A Thief Hatch is a closable aperture in a tank or vessel.  In the oilfield Thief Hatches are normally used on low pressure and atmospheric tanks. Thief Hatches are used to take samples of the tanks contents, determining the level of the tank and protect the tank from over pressure and excessive vacuum.

BS&B Oilfield Parts Express can supply immediately aluminum Thief Hatches, but other materials are available.These Thief Hatches are also low pressure/vacuum relief vents. 

Some of the applications that Thief Hatches are used for include:

Thief Hatches for Dump Tanks
Thief Hatches for Metering Tanks
Thief Hatches for Gauge Tanks
Thief Hatches for Feeder Tanks
Thief Hatches for Storage Tanks
Thief Hatches for Sour Service
Thief Hatches for Atmospheric Process Vessels