Flat bottomed tanks and storage systems

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The need for storage systems of cryogenic products occurs when it is required to accumulate cryogenic products, or to have an emergency reserve of cryogenic products. For example, to accumulate oxygen for the day of work in case of emergency cutoff of the ASU, or to accumulate nitrogen in order to use it in case of firefighting. In addition, when it is required to accumulate air separation cryogenic products for forwarding to the client, for example, to the client’s semitrailer.

Following main equipment is the part of the storage system of air separation cryogenic products:
1. Flat-bottomed tanks for the storage of cryogenic products, that consist of inner and external tanks with the evacuated perlite insulation, effective volume up to 5000m3, and operating pressure – 0,2 bar.
2. Vertical centrifugal cryogenic pumps (2 sets – one working pump and one standby pump) with 380В/50Hz/3F variable frequency drive.
3. The systems of ambient vaporizers consisting of: ambient vaporizers, steam heater, intermediate condensate trap, condensate pump, condensate tank.
4. Vaporization systems with steam heating consisting of: steam heater, intermediate condensate trap, condensate pump, condensate tank.
5. Cryogenic pipelines with evacuated perlite insulation.
6. Cryogenic pipeline fittings.
7. Automated control system without integration into ASU DCS or suitable for integration into ASU DCS. Automated control system also includes auxiliary systems: firefighting system, video control system, access control system.

Tank consists of two vessels. A cryogenic liquid is stored in the inner vessel made of stainless steel. The external vessel (casing), made of carbon steel, serves as a protective casing. The inner vessel and the casing are attached to the foundation with anchors connected to the reinforcement mesh of the foundation. Interstitial space – the thermally insulated space (TIS) between the inner vessel and the casing is filled with insulating material – perlite, which is traditionally used to isolate cryogenic equipment. Thermal insulation between the bottoms of the inner vessel and the casing is made of foam glass of necessary strength and thermal conductivity. In order to prevent atmospheric moisture from getting into the insulation, the TIS is blown with nitrogen gas from a collector located in the bottom of the tank casing. The tank is equipped with manholes for various purposes: – one manhole on the roof of the tank to access the inner vessel; – from two to eight manholes (depending on the capacity of the tank) on the roof of the tank for access to the TIS and backfilling of perlite; – one manhole in the side of the housing for access to the TIS. Platforms with fencing are installed on the top of the casing roof for maintenance of the manholes, valves and safety devices. Access to the site is provided by either a spiral staircase on the side wall of the casing, or a mine ladder, installed next to the tank on a separate foundation.
Estimated loss of liquid oxygen with a tank capacity of 1000 m3 is 0.144% per day. All flat steel and profile iron for the installation of external and internal tanks must undergo preliminary mechanical processing at the factory in the volume of the specified requirements, including the operations of edge cutting for welding.
The tank is constructed from prepared sheets, which are delivered to the site on the racks for transportation and storage, in the following order: first, an external tank is constructed, then foam glass insulation is laid, after which an internal tank is constructed.

Stainless steel anchor brackets of the inner tank are embedded in concrete.
Stainless steel anchor brackets of the external tank are embedded in concrete plate. Anchoring of the outer tank can be performed both from the inner side of the wall and from the outer side. The load distribution plate is made of reinforced cryogenic concrete, ensuring a stable distribution of loads on the thermal insulation layer. All layers of foam glass must be laid with layers of mineral felt. Mineral felt is also laid between a layer of concrete and foam glass.
The tank is equipped with instrumentation devices such as local level measurement sensors, nitrogen flow sensors in the insulation space, self-acting pressure sensor in the insulation space, insulation space purge pressure sensor, internal tank pressure sensor, internal tank wall temperature sensor, insulation space gas sensor, tank bottom gas sensor.
The outer surface of the external tank is prepared by sandblasting up to Sa 2½Rz 40–60 µm, then 2K-zinc epoxide or 2K-zinc ethyl silicate 80 µm NDFT is primed, then an intermediate layer of 2 µm NDFT 2K-epoxide and an outer layer of 2K-polyurethane 60 microns NDFT is applied.