© Bruce D. Bullough, Process Engineering Specialist Sebesta Blomberg & Associates, Inc. Roseville, MN, USA
Design of instrument air distribution systems is not a clear science, and there are myriad of ways to approach it. Even most experienced piping system designers take the easy way out and over design, because doing so is rarely costlyand instrument air delivery systems are rarely a major cost factor in the design or installation of the plant. Standard Assumptions and Terms • Pressure: Typical instrument air pressure in a chemical or manufacturing plant must be at 6 bar* (or about 90psi); maximum ratings are seldom over 8 bar (120 psi). There are other uses and facilities, such as hospitals, that may require instrument airpressure of up to 10 bar (150 psi) and have much tighter specifications for air quality. • Volume and volumetric flowrate units – o nmch is normal meters-cubed per hour, o scfm is standard cubic feet per minute. o fpm is feet per minute. Normal and Standard both refer to a volume at their respective standard reference conditions. • • • Air to Open (ATO) - air pressure (and the small volume to fillaccompanying chambers) are required to open the valve; Air to Close (ATC) - air pressure must be applied to close it; Fail Open (FO) - when air pressure drops below the minimum required, the valve opens (this normally requires a spring return, and is not necessarily related to ATO or ATC). Fail Close (FC) – when air pressure drops, the valve closes (spring close) Normally Open (or Closed) – this isthe position of the valve during normal operation (under some circumstances, it might be ATO, so it is FC).
Instrument air is often specified to have a maximum dew point of -40˚ C (-40˚ F), and is very seldom allowed any higher than -20˚ C (-4˚ F). Even in climates where ambient temperatures can never be so low, the expansion cooling effect of air leakage through a leak or in an equipmentbleed can result in condensate and ice buildup. There are many good reasons for very dry air; even very dry air from a compressor system has some moisture in it, and therefore can, over time, allow buildup of condensate in the system. Moisture/condensate buildup will result in sluggish performance and even damage to internal parts of actuators and devices. Background Typical instrument air devicesare pressure based, rarely flow based. However, each valve actuator, sensor, signal converter or other device requires some finite volume of air
at pressure to operate. For instance, control valves always require some flow, unless fully closed (or fully open, depending on type), and signal conditioners (including devices commonly called I to P converters, for instance) have a normal andvarying amount of air bleed. Some leakage or venting is inherent in many devices to assure pressure. There are also devices such as a "normally open/fail closed" valve, that require constant but finite flow availability during steady-state operation. Other uses of instrument quality air include pneumatic motors, pumps, chucks, and convenience outlets for tools and other irregularly used devices. Thefollowing Table lists some common instrumentation and devices that use instrument air, and the pressure and flow requirements that might be expected. Please note that these data are used here for example only, as the pressure and flow or volume requirements of any device are a function of its design (manufacturer and/or type), size, operating range, process use, and other factors. Specialty airusers or systems each have their own air consumption or bleed rates (including box folders, web tension controllers/dancers, air knives, punches, etc). This information is available from the supplier.
Table 1 Device type Signal Converter (I/P, P/I, E/P, etc.) Ball Valve Actuator Valve Positioner solenoid Air required 1.02 nmch 0.60 0.59 0.93 0.0012 1.21 21.4 0.0002 nmch nmch m3 / operation nmch...