Analysis of rigid and flexible pipes for central drainage in crude oil storage tanks

The central drainage systems commonly used in traditional large domestic floating roof oil tanks are of the pivot type and the all-hose type. The crude oil tank farms of Lanzhou Petrochemical Company currently all use a fully hose-based central drainage system. This paper will discuss the feasibility of different drainage pipes in crude oil tank farms by analyzing two types of drainage systems. 　　2 Characteristics of Pivot-type and Fully Hose-type Central Drainage Systems 2.1 Pivot-type Central Drainage System This type of central drainage system uses four pivots in combination with steel pipes; the pipes inside the pivots are flexible short tubes wrapped with stainless steel wire on both the inner and outer surfaces, and there are reinforcing side plates on each side. 　　The continuous slope design of this structure ensures smooth flow, no retention of sediment, and stable operating trajectory. Since the system mainly consists of rigid straight-through fittings, it has a lesser impact on fluid retention compared to flexible hoses. 　　2.2 Fully hose-type central drainage system: The drainage pipes are made directly from hoses, which can be divided into two types: non-metallic hoses such as rubber, and metal hoses. The entire drainage system has only two flange connections, resulting in few potential leakage points; it is not affected by the drift of the floating disk, so the likelihood of leakage is low. The drainage system has a simple structure and operates reliably, exerting almost no force on the floating disk. However, when crude oil flows into or out of the storage tank, it creates vortices inside the tank. Due to the uncertainty of these vortices’ paths, once the hose is drawn towards the heating coil or the buoy support by these vortices, it can easily be scalded, aged, or crushed, thereby losing its ability to drain water. 　　2.3 Comparison of the two drainage systems The pivot-type central drainage system features stable spatial trajectory, low likelihood of collision with other structures inside the tank, minimal impact on fluid retention, and low cost. The all-hose drainage system, on the other hand, has fewer leakage points, is not affected by lateral forces from the floating pad, and has a longer service life. 　　3 Analysis of the actual application of the two drainage systems at Lanzhou Petrochemical Company The crude oil storage area of Lanzhou Petrochemical Company currently has 12 crude oil storage tanks, including 6 tanks with a capacity of 30,000 m3. These tanks are primarily used for the transfer and storage of certain types of heavy crude oil, and they are characterized by fewer operations related to receiving and dispatching oil, as well as infrequent transfers ； There are 6 storage tanks with a capacity of over 50,000 m3, which are used for the transfer and storage of crude oil; they undergo numerous oil receipt and dispatch operations as well as frequent transfers. This requires us to consider selecting a drainage system with a long service life and low failure rate, so as to minimize the impact of tank cleaning and maintenance on the device’s production. Taking these characteristics into account, the use of a fully hose-based drainage system allows for fewer sealing points and leakage points, thereby reducing the likelihood of leaks in the drainage pipes.

The French COFLE*P drainage pipes distributed by Beijing Langsi Century have no joints in their structure, offering a long service life.