High and low pressure systems

 In pneumatic conveying, high pressure
typically means any pressure above about 15 lbf/in2 gauge. For systems
delivering materials to reception points at atmospheric pressure, 100
lbf/in gauge is typically the upper limit, due to the problems of air expansion.
Very much higher pressures (typically 300 to 400 lbf/in2) can be employed
if delivering materials to reception points maintained at pressure,
such as chemical reactors and fluidized bed combustion systems.