Thief Hatch Static Protection
Thief Hatch Fire Prevention
Thief hatches are used to relieve excess pressure that can build up inside low-pressure, steel, and fiberglass tanks. The thief hatch hinge opens at a predetermined set pressure and, once the overpressure has dissipated, the cover reseats onto the base. When excessive vacuum builds within a storage tank, the spring-loaded pallet lifts, breaking the seal between the seat and pallet, allowing vapors to pass through the valve orifice, and relieving the vacuum buildup.
During a lightning event, the current can arc over the pallet and the seat if they happen to be separated at that moment. This will ignite fires in both steel and fiberglass storage tanks. Sudden separation of the thief hatch lid from the base can generate a static discharge if the lid and base are inadequately bonded. Very low or high temperatures, with resulting low humidity, also increase the accumulation of electrostatic charge both while it is being generated and during the normal relaxation period.
Flammable vapors can ignite from sparks caused by static discharge at the thief hatch causing fire and spills. A static discharge could occur when a pumper opens the thief hatch for maintenance operations resulting in physical injuries. Accumulated static on the pallet and seat can also arc over during hot and dry days, causing fires and explosions similar to lightning.