Hydrocarbon Processing - October 2021 - 85

Valves, Pumps and
Turbomachinery
M. AGUIRRE, Cowan Dynamics,
Montreal, Quebec, Canada
Air-to-air pneumatic booster fail-safe system
for valves
In pneumatic valve automation systems,
it is common to hear the following
terms for valve positions: " fail open (FO), "
" fail close (FC) " and " fail last (FL) " . These
terms refer to the position the valve must
take in the event of a failure in the system.
In cases where the failure is loss of energy
used to move the valve actuator, what do
we use to move the valve to the desired
failure position? The most common systems
are via energy contained in pre-compressed
springs. These are known as single-acting
actuators. These systems have
limitations in the amount of energy they
can deliver and the space they occupy.
This article explains how pressurized
gas can be stored to be used as a backup
system for failure actions and how to do
this more efficiently with the use of pneumatic
pressure boosters.
Why is it needed to store energy in
a valve actuation system? Valves are
major elements of piping systems and
are often used as safety equipment in the
event of system failures. In a pneumatic
valve automation system, two main types
of failure can occur: loss of signal and
loss of energy. When there is a failure in
the signal to the controls, which is usually
an electrical signal of low voltage (e.g.,
an electrical signal to the solenoids), this
type of failure is known as a loss of signal.
When there is a failure in the source
of energy used to move the valve actuator
(which, in the case of pneumatic systems,
is the pressurized gas), this is known as a
loss of power or loss of energy.
In normal operation, the system receives
a signal to the controls, and a pressurized
gas-normally dry, clean air-is
sent to the actuator and any pilots present
in the system. FIG. 1 shows a standard
schematic of a pneumatic control system
for a double-acting actuator with an FC
failure action because of a loss of signal.
In the event of a loss of signal, air can
be routed to meet an FO or FC requirement,
or controls can be selected to trap
the compressed gas in the actuator to
achieve an FL valve position. In FIG. 1, the
system routes the air to make an FC in
case of a loss of signal.
What happens in cases where the
failure is the result of a loss of
power? If the energy used to move the
actuator is lost, then an FL position can be
achieved; however, if opening or closing
is needed after this failure action, there is
no energy to move the actuator and, therefore,
the valve. In these cases, an external
source of energy is needed. The most
common stored energy systems in valve
automation are pre-compressed springs,
also called single-acting actuators. When
using these actuators, if a power failure
occurs, the spring loses resistance and actuates
the valve to an open or closed position,
depending on the configuration.
Single-acting actuators (FIG. 2) have
limitations in terms of the amount of energy
they can store, which is proportional
to the spring elasticity constant and the
pre-compression. Therefore, for systems
where a high amount of energy is needed
(e.g., high-thrust or torque valves), springs
are no longer a practical option. In such
cases (e.g., for pneumatic systems), stored
pressurized fluids are used. The idea behind
these systems is to route pressurized
gas to move the actuator to an open
or closed position in the event of a loss of
power or plant air supply pressure loss.
This pressurized air is stored in tanks
or vessels. The size of these tanks is determined
by the amount of air that is needed
to be stored, which depends on the volume
of air displaced by the actuator, the
maximum air supply pressure and the
minimum pressure required to operate
the actuator. In systems where large volumes
of air need to be stored, the tanks to
store this air would also need to be large.
To solve this problem, pneumatic pressure
boosters can be used.
Pneumatic pressure boosters. Pneumatic
pressure boosters are 100% mechanical
and are used to boost pneumatic
pressure. When air at X pressure is introduced
to the booster, the pressure activates
the mechanical components of the
booster. Employing an internal piston,
the booster is sectioned into two volumes
(i.e., thrust and compression). The thrust
volume is the section that generates the
FIG. 1. Pneumatic control system: Doubleacting,
on-off, fail in last (FL) position.
FIG. 2. Single-acting actuator 14-in. tandem
mounted on a knife gate valve.
Hydrocarbon Processing | OCTOBER 2021 85
Hydrocarbon Processing - October 2021

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