American Oil and Gas Reporter - January 2022 - 79

systems that integrate digital solutions
such as programmable logic controllers
and variable speed drives with engineered
mechanical components to achieve high
run times and compliance with U.S. EPA
" Quad-O " emission regulations (40 CFR
60 Subpart 0000a).
Senor says the systems are technology
and brand " agnostic " and can use rotary
screw (Leroi, Howden, Mycom, Sullair
or Gardner Denver), rotary vane (Roflo
or Mattei) or reciprocating (Corken, Blackmer
or Ariel) compressors matched with
a PLC-controlled bypass/recycle valve,
oversized scrubbers and filters, and a
dual-fan aftercooler to achieve run time
targets well above industry averages.
" Our VRUs primarily use screw compressor
technology, but rotary vanes and
recips definitely have preferred applications
based on the gas composition, flow
and pressure ratios required, " he explains.
" Rotary vanes are good for sour gas, saturated
streams and other critical service
applications. We recently sold a rotary
vane package with 316 stainless steel
process piping and coolers along with
corrosion inhibitors in the lube oil to run
on up to 8% hydrogen sulfide. "
SCS Technologies' goal for any VRU
is to achieve a run time of 98% or greater.
" Run time is everything, " Senor emphasizes.
" If a VRU is down, the gas has to be
flared instead of going to the sales line. "
Typically, VRUs must be periodically
shutdown for scheduled maintenance tasks,
such as changing the lube oil. But run
time is so critical that Senor says operators
are devising solutions to maintain the
equipment without shutdowns and restarts.
" One of our customers prefers to use a
lube oil makeup system external to the
VRU; our packages come standard with
redundant filter manifolds, allowing the
VRU to run without having to stop to
change lube oil and filters, " he points out.
For new projects, Senor advises that
operators install a vapor recovery tower
(VRT) to optimize vapor recovery flash
gas at a higher pressure than storage tanks.
" A facility designed without a VRT will
operate at very low pressures making oxygen
ingress a major concern, " he relates.
" Feeding the VRU off the VRT alleviates
oxygen ingress and helps consolidate gas
before it enters the sales line. "
Tank battery VRU applications typically
The ability to eliminate fugitive emissions and minimize flaring while capturing an otherwise
lost revenue stream makes vapor recovery units a strategic fit for operators looking
to improve both financial and ESG performance. SCS Technologies describes its
VRUs as turnkey systems that integrate programmable logic controllers and variable
speed drives with rotary screw, rotary vane or reciprocating compressors matched with
a bypass/recycle valve, oversized scrubbers and filters, and a dual-fan aftercooler to
achieve run time targets of 98% or greater.
JANUARY 2022 79
try to maintain the highest possible pressure
so that oxygen cannot be pulled in
through thief hatches or fiberglass piping,
he goes on. " If you operate too close to 0
psi, there is greater potential to pull in
oxygen. The gas sales meter has a shutdown
valve and an O2 meter, so if any
oxygen is being pushed into the sales
line, the pipeline will reject it. "
Monitoring And Control
Senor describes the VRU as " the most
automated piece of equipment on a production
site. " As opposed to a wellhead
compressor that has a suction range measured
in pounds of pressure, a VRU at a
tank battery operates within the margin
of ounces of pressure, although he says
one trend that is quickly gaining traction
is tankless battery designs. " Instead of
having tank gas pressure held at a few
ounces, end-users have production vessels
holding several pounds of pressure.
" For tank applications, the VRU must
precisely control the bypass valve's opening
and closing to maintain pressure in
the range of a few ounces, " Senor states.
" Multiple PID loops run simultaneously,
and there must be a smooth handoff between
them so it doesn't create sudden
bursts of gas going to the scrubber to
send the VFD out of control to pull all
the vapors down. You cannot pull a
vacuum on a storage tank or it could collapse
the tank, so it is mission critical to
keep pressure positive. Maintaining that
narrow window requires a lot of monitoring
and control. "
That is where PLCs and VFDs come
into play, helping extend turndown capacity
and automate operations for consistent
performance. " A VRU with a VFD
and recycle/bypass valve has more turndown
capability and can run a lot longer
than a VRU with just a simple on/off or
lead/lag control, " he adds. " It has more
flexibility to adapt as conditions change
over time. "
One of the challenges for sizing any
piece of equipment in tight oil plays is
varying production rates, and the possibility
that gas percentages and compositions
will change as wells mature. To engineer
around that challenge, Senor says
SCS units are sized using a fieldwide
" worst-case scenario " approach to account
for all potential conditions that may be
encountered in the future.
" We make performance runs using,
say, a pure methane for the lube oil
section and perhaps a 1.6 specific gravity
heavy gas for the gas section of the cooler

American Oil and Gas Reporter - January 2022

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