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P&ID – Piping and Instrument Diagram:
A schematic diagram showing the position of valves, switches, pumps etc in a flow or process.

Package Power:
The total power absorbed by a compressor, including the power absorbed by all pumps, fans, coolers and the like. This is the figure to look for when buying a compressor.

Particulate Matter.
BS3045 defines particle sizes as follows.

* Grit greater than 75 microns
* Dust between 1 and 75 microns
* Fume smaller than 1 micron.

Pascal:
SI unit of pressure equal to one Newton per square metre. 1 Pa is a very small unit of pressure. For this reason kPa (1000 Pascal’s) is more often used. 1 psi = 6.8948 kPa. Also see bar.

Pipe:
This may seem a funny thing to define but new legislation requires it. The pipe is the tube which transmits compressed air (amongst other things) from A to B, but not across a boundary.

Pipeline:
The new legislation in the UK defines a pipeline as a pipe which conveys a liquid or gas across a boundary. This means that the pipe around a factory wall is not a pipeline. See Transportable Gas Containers Regulations.

Pipework:
The new legislation in the UK defines pipework as the whole pressurised system, excluding a couple of items such as air receivers and protective devices. See Transportable Gas Containers Regulations.

Planning Supervisor:
The CDM Regulations in the UK, requires the appointment of a person (or company) who is competent in the area of work applicable to that project. This appointment carries the title ‘Planning Supervisor’, and also carries the duty to advise the client upon specialist areas of Health & Safety. Major compressed air projects should engage a Planning Supervisor who has specialist compressed air expertise. See Construction (Design & Management) (Amendment) Regulations 2000.

PM10:
Particulate Matter below 10 microns in aerodynamic diameter.

Positive Displacement:
This describes reciprocating, rotary lobe, rotary vane and screw type compressors. This type of compressor does not ‘stall’ and potentially can reach pressures well in excess of the design pressure.

Pressure:
This is stated as absolute or gauge. It is most important to ensure any tender documents specify gauge rather than absolute, unless you know what you’re doing. It should also be the total of static head and velocity head and several different units of measurement are used throughout the EEC.

Pressure Dewpoint:
In some sales leaflets this is referred to as PDP and you’re supposed to guess what it means. The PDP is the temperature at which water will condense out of the pressurised compressed air.

Pressure Swing Dryer:
A desiccant dryer either heatless or heat reactivated. See dryers.

Pressure Equipment Directive 97/23/EC (PED)
This piece of legislation was formalised on 29th May 1997 and will be implemented in the United Kingdom via the Pressure Equipment Regulations 1999. Other EEC member states must introduce their own statutory legislation to enable the Directive to be implemented in their own National Countries.

Pressure Equipment Regulations 1999 (SI1999/2001).
PED is the EEC Directive, and the UK conduit for implementing the directive is the Pressure Equipment Regulations 1999. The regulation became law in the UK during November 1999, however a transitional period has been granted so that it will not become mandatory until May 2002. PER 1999 will require a supplier to document and demonstrate that ‘pressurised assemblies’ are safe, have been designed & manufactured according to sound engineering practice, and bear specified markings. This regulation may curtail the activities of many ‘plumbers’, because depending upon the size of the pipe and the pressure of the system, they will be required by law to have in place a formally certified ISO9000 Quality Control System administered by a UKAS accredited inspection body. Indeed, some even larger systems will require the use of ‘Notified Bodies’ to ensure the final safety of the assembly.

Pressure Systems Safety Regulations 2000:
This updates and replaces the old Pressure Systems and Transportable Gas Containers Regulations 1989 and irons out some of the irregularities. The regulations are mandatory and require the user to have a Written Scheme of Examination which has been signed by a Competent Person. This Person is not usually an employee of the user because conflict can arise. Can you imagine the Competent Person approaching his employer to tell him to shut the factory down because he is unhappy with a component within it?

Protective Device:
The new legislation in the UK defines a protective device as an item which keeps a system within it’s operating limits. This includes such things as pressure gauges, relief valves and the like.

PSIA:
Pounds per square inch, absolute. This is the pressure of a system measured from absolute zero. It is exactly 1 atmosphere less than PSIG.

PSIG:
Pounds per square inch, gauge. This is the pressure of a system which you would see displayed on a normal pressure gauge. It is the pressure of the system, over and above atmospheric pressure.

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