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Useful information on pressure terms

Many terms, abbreviations and acronyms are used to describe pressure and values can be quoted in a host of different units. This wide variation is partly down to historical or cultural differences, or a particular method of defining and measuring pressure is more convenient, intuitive and useful in some applications but not in others.

What is the SI System?

The SI system of units is the International System of Units (Système International) derived from the metric system and is based on the kilogram and the metre. It is widely accepted and used across the world. The basic unit of pressure is the pascal, defined as the pressure exerted by a force of one newton perpendicularly upon an area of one square metre.

In North America, however, the US Customary System is preferred. This is based on Imperial units such as the pound (lb) and inch (in) or foot (ft). The standard unit of pressure in this system is pound per square inch (PSI): the pressure resulting from a force of one pound applied to an area of one square inch. 1 PSI is approximately equal to 6895 Pa.

Table 1. Pressure units of the SI and USCS systems

Measurement System Base Units Standard Pressure Unit Abbreviation
SI kg, m, cm, mm, s Pascal Pa, N/m­²
US customary system
(UCS or USCS)
lb, ft, in, s Pounds per square inch PSI, lb/in²

 

How is Pressure Measured?

Pressure values can be stated in three ways:

  • Relative
    Most pressure measurements (gauge pressures) are made relative to ambient air pressure – the gauge shows a zero reading when exposed to atmospheric pressure.

  • Absolute
    An absolute pressure is referenced against a perfect vacuum, using an absolute scale, so it is equal to gauge pressure plus atmospheric pressure (Torr is an absolute unit).

  • Differential
    A differential pressure is the difference in pressure between two places in a system (Head values are differential pressures).

Sometimes, pressure units are appended with letters to show how the value has been measured. For example, in the USCS system, lbf/in2 (the ‘f’ stands for force) or psig (the ‘g’ stands for gauge) shows that the value is relative to ambient atmospheric pressure. This distinguishes it from an absolute pressure measurement (lba/in2, psia), which is relative to vacuum. Similar suffixes and notes are sometimes applied to SI units, for example 101 kPa (abs).

As the pascal is a very small unit, it is commonly quoted in vacuum applications. For specifying industrial pressures, the kilopascal is used when SI units are preferred (1000 kPa = 145 psi). From the original definition, other units can be substituted (g for kg; cm or mm for m) to produce a whole range of combinations such as gf/m², kgf/cm², and gf/mm².

What is an Atmosphere?

The standard ‘atmosphere’ (abbreviated to atm) is a convenient unit for measuring pressures. 1 atm is equal to 101.325 kPa or 14.7 psi, which corresponds to atmospheric pressure at mean sea level. In reality, atmospheric pressure varies quite widely with elevation, weather, temperature and humidity. For example, the atmospheric pressure in Denver, Colorado, is only approximately 12.1 psi.

The abbreviation ata denotes an absolute measurement of the total pressure of the system, including atmospheric pressure. For example, a water pressure of 3 ata consists of 1 atm of air pressure and 2 atm of water pressure.

The pressure exerted by a 10m column of fresh water is roughly equal to atmospheric pressure and this is the maximum height to which water can be raised by a pump using suction. In reality, the limit is only about 7-8m because of pump inefficiencies, frictional losses, elevation and temperature differences. This suction limit can only be overcome by pressurising the supply vessel or by using multiple pumps and intermediate reservoirs.

What is a Bar?

A bar is defined as 100,000 Pa (100 kPa). This is slightly lower than standard atmospheric pressure (101325 Pa). The bar is commonly used in weather forecasting and engineering. In vacuum measurement, pressures are typically given in millibar (mbar) although Torr or millimetre of mercury (mmHg) are also used (see below).

What is a Torr?

Atmospheric pressure was first measured by the Italian scientist, Evangelista Torricelli, using a mercury-filled glass tube. He found that atmospheric pressure could sustain a column of mercury of about 760mm. The extensive early use of mercury in manometers led to the widespread adoption of mmHg as a convenient unit of pressure. In North America, ‘inch of mercury’, inHg, is preferred. In honour of Torricelli’s work, a pressure of 1 mmHg became known as 1 Torr. These units continue to be used widely in many other scientific and engineering fields.

What is Head?

Historically, pumps were first used to raise water for irrigation or drainage purposes. It was important that the pump was capable of lifting the water from the lower to the higher level. The delivery height became known as the Head and, despite the vastly extended range of modern-day pumping applications, this term is still used to characterise rotodynamic pump performance. Head is specified as a height, in metres (m) or feet (ft), rather than as an actual pressure. Often it is discussed in two parts: Suction Head – the vertical lift from the source reservoir to the pump, and Discharge Head, the vertical lift from the pump to the point discharge. The following table lists some of the common terms used to describe head pressures in pumps.

Table 2. Definitions of terms used to describe Head values

Head Term Meaning
Suction head
(also static suction head)
The vertical distance between the liquid level in the supply tank and the centreline of the pump suction port when the liquid is above the pump. 
Static suction lift Vertical distance between the liquid level in the supply tank and the centreline of the pump suction port when the liquid is below the pump
Net positive suction head Reduction in Suction Head caused by losses in the system such as the liquid vapour pressure and frictional losses in the pipework
Total static head Vertical difference between the liquid level at discharge and the level in the supply tank
Friction head Pressure loss in pipework caused by friction of the fluid.
Occurs on both suction and discharge sides of a pump
Discharge head Discharge pressure a pump must develop to meet the requirements of the system
Static discharge head Pressure at the discharge port when the pump is not operating. This head or pressure is equal to the difference in elevation between the discharge port and the point of free discharge of the liquid
Total discharge head Sum of static discharge head and friction head (line loss in the discharge piping). This is often minor and Total Discharge Head is effectively the same as Discharge Head. 

 

What is NPSH?

NPSH (Net Positive Suction Head) is a measure of the pressure experienced by a fluid on the suction side of a centrifugal pump. It is used to avoid running a pump under conditions which favour cavitation. NPSH-R (NPSH Required) and NPSH-A (NPSH Available) are two key NPSH values:

  • NPSH-R is a pump property quoted by pump manufacturers as the suction pressure at which cavitation has already reduced pump performance by 3%.

  • NPSH-A is a system property calculated from the suction-side system configuration. It is essentially the suction-side pressure less the vapour pressure of the pumped fluid at that point.

To avoid cavitation, it is necessary to ensure that NPSH-A exceeds NPSH-R by a sufficient safety margin, for example: NPSH-A ³ NPSH-R + 0.5m. This margin depends on the type of pump and application and may be quoted as a ratio or a head difference.

What is NPIP?

Positive displacement pumps operate on completely different principles to centrifugal pumps. Fluid is transferred from inlet to discharge by repeatedly enclosing a fixed volume, with the aid of seals or valves, and moving it mechanically through the system.

Pumps of this type also require an inlet pressure greater than the vapour pressure of the fluid to avoid cavitation during the suction phase and this is discussed in terms of Net Positive Inlet Pressure (NPIP) in a similar manner to NPSH for centrifugal pumps. Whereas NPSH is measured in metres or feet, NPIP is measured in pressure units: Pa, psi, or bar. When converted to the same units, NPSH and NPIP are the same. The following formula can be used to convert between a head value (m) and pressure (bar):

where:                                                 h = 10.197 x (p/d)

or                                                        p = 0.0981 x h x d

h = head (m)
p = pressure (bar)
d = density of the fluid (kg/dm3)

Manufacturers may quote NPIP-R as the recommended inlet pressure and provide charts showing how it varies with pump speed. The available or actual inlet pressure on an operating system is termed NPIP-A.

Conversion Factors

It is often necessary to convert pressure values from one unit system to another. This can avoid any confusion or misunderstandings but is particularly important when feeding values into calculations. It is essential that all the values in an equation are in compatible units. Look out for discrepancies in absolute or relative values. When converting Head values to other pressure units with liquids other than water, it is necessary to take into account the specific gravity of the liquid.

Table 3. Conversion factors for commonly used pressure units.

 

 *1 Torr was originally the same as 1 mmHg. However, redefinitions of the two units have made them slightly different by a margin too small to show in this table.