Pressure testing verifies the pressure capability or current pressure of a system. Pneumatic testing uses compressed gases and is limited to lower pressures or smaller systems due to the danger of explosions. Hydrostatic testing uses liquid and can introduce water into systems. Pressure is measured in pounds per square inch (psig) using a pressure gauge. Safety procedures are extensively documented, and the American Society of Mechanical Engineers provides training and certification programs.
A pressure test verifies the pressure capability or verifies the current pressure of a system. A pneumatic test is a type of pressure test. Common pneumatic measurement test equipment includes gauges that read pressure and a source of compressed gases. Pressure testing is essential in a number of industrial areas for safety reasons and for the proper functioning of the equipment.
In pressure testing, if the fluid used to pressurize the system is liquid, usually water, the test is called a hydrostatic test. A pneumatic test implies that the fluid used is a gas, usually air or an inert gas. In determining the pressure capability of a system, all ports in the system are closed except one through which fluid is added until the pressure rating for the system is reached, the pressure cannot be achieved due to leaks in the system or the system fails catastrophically by exploding.
A pneumatic test is inherently more dangerous than a hydrostatic test due to the higher energy content of a compressed gas and this type of testing is limited to lower pressures or smaller systems. Serious injury or death can occur from explosions during a pneumatic test. Safety procedures for pneumatic testing are extensively documented in technical references and codes. The American Society of Mechanical Engineers (ASME) prints the Boiler and Pressure Vessel Code (BPVC) and provides training and certification programs for engineers around the world.
A disadvantage of hydrostatic testing is the introduction of water into systems that must be water-free. For testing large, high-pressure systems, a low-pressure pneumatic test can be used to detect leaks, followed by a high-pressure hydrostatic test at the manufacturer’s machine shop prior to installation. Alternatively, methanol or other hydrocarbon liquid may be used in place of a pneumatic test. Vacuum testing is also limited to low pressures, but is less dangerous than pneumatic testing and is often used in laboratories to check for leaks in glassware installations.
A common example of measuring tire pressure is using a tire pressure gauge to measure the pressure in car tires. A mechanic also measures the compression on an automobile engine by running the vehicle and measuring the pressure at each of the pistons. Cryogenic systems, such as those used in surgery and refrigeration, are also often tested pneumatically.
To measure the pressure in systems with pneumatic testing, a pressure gauge with a deformable internal diaphragm is used. Pressure is the difference between system pressure and atmospheric pressure and is often stated in pounds per square inch, psig (kPa/cm2). In reactor design, where it may be necessary to know the working pressure from a kinetic or thermodynamic point of view, or the use of gas law calculations is required, the atmospheric pressure, 14.7 psi (101.3 kPa) must be added to gauge pressure to obtain total system pressure.
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