TURBOTIPS

PNEUMATIC VERSUS HYDRAULIC

T
he choice between pneumatic and circuit, but their operating costs can be matic networks and systems are usually
hydraulic actuators is dependent higher. Compressing atmospheric air to a required to develop stable responses in
upon the application. Both types nominal working pressure requires a lot of closed-loop systems. On the other hand,
have been used in different tur- power. Thus, pneumatic actuators are usu- there are no harmful shock waves analo-
bomachinery systems. ally expensive to operate. Air/pneumatic gous to the transients, which can occur in
Both types of actuator are methods of actuators, for example, are one of the cost- hydraulic systems, and pneumatic system
applying fluid power. Pneumatics uses an liest components to operate. On the other components last comparatively longer.
easily compressible gas such as air or a hand, air-driven actuators are usually qui- The low stiffness of pneumatic systems
suitable pure gas. Hydraulics uses a rela- eter and safer than their hydraulic coun- is another indicator of relatively long
tively incompressible liquid such as terparts. Lower noise is generated primar- response time. Resonances can occur
hydraulic oil. ily due to the power source (air compres- between the compressible gas and equiva-
Hydraulic applications commonly use sor system) being installed remotely from lent system inertias at lower frequencies.
pressures from 50 to 350 Barg and special- the turbomachinery. Even the relatively low speed of sound in
ized hydraulic systems may exceed 400 connecting lines contributes to response
Barg. Pneumatic systems, on the other delay, adding to the difficulty of closed-
hand, use operating pressures far lower
than those used by hydraulics (somewhere
Pneumatic systems loop stabilization. Fortunately, it is possible
to construct sophisticated pneumatic sys-
between 6 to 14 Barg).
Considering operating pressure range,
offer specific tems to achieve stabilization. Such means
of pneumatic stabilization are commer-
hydraulic systems are more compact with
a higher power-to-weight ratio. Pneumatic
benefits which make cially available and are important elements
of closed-loop pneumatic control systems.
systems, therefore, require larger actuators
than hydraulic systems for the same load
them better options Pneumatic system control is almost
exclusively accomplished by valves, which
(although in many cases pneumatic actua-
tors are still smaller than electrical ones).
than hydraulic control the flow from a pneumatic pressure
source. Low-pressure outlet ports should
Despite this, pneumatic systems offer spe-
cific benefits which make them better systems for certain be large enough to accommodate the high
volume of the expanded gas.
options for certain applications.
Simplicity of design and control is one applications. One issue with some pneumatic sys-
tems is caused by sluggish pneumatic
advantage of pneumatic systems. Mecha- actuators. This can limit the control band-
nisms and actuators are easily provided As air is compressible, a pneumatic width of actuated systems. Too often,
using standard pneumatic cylinders, valves, actuator cannot hold a load rigidly in place speed of response in pneumatic systems
actuators and other components. They in the way that a hydraulic actuator can. An is an issue and should be taken into
operate via simple controls (often very sim- air-driven device can use a combination of account for circuits and applications
ple on-off logic). Although there has been air for power and oil as the driving medium which require fast response.
some standardization of hydraulic systems, in order to overcome this problem. But this Overall, gas compressibility makes
they are usually designed and assembled combination adds cost to the circuit, and is pneumatic systems one or two orders of
for each application. only used in special applications. magnitude slower than hydraulic systems.
Pneumatic systems usually have long Further, pneumatic systems are usu- Thus, there are applications where
operating lives and require little mainte- ally cleaner than hydraulic systems. Leaks hydraulic systems were used instead of
nance. As the gas is compressible, equip- in an air circuit do not cause housekeep- pneumatic systems purely on the basis of
ment is less subject to shock damage. The ing problems. However, such leaks can be speed of response. ■
gas absorbs excessive force, whereas the expensive
liquid in hydraulic systems directly trans-
fers force. Although accumulators and Speed of response
other types of storage/intermediate vessels Pneumatic systems use compressible gas; Amin Almasi is a Char-
have been used in hydraulic systems; the for this reason, a pneumatic system is tered Professional Engi-
stored energy is limited. slower in responding to loads, especially neer in Australia and
Additionally, there is a very low chance sudden output loads, compared with U.K. (M.Sc. and B.Sc.
of fire in pneumatic systems compared to hydraulic systems. Similarly, torque or in mechanical engineer-
hydraulic oil. Gas viscosity is virtually force requires time and output motion to ing). He is a senior con-
negligible with pneumatic systems. build-up. Responses to sudden output loads sultant specializing in
How about first costs? The first cost of often show initial overshoot. rotating equipment, condition monitoring
an air/gas circuit is less than a hydraulic That’s why much more complex pneu- and reliability.
14 Turbomachinery International • September/October 2020 www.turbomachinerymag.com