By: Chris Pasquali; President & CEO of Factory Direct
Pipeline Products, Inc.
Gas/Liquid separators are pressure vessels designed to remove entrained
particles and droplets from gaseous processes.
Principle of Operation
Moving air, steam or other gas carrying particles of condensed vapor or
foreign matter are directed in a straight stream against a baffle such that its
flow is suddenly diverted. The moving steam/air/gas being much lighter
than the condensation, will flow around the baffle easily, however the heavier
particles of moisture and foreign matter, striking the baffle forcibly, are
stopped and fall to a common coalescing area out of the path of the gas flow.
A well designed separator must be properly proportioned to provide 99%
removal efficiency of particles and droplets >10 microns, meaning the baffle
must be of sufficient size and area to collect and carry away the moisture, oil
or solid matter in such a manner that it is not re-entrained into the moving
Gravity does most of the work, with the gas velocity forcing the separated
moisture to cling to the vessel surface where it slowly descends to the drain.
The baffle between the body of the separator and drain is referred to as a
vortex containment plate and it prevents re-entrainment and further agitation of
the retained fluid.
The physical size of the separator must accommodate the proper baffle size
in addition to accommodating the sudden change of direction of the gas without
causing significant friction or backpressure. Thus, a properly designed
gas/liquid separator has a substantial area at right angles to the flow of the
Physics Involved with our Gas/Liquid Separators
- Centrifugal Force The internal geometry of a properly sized vessel
results in centrifugal force sufficient to propel droplets and particles >10
microns to surfaces where they coalesce to a common drain.
Velocity Drop The cross-sectional volume of the vessel exceeds that
of the feed pipe resulting in reduced velocity and thus the mass of material >10
microns impinges on the internal surfaces, coalescing to a common drain.
Torturous Path The geometry between the inlet and outlet nozzles is
sufficiently complex such that droplets and particles >10 microns impinge on the
internal surfaces, coalescing to a common drain.
Our gas/liquid separators have an infinite turndown ratio, meaning they
maintain their efficiency even when the flow is drastically reduced. This
is beneficial in situations where the start-up or shutdown portion of a process
results in lower flow rates.
The reason we maintain our efficiency even though centrifugal force
decreases with a reduced flow rate, is that the resulting reduction in velocity
compensates accordingly for the reduced centrifugal force.
Most gas/liquid separators may be in service for several decades requiring
zero maintenance as they have no moving, serviceable or replaceable components.
They are a fully welded pressure vessel and considered self-cleaning due to the
continuous centrifugal swirling of the gas within the vessel. It is still
advisable to include an inspection port for visual confirmation of any internal
contamination, erosion or corrosion.
Not all separators employ the same design, most require maintenance for
internal mesh pads or fine channels. Likewise, these other designs lose
efficiency beyond a specified turndown ratio. You will also discover that
their physical size and cost increases to provide an equivalent pressure drop.
Drainage of Condensate
The condensate may be drained from the separator without loss of any
process gas using a float style drain trap. This mechanical method of
drainage simplifies installation and is extremely reliable.
Our float drain traps consist of a cast body containing a valve, valve
seat and hollow stainless steel ball (“float”). The float is connected to
the valve via a fulcrum and lever. The separated condensate drains into
the body of the drain trap, eventually causing the float to rise sufficiently
and lift the valve from its seat. The system pressure pushes against the
condensate and it drains through the drain trap outlet orifice.
Once the condensate level within the trap lowers sufficiently, the float
lowers and reseats the valve against its seat. Thus, condensate within the
float drain trap body acts as a buffer or seal, preventing any process gas from
escaping during drainage.
It is possible to use a manual or actuated valve in
conjunction with a sight gauge to achieve the same effect, however that is more
complicated and expensive for most applications.
Float drain traps have a drain port for occasional blow-down if
particulate is being removed; alternately, it may be protected with an
inexpensive Y strainer.
Since float drain traps always contain liquid condensate, special heaters
are inserted for locations where freezing is a concern.
The most common application for gas/liquid separators involves steam
systems to protect downstream equipment from condensate and pipe scale.
Compressed air systems also use multiple separators for condensate removal.
Other industries, such as oil refineries, use gas/liquid separators
throughout their process to recover product at specific temperature and pressure
Gas/liquid separators are sometimes used to remove a heavy load of
particulates, resulting in a sludge-like discharge. The drain section of
the separator changes to facilitate better drainage and a rotary valve is used
to minimize loss of process gas.
The rate of liquid removal, expressed as a percentage of the process gas
weight flow, is determined by the separator size and design. The range of
removal rates vary from 5% to 90% of the weight flow. The size and
complexity of the separator design increases along with higher removal rates.
The amount of liquid to be removed affects the drain orifice size used in the
Removing droplets finer than 10 microns is possible by adding a coalescing
stage to the separator design. The coalescing stage consists of a mesh pad
for >4 micron removal. The fine droplets impinge within the mesh pad,
coalescing to >10 micron droplets which are subsequently removed by the
The addition of a coalescing stage requires a two-piece vessel to enable
access to the mesh pad for periodic cleaning or replacement.
It is possible to remove droplets smaller than 1 micron with the addition
of a filter element stage after the separation stage. These specially
designed borosilicate microglass filter elements have a glass matrix drainage
layer in support of retentions as fine as 0.30 microns.
The orientation of the inlet and outlet nozzles are another customizable
characteristic to support a variety of flow paths. While horizontal piping
is commonplace, sometimes the flow path is vertically UP, vertically DOWN or
some combination of horizontal and vertical.
An exhaust head is a special version of a gas/liquid separator used at the
end of a ventilation pipe; most commonly for steam applications although
sometimes there are exhaust systems other than steam for which it is desirable
to remove droplets >10 microns.
One side of an exhaust head is always “open” to atmosphere, which means
that it is NOT a pressure vessel; its design pressure is atmospheric despite it
being a fully welded item having a flanged inlet matching the class of the
Exhaust heads reduce the opacity and volume of the condensate plume
exiting the exhaust pipe. Unlike a pressure vessel separator, there is no
requirement for a float drain trap and often the condensate is recycled to
reduce the consumption of boiler treatment chemicals.
Exhaust heads are sometimes referred to as “silencers” because, like a
muffler, they do reduce the exhaust sound. However, this is not their
purpose, any reduction of sound is attributed to the indirect path required for
droplet removal and thus there are no design standards for sizing exhaust heads
specifically for noise reduction.
Proper sizing of gas/liquid separators requires only a few data points:
The initial cost of gas/separators are in many ways the final cost, given
they operate reliably for decades protecting equipment and improving process
efficiencies without need for maintenance.
Visit us at http://www.fdpp.com
us know how we can assist you with your gas/liquid separation application!
Chris Pasquali has provided sales and engineering support for the
Wright-Austin product line since 2001, trained by both Hayward and Eaton.