Pressure, abbreviated as \(P\) or \(p\), is one of the most important concepts in piping design. Pressure is a measure of the force exerted per unit area on a surface. It is defined as the force per unit area perpendicular to the surface over which the force is distributed.
Pressure Formula |
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\( p \;=\; \dfrac{ F }{ A } \) (Pressure) \( F \;=\; p \cdot A \) \( A \;=\; \dfrac{ F }{ p } \) |
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| Symbol | English | Metric |
| \( p \) = Pressure (psi) | \(lbf \;/\; in^2\) | \(Pa\) |
| \( F \) = Force Acting on the Surface | \(lbf\) | \(N\) |
| \( A \) = Surface Area | \(ft^2\) | \(m^2\) |
Pressure can be expressed as either absolute pressure or gauge pressure. Absolute pressure is the total pressure measured relative to a perfect vacuum, while gauge pressure is the difference between the absolute pressure and the ambient pressure of the surrounding environment. Pressure is a scalar quantity having magnitude and is independent of any specific direction.
Pressure Types
Absolute Pressure - The pressure referenced to an absolute vacuum.
Atmospheric Pressure - The pressure exerted upon the earth's surface by the air because of the gravitational attraction of the earth.
Back Pressure - The pressure held back on the upstream side of a system.
Breakout Pressure - The minimum pressure which starts moving an actuator.
Cracking Pressure - The pressure at which the fluid starts to leak through the valve before the set pressure is reached.
Critical Pressure - The highest temperature at which well defined liquid and vapor states exist. It may be defined as the highest temperature at which it is possible to liquefy a gas by pressure alone.
Discharge Pressure - The total gas pressure (static plus velocity) at the discharge flange of the compressor. Velocity pressure usually is considered only with dynamic pressure.
Full-flow Pressure - The pressure at which a valve is wide open and passes its full flow.
Gauge Pressure - Measures pressure referenced to local atmospheric pressure and is vented to the atmosphere.
Inlet Pressure - Pressure at the intake to the compressor. Usually expressed as absolute pressure.
Kick-in Pressure - Factory set low pressure point of the pressure switch that starts the compressor to re-pressurize the tank to a higher pressure.
Kick-out Pressure - Factory set high pressure point of the pressure switch that stops the compressor from increasing the pressure in the tank above a certain level.
Negative Pressure - Pressure is normally positive, but negative pressure is when the enclosed pressure is lower than the area around it.
Operating Pressure (Working Pressure) - The pressure at which a system is normally operated.
Osmotic Pressure - The pressure created by the difference in concentration of the constituents on either side of the membrane, and this pressure drives the osmosis process.
Pilot Pressure - Auxiliary pressure used to actuate or control a component.
Pressure Differential - Used to describe the amount of pressure created by a pump or compressor. It also is used for pressure drop in a pipe, piece of equipment or an orifice plate.
Pressure Loss - The difference in pressure between two points, usually caused by friction resistance in the system,
Rated Pressure - The operating pressure which is recommended for a component or a system by the manufacturer.
Relative Pressure (Overpressure) - The difference between the process pressure and the prevailing atmospheric pressure.
Sealed Pressure - Measures pressure referenced to the prevailing atmospheric pressure hermetically sealed within the transducer.
Set Pressure - The point where a pressure relief valve (PRV) is set to start opening.
Static Pressure - The difference in pressure between the suction side and pressure side. The higher the static pressure or resistance, the more energy it takes to move the object.
Suction Pressure - The absolute pressure of the fluid at the inlet side of the pump.
Surge Pressure - The pressure changes caused in a system from a rapidly acceleration. The surge includes the span of the changes, from high to low.
System Pressure - The pressure which overcomes the total resistances in a system.
Pressure Instruments
In piping design, pressure is measured several different ways. On a Piping & Instrumentation Diagram, the typical instruments are:
Pressure Indicator - It is a mechanical device, that is calibrated to display a pressure.
Pressure Transmitter - This is used to display the pressure in the equipment and send an analog signal to a computer for futher processing. It might be used as an alarm in case the pressure gets outside normal operating conditions.
Pressure Switch - Used to send a digital signal (yes or no, 1 or 0) to a computer for an action to be performed. E.g. send an alarm, turn off a pump, etc.
