were replaced by Squadron 9 of the special fire brigade that had been newly deployed. At around 18:40, during the rescue efforts, the storage tank suddenly exploded and caught fire; the flames reached a height of five or six stories. Alarms of agony could be heard amidst the fire, and most of the people were killed or injured in this explosion. More than 30 people managed to escape from the sea of fire; many of them had no clothes left on their bodies, as the shock waves had ripped them off ； Some people’s remaining clothes had caught fire, and most of them ran outside and rolled on the ground to put out the flames. Some rescuers tried to rush through the gate to save people, but the intense fire made it impossible to get close; they could only pull those who had fallen at the entrance to safety from a nearby distance. Soon, no one’s cries could be heard amidst the sea of flames; those outside could only watch as the fire burned violently, shooting high into the sky⋯ About 10 minutes later, a second explosion occurred, with red and yellow flames accompanied by black smoke rising once again. At 19:12 and 20:01, the third and fourth explosions occurred respectively, generating mushroom clouds of fire that illuminated the western suburbs, each lasting for about ten seconds. After the explosion, 100,000 residents living in the vicinity began to flee in panic; many didn’t even have time to lock their doors or security gates, creating a highly chaotic situation. From the time of the explosion until the following morning, thousands of vehicles were stuck in traffic in the western suburbs, and the police established an evacuation zone with a radius of 3 km. As many as four to five hundred vehicles arrived at the scene to assist with firefighting and rescue efforts. Xi’an deployed all its firefighting resources, and backup vehicles from nearby areas such as Xianyang, Baoji, and Weinan also made their way to the scene. 　　On the morning of the second day, observations showed that two 400m3 spherical tanks at the accident site had burst; a large crack appeared on the top of the tanks, and gas rushed out through that crack, causing a massive fire to break out. To the east, there was a row of 10 small horizontal tanks, each with a capacity of 100 m3; they were only six or seven meters away from the damaged large tanks No. 11 and No. 12, and the tops of the 4 horizontal tanks located near those large tanks caught fire violently ； Two exploded 400m3 spherical tanks are lying at an angle to the east side of the horizontal tanks; another 6 horizontal tanks along with 2 even larger 1000m3 spherical tanks are under the threat of the flames and could experience secondary explosions at any time. The emergency rescue and disaster relief command center decided to immediately dispatch forces to the accident site to apply water spray cooling to the unexploded gas tanks in order to prevent the fire from spreading. At 23:20 that night, a total of 40 fire trucks from four fire departments in Xi’an, Xianyang, Baoji, and Weinan, along with over 300 firefighters, were deployed to fight the fire; each fire truck sprayed water dozens or even hundreds of times. The fire continued to burn for another 37 hours, and at 19:05 on March 7, it was completely extinguished. The fires in tanks 10 and 7 went out on their own—one due to the fuel running out, the other due to the wind—while the fires in tanks 8 and 9 were extinguished manually. According to the firefighters who were involved in the firefighting efforts, tank number 11 was the first to explode, followed by adjacent tank number 12. After the explosions, the two tanks tilted eastward rather than collapsing to the west, which was a great relief. At the time of the explosion and fire that day, the wind was blowing from west to east, which objectively protected the two 1000m3 large tanks on the west side. The area around the center of the explosion was home to numerous factories and residential zones; opposite the entrance to the liquefied gas management facility was Factory 3057. When the explosion occurred, a massive ball of fire descended from the sky, completely destroying the cotton processing workshop at that factory. Since it was a weekend and no one was working there, there were no casualties. Adjacent to it are Xi’an Daily Chemical Factory, Xi’an Coking Plant, and others; if the flames spread to these areas, they too could become hidden bombs, potentially causing even greater damage. 　　II. Analysis of the accident cause After investigation by the task force following the accident, the technical analysis and conclusions regarding the cause of the liquefied gas leak are as follows: (1) Based on the fact that the external shape of the drain valve was largely intact and its outer surface color remained normal, it can be inferred that this valve was not subjected to severe burning ； The liquid phase valve has been distorted and deformed, entirely as a result of severe high-temperature scorching and collisions. When LPG leaks in its liquid phase, endothermic vaporization occurs, causing the valve body to cool down; the drain valve and the connected flanges remain in their usual rust-colored appearance as an inevitable result of the leakage in a fire situation. 　　(2) On the upper and lower surfaces of the gasket on the discharge valve flange, as well as on the pipe flange and the upper flange sealing surface, there are areas where no contact exists (the lack of contact on the upper surface of the gasket is particularly severe), and the areas without contact are roughly the same in size, which creates the necessary conditions for leakage. 　　(3) The area where there was no seal and a liquefied petroleum gas leak occurred is in the due south direction, directly facing the location where the lower connection section of the liquid-phase valve (located south of the drain valve) exploded and was severely burned (in a direction slightly east of north). When fire broke out at the site of the liquefied petroleum gas emission, it created a fire environment that caused severe burning of the liquid-phase valve and its lower connecting pipes, which is consistent with the witnesses’ testimony that the leak was in the southern direction. 　　(4) The gasket on the flange of the sewage valve is about 650 mm above the ground, indicating that the location of the leakage is consistent with the emergency responders’ account that \"there were frostbites from above the knees to a height of 77 cm on the thighs, with obvious swelling due to frostbite; no frostbites were found below the knees.\" In summary, the flange gasket on the drain valve, due to uneven stress resulting from long-term operation, fails to fit perfectly against the flange sealing surface, losing its sealing function in certain areas (becoming defective), which in turn leads to the leakage of liquefied petroleum gas. 　　III. Lessons from the Accident and Preventive Measures There are two important lessons to be learned from this accident. One of the lessons is the failure to detect the problems with the discharge valve in a timely manner, and the failure to replace the flange gaskets promptly. There are several possible reasons for the failure to replace it in a timely manner; one of them is that the quality of the flange gasket is poor, and it has not reached the end of its useful life, which leads to carelessness ； Another possibility is poor management, resulting in failure to replace them when they reach their useful life. In any case, there should be precursors before an accident occurs, as the discharge valve is in constant use; changes in the valve body can be detected during use, or such changes may be noticed but not given enough attention. The second lesson is the method of sealing leaks after a liquefied gas spill. After detecting the liquefied gas leak, the management team used freezing as a method to seal it. The freezing method is suitable for low-pressure situations but not for high-pressure ones, and facts have confirmed this. So, what methods should be used to seal leaks under high-pressure conditions? The management had not prepared any corresponding rescue plans in advance, which led to severe casualties. Therefore, as part of preventive measures, it is necessary to strengthen safety management; worn parts that are prone to causing leakage accidents should be replaced in a timely manner, without hesitation ； The established rescue plan must include detailed procedures for sealing leaks under high-pressure conditions, and these procedures should be thorough and practical. The main preventive measures that should be taken include: the degradation of the physical properties of the gaskets, the fact that the valve bodies and pipes connected to the spherical tank are in a suspended state, as well as the periodic impacts and vibrations resulting from the opening and closing of the valve bodies. All these factors can cause changes in the stress conditions on various parts of the flange sealing surfaces and the bolts. To address this, it is necessary to first improve the design of the flange sealing surfaces and gaskets; secondly, replace the gaskets regularly and check for cracks on the surfaces of the bolts; thirdly, pay attention to the relative stability of attachments such as the pipes at the bottom of the spherical tank; and fourthly, avoid periodic impacts and vibrations.

This post was last edited by Douwan on 10/30/2017 at 10:06. This requirement was not present in the 89th version of the regulations; however, due to the accident that occurred in Xi’an, the General Administration for Quality Supervision, Inspection and Quarantine organized experts to conduct investigations. In order to prevent such accidents from happening again, it issued Notice No. 143 on Strengthening the Safety Supervision and Management of Liquefied Petroleum Gas stations. This notice emphasizes the need to “improve the safety measures and methods for handling liquefied petroleum gas,” which is why the use of high-diameter butt-welded flanges, metal wound gaskets, and high-strength bolts is required. It was precisely during the update of the 1999 version of the regulatory standards that the **Prime Minister demanded that this requirement be included in the safety technical specifications as a mandatory clause. Of course, this provision was continued in the 2009 version of the Fixed-Content Regulations; the considerations regarding the toxicity of the media saw an expansion in the scope of requirements ；

The 1999 version of the regulations specifies requirements only for containers whose medium is liquefied petroleum gas； The 09 edition of the regulations adds media with extreme and high hazard levels of toxicity, as well as highly permeable media with moderate hazard levels ； The 09 edition of the tolerance specifications emphasizes that special-grade bolts must be used ； And the description of ‘first flange sealing surface’ was removed ； --------------------------- Version 16 of the code emphasizes the use of special-grade studs, and adds that for certain special media that are incompatible with gaskets, the designer may determine the flange connection type based on the actual operating conditions.