# Pressure

Written by Jerry Ratzlaff on . Posted in Classical Mechanics 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

• Back Pressure  -  The pressure held back on the upstream side of a system.
• Breakout Pressure  -  The minimum pressure which starts moving an actuator.
• 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 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 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 Foot2, KSF
Pascals, Pa Kip per Inch2, KSI
Pound per Foot2, PSF
Pound per Inch2, 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 