Direct-operated regulators are the first choice unless the flow or accuracy requirements cannot be met.
Accuracy - Direct-operated regulators are much less accurate than pilot-operated regulators. Some direct-op products designed for low pressure settings (less than 1 psi) achieve high accuracy by taking advantage of boost. However, in general direct-operated regulators require 20%-40% offset from setpoint whereas pilot-operated regulators require 1%-5%.
Capacity - Pilot-operated regulators are available in larger sizes and therefore higher capacity. Because of their superior accuracy, essentially all of a pilot-operated regulator’s flow capability is usable whereas direct-operated regulator’s useful flow capacity is limited due to excessive droop.
Response Time - Direct-operated regulators have the fastest speed-of-response because changes in outlet pressure move the diaphragm which is tied directly to valve plug travel. Pilot-operated regulators go through the same chain of events but then loading pressure must change prior to repositioning the main valve. Loading-style pilot-ops are slightly faster than unloading-style because the main valve diaphragm senses outlet pressure giving the main valve a head start for outlet pressure changes.
Minimum Differential Requirement - Direct-ops don’t have a required minimum differential. They can continue controlling regardless of pressure differential whereas pilot-operated regulators require a pressure differential between their inlet and outlet in order to open the main valve. Unloading-style pilot-ops have small diaphragms which require higher differential pressures to fully open the main valve.
Cost/Maintenance - Direct-operated regulators are the least complex and most compact of the regulator styles making them the least expensive and easiest to maintain. Unloading-style pilot-ops are the next least costly because of their simplistic main valve design which merges the diaphragm and valve plug into one component, the boot.