American Oil and Gas Reporter - May 2019 - 67

SpecialReport: Gas Compression & Vapor Recovery
hatches are properly closed after vessel
gauging, sampling and unloading.
With respect to proper sizing of emission
controls, operators should ensure that vent
gas control devices are designed for the
specific facility's operations. The design
should be sized and operated to control
for the full range of expected gas flow
rates. Key to ensuring proper sizing of
emission controls is appropriate sampling,
measurement and modeling to estimate
potential maximum vent gas flow from
storage tanks. Process simulation calculations can be run to estimate the potential
range of vent gas flow rates for various
operating scenarios, and these assessments
then can be used in sizing the VRU.
Other Recommendations
There are a number of other design
and operating recommendations beyond
the best practices listed in the EPA alert.
For example, the flow rate of vent gas
from the emission source should be measured whenever possible using turbine meters or thermal mass flowmeters in conjunction with infrared optical gas imaging
cameras. Actual vent gas flow rate measurements are valuable for the optimal sizing, design and operation of a VRU by
determining the minimum and maximum
flow rates over a 12- to 24-hour period.
In addition, samples of the vent gas
taken during the measurement process
can supply data on the chemical makeup
(molecular weight, VOC, methane, etc.)
and BTU content of the gas. This is
useful data for air permitting, emission
inventories and calculating the value of
the vent gas recovered by the VRU.
Another recommended practice is to
install a VRU scrubber vessel in applications with wet vent gas sources to collect
free natural gas liquids that might condense
out upstream of the compressor inlet.
Crude oil/condensate storage tanks typically have high-BTU content vent gas
(greater than 1,500 BTU/cubic foot) and
will have more free liquids that should
be removed prior to sending the gas to
the compressor.
When VRUs are used to recover vapors
from storage tanks, the vent line from
the storage tank or the common manifold
suction line for multiple storage tanks,
should be piped to the suction scrubber
on the VRU. The suction scrubber is a
two-phase separator that removes some
of the free oil in the vent gas. The recovered oil is pumped back to the storage
tank, and the gas from the suction scrubber

flows to the VRU for compression. The
discharge piping from the VRU can be
routed to the gas gathering line, a meter
run, or the suction of a gas compressor.
The piping connecting the storage
tanks and VRU or vapor recovery towers
(VRT) should operate with no detectable
emissions (VRTs are tall vertical separators
used to recover most of the flash gas that
would be generated in a crude oil storage
tank, with the VRT routing lower pressure
oil to the storage tank and recovered gas
to the VRU). This no-emissions VRU/VRT
piping connection is referred to as a
closed vent system (CVS) in NSPS
OOOO/OOOOa. CVS piping from the
storage tank to the VRU should slope
downward 20 degrees or greater toward
the VRU/VRT to ensure that any liquids
that condense in the piping can be gravity-fed back to the VRU inlet scrubber to
keep the piping clear.
VRUs are designed to turn on and off
based on the pressure of the gas in the
storage tank vapor space. To detect the
pressure in the storage tanks, a sensing
line is run to a tee or chamber on-skid
and monitored by a transducer, which in
turn, is connected to the control panel.
The sensor should be located in the most
active storage tank or located at a distance
as far as practical from the VRU suction
line if all tanks are equally active in receiving oil. Some systems use a tankmounted pressure transducer with an analog line to the control panel to detect
pressure in the storage tank vapor space.
The VRU is configured to stop and
start automatically, depending on pressure
in the tanks. A properly designed VRU
must include a bypass system that will
initiate automatically and divert the discharge volume back to the suction scrubber.
This process allows tank pressure to build
back to the point at which compression
occurs. If the pressure continues to decrease
while in the bypass mode, the unit will
shut down and wait in standby mode for
the start pressure to be obtained.
Properly designed VRUs are configured
to shut down before any type of vacuum
is reached to avoid pulling oxygen into
the tanks, or imploding them. If air is
pulled into the system, it is typically
caused by an improperly designed package
(no bypass system or improper pressure
settings), improperly sealed tank hatches,
or leaking relief valves. These units actuate
on pressures as small as 0.5 inches of
water column (0.019 psi), and most packages used in typical oil field compression

are simply not able to calibrate to these
minute pressures. With a correctly designed
package, the addition of a gas blanketing
system on the tanks can alleviate the majority of any other issues that can cause
oxygen ingress.
Either rotary screw or rotary vane
compressors are strongly recommended
for crude oil/condensate storage tank
VRU applications because of their ability
to effectively handle wet gas. Reciprocating compressors are good machines
in dry gas applications, but they are not
recommended for wet gas VRU service.
Wet gas tends to foul the valves and seats
in reciprocating compressors, and condensate can fall out in the crankcase and
compromise the lubricating oil, resulting
in component failure.
A final recommendation is to use an
enclosed combustion device or flare as a
backup control device to a VRU. The
combustion device can combust gas during
times when it is not recovered by the
VRU. Backup emission controls help ensure that a facility can continuously meet
NSPS OOOO/OOOOa limits for storage
tanks and state site-specific facility air
permit limits. They can also help avoid
the need to report upset, bypass and emergency air emissions that will occur if the
gas is not combusted.
❒

JEFF
VOORHIS

Jeff Voorhis is a corporate engineer at Midland, Tx.-based HYBON/EDI, where he is involved in
designing vapor recovery unit installations to prevent valuable resources from venting to the
atmosphere. Before joining HYBON/EDI in 2012, Voorhis served
for 22 years as an engineer at the
Texas Commission on Environmental Quality, including engineering
functions related to oil and gas and
air permitting processes. He holds a
B.S. in petroleum and natural gas
engineering from Texas A&M University-Kingsville.
MAY 2019 67

American Oil and Gas Reporter - May 2019

Table of Contents for the Digital Edition of American Oil and Gas Reporter - May 2019