TABLE OF CONTENTS Pneumatic instruments and explosionproof electrical systems were used in hazardous locations in the early years. New ideas, such as intrinsic safety and sealed and purged electronic instruments, are now being used. Yet, many plants still use pneumatic instrumentation, mostly to move large control valves through an I/P transmitter.
With all the electronic and electrical controls in place or on their way, one can get rid of the pesky high maintenance air compressors, right? Wrong! The prime movers for most control valves, damper drives, and fan louvers are still pneumatically operated. They are likely to be so for some time. Many current-to-pneumatic (I/P) and pneumatic-to-current (P/I) transducers are still in place, and many pneumatic control loops are still in service. Compressed air as a control necessity is going to be around for a while.
The two worst enemies of compressed air for control service are moisture and oil. Moisture is always a potential problem because the cooling of compressed air below its dew point causes moisture to condense. Oil can be dealt with more easily because of nonlubricated compressors. Precautions must still be taken to condition compressed air to be of instrument quality.
Types of compressors found in service for instrument air are rotary vane, rotary liquid ring, diaphragm, rotary screw, and centrifugal. Some reciprocating oiled piston compressors are still in service, but it would not be advisable to start a new installation with one because of the added concern for oil.
Compressor intakes should be well...
