The
first thing to do is to make very sure you know all of the possible inlet situations
- maximum flow rate, minimum temperature and oil types and concentrations that
may occur in the inlet.
Make
a schematic flow diagram of the system and identify possible pressure drop causing
equipment such as pumps and valves.
With
this information, MSR can estimate the oil droplet sizes that are likely to be
present in the inlet water.
We
use this information in our proprietary computer program to predict operations
of the system. We estimate a separator size and configuration and do a simulation
calculation to see if the performance will meet the required effluent concentration.
We then adjust the size larger or smaller to ensure a proper fit.
Because
the separation depends on Stokes's Law and we are careful to ensure (in the separator
design) that the conditions of this physical principle are met, the final design
system will meet the required effluent. We try to be very conservative in estimating
the droplet sizes so that the final design will be conservative and capable of
handling variations in flow and other conditions. The physics work and Sir Isaac
Newton's gravity can be trusted to operate correctly and predictably every time.
A
very convenient property of coalescing plate separators not shared by some types
of oil-water separators is that at flow rates lower than the design flow they
just work better!
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Why is it better
to direct the flow into a separator by gravity flow than pumped flow?
Anything
that decreases the average droplet size in the inlet of a separator will decrease
the performance of the separator. The type of equipment that decreases droplet
size is anything that causes shear in the inlet stream. This includes pumps, especially
centrifugal pumps, valves, especially globe and other control valves, small or
rough piping and undue amounts of elbows or other fittings in the inlet pipe.
If
I have to pump into a separator, what is the best kind of pump to use?
The
best type of pump to use is a positive displacement pump such as a diaphragm pump
or screw pump. The worst are centrifugal pumps.
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What
information do I need to gather for designing an effective, reliable separator?
Numerous factors
must be considered in the selection and design of oil-water separation system.
Among these are:
1.
Flow rate and conditions
2. Degree of separation required - effluent quality
3. Amount of oil in the water
4. Existing equipment
5. Emulsification
of the oil
6. Treated water facilities
7. Recovered oil disposal method
For industrial
and some municipal applications, flow rate, amount of oil, flowing temperature,
and other conditions affecting separation such as whether flow is laminar or turbulent
may be easily determined. For stormwater applications, however, it may be necessary
to estimate water flows. The degree of separation required is usually a matter
of statutory or regulatory requirements, but if the water is discharged to a sanitary
or industrial treatment plant it may be negotiable.
The
amount of oil in the water may be known, especially in industrial applications,
but it will often be necessary to estimate the quantity in stormwater applications.
MSR can provide guidance about quantities to be expected, and some information
has been published about stormwater quality
Existing
equipment such as API separators may affect the design of equipment to be used.
Often it is possible to retrofit existing equipment with more sophisticated internals
to enhance separation quality.
It
is necessary to ensure that adequate size piping is provided for downstream treated
water removal to avoid flooding the separator and perhaps filling the oil reservoir
with water. A downstream test point should be provided to allow for effluent testing.
Adequate storage facilities for the removed oil should be provided and means for
recycling the oil included. Careful records of removed and recycled oil should
be kept to avoid possible future regulatory problems.
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Can
I use soaps and detergents and still have my separator work well?
Soaps and detergents
(known collective as surfactants, which is a contraction of surface active agent)
can cause emulsions that are very difficult to remove. Without careful attention
to the amount and type of soaps and detergents used it is likely that a system
will NOT operate satisfactorily if they are used. The degree of emulsification
of the oil is difficult to assess, but steps can be taken to discourage the formation
of emulsions and encourage the breakup of emulsions that are inadvertently created.
It may be necessary to substitute quick-break detergents for conventional detergents
that are also emulsion causing. Quick-break detergents are those detergents designed
to remove the oil (or grease) from the item to be cleaned and then quickly dissociate
again from the oil, leaving the oil as free hydrocarbon droplets in the water.
MSR will be glad to make design suggestions and / or send our list of acceptable
quick-break detergents.
Note:
the small amount of detergents in detergent motor oils will not cause problems
with the operations of oil water separators.
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I
have dirt and / or grit in my flow stream - what will that do to the system?
Small
quantities of dirt and grit will not harm the operations of the separator; they
will only cause some eventual plugging that must be removed periodically. If large
quantities of solid particles are expected, it is wise to provide a grit removal
chamber before the separator. These chambers should be designed according to normal
design parameters for grit removal as used in sanitary sewer plant design. MSR
will be glad to offer suggestions on this design.
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What
is the benefit of using separators over absorbents?
The
biggest problem with the use of absorbents is that they are fairly quickly used
up, and since there no way to determine if they are exhausted without laboratory
testing, they are often left in place long after their useful life is done. The
used absorbents are sometimes hazardous waste and will certainly cause a disposal
cost.
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What can
I do about dissolved hydrocarbons?
In
general, hydrocarbons are only marginally soluble with the notable exception of
Benzene and some other aromatic hydrocarbons. Even these are removable using coalescing
plate separators or other physical means so long as they are either removed so
quickly there is not time for the dissolution to occur or if they are present
in concentrations greater than the solubility.
Dissolved
hydrocarbons are, strictly speaking, not covered by the Clean Water Act since
they do not cause a sheen on the water. Many analysis methods will detect dissolved
hydrocarbons and some jurisdictions are concerned about them, so it may be necessary
in some cases to treat for their removal. Physical methods such as coalescing
plate separators will not remove dissolved hydrocarbons and other methods such
as biological treatment or absorbents are required.
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What
can I do with the oil recovered in a separator?
Removing
the oil from the separators is not enough to protect the environment, it must
also be re- cycled to ensure that it is disposed of properly. Current U.S. law
can hold the owner of the oil-water separator responsible if this oil is not properly
disposed of, even if the owner has paid for proper disposal.
There
are many local firms involved in recycling hydrocarbons. Most will be able to
tell you what the local regulations are and usually offer a pick-up service.
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What
are the ideal inlet conditions for an oil water separator? I'd like to make it
work as well as possible.
Ideal
inlet conditions for an oil-water separator are:
1. Gravity flow (not pumped)
in the inlet piping
2. Inlet piping sized for minimum pressure drop
3. Inlet piping straight for at least ten pipe diameters upstream of the separator
(directly into nozzle)
4. Inlet piping containing a minimum of elbows, tees,
valves, and other fittings.
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