Pressure
Pressure, abbreviated as p, is one of the most important concepts in piping design. Simply put, it is the force exerted perpendicular to the surface of an object and is expressed as force per unit area. Why is this important? The purpose of a pipe or pressure vessel is to keep pressure contained in a specific location. Differential pressure is also what causes fluids to move. It will always flow from high to low pressure. Pressure differences introduced by a pump or compressor will cause the fluid to flow.
Pressure is a scalar quantity having direction, some of these include area, density, energy, entropy, length, mass, power, speed, temperature, volume, and work.
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.
 Fullflow 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.

Kickin Pressure  Factory set low pressure point of the pressure switch that starts the compressor to repressurize the tank to a higher pressure.
 Kickout 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 Calculator

Pressure formula
\(\large{ p = \frac{F}{A} }\) 
Where:
Units  English  Metric 
\(\large{ p }\) = pressure  \(\large{\frac{lbf}{in^2}}\)  \(\large{Pa}\) 
\(\large{ A }\) = area  \(\large{ft^2}\)  \(\large{m^2}\) 
\(\large{ F }\) = force  \(\large{lbf}\)  \(\large{N}\) 
Related formulas
\(\large{ p = \rho \; g \; h }\)  (caused by liquid) 
\(\large{ p = K \; \frac{\Delta V}{V_i} }\)  (Bulk Modulus) 
\(\large{ p = \frac {Ca\; \rho\; U^2} {2} + p_v }\)  (Cavitation number) 
\(\large{ p = \frac{ n \; R* \; T }{ V } }\)  (Ideal Gas Law) 
\(\large{ p = \rho \; R \; T }\)  (Ideal Gas Law) 
\(\large{ p = \gamma \; \left( NPSH  \frac{ v^2 }{ 2 \; g } + \frac{ p_v }{ \gamma } \right) }\)  (Net Positive Suction Head) 
\(\large{ p_b = p_t + \rho \;g\; h }\)  (Pascal's law) 
\(\large{ p_t = \rho \; g\; h  p_b }\)  (Pascal's law) 
Where:
\(\large{ p }\) = pressure
\(\large{ K }\) = bulk modulus
\(\large{ Ca }\) = Cavitation number
\(\large{ U }\) = characteristic velocity
\(\large{ \rho }\) (Greek symbol rho) = density
\(\large{ g }\) = gravitational acceleration
\(\large{ h }\) = height of the liquid column
\(\large{ n }\) = mole
\(\large{ NPSH }\) = net positive suction head
\(\large{ p_b }\) = pressure at bottom of column
\(\large{ p_t }\) = pressure at top of column
\(\large{ R }\) = specific gas constant
\(\large{ \gamma }\) (Greek symbol gamma) = specific weight
\(\large{ T }\) = temperature
\(\large{ R* }\) = universal gas constant
\(\large{ p_v }\) = vapor pressure
\(\large{ v }\) = velocity
\(\large{ V }\) = volume
\(\large{ V_i }\) = initial volume
\(\large{ \Delta V }\) = volume differential
Typical Units
Some common units for pressure are as follows. For a more complete list, visit the pressure conversion page.
INTERNATIONAL SYSTEM OF UNITS, SI 
ENGLISH UNITS 

Bar  Inches of Mercury 
Centimeter of Mercury  Inches of Water 
Centimeter of Water  Kip per Foot^{2}, KSF 
Pascals, Pa  Kip per Inch^{2}, KSI 
Pound per Foot^{2}, PSF  
Pound per Inch^{2}, PSI 
Pressure Instruments
In piping design, pressure is measured several different ways. On a Piping & Instrumentation Diagram, the typical instruments are:
 Pressure Indicator  A pressure indicator is a pressure gauge. It is a mechanical device, that is calibrated to display a pressure.
 Pressure Transmitter or pressure indicating 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  A pressure switch is 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.
Pressure Temperature
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