Hydrocarbon Processing - August 2022 - 25

Special Focus Refining Technology
S. ARUMPANAYIL, Swagelok Company, Cleveland, Ohio
Choosing the proper regulator configuration
can reduce droop
Many factors go into selecting the proper
regulator for your system. One of those
factors is sensitivity to changes in outlet
pressure as downstream flow fluctuates,
which is often characterized as " droop " as
illustrated within a regulator's flow curve.
What is droop? Droop is defined as the
reduction of outlet pressure experienced
by pressure-reducing regulators as the
downstream flowrate increases. While
every pressure-reducing regulator will exhibit
some droop, external components
can be added to the regulator configuration
to help maintain reliable downstream
pressure and minimize droop.
Flow curves are a useful tool in understanding
the range of outlet pressures
a regulator will maintain based on various
system flowrates. They are created
through product testing and represent
the real performance of a regulator for a
given set of system parameters.
FIG. 1 is an example of a flow curve
with four different curves shown to demonstrate
how different pressure regulation
configurations can help to reduce
droop. The vertical axis displays outlet
pressure, with the horizontal axis showing
downstream flowrate. The flatter or
more horizontal the curve, the more the
regulator preserves consistent pressure.
For example, the purple curve-which is
a result of a configuration using a domeloaded
regulator and a pilot regulator with
an added feedback line to the pilot regulator-shows
the best performance. The far
right of each curve indicates that the pressure
will quickly decline toward zero when
the regulator is completely open. Under
these circumstances, the poppet is pushed
open toward its stroke limit. At this point,
the regulator acts more like a restricting
orifice than a pressure control device.
No matter how good your regulator is,
some droop will be experienced. However,
the goal is to flatten the curve as much
as possible. Selecting the proper regulator
configuration for your system will enable
you to achieve this goal.
Option A: A simple spring-loaded
regulator. The most common type of
pressure-reducing regulator is a springloaded
regulator (FIG. 2A). In this design,
a spring applies force on a sensing
element-either a diaphragm or a piston-which
moves the poppet closer to
(FIG. 2B) or farther away from the orifice.
FIG. 1. This chart illustrates different flow
curves using different pressure regulation
configurations: a simple spring-loaded regulator
(Option A-Baseline); a dome-loaded regulator
and a pilot regulator (Option B); a domeloaded
regulator and a pilot regulator with
an added feedback line to the dome-loaded
regulator (Option C); and a dome-loaded
regulator and a pilot regulator with an added
feedback line to the pilot regulator (Option D).
FIG. 2. A spring-loaded regulator (A), which uses a spring to control flow and pressure by
moving the poppet either closer to or farther away from the orifice (B), provides a baseline
(Option A) for the flow curve comparison.
Hydrocarbon Processing | AUGUST 2022 25

Hydrocarbon Processing - August 2022

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