Causes of puncture and leakage of blind flange of butane storage tank

Causes of puncture and leakage of blind flange of butane storage tank

There are 8 tanks in the butane plant of the natural gas treatment plant. The tank bottom is 5m high from the ground and of the same size, belonging to the pressure vessel. The storage medium is mixed butane (mixture of n-butane and isobutane) and isobutane (product), which was put into use in March 2008, and is a recycled pressure vessel. The tank parameters are as follows:

• Inner diameter of tank body: 2400mm;
• Tank length: 11542mm;
• Tank volume: 49.50m3;
• Design pressure: 1.58MPa;
• Blind flange size: DN50.

In October 2008, the whole plant was shut down for internal and external inspection of the tank and strength test. The test medium was water. After testing, drain the water in the tank from the drain outlet, and then purge and replace it with nitrogen before putting it into use. At 3:10 p.m. on December 6, 2008, it was found that the DN50 blind flange at the bottom of 3 # storage tank with isobutane medium was punctured and leaked. At the same time, it was found that 2 # isobutane storage tank and 6 # isobutane storage tank had different degrees of leakage at the same location. At that time, the situation on the scene was very critical. 2 # storage tank has 10 tons of isobutane, 3 # storage tank has 15 tons of isobutane, and 6 # storage tank has 12 tons of isobutane. The medium in the storage tank is in gas-liquid equilibrium. The three tanks all leak at the same blind flange at the bottom of the tank, and the leakage tends to increase. Fortunately, it was found that the leakage was timely and properly handled, avoiding a possible explosion accident caused by leakage.

1. Characteristics of storage medium in storage tank

1) See Table 1 and Table 2 for physical and chemical properties and detonation characteristics of n-butane and isobutane
Table.1 Physical and chemical properties

Physical and chemical properties			Melting point/°C	Boiling point/°C	Relative density water=1	Saturated vapor pressure/kPa	Heat of combustion/(kJ/mol)	Critical temperature/°C	Critical pressure/MPa	Solubility
Conductive butane			-159.6	11.8	0.56	160.09	2856.6	135	3.65	Slightly soluble water
N-Butane			-138.4	-0.5	0.56	106.39	2653	151.9	3.79	Slightly soluble water

Table.2 Combustion and explosion characteristics

Deflagration characteristic			Flammability	Flash point/°C	Ignition temperature/°C	Lower explosive limit/%	Upper explosion limit/%	Minimum ignition energy/MJ	Hazard characteristics
Conductive butane			Inflammable	-82.8	460	1.8	8.5	0.52	Flammable gas
N-Butane			Inflammable	-60	287	1.5	8.5	0.25	Flammable gas

2) The mixture of butane and air can form explosive mixture, which may cause combustion and explosion in case of heat source and open fire. The gas is heavier than air and can diffuse to a relatively far place at a lower place. It will lead to back combustion in case of open fire.

2. Cause analysis

The 8 pressure vessels in the tank farm of the butane plant are designed to use old tanks, and the internal structure remains unchanged during reuse. There are 6 flange connections at the bottom of the tank body, as shown in Figure 1.

Fig.1 Schematic diagram of storage tank

2.1 Blind flange puncture analysis

After the danger was eliminated, when the maintenance personnel removed the 2 # blind flange of the 3 leaking pressure vessels as shown in Figure 1, they immediately sprayed a lot of liquid and saw a little ice around the flange mouth, which was analyzed to be water. It was found that the original flange sealing gasket was made of 3mm thick asbestos pad, which had been soaked in ice water and used for a long time, and had decayed and adhered to the flange sealing surface. During the period from the reuse of 8 old pressure vessels in March 2008 to the leakage on December 6, 2008, no structural modification was carried out, no water was injected, and the flange gasket at the puncture point was not replaced.

2.2 Water content analysis

The strength test of 8 pressure vessels was conducted from November 13, 2018 to the end of November 15, 2018. The water in the tank was discharged through the drain outlet. The compressed air with pressure of 0.4MPa was used for purging, and then the 8 butane tanks were replaced with 0.2MPa nitrogen and put into use. Why is there a large amount of water discharged? After careful analysis, it turns out that there is a DN50 diameter and 2000mm high lead pipe (idle) in the tank at the flange opening of the No. 2 blind plate as shown in Figure 1. Where the water is, it will not be discharged from the drain pipe. In this case, it is easy to be neglected.

2.3 Ambient temperature change

Due to the water storage in the lead pipe in the tank for 23 days, there were 4 days of low temperature (ambient temperature – 5 ℃) from the time the tank was filled with water to the time it was started to run and before the leakage occurred. On December 6, 2008, the weather suddenly changed, with the lowest ambient temperature of – 8 ℃ and the highest temperature of about 6 ℃. Due to the repeated changes of the ambient temperature, the accumulated water in the pipe freezes at the flange connection and melts again when the temperature rises. As a result, the asbestos mat is soaked in water for 23 days, the strength is seriously damaged, and the leakage is easy to occur.

3. Solutions and preventive measures

Emergency stop in case of puncture and leakage. Carry out tank pouring, replacement and emergency treatment according to the operating procedures.

• 1) Completely drain the water in the lead pipe of the pressure vessel. Since the structures of the 8 pressure vessels are identical and there are lead pipes in the tanks, remove the bottom flanges at the lead pipes of the 8 pressure vessels and drain the water in the lead pipes of the pressure vessels.
• 2) Replace all metal wound gaskets. Carefully inspect all flange connection parts of the 8 pressure vessels. Only the 2 # blind plate as shown in Figure 1 of each pressure vessel is asbestos pad. Then remove the flange of the 8 pressure vessels, remove the original asbestos pad, and clean the flange sealing surface. The replacement of DN50 metal spiral wound gasket also complies with the provisions of GB 50160-92: all flammable and explosive pressure flange connections shall be sealed with metal spiral wound gasket.
• 3) Conduct strength and tightness test with compressed air. Due to the special internal structure of 8 pressure vessels, there are 3 lead pipes (original design) in the tank. For pressure test with water, the flange at the lead pipe of each pressure vessel needs to be removed for drainage. In addition, the height from the bottom of each pressure vessel to the ground is 5m, which is difficult to operate and heavy workload. According to the relevant provisions of the Supervision Regulation on Safety Technology of Pressure Vessels, compressed air is used for pressure test and nitrogen is used for replacement. The recurrence of such accidents was avoided.
• 4) It is suggested that during the next tank inspection, the lead pipe in the tank should be modified, and other parts should remain unchanged to prevent water storage.

4. Conclusion

After taking preventive measures, the occurrence of puncture and leakage at the flange connection was eliminated, and the explosion accident caused by leakage of pressure vessel was avoided. 8 pressure vessels have been in safe operation so far.

Source: China Blind Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)