Partner Reported Opportunities (PROs) NatllHilGciS T
for Reducing Methane Emissions EPA POLLUTION PREVENTER
Wet Seal Degassing Recovery System
for Centrifugal Compressors
Technology/Practice Overview
Description
Centrifugal compressors are used
throughout the natural gas industry to
compress gas for processing,
movement through pipelines, and
other needs. These compressors
require seals around the rotating shaft
to prevent high pressure gases from
escaping where the shaft exits the
compressor casing. Seals may be
either wet seals using oil as a barrier
or dry seals that use a mechanical
barrier with most new compressors
equipped with dry seal. However,
there are a large number of
centrifugal compressors with wet seals
in operation. Wet seals use specialty
oil which is circulated under higher
pressure than the gas in the adjacent
compressor case, between rings
around the compressor shaft, forming
a barrier against the compressed gas
leakage. In the wet seal design shown
in Exhibit 1, the center ring is
attached to the rotating shaft, while
the two rings on each side are
stationary in the seal housing, pressed
against a thin film of oil flowing
between the rings to both lubricate
and act as a leak barrier. "O-ring"
rubber seals prevent leakage around
the stationary rings. Very little gas
escapes through the oil barrier;
considerably more gas is entrained in
the oil that comes in direct contact
with the high pressures gas at the
"inboard" (compressor side) interface,
thus contaminating the seal oil. Prior
to recirculation, seal oil must be
purged of natural gas (which is
primarily methane) in a process called
seal oil "degassing." Methane released
during degassing is commonly vented
to the atmosphere with vent outlets
usually located in elevated areas that
are not easily accessible to operators
for inspection or maintenance (e.g.,
roof vent stacks). As a result , the
majority of methane emissions from a
centrifugal compressor often go
undetected by operators.
Compressors/Engines
Dehydrators
Directed Inspection &
Maintenance
Pipelines
Pneumatics/Controls
Tanks
Valves
Wells
Other
Applicable Sector(s)
Production
Processing
Transmission
Distribution
Economic and Environmental Benefits
Economics Evaluation
Estimated
Gas Price
Annual Methane
Savings1
Value of Annual
Gas Savings
Estimated Implementation
Cost
Incremental
Operating Cost
Payback
$3.00/ Mcf
30,000 Mcf
120,000 Mcf
$90,000
$360,000
$33,000
$90,000
Minimal
5 months
3 months
$5.00/ Mcf
30,000 Mcf
120,000 Mcf
$150,000
$600,000
$33,000
$90,000
Minimal
3 months
2 months
$7.00/Mcf
30,000 Mcf
120,000 Mcf
$210,000
$840,000
$33,000
$90,000
Minimal
2 months
1 months
1 At each gas price, the costs and savings are for one and four compressors at a station.

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Wet Seat Degassing Recovery System for Centrifugal Compressors
Exhibit 1: A typical wet seal setup,
the seal oil enters through the inlet
(top) and provides a barrier to gas
attempting to escape by forming two
thin films under higher pressure
between the center rotating ring and
the two stationary rings (seen with
surrounding o-rings).
Exhibit 1: Wet Seals
Seal Housing
\
Seal Oil Inlet
Motor and
Shalt Bearing Side
"Outkoard"
"Outboard"
Lakyrlnth
Mi
Process Gas
Leaks Through "Inkoard"
Lakyrlnth Seal
Compressor Side
"Inboard"
f\
Seal Oil
(llncontamlnated)	Seal ®"
(Contaminated with Gas)
Spinning Shall
One Partner has reduced methane emissions from
centrifugal compressor seal oil degassing by separating
gas from seal oil in a small separator/disengagement
vessel and routing it back into the compressor suction,
to high pressure turbine fuel gas, or to low pressure fuel
gas for heaters/burners, A diagram illustrating this
partner's approach is shown in Exhibit 2. Seal oil with
entrained gas is typically routed directly to an
atmospheric pressure degassing tank from which
disengaged gas is vented to the atmosphere. In the
capture system, the contaminated seal oil is instead
separated from entrained gas in a separator operating
at seal oil pressure with gas flow control by a critical
orifice. The entrained gas captured from the seal oil is
Sell Htnng
SMft Burnt Silt
Seal Oil laltt
OltkMll
LakytMh
Seal Oil
{UoctitaraiulMl
Seal Oil
(CertamlnatMl with Git)
New seal oil high
pressure gas
disengaging separator*"
Ptocmj Cat
letkt Ikroeik likuid'
LibinnPi Sell
Cotsmmt JHSe
-lnttotU
Siimim Skin
Atmospheric
seal oil
degassing
separator
Less gas
vented to
atmosphere
•Seal oil is at compressor discharge pressure

Note: New equipment in red
Compressor
suction/
recycle
Gas Demister
High pressure
turbine fuel
gas
Boiler
4 OPTIONS
Low pressure
fuel gas
FLARE
PURGE
Seal oil circulation pump
Exhibit 2: General process flow of wet seal degassing recovery system. Existing equipment is shown in blue or black and new
equipment for seal oil degassing recovery is shown in red.
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Wet Seal Degassing Recovery System for Centrifugal Compressors
routed to a seal oil clemister to remove entrained seal oil
before routing to beneficial use. The seal oil then flows
from the bottom of the seal oil degassing separator to
the atmospheric degassing separator where the
remaining minimal volume of entrained/dissolved gas is
removed and vented to the atmosphere. The
regenerated seal oil is then recirculated back to the
compressor seal oil system.
Operating Requirements
In order to implement this recovery and use of gas
separated from seal oil there must be a use for the
recovered gas. Operators may configure the wet seal
degassing recovery system to route the recovered gas as
low pressure heater/boiler fuel (approximately 50
pounds per square inch gauge [psig]), high pressure
turbine fuel (approximately 250 psig), to compressor
suction, flare sweep gas, or any combination of the four
options. One Partner has configured their wet seal
degassing recovery systems to route recovered gas from
multiple centrifugal compressors to all four of these
choices. If routed to fuel gas use, the small amount of
entrained seal oil mist in the gas stream from the new
seal oil high pressure gas disengagement separator
(shown in red in Exhibit 2 above) requires a
demister/filter knock-out vessel (s) be installed to
remove residual oil.
Applicability
Wet seal degassing recovery systems could be installed
at most locations with wet seal centrifugal compressors,
particularly in lieu of retrofitting compressors with dry
seals. Retrofitting wet seal centrifugal compressors with
dry seals is not always feasible due to operating
conditions/requirements, substantial costs, and
compressor downtime. Wet seal degassing recovery
systems offer an economic alternative to installing dry
seals to mitigate methane emissions from wet seal
degassing with very little downtime.
Methane Emissions
According to previous measurements of seal oil
degassing reported in Bylin, et al. (2009), emissions
from degassing centrifugal compressor seal oil can be as
high as 185 standard cubic feet per minute (scfm) (5.2
cubic meters (m3) per minute) of gas per compressor,
with an average value of 63 scfm (1.8 m3 per minute) of
gas per compressor. Gas volume from degassing
centrifugal compressor seal oil varies primarily due to:
the number of seals per centrifugal compressor, the rate
of seal oil circulation, the size of the compressor, and
the compressor discharge pressure. One Partner
currently operates wet seal degassing recovery systems
that recover 99 percent of the degassing emissions from
fifteen centrifugal compressors - four low pressure
compressors, nine high pressure compressors, and two
tandem compressors at one facility. Based on a study
conducted at this particular facility, the wet seal gas
recovery system captures 3,300 scfm of gas; that
equates to 1.6 billion cubic feet (Bel) of gas per year
(8,500 hours of operation per year as indicated by the
Partner). This Partner reports another approximately
85 wet seal centrifugal compressors with the same or
similar seal oil gas recovery systems in this particular
operation.
Economic Analysis
The analysis for installing a wet seal degassing recovery
system should consider the capital and operational costs
along with the revenue generated by recovered gas. As
shown in the overview table (Economic and
Environmental Benefits) above, the economics for
recovering wet seal degassing losses are compelling at
even low gas prices. The detailed economic analysis in
Exhibit 3 below presents two scenarios. The first
scenario depicts the wet seal degassing recovery system
installed for one centrifugal compressor, and the second
scenario illustrates a wet seal degassing recovery
system installed at a centrifugal compressor station
with four centrifugal compressors. These examples are
based on one Partner's emissions measurements,
savings, and installed equipment costs which have high
pressure gas disengagement separators on each seal of
15 compressors, with one recovered gas two stage
filter/demister system.
Capital Costs
The investment per compressor to recover seal oil
degassing emissions using this technology includes the
cost of a high pressure seal oil gas disengagement
separator for each compressor seal or pair of seals
operating at the same seal oil pressure, new piping, the
appropriate pressure and flow controls, and the labor to
design and install the equipment. Operating and
maintenance (O&M) costs are expected to be minimal.
The investment per station to condition the recovered
degassing emissions for injection into a low pressure
fuel system would include the cost of a gas demister to
remove residual seal oil from the recovered gas. If the
recovered gas is routed to a higher pressure turbine fuel
system, a second seal oil high efficiency filter/separator
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Wet Seal Degassing Recovery System for Centrifugal Compressors
is likely required to ensure all remaining seal oil mist is
removed.
The capital cost (estimated) for each separator includes
the purchase cost of the process vessel, piping,
instrumentation, structural support, electrical,
painting, shipping, insurance, and installation. Exhibit
3 summarizes the estimated capital costs for each piece
of equipment installed in a wet seal degassing recovery
system.
High Pressure Seal Oil Gas Disengagement Separators
For each centrifugal compressor connected to the wet
seal degassing recovery system, a seal oil gas separator
will be installed for each seal or pair of seals if
operating at the same seal oil pressure. The seal oil gas
separators can be designed to operate at the same
pressure as the seal oil exiting the seal housing. Based
on a Partner's installation, the size of these separators
assumes 1-foot diameter, 3-foot height seal oil gas
separators with a 1,125 psig design pressure and made
of carbon steel. The total cost of each seal oil gas
separator is $9,500; assuming two seal-oil/gas
separators per compressor, the total cost per compressor
is $19,000.
Seal Oil Recovered Gas Demister/Filter
Before the recovered gas can be sent to a fuel line it
must pass through at least one high-efficiency demister
to remove entrained seal oil that may foul burners and
potentially clog fuel injectors. The seal oil gas demister
can be designed to receive recovered gas from multiple
centrifugal compressors with wet seals. Therefore, the
design characteristics of this vessel will vary depending
on the number of centrifugal compressors connected to
the vessel. Based on a Partner's installation, this
economic analysis assumes, for a single centrifugal
compressor, the seal oil gas demister will be vertical
and have a 1-foot diameter and 4-foot height with a
design pressure of 720 psig. The cost for a seal oil
single-stage demister/filter system designed for a single
centrifugal compressor or a compressor station with
four centrifugal compressors is $9,000.
If any of the recovered gas is being used as high
pressure turbine fuel, the recovery system may include
a second seal oil gas high-efficiency filter to ensure trace
amounts of seal oil do not foul the turbine fuel injectors.
This economic analysis assumes the seal oil gas filter
for turbine fuel quality gas will be vertical and have a 1-
foot diameter and 3-foot height with a design pressure
of 300 psig and be made of carbon steel. The cost for the
second seal oil high-efficiency filter for a single
centrifugal compressor or a compressor station is
$5,000.
Piping and Instrumentation
Operators will need to route seal oil exiting the
compressor seal to the new seal-oil/gas separator and
then to the degassing tank which will require piping
modifications. Additionally, pipes and valves to
transport the recovered gas from the seal oil gas
disengagement separators to the seal oil gas
demister/filters and further to the fuel gas, compressor
suction or flare line(s) will be required. Pressure and
flow controls will also need to be installed to regulate
the pressure and flow of the recovered gas from each
high pressure seal oil degassing separator. In this
Partner's installations this is accomplished with a
critical orifice (1/16 inch/1.59 mm) on the gas outlet
from each seal-oil/gas separator which restricts gas flow
using choke flow effects. For example, this Partner has
set up the wet seal degassing recovery system to route
captured gas to a high pressure turbine fuel line, low
pressure process heater fuel line, and flare purge which
requires additional instrumentation and flow control.
The majority of the gas is routed to the high pressure
Exhibit 3: Wet Seal Degassing Recovery System Installation and Equipment Costs
Equipment
One Centrifugal Compressor
Capital Cost ($2011)
Centrifugal Compressor
Station Capital Cost ($2011)
Seal-Oil/Gas Separator1
$19,000
$76,000
Seal Oil Gas Demister - Low Quality Gas
$9,000
$9,000
Seal Oil Gas Demister - High Quality Gas
$5,000
$5,000
Total
$33,000
$90,000
1 Assuming two seals per centrifugal compressor and four centrifugal compressors at the station. An individual high pressure seal oil gas separator
costs $9,500 per seal.
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Wet Seal Degassing Recovery System for Centrifugal Compressors
turbine fuel line with excess gas routed to the low
pressure process heater fuel line or flare, depending on
fuel requirements. The piping and instrumentation
costs are included in the installation and capital costs
for each individual piece of equipment listed in
Exhibit 3.
Estimated Sa vings
The savings for capturing seal oil degassing emissions
are realized by generating additional gas sales and
revenue through using recovered gas for site fuel gas or
recycling directly to suction and sales or processing.
Based on the measurement studies discussed in Bylin,
et al. (2009), the average methane emissions from
centrifugal compressor wet seal degassing is 63 scfm
(1.8 m3 per minute) of gas per compressor. Assuming
8,000 hours of operation per year, the total gas
emissions per year is 30 million cubic feet (MMcl) per
compressor. Based on one Partner's experience, over 99
percent of the entrained gas in the seal oil was captured
and used as fuel - proving this technology performs as
well as dry seal systems from an emission perspective.
This analysis assumes 99 percent of the potential
degassing emissions are captured and 1 percent is
vented. Therefore, 30 MMcf of gas is recovered per
centrifugal compressor and displaces the same volume
in fuel gas or is routed to compressor suction and sales.
At $3.00 per thousand cubic feet (Mcl), savings from
reduced fuel gas consumption is estimated to be $90,000
per year per centrifugal compressor connected to a wet
seal degassing recovery system. A station with four
centrifugal compressors connected to a wet seal
degassing recovery system could potentially generate
$360,000 per year of additional revenue.
Comparing Costs to Savings
The economics of implementing a wet seal degassing
recovery system is shown below using a five-year cash
flow table. This analysis presented in Exhibit 4 and
Exhibit 5 considers capital costs and methane emissions
savings. The capital and installation costs in this
economic analysis assume the operator installs both the
seal oil gas demister/filter separator for low pressure
fuel gas and seal oil gas high efficiency filter for high
pressure turbine fuel gas, which will allow the operator
to route to a flare, low pressure burner, or turbine fuel
line. Exhibit 3 shows the detailed breakdown of the
equipment and capital cost used in this economic
analysis. It is important to note that all analyses will be
highly site-specific, but the economics of installing a wet
Exhibit 4: Wet Seal Degassing Recovery System Costs and Savings for One Compressor
Costs and Savings ($)
YearO
Year 1
Year 2
Year 3 Year 4 Year 5
Wet seal recovery system capital &
installation costs
($33,000)



Annual natural gas savings
($3.00/Mcf of Methane)

$90,000
$90,000
$90,000 $90,000 $90,000
NPV (Net Present Value) = $280,000
IRR (Internal Rate of Return) = 270%
Payback Period = 5 months
Costs and Savings ($)
YearO
Year 1
Year 2
Year 3 Year 4 Year 5
Wet seal recovery system capital & installation
costs
($90,000)



Annual natural gas savings
($3.00/Mcf of Methane)

$360,000
$360,000
$360,000 $360,000 $360,000
NPV (Net Present Value) = $1,200,000
IRR (Internal Rate of Return) = 400%
Payback Period = 3 months
Exhibit 5: Wet Seal Degassing Recovery System Costs and Savings for Four Compressors at a Station
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Wet Seal Degassing Recovery System for Centrifugal Compressors
seal gas recovery system are so attractive that
companies should consider implementing this
technology at any and all facilities with at least one
centrifugal compressor with wet seals.
Exhibit 4 presents the economics of installing a wet seal
degassing recovery system for a single centrifugal
compressor. Exhibit 5 presents the economics of
installing a wet seal degassing recovery system at a
station with four centrifugal compressors. The net
present value and payback period are calculated using a
10 percent discount rate.
Operational Reliability
The Partner reporting the seal oil degassing systems
reports them being in place with constant use since
1977 and no failures due to the degassing systems.
With about 100 such systems installed and operating
from very low pressures to very high pressures across a
range of gas compositions, the operational reliability of
seal oil degassing systems has been clearly
demonstrated.
Discussion
A wet seal oil degassing recovery system is likely to
provide the lowest cost for retrofit with the quickest
payback for compressor stations, offshore production
platforms or gas processing plants with single or
multiple wet seal centrifugal compressors. All options
pay back in less than a year. The economics are
compelling, but the installation may require a brief
shut-down of each compressor to tie the seal oil
circulation piping into the new gas disengagement
vessels. New facilities requiring the installation of wet
seal centrifugal compressors can also integrate wet seal
degassing recovery systems into their process design at
the same costs. As demonstrated by a Partner facility,
wet seal degassing recovery systems are highly effective
at capturing degassing emissions from wet seal
centrifugal compressors and subsequently reducing fuel
gas purchases and/or increasing gas sales with a high
rate of return.
References
Biegler, et al. "4.3.1 Guthrie's Modular Method."
Systematic Methods of Chemical Process Design.
Ed. Neal R. Amundson. Saddlewood: Pearson,
pages 133 to 135. 1997.
Bylin, C., et al. Methane's Role in Promoting
Sustainable Developmen t in the Oil and Natural
Gas Industry. 24th World Gas Conference Paper.
Buenos Aires, Argentina. 5-9 October 2009.
www.epa.gov/gasstar/documents/
best_paper_award.pdf.
EPA. Natural Gas STAR Lessons Learned: Replacing
Wet Seals with Dry Seals in Cen trifugal
Compressors. October 2006.
www.epa.gov/gasstar/documents/ll_wetseals.pdf.
Turton, et al. "7.3.2 Module Costing Technique."
Analysis, Syn thesis, and Design of Chemical
Processes. 3rd edition. Pearson Education, Inc.,
pages 192 to 209. 2008.
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