Passivation - A process which encourages the corrosion resistance of stainless steel by the forming of chrome oxide layer on the steel surface.
Payload - Mass of cargo that can be carried
PCD - Pitch Circle Diameter. Diameter of circle upon which bolt holes are positioned.
Periodic Tests - These are performed every 2.5 years, The depot ensures the tank is repaired and ready for the test and then certified test authorities (BV, DNV, LR etc) perform the tests In the 2.5 year test the tank is filled with air under pressure and checked to see it is completely airtight. The 5 year periodic test known as a Hydrostatic Test, must be performed every 5 years and in this test the tank is filled with water and checked for leaks. Hazardous cargoes may only be transported in tank containers which have a valid test certificate.
Pickling - A chemical or electrochemical method of removing mill scale, rust, metal oxides and similar coatings from steel.
Plate valve - A valve consisting of a plug which is moved in and out of its seating by an operated mechanism.
PSI (Ib/in2) - Pound per square inch (unit of pressure)
PTFE - Polytetrafluorethylene. A plastic used for seals and bearings. Has a wide temperature tolerance and is resistant to most chemicals.
P/V Valve - A combined pressure and vacuum-relief valve to protect the tank against excessive overpressure and vacuum.
PR Valve - A pressure relief valve to protect the tank against accidental overpressure. As the P/V valve (above) but without the vacuum relief facility.