Failures of Sealing of Flanges on the Top of the Cryogenic Tank's Pump Well (Part One)

Abstract
Flanges on the top of pump wells of cryogenic tanks not only are subjected to periodic pressure and temperature changes during the normal operation of the cryogenic tank, but also the vibration load of the pump during operation. If the design is unreasonable, it may cause the sealing of the flange to fail and cause the cryogenic medium to leak at the sealing of the flange, resulting in partial cold brittleness cracking of the tank roof near the pump well casing and causing great harm. The temperature, pressure, load and other changes of the top flange of the pump well under various working conditions are analyzed; find out the reasons that may lead to the failure of the sealing of the flange's top, and proposes measures to avoid leakage at the flange's sealing surface; give advice for rationally designing the flanges and fasteners of the top pump well in this article.
 
The material at the top design is adopted for a large number of steel cryogenic tanks on the market every year based on safety. An immersed pump is provided at the bottom of the inner tank, and the cryogenic medium in the tank can be transported to outside the cryogenic tank.
 
Once the large flange on the top of the pump well leaks, even a small amount of leakage will seriously damage the cryogenic tank top, especially the non-continuous of tiny amounts of leakage. It may only take a few minutes to quickly atomize the leaked low-temperature medium. If a conventional catalytic combustion alarm is used, the alarm won't go off due to the concentration of the leaked air mass not exceeding the upper limit of the explosion limit, posing a hidden danger. In addition, most devices have a drip pan at the top flange of the pump well. However, the leaking air mass containing liquid droplets is blown to other places by the wind or there is a defect in the design structure. The low-temperature medium is not effectively avoided from dripping on the steel plate of the room temperature tank top, eventually causing low temperature cold brittle fracture. When a crack is found, the flange of the pump well seems to be fine. However, its sealing surface has failed but it is not easy to be noticed, posing a great accident hazard for subsequent damage to components of the equipment.
 
The causes of leakage caused by the failure of the flange's surface due to improper design are analyzed, and the problems that should be paid attention to and the measures to be taken in the design process are put forward.
 
1. Analysis of the working conditions of the top flange of the pump well
The pump well is a pressure vessel and is designed according to GB 150-2011 Pressure Vessels. It goes from the top of the tank to the bottom of the inner tank. The immersed pump is installed at the bottom. The top of the pump well extends out of the tank top plate and is sealed with a flange cover. Lateral medium outlets and instrument ports are installed at the top side (Figure 1). The medium in the pump well is the low-temperature flammable and explosive liquid medium in the cryogenic tank, such as LNG, ethylene, etc.

 
Figure 1 The schematic diagram of the structure at the top flange of the pump well

The nominal diameter of the top flange is generally 500 mm or 600 mm. The design pressure of the flange is different depending on the medium output destination, generally around 2MPa. The design temperature is the medium temperature. For example, the design temperature of LNG is -165℃, ethylene -105°C and propane -45°C. The vibration of the pump well caused by the operation of the immersed pump is usually not obvious. When the flow is small, the vibration of the pump will become serious. When it is working at the allowable flow value, the pump will surge and its vibration will increase.
 
Most of the discharge pumps in the cryogenic tank are operated intermittently, and each pump is often turned on and off. Therefore, when the cryogenic tank is in normal operation, the flange on the top of the pump well will undergo periodic pressure and temperature changes. When the immersed pump is turned on, it is subjected to low temperature and high pressure. When the immersed pump stops conveying, the top flange returns to the ambient temperature, and the gauge pressure maintains a few kilopascals. For example, for a common LNG cryogenic tank, the working temperature of the pump flange varies from -165 to 30°C, and the gauge pressure varies from 0.012 to 0.052MPa. At the same time, it bears the slight vibration load of the pump. In addition, the lateral outlet on the top of the pump well is a low-temperature pipeline, which causes the pump well to withstand greater pipeline stress.