Third Edition

The loading/unloading fittings and the PRD (PRV
or combination PRD) will be mounted on a manway
cover or pressure plate and enclosed within the
protective housing. Some tank cars in refrigerant gas or
certain corrosive service may have a bottom outlet,
bottom washout, and a hinged and bolted manway cover
enclosed within a dome-shaped housing.
Unlike nonpressure tank cars, which may be loaded
or unloaded using an open system, pressure tank cars are
loaded and unloaded using a closed system; i.e., the tank
is not opened to the atmosphere during the process. The
manway cover or pressure plate is removed only for
maintenance purposes at a tank car repair facility.
Although rare, pressure tank cars may be equipped
with heater coil systems.
Typically, in addition to the PRDs, a pressure tank
car will have two liquid eduction lines running along the
tank’s longitudinal centerline with the valves facing the
ends with eduction pipes reaching to the bottom of the
tank and at least one vapor eduction line with the valve
facing the side of the tank car with an eduction pipe
extending only into the top of the tank.
The valves for liquid and vapor eduction lines are
commonly called angle valves (so called because product
flow through the valve is at a right angle to the flow
through the eduction line).
The angle valves may be either plug or quarter-turn
ball style valves. Instead of angle valves, some pressure
tank cars are equipped with ball valves designed for
vertical flow of the product.
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