Petroleum Instrumentation NGT 160 Basics Instrumentation Chapter 4

Petroleum Instrumentation NGT 160 Basics Instrumentation Chapter 4 Pneumatic Automatic Controls “This product was funded by a grant awarded by the U. S. Department of Labor’s Employment and Training Administration. The product was created by the grantee and does not necessarily reflect the official position of the U. S. Department of Labor. The Department of Labor makes no guarantees, warranties, or assurances of any kind, express or implied, with respect to such information, including any information on linked sites and including, but not limited to, accuracy of the information or its completeness, timeliness, usefulness, adequacy, continued availability, or ownership. ” Unless otherwise specified, this work by Shale. NET U. S. is licensed under a Creative Commons Attribution 4. 0 International License.

Pneumatic Controls • Electronic controls, sensors, and measuring devices are significant developments that have advanced automation to a new level. • However, prior to electronic control mechanical and pneumatic controls were used. • We have already seen simple examples of mechanical controls. • Pneumatic controls are still used and learning about pneumatic controls leads to a better understanding of electronic controls.

Pneumatic Controls • Automatic controls perform self-correcting functions • Examples – cruise control and heating and cooling thermostats • On the pneumatic side we have pressure regulators for example • Pneumatic devices depend on pressure from an air supply. • The air supply must be held at a steady value or other words – the pressure must be regulated • Two types are used: weight-loaded and springloaded

Weight-loaded regulators • The diaphragm supports a properly sized weight which operates a double ported valve plug.

Spring-loaded regulators • A spring under compression replaces the weight. • These regulators are simple, light in weight, low maintenance and reliable.

Pneumatic Controllers (pg. 52 -55)

Valve Positioners (pg. 65 -67) • Valve positioners accurately position valves. • Likewise the force valves with sticky stems or other problems to assume a precise position. • An air relay is used to accomplish this. • A change in the air relay output pressure repositions the control valve plug. • In an air relay – a small volume of air actuates the air relay valve which in turn controls large volume air impulses to drive the actuator. • A small volume bellows is used. Nozzle back pressure actuates the bellows. Which operates a push rod that operates a special ball-plug valve.

Volume Booster Relays (pg 64) • There may be a considerable distance between a controller and the valve to be pneumatically operated • Air line resistance and capacity of long connecting lines can slow valve action to a considerable degree. • A booster relay installed at the valve location overcomes most of the response lag.

Valve Positioners (pg. 65 -67)