• Practice and Analysis of DBB Valves for Tank Valves (Part Two)

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Practice and Analysis of DBB Valves for Tank Valves (Part Two)

  • Sep 24, 2022
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2.2 Closing of the tank root valve (DBB) and opening of the operating valve (gate valves)
According to the source of unloading, the pipeline for receiving oil for the tank usually has two or more channels. When the oil enters, it is necessary to close one gate valve and open the tank root valve, forming a situation where the front and rear are both operating valves, and there are still problems such as wear and maintenance due to frequent operation of the gate valve. In addition, the safety valve of the pipe gallery cannot be released into the tank, and the role of the light slab gate valve is weakened (only used for maintenance). This is not desirable. If this mode is not consistent with the original tank equipment’s location and operation mode, it will complicate the operation and management of the oil depot, including numbering rules of valves, post-maintenance, spare parts and safe discharge. The risk of operation is greater than that of the equipment. Attention should be paid to the function of the emergency valve. In addition to the sealing of the valve, the more important thing is whether the storage tank can be cut off quickly in the event of an accident. After practice, most electro-hydraulic valves use slab gate valves, which are relatively easy to achieve fast closing after improvement. The related products made in China can be closed within 10 seconds.
 
Even if electric actuators are used, the current electric gate valves of DN600 made in China can also be completely closed within 30 seconds; while the DBB valve is limited by the running trajectory; it is more suitable for the position of frequent daily operations, and it is not suitable to close quickly; most of the operation time is controlled within 1 minute.
 
2.3 The tank root valve and operation valve with DBB valve modes
In daily operation, airport aviation kerosene storage tanks will encounter operations such as empty tank oil entering control flow rates, reverse static pressure exhaust and other operations. However, the DBB valve is not suitable for the half-open mode due to the structure of the plug. Even the impact force of static crushing oil should not be underestimated. The gate valve is not limited, and it is easy to adjust the valve’s opening degree to achieve the purpose of controlling the flow rate. Especially in the electric drive mode, matching with the servo level gauge and PLC control system can monitor and adjust the flow rate of the tank in real-time.
 
3. The big risk of quality of the tank root valve
The weight of DBB (DN500) valve is generally in the range of 1.2 to 1.3 tons, and that of a light slab gate valve of the same size is 600 to 700 kg. The pressure-bearing risk of the tank wall cannot be ignored. In severe cases, it may cause fatigue and cracks in the fillet welding seam at the bottom of the tank, causing leakages of oil and environmental pollution and other consequences, causing major safety incidents and regional impacts.
 
During the design stage, special attention should be paid to the consideration of factors of the valve’s quality, and the corresponding steel plates, reinforcing rings and valve spring supports should be selected; spring supports with a certain safety margin should be selected to avoid accidents caused by factors such as overweight equipment.
 
During the construction stage, the steel plate should be accurately cut, and there should be enough edge plates around the storage tank to ensure the movable distance between the water retaining plate and the surrounding wall to avoid the deformation of the edge plate and the cracking of the bottom fillet welding seam due to the excessive distance. Affected by the gravity change of different liquid levels in the oil tank and the deformation of thermal expansion and contraction, the ring wall blocks the joint displacement and deformation of the water retaining plate and the bottom plate. After long-term accumulation, the edge plate of the tank bottom plate will be deformed. If the spring support does not work, the clearance of the tank bottom caused by the subsidence of the ring wall packing or the foundation subsidence will also cause deformation of the tank wall or bottom, and this risk should be avoided as much as possible.
 
During the construction of the storage tank, the welding deformation of the tank bottom plate is easy to occur in the welding and installation process, and it is difficult to avoid it, resulting in a certain displacement space between the tank bottom plate and the surface of the ring wall. Because the DBB tank root valve is not effectively supported, the heavy equipment weight is transmitted to the tank bottom through the tank wall, and the gap between the bottom plate and the surface of the ring wall is compressed for a long time; the oil cannot be rebounded to the original position after unloading. Other issues cannot be ignored.
 
For the storage tanks that have been built, attention should be paid to whether there is a too small distance between the water retaining plate and the ring wall. Find solutions timely to avoid further increases in risks. In terms of daily inspection, it should not be limited to leakages, but should pay close attention to the abnormal situation of the edge plate, pipeline and leakage inspection hole of the storage tank, and regularly test the effectiveness of the spring support.
 
4. Conclusion
Equipment procurement, engineering construction, production operation, equipment management, fixed assets, technological innovation and applications and experience have formed an ecological environment for the entire chain of safe production, and technology is crucial.