The attention in process control instrumentation today seems to focus upon the glamour of microprocessor based intelligent devices. However, when the lights go out (power failure) you need something absolutely dependable – and these are the direct connected, self-powered devices, such as gauges and regulators.
Although seen as more of a commodity item, these devices actually deserve serious attention, as often they are the ultimate safety devices in the plant. While today’s sophisticated digital devices are feature-rich and provide high flexibility that makes for easy selection, this is not the case with these basic devices. Significant differences in quality and features need to be correctly specified for each application.
For gauges, a quality issue on some items is the zero adjustment. Some gauges drive the pointer hard against a zero pin and look nicely calibrated at zero. However, when applying 2-3% of full scale pressure, the pointer does not budge off the stop. Gauges, such as the Ashcroft Duragauge, are provided with a live zero, which allows for accurate zero adjustment.
Next, isolation of pressure measuring devices from process fluids – using chemical seals. These are often used for corrosive fluids, but seals are also used for solids in suspension, crystallisation, viscosity, hygiene and for remote reading.
No one fill-fluid will meet the wide variety of conditions imposed by many installations. A fill-fluid should: have a specific gravity higher than the measured fluid; be inert to attack by, immiscible with, and not a solvent for, the measured fluid; be stable at high ambient temperatures; have a high boiling point, low vapour pressure; have a low pour or freeze point, low viscosity and flat viscosity curve; be non-toxic on contact or inhalation; and be free from obnoxious odours.
Another factor, often overlooked, is the correct matching of displacement volumes of the seal and the measuring instrument. This factor may limit the minimum range that an instrument can be calibrated to when coupled to a seal. Fill pressure is a further consideration, especially in compound ranges where a `can’ effect may happen, as the diaphragm moves from vacuum to positive pressure. Over-pressure, temperature extremes, material of diaphragm and housing, type of connection, physical locations and pressure range are all essential specifications for the selection of a suitable seal.
Moving on, regulators or self-operated control valves use the flowing media to control the position of the plug in the seat. There are two basic types of regulators:
* Direct operated regulators – a spring acts on one side of a diaphragm assembly holding the plug off the seat, and the process pressure acts on the opposite side of the diaphragm assembly to overcome the spring force and close the valve. In the case of pressure reducing valves, the process pressure is obtained from the downstream side of the valve, whilst in the back pressure regulator the process pressure is obtained from the upstream side of the valve.
Mechanical adjustment of the spring compression provides the control setting. Direct operated valves will exhibit offset or a droop characteristic. Spring selection is critical and the lowest range spring should be selected that can be adjusted to the required setpoint. Capacity is a trade-off between valve size and acceptable offset.
* Pressure loaded or pilot operated regulators – these use a pressure source instead of a pressure spring to set the main valve. Pressure loaded valves use a remote loading pressure as its setting. The pilot assisted valve uses a small direct actuated regulator powered from the line and mechanically set to the desired pressure. The pilot acts as a power amplifier and improves valve operating accuracy by eliminating the droop characteristics manifest in spring-loaded valves.
Regulators are used in many applications requiring reliable and low cost pressure control without dependency on auxiliary energy. Air supply regulators, steam pressure regulators, vacuum breakers, fuel oil regulators and the constant pressure regulators used in gas flow measurement systems substituting for pressure compensation, are just some examples.
* The Author is with KC Controls.