Jargon Buster - P

An acronym for; piping and instrument diagram.
This is the process of making a material resistant to corrosion. It is achieved by the formation of surface film that prevents and inhibits any further corrosion. The surface film is normally a layer of oxide or nitride that is a few molecules thick.
An acronym which stands for Pitch Circle Diameter. It can be described as the diameter of the circle which passes through the centre of all the holes or bolts, on a valve, flange, or other type of fitting.
A European Union directive; Pressure Equipment Directive.
An indication of the acidity or alkalinity of a solution in units ranging from 0 (most acidic), to 7 (neutral), to 14 (most alkaline).
PID control
A form of control in which the proportional, integral and derivative actions are combined.
Pilot operated pressure reducing valve
A type of valve used to reduce the pressure of the medium going through it. It controls the downstream pressure to a set level. The main valve is either assisted or completely controlled by the operation of the pilot valve, which may itself be a small direct acting reducing valve. The pilot acts as to regulate the amount of opening of the main valve in a way that will maintain the flow at the desired level of pressure. Pilot operated valves provide very close accuracy of pressure control, are compact in design and are usually smaller than direct acting valves for the same duty. The pilot valve may be integral with the main valve or may be a separate unit suitable for remote pressure sensing. It can also be used as part of a complex system governed by central control. Because of the complexity of design, pilot operated valves require regular maintenance and clean working conditions.
It is a round tubular hollow section which is used to flow a medium through it. It can also be used for structural purposes. Pipe is specified in a number of different ways. These being: Internal diameter (ID), and a wall thickness, Nominal diameter and a wall thickness or Outside diameter (OD) and a wall thickness. Pipes can be constructed from a number of different materials.
Pipe fittings
These are items which are used for coupling and connecting pipe and equipment. These could include; bends, caps, couplings, elbows, flanges, nipples, plugs, reducers, and tees.
Plate heat exchanger
This is a type of heat exchanger which uses metal plates to transfer heat between the two mediums in it. The heat exchange surface area can easily be made very large when compared to conventional type heat exchangers (shell and tube type). This means a plate type heat exchanger is very compact in comparison. Plate heat exchangers can take to form of welded, braised or gasketed types.
An acronym for Programmable Logic Controller. This is a digital controller used for applications such as on/off control, timing, logic, counting and sequencing.
A type of pipe fitting. It is used on the end of a piece of pipe to close it off. It fits inside the pipe that it is being mated to.
Is a device into which a temperature sensor is fitted. The pocket allows the sensor to be removed from a system without leakage.
The force exerted per unit area by a medium. It can be expressed in many different units. For example; pounds per square inch (psi), or bar.
Pressure drop
The difference in upstream and downstream pressure of a medium flowing through a valve or orifice.
Pressure reducing valve
A type of valve which is installed where it is required to reduce from one level of pressure to another and to maintain the reduced pressure on the downstream side within limits, irrespective of fluctuations in the inlet pressure or change in flow demands the valve is automatic in operation.
Pressure retaining valve
The pressure retaining valve, also known as a surplus valve is used to maintain a level of pressure in the line upstream of the valve, the valve opening with rising upstream pressure. It is usually a reverse acting version of a pressure reducing valve.
This occurs in a steam shell boiler when a stable layer of foam above the water surface is formed caused by impurities in the water. This leads to carryover.
Psychrometric chart
A chart that shows the relationship between temperature, pressure, and moisture content of the air.
Polytetrafluoroethylene is a plastic material used for valve seats, diaphragms, gaskets, seals, packing, etc. PTFE is unique in being resistant to practically all fluids, has very low frictional properties and can be used at temperatures up to 250 °C. It can be used in its pure form but to improve its compression set properties can be filled with a proportion of relatively inert filler, such as glass.
An acronym for Polyvinyl chloride. It is a polymer of thermoplastic type. It is commonly used to make flexible hoses and tubing.