Chemical Engineering June 2020 - 30

110
30% open
95% open
100
90
80
700
100
200
Length, ft
Figure 4. The graph shows the pressure profiles for two cases, one in which a valve is more
open (dashed line) and one with the valve more closed (solid line). The pressure drop across the
valve is higher with the more closed valve, but the starting and ending pressures are the same
nying noise levels.
valves are a source of pressure loss in the
system. this gives engineers a solid foundation
to design, and they can use that to
their advantage to throttle flow. in most processes,
there are two main categories of
pressure control valves: pressure-reducing
and pressure-sustaining. relief valves are a
common component in gas handling, but
they are meant more for safety precautions
than for controlling pressure. pressure-reducing
valves control the pressure downstream,
while pressure-sustaining valves
control pressure upstream.
there is a unique aspect to this type of
control. many control valves operate on a
pneumatic or hydraulic pressure signal from
the controller for actuation. in this special
case, pressure is both the controlled variable
and the actuating signal, allowing design for
a self-actuated pressure control valve. these
do not go through a typical control loop with
an outside signal to a controller. in industry,
self-actuated pressure control valves and
pressure regulators are used synonymously.
not all pressure-control valves are self-actuating,
but it is helpful to understand the potential
for process simplification.
pressure-reducing valves exercise control
by closing to create more frictional loss and
pressure drop, or by opening to do the opposite.
a pressure-sustaining valve will operate
similarly (by opening and closing), but
for controlling the upstream pressure. many
people understand pressure-reducing valves
because valves are said to cause pressure
drop. but the flipside to that pressure drop is
that a higher pressure is sustained upstream.
the same opening and closing can achieve
both goals.
this is an easier concept to grasp when
30
300
400
500
looking at an example
of flow going
from a 100-psig tank
to an 80-psig tank,
with a valve directly
in the middle of a
500-ft line. figure 4
shows the pressure
profile of two cases.
when a valve is in a
more closed position,
there is more pressure
drop across the
valve; there is less
pressure drop with an
open valve. however,
the flow from tank to
tank will always have
a profile going from 100 psig to 80 psig, no
matter what losses are present in the line.
by closing a valve, you both decrease
the downstream pressure and increase the
upstream pressure. it is up to the engineer
to decide what is the controlled variable of
interest. don't forget that there will be less
flow in the scenario with a more closed valve;
there is more resistance in that line. when
you see a pressure profile such as this in
a gas-handling process, also keep in mind
that density, velocity, volumetric flowrate and
temperature are also changing down the
line, even though mass flowrate is constant.
the other common type of pressure valve
is a relief valve. this type of valve does not
typically operate as part of a control loop.
however, it is not unheard of for relief valves
to be actuated by an outside controller communicating
with a separate part of the process.
most often, pressure-relief valves are
mechanically operated valves that crack
open when the upstream pressure is past
a set threshold. this valve relieves an upstream
pressure, but it is not meant to control
the pressure to a certain setpoint.
relief valves are used more as safety devices
in gas handling processes. with the
multi-faceted coupling of parameters in compressible
flow, pressure can change for many
different reasons. this extra complication
makes controlling pressure a more difficult
task, and thus the common remedy to pop
open valves when pressure gets too high.
the traditional relief valve operates on a
pressure differential across the valve. if the
difference is greater than the setpoint, the
valve will crack to relieve upstream pressure.
a balanced relief valve is the kind that operates
only on the upstream pressure, not a
differential. it has means of cushioning the
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Static pressure (psig)

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