Control loops can be roughly divided into loops that respond relatively quickly (fast loops) and those loops that respond sluggishly (slow loops).
Most flow control loops are fast loops: when the control valve (or variable speed drive) is manipulated, the flowmeter usually senses the change almost immediately. Some gas flow control loops can have damped responses due to long pipe lengths, fluctuating gas pressure, large pipe size, and flowmeter technology. Despite this sluggishness, even these flow loops are usually the fastest loops in the process and can be considered to be fast. See Figure 11-1.