Comprehensive protection includes inhibitory protection of the roof, upper belts of the tank wall in the gas-vapor space, cathodic protection of the bottom and lower belts of the VST. The central stand of the tank is also exposed to cathodic and inhibitory protection.
The cathodic protection is carried out by applying direct current from the cathodic protection station (CPS).The tank body is connected to the negative pole of the CPS and is the cathode, and the anode earthing devices are connected to the positive pole. The anode earthing devices are arranged vertically or horizontally and placed inside the tank symmetrically relative to the mounting hatch, where the potential measuring unit and the input box are located. The anode terminals are interconnected by a drainage cable in the junction box. The supply cable from the CPS is connected to the tank body in the inlet box. The effectiveness of cathodic protection is determined by the magnitude of the cathodic polarization (the displacement of the reservoir potential measured using a potential measuring unit and/or a steel measuring electrode).
Inhibition is carried out by spraying the inhibitor through nozzles evenly distributed over the surface of the tank roof. The inhibitor solution is supplied to the sprayers via a pressure inhibitor line through a filter. The effectiveness of inhibitory protection is determined gravimetrically by the witness samples installed in the control units. The control nodes are located at the points of the roof of the VST farthest from the spray nodes in an amount of at least three.
The proposed technology provides at least 90% cathodic protection of the inner surface of the tank from corrosion, and at least 80% inhibitory protection.
The scheme of integrated protection of VST from internal corrosion
4 – The mounting hatch. 5 – A potential measurement node. 6 – The introductory box.
7 – Anode terminals. 8 – Drainage cable. 9 –The junction box.
10 – The power cable. 11 – Steel reference electrode. 12 – Pressure inhibitor line.
13 –Filter. 14 – The control node. 15 – The limit level sensor. 16 – The spraying unit.