Oil/Water Separator Theory
Parallel plate oil/water separators are devices that permit the
removal of oil from a waste stream by allowing the oil droplets to
rise out of the hydraulic flow path of the separator, thereby
extracting them from the waste flow. In theory, the flow through
velocity (V) of a vessel is a function of the vessel size and flow
rate (gpm). The velocity is then compared to the rate of rise of
the oil droplet and the length of the vessel. If the resulting
calculated trajectory (T) of a given droplet will allow it to rise
out of the effluent flow path before it reaches the vessels exit, it
is removed. This is an application of Stokes' Law and terminal
velocity to the rate of rise of a particle in a liquid medium.
In 1845, an English
mathematician named George Stokes first described the physical
relationship that governs the settling solid particles in a liquid
(Stokes' Law, 1845). This same relationship also governs the rising
of light liquid droplets within a different, heavier liquid. This
function, simply stated is:
terminal, fall or settling velocity
acceleration of gravity
density of medium (e.g. water, air, oil)
viscosity of medium
viscosity of medium (μ)
settling or terminal velocity (Vt)
acceleration of gravity (g)
density of particle (ρp)
density of medium (ρm)
particle diameter (d)
negative velocity is referred to as the particle (or droplet) rise
Assumptions Stokes made in this calculation are:
Particles are spherical
Particles are the same size
is laminar, both horizontally and vertically. Laminar flow in
this context is equal to a Reynolds number less than 500.
variables are the viscosity of the continuous liquid, specific
gravity difference between the continuous liquid and the particle,
and the particle size.
rate of oil droplets is also governed by Stokes' Law. If the
droplet size, specific gravity, and viscosity of the continuous
liquid are known, the rise rate may be calculated.
Calculation of rise rate by this method is a gross simplification of
actual field conditions because oil droplets are not all the same
size, and they tend to coalesce into larger droplets. Furthermore,
inevitable turbulence within a separator makes an orderly rise of
very small droplets impossible.
will rise following Stokes' Law so long as laminar flow conditions
prevail. When the particle size exceeds that which causes a rise
rate greater than the velocity of laminar flow, the flow around the
droplet (as they rise) begins to be turbulent. Particles of this
size and larger do not rise as rapidly as would be expected from
calculations based on Stokes' Law because of the hydrodynamic drag.
They do, however, rise very quickly in relationship to smaller
droplets, and will be removed by a properly designed separator.
small particles, such as those of 10 microns (micrometers) and less
in diameter, do not rise according to Stokes’ Law (or hardly at all)
because the random motion of the molecules of the water is
sufficient to overcome the force of gravity and therefore they move
in random directions. This random motion is known as Brownian
Motion. Fortunately, the volume of a droplet decreases according to
the cube of the diameter, so these very small droplets tend to
contain very little oil by volume. And unless there are large,
large quantities of very small droplets (such as would be present
with an emulsion or created by using a centrifugal pump to pump the
water) they contain negligible amounts of oil.
Rate of Rise
The separation process can be accomplished and enhanced in a variety
of ways and with a variety of equipment configurations. One common
way to improve separation without increasing the need for floor
space is to install a multiple plate pack that will create many
separation chambers in one vessel, each with a shallow depth. This
is done by adding a series of appropriately spaced plates. The flow
is distributed through the plates and the rate of rise of the
droplet is applied to the application. The advantages of multiple
plates is that surface area is increased without requiring
additional floor space.
The most efficient oil/water separators are
designed to exploit Stokes' Law and the rate of rise for a given
droplet. In order for a particle to be removed according to Stokes'
Law, the separator must conform to several critical design criteria:
flow conditions must be achieved (Reynolds “Re” number less than
500) in order to allow a droplet to rise.
flow path must distribute influent AND effluent flow in such a way
as to ensure complete utilization of the coalescing surface area, in
order to take full advantage of the plate pack coalescing surface
area. Design of the flow distribution must be such as to prevent any
hydraulic short circuiting of the plate pack, which would be
flow-through velocities in the separator must not exceed 3 feet per
minute, or 15 times the rate of rise of the droplets - which ever is
smaller - per the American Petroleum Institute’s Publication 421 of
surface area must not become clogged during use, which would
adversely alter flow characteristics, possibly creating hydraulic
short circuiting and increasing the “Re” number past 500.
inclined parallel plates are used, they must be at the proper angle
of repose to allow solids to settle in a liquid medium (ideally
55-60 degrees from horizontal) , and they must be smooth enough to
allow the unhindered migration of a solid particle to the bottom of
the plate pack and an oil droplet to the top of the plate pack, where
they will exit the waste stream.
There are several important factors to
consider in efficient parallel plate oil/water separator design. As
stated earlier, the parallel plate must have a smooth surface in
order to allow unhindered migration of the droplet to the top of the
pack and solid particles to the bottom. Another enhancement is to
use cross corrugated plates. Corrugated plates provide additional
coalescing surface area, within the same volume, in the form of
crests and valleys that aid in the migration of the droplet to the
top of the pack. As the droplets impinge on the crests and valleys
and begin to migrate toward the top of the plate pack, they will
coalesce with other droplets, thus creating larger droplets with
increased mass which will improve their rate of rise.
Technologies employs parallel plates in a most
efficient manner. We use ultra-smooth surface, cross corrugated
plates, that are arranged at a 60 degree angle of inclination. This
promotes self- flushing and efficient droplet agglomeration, which
improves the migration of droplets toward the top of the plate pack,
and sludge to the bottom of the plate pack and out of the waste
stream into the sludge settling chamber. Our influent and effluent
flow distribution systems are carefully designed to ensure
efficient, even and complete usage of the entire plate pack and to
prevent short circuiting. In summary, our parallel plate oil/water
separators are thoughtfully designed to ensure reliability and
performance. We also offer custom design services to meet specific
Inlet Flow (Influent)
Much of the performance of an
oil/water separator depends on the influent conditions, because
equipment or conditions that cause small droplet sizes in the
influent to the separator will cause requirements for a larger
separator to accommodate the additional time required for the
smaller droplets to coalesce.
Conditions that cause small droplets are any conditions that cause
shear in the incoming water. The following are, more or less in
order of severity, some factors that can cause small droplet sizes:
Pumps, especially centrifugal pumps.
Valves, especially globe valves.
restrictions in flow such as elbows, tees, other fittings, or
simply unduly small line sizes.
Vertical piping (horizontal is better)
Emulsifying chemical agents (soaps and detergents)
Emulsifying agents such as soaps and detergents greatly contribute
to small droplet sizes, in addition to disarming coalescing plates
and discouraging coalescing.
inlet conditions for an
oil/water separator are:
Gravity flow (not pumped) in the inlet piping.
piping sized for minimum pressure drop.
piping straight for at least ten pipe diameters upstream of the
separator (directly into nozzle)
piping containing a minimum of elbows, tees, valves, and other
Most separators are provided with an inlet elbow or tee inside the
separator, pointing down. This is an exception to the above rules and
is intended to introduce the influent water below the oil layer on
the surface, thus avoiding disturbances of the surface oil and
possible re-entrainment of some of the already separated oil.
Gravity flow conditions are not often obtained, except in sanitary
sewer systems. In stormwater, or some process water applications, a
positive displacement pump such as a progressive cavity type pump
may be used because they provide minimum disturbance of the fluid.
The best choice, if gravity flow is not available, is a progressive
piping should be as smooth as possible to avoid turbulence caused by
pipe roughness. Smooth PVC is preferable to rough concrete.
anti-emulsion chemicals are utilized, but extreme care must be
exercised in the use of these chemicals to ensure that they do not
make the emulsion worse, instead of improving it. Plant operators
have a tendency to believe that if a small amount of anti-emulsion
chemical is good, then a really large
amount is better. It is necessary to provide sufficient operator
education to avoid this problem, and best to avoid use of such
quantities of solid particles are expected, it is wise to provide a
grit removal chamber before the stream enters the separator. These
chambers should be designed according to normal design parameters
for grit removal as used in sanitary sewer plant design.
Outlet Flow (Effluent)
Effluent designs are also
important in the operation of oil/water separators.
Downstream piping and other facilities must be adequately sized to
process the quantity of water (and oil) from any likely event.
Manholes overflowing during a heavy rainstorm will surely cause any
oil caught in them to be re-released into the environment.
piping must be designed with siphon breaks so that it is not
possible to siphon oil and water out of the separator during low
be removed from the separator on a regular basis, preferably
continuously. If not removed in a timely manner, this oil may fill
the separator, blinding the media and causing high effluent oil
contents. It may eventually become re-entrained at the next rainfall
event and reintroduced into the environment.
the oil from the separators is not enough to protect the
environment; it must also be recycled to ensure that it is not
merely treated as a waste and to avoid possible problems elsewhere from
improper disposal. Current law can hold the owner of the oil/water
separator responsible if the oil is not properly disposed of, even
if the owner had paid for proper disposal.
Stokes, George Gabriel. 1845.
Transactions, Cambridge Philosophical Society 8, no. 287.
Petroleum Institute. 1990. "Design and Operation of Oil-Water
Separators, Publication 421, American Petroleum Institute. American
Petroleum Institute, Washington, D.C.