Pressure Measurement for Industrial Applications

digital pressure gauge
Digital Pressure Gauge
Pressure measurement is critical in many processes and essential in many industries. There are many techniques used to measurement pressure and vacuum (negative pressure). Instruments used to measure pressure are called pressure gauges, switches and transmitters.

Fluid pressure is defined as the measure of force on a surface, per some unit of area, perpendicular to the surface. The standard unit of measurement for pressure measurement in the English system is PSI, or pounds per square inch. In countries that use the Metric system, the Pascal (Pa) or the Newton/meter (N/m2) is used.

Level Measurement

level measurement
Level Indicator
Level measurement is an important process measurement. In many processes, it is important to measure the level of various liquid and solid materials. Effective level measurement provides reliable and continuous operations by maintaining optimum material inventory, by maximizing plant availability, and the prevention of spillages and other process disturbances.

Level is measured at the interface between two material "phases", that is, between liquid and air or solid and air, or other different density materials. The different phases must be clearly separated. When liquids are thoroughly mixed together, level cannot be measured because there is no separation of phases. The interface must be definite in nature.

In the case of an open tank filled with water, the point at which water and air contact is the definite interface, or phase change. Another interface example would be between oil and water. In this example, the specific gravity of oil is less than water, so oil floats on top, but in separate form. Liquids that mix well together do not have a clear interface, so it is difficult to determine level of each.

Introduction to Electric Valve Actuators

electric actuator
Crane 44000 Series Electric Actuator
In many industrial control applications, a motorized valve can be used in place of a pneumatically operated valve. Motorized valves use electricity and an electric motor to operate a valve. The spindle of the valve is connected to the motorized (electric) actuator via linkages (brackets and couplings) or are directly mounted to the shaft of the valve. Valves most commonly operated by electric actuators are globe, gate ball or butterfly valves.

Electric actuators are widely used in control systems because they are easy to interface with control systems. Since the control signals are also electric, the same conduit or conduit paths can be used. Electricity is normally available and can be run over long distances, unlike fluid power (pneumatic) which requires pumps and compressors. Special construction and wiring options must be considered when working in hazardous location though.

Introduction to Actuators

pneumatic actuator
Pneumatic, quarter-turn
actuator with accessories

An actuator is a pneumatic, hydraulic or electrically operated device that supplies force and motion to open or close a valve.

In a typical control loop, the controller compares a signal parameter from the process to a desired set parameter (set point) and then provides some output which drives some control element, so that the process signal parameter eventually achieves the set point parameter. In most cases, the device that corrects the control element is an actuator. 

Actuators are manually operated, pneumatically operated, electrically operated or hydraulically operated. The primary function of any actuator is to adjust position as to control and regulate the process parameter through some type of control element, such as a valve, to regulate temperature, pressure or flow.

Introduction to Ball Valves

blaa valve
Trunnion type ball valve
Industrial ball valves are among the most cost-effective and most widely used type of industrial valve. A ball valve is a quarter-turn rotational motion valve, that uses a ball shaped disk to stop or start flow thorough the valve. When ball valve is opened, the ball rotates such that the hole through the ball is in line with the valve body inlet and outlet. If the valve is closed, the ball is rotated so that the hole is perpendicular to the flow openings of the valve body and the flow is stopped.

The basic design of a ball valve is similar to that of a plug valve. Instead of a plug though, a ball valve uses a spherical ball sandwiched between two sealing rings in a the valve body. This ball has a hole to allow the fluid to flow through. When the valve is closed, isolation is provided by the sealing rings and the solid side of the ball.