#insights

A very particular case in storm drainage networks is the drainage of structures associated with oil transformers. This type of transformer is characterized by having its magnetic circuit and windings completely submerged in oil, which is used as an insulating liquid.

Normally, transformers are installed outdoors, so that, in the event of rain and oil leakage simultaneously, the drainage system must be able to retain and prevent the oil from spilling into the environment. Whether under normal operating conditions, in which oily particles will be carried by the normal operation of the installation, or under eventual conditions, in the event of a leak due to a breakdown, the drainage system must be able to retain the entire oil content in the transformer.

Eventual operation: breakdown situation

Oil transformers are installed on concrete foundations that form an anti-spill retention cube. This tank is sized so that, in the event of an oil spill due to a fault, the entire volume of oil contained in a transformer can be housed inside it. In addition, the tank is equipped, on its entire surface, with fire-breaking metal gratings and a layer of gravel to prevent the spread of fire in the event of a fire.

As they are installed outdoors, the retention tank has a connection to the storm drainage network to drain the rainwater that falls inside it.

The storm drainage pipe exits the tank towards the drainage network at a height H above the bottom of the tank. The volume defined by the surface of the tank and the height H is greater than the volume of oil stored by a transformer. In this way, there is a sufficient volume to accommodate a possible oil leak from the transformer.

To achieve oil retention, the water intake from the drainage pipe inside the tank is done through the bottom of the tank, and not at height H, by means of a vertical pipe section. This facilitates the drainage of rainwater, but not of the oil, which will accumulate in the tank floating on the rainwater due to the difference in density between the two liquids.

The removal of the oil spilled inside the tank must be carried out by an authorised waste manager.

Normal operation

The storm drainage network from the transformer spill tank is connected to a hydrocarbon separator. This equipment is installed in line with the same drainage network and buried at the depth of the pipeline. The purpose of the hydrocarbon separator is to retain small oil particles that may be carried towards the storm drainage network.

The hydrocarbon separator is designed in accordance with standard EN 858. The same standard establishes two types of separators: Class I and Class II. Each type is defined based on the maximum permissible residual oil content at the point of discharge, being:

• Class I (coalescence separator): 5 mg/l
• Class II (gravity separator): 10 mg/l

The combined operation of the anti-spill tank, equipped with a properly designed emptying system, and the hydrocarbon separator, located downstream in the drainage network, guarantee the minimum discharge of oils and hydrocarbons into the environment in the storm drainage system of the transformer station.

From the Water Technologies team at Sener, we are used to incorporating this type of design in our projects; in all types of electrical transformation substations, located in many of the projects in which we work.