American Oil and Gas Reporter - August 2019 - 57

and saturated with water vapor. Many
sites in cold climates run into issues with
undesired condensation of water or hydrocarbons and/or formation of solids
that can include ice, paraffins and hydrates.
These issues must be managed on a casespecific basis.
Single-Stage Separator
With a single-stage production separator, vapor control is simple, but not
necessarily optimal. Gas in the separator
goes into the sales gas pipeline, water
goes to a water storage tank (or
vault/sump), and oil dumps to the storage
tank. When the oil dumps to the storage
tank, some of the liquid flashes to vapor
and all of the vapors to be controlled end
up in the storage tank. In addition to the
flash gas, this also includes working and
breathing losses.
The control device is typically one or
more flares, but a VRU can be used in this
application. Oxygen can be present in the
storage tank from air ingress, which can
complicate VRU operations fed from storage tanks unless a gas blanketing or
catalytic oxidation system is added.
All of the vapor to be controlled will
be near atmospheric pressure (< 1 psig).
Without any equipment between the single-stage heater treater and storage tanks,
there is no means of moderating the vapor
generation rate. Every time the dump valve
opens, more oil flashes and more vapors
enter the storage tank with the oil. The
control device(s) must be sized to handle
this peak rate or the pressure relief valve
on top of the storage tank (typically set
around 12 ounces or 0.75 psig) will vent
excess vapor to the atmosphere.
The thief hatch used to gauge tank
levels and collect oil samples has a twoway "breather" valve to keep the pressure
in tanks designed for near atmospheric
pressure operations from becoming too
low or too high. Pressure can be too low
when the tanks cool for diurnal or other
reasons, which could collapse the tank.
Pressure can be too high when levels or
temperatures rise or when a lot of flash
gas is being formed, which potentially
could rupture the tank. Usually, the thief
hatch vents at pressures ranging from
12-16 ounces (0.75 to 1 psig). It can be
very costly to size control devices to
handle all possible peak generation rates
in this scenario in order to avoid venting
vapors to the atmosphere. This leads to
some interesting (and complicated) options.
Two-Stage Separator
Figure 3 shows a diagram for a twostage separator. A key operational advantage is that the overall amount of

flash gas formed is reduced and some of
it is available at an intermediate pressure.
The intermediate pressure flash gas is
easier to recover with a VRU, while the
amount of atmospheric pressure flash gas
that needs to be controlled from the
storage tanks is reduced. The intermediate
pressure flash gas is much richer than the
sales gas with heating values as high as
2,500 Btu/cubic foot, which results in
higher revenue if the recovered gas is
sold on a heating value basis.
With a two-stage separator, sales gas is
taken from the top of the high-pressure
(HP) separator and oil flashes across a
dump valve into a low-pressure (LP) separator. Water also is concurrently dumped to
the LP separator. More hydrocarbons stay
in the liquid phase with this step, since the
downstream pressure is an intermediate
pressure, perhaps around 40 psig, instead
of near atmospheric. An intermediate pressure gas stream also is formed that needs
to be removed from the LP separator.
Temperature settings are important in
the HP separator. Higher temperatures
cause more of the valuable hydrocarbons
to end up in the sales gas line, while
lower temperatures cause more of the
lighter hydrocarbons to dump to the LP
separator with the oil/water. The light
hydrocarbons then will flash out in the
LP separator, stripping along with them
some of the heavier hydrocarbons that
otherwise would have ended up in the oil
storage tank. The HP separator typically
rides the sales gas pipeline pressure, so
this is not usually a variable that can be
controlled. Temperature control methods
in the HP separator vary depending on
the separator design and manufacturer.
Routing the intermediate pressure gas
from the LP separator to the storage tank
is one option, but then the gas drops to
atmospheric pressure with the same associated vapor control challenges as described for the single-stage separator.
This gas can be routed from the LP separator to a VRU that compresses the flash
gas to sales gas pressure and combines
with the sales gas in or downstream of
the high stage separator before going
through a flowmeter to the pipeline.
The challenge here is specifying a
VRU with sufficient capacity for the peak
gas generation rate and that also is suited
for intermittent operation and variable
suction flow rates. A backup vapor vent
line from the LP separator to the storage
tank or to a separate flare (ECD) typically
is provided for times when the VRU is
down or when it is not able to keep up
with the peak vapor generation rate in
the LP separator.
It also is important to manage the
temperature and pressure in the LP sepa-

rator to the extent possible for stable operations and maximum oil production.
Heating is needed primarily to prevent
the formation or buildup of paraffins and
is often done only in winter months. It
also helps break oil/water emulsions and
facilitates oil/water separation.
A temperature higher than needed will
result in more light and intermediate hydrocarbons in the intermediate pressure
gas formed in the LP separator, when
they could have otherwise ended up with
the sales oil. A temperature too low or a
pressure too high in the LP separator
will cause more of the light hydrocarbons
to dump with the oil to the storage tank.
These will readily flash to vapor across
the dump valve, which will strip intermediate hydrocarbons in the process, resulting in more vapor to be controlled
and lower oil production. A pressure too
low in LP separator results in more hydrocarbons in the LP separator gas and
also could result in problems with draining
the oil to the storage tanks.
Making sure that each pumper understands that managing temperatures and
pressures in each stage of the separator
is a balancing act that impacts operations,
revenue and the amount of vapors that
need to be controlled can lead to benefits
across all production sites. Adjustments
may be needed seasonally, when production changes, or for other reasons.
❒
Editor's Note: This article is the first
in a two-part feature on vapor control
and recovery in oil and gas production
operations. Part II will appear in AOGR's
September issue.
RAY MCKASKLE is a principal engineer at Trimeric Corporation in
Buda, Tx. With 27 years of industry
experience, he joined Trimeric in 2005
after serving as a senior field process
engineer at Novellus Systems and as
a staff engineer at Radian International.
McKaskle holds a B.S. in chemical
engineering from Oklahoma State University.
KEVIN FISHER is vice president
at Trimeric Corporation. Before joining
the company in 2003, he was a principal engineer at CrystaTech, a senior
engineer at Radian International, and
a chemical engineer at JFW Development. Fisher holds a B.S. in chemistry
from Sam Houston State, a B.S. in
chemical engineering from Texas A&M
University, and an M.S. in chemical
engineering from the University of
Texas at Austin.
AUGUST 2019 57

American Oil and Gas Reporter - August 2019

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