Lessons
Learned
NaturalGas
EPA POLLUTION PREVENTER
From Natural Gas STAR Partners
REPLACING WET SEALS WITH DRY SEALS IN CENTRIFUGAL
COMPRESSORS
Executive Summary
Centrifugal compressors are widely used in production and transmission of natural gas. Seals on the rotating
shafts prevent the high-pressure natural gas from escaping the compressor casing. Traditionally, these seals used
high-pressure oil as a barrier against escaping gas. Natural Gas STAR partners have found that replacing these
"wet" (oil) seals with dry seals significantly reduces operating costs and methane emissions.
Methane emissions from wet seals typically range from 40 to 200 standard cubic feet per minute (scfm). Most of
these emissions occur when the circulating oil is stripped of the gas it absorbs at the high-pressure seal face. Dry
seals, which use high-pressure gas to seal the compressor, emit less natural gas (up to 6 scfm), have lower
power requirements, improve compressor and pipeline operating efficiency and performance, enhance compres-
sor reliability, and require significantly less maintenance.
Although dry seal conversions might not be possible on some compressors because of housing design or opera-
tional requirements, partners should select dry seals over wet seals whenever they replace or install centrifugal
compressors where possible. A dry seal can save about $315,000 per year and pay for itself in as little as 11
months. One Natural Gas STAR partner who installed a dry seal on an existing compressor, for example, reduced
emissions by 97 percent, from 75 to 2 Mcf per day, saving almost $187,000 per year in gas alone.
Method for Reducing
Methane Emmissons
Replacing wet oil seals with dry
seals in centrifugal compressors
Volume of
Natural Gas
Savings (Mcf/yr)
45,1 201
Value of
Natural Gas
Savings ($/yr)
$315,840
Cost of
Implementation
($)
$324,0002
Payback
(months)
103
0
1 Based on the difference between typical vent rates of wet and dry seals (i.e., 100 scfm versus 6 scfm) on a "beam" type compressor operating 8,000 hrs/yr.
2Valueofgas = $7.00/Mcf.
3 Based on replacement of a fully functioning wet seal with additional $102,400 in O&M cost reductions.
This is one of a series of Lessons Learned Summaries developed by EPA in cooperation with the natural gas industry on superior
applications of Natural Gas STAR Program Best Management Practices (BMPs) and Partner Reported Opportunities (PROs).
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Technology
Background
Wet Seals
Centrifugal compressors require seals around the rotating shaft to prevent
gases from escaping where the shaft exits the compressor casing. The more
common "beam" type compressors have two seals, one on each end of the
compressor, while "over-hung" compressors have a seal on only the
"inboard" (motor) side. As shown in Exhibit 1, these seals use oil, which is
circulated under high pressure between three rings around the compressor
shaft, forming a barrier against the compressed gas leakage. The center ring
is attached to the rotating shaft, while the two rings on each side are station-
ary 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 pre-
vent leakage around the stationary rings. Very little gas escapes through the
oil barrier; considerably more gas is absorbed by the oil under the high pres-
sures at the "inboard" (compressor side) seal oil/gas interface, thus contami-
nating the seal oil. Seal oil is purged of the absorbed gas (using heaters,
flash tanks, and degassing techniques) and recirculated. The recovered
methane is commonly vented to the atmosphere.
Exhibit 1: Wet Seals
Seal Housing
Motor and
Shaft Bearing Side
"Outboard"
Seal Oil Inlet
"Outboard"
Labyrinth
Seal Oil
(Uncontaminated)
Process Gas
Leaks Through "Inboard"
Labyrinth Seal
Compressor Side
"Inboard"
Spinning Shaft
Seal Oil
(Contaminated with Gas)
Dry Seals
An alternative to the traditional wet (oil) seal system is the mechanical dry
seal system. This seal system does not use any circulating seal oil. Dry seals
operate mechanically under the opposing force created by hydrodynamic
grooves and static pressure.
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As shown in Exhibits 2a and 2b, hydrodynamic grooves are etched into the
surface of the rotating ring affixed to the compressor shaft. When the com-
pressor is not rotating, the stationary ring in the seal housing is pressed
against the rotating ring by springs. When the compressor shaft rotates at
high speed, compressed gas has only one pathway to leak down the shaft,
and that is between the rotating and stationary rings. This gas is pumped
between the rings by grooves in the rotating ring.
The opposing force of high-pressure gas pumped between the rings and
springs trying to push the rings together creates a very thin gap between the
rings through which little gas can leak. While the compressor is operating,
the rings are not in contact with each other, and therefore, do not wear or
need lubrication. O-rings seal the stationary rings in the seal case.
Exhibit 2a: Dry Seal
Rotating
Ring
Compressor End
Process Gas Tries
to Leak Between Rings
Stationary Ring
Spinning Shaft
Lr
Exhibit 2b: Dry Seal
Tandem Rotating Rings
with Grooves
Process Gas
Leaks Through
Labyrinth
Very Little
Process
Gas Leakage
(Fugitive)
Motor
End
Spinning
Shaft
Gas Pressure Between
Rings Prevents Process Gas
From Leaking
Spring Pushes Stationary
Ring Against Rotating Ring
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Economic and
Environmental
Benefits
Decision
Process
Putting two or more of these dry seals together in series, as shown in Exhibit
2b, is called "tandem dry seals," and is very effective in reducing gas leak-
age. This type of seal has less than one percent of the leakage of a wet seal
system vented into the atmosphere and costs considerably less to operate.
Dry gas seals substantially reduce methane emissions. At the same time,
they significantly reduce operating costs and enhance compressor efficiency.
Economic and environmental benefits of dry seals include:
* Gas Leak Rates. During normal operation, dry seals leak at a rate of
0.5 to 3 scfm across each seal, depending on the size of the seal and
operating pressure. While this is equivalent to a wet seal's leakage rate
at the seal face, wet seals generate additional emissions during
degassing of the circulating oil. Gas from the oil is usually vented to
the atmosphere, bringing the total leakage rate for dual wet seals to
between 40 and 200 scfm, depending on the size and pressure of the
compressor.
* Mechanically Simpler. Dry seal systems do not require elaborate oil cir-
culation components and treatment facilities.
* Reduced Power Consumption. Because dry seals have no accessory
oil circulation pumps and systems, they avoid "parasitic" equipment
power losses. Wet systems require 50 to 100 kW per hour, while dry
seal systems need about 5 kW of power per hour.
* Improved Reliability. The highest percentage of downtime for a com-
pressor using wet seals is due to seal system problems. Dry seals
have fewer ancillary components, which translates into higher overall
reliability and less compressor downtime.
* Lower Maintenance. Dry seal systems have lower maintenance costs
than wet seals because they do not have moving parts associated
with oil circulation (e.g., pumps, control valves, relief valves).
* Elimination of Oil Leakage from Wet Seals. Substituting dry seals for
wet seals eliminates seal oil leakage into the pipeline, thus avoiding
contamination of the gas and degradation of the pipeline.
Partners usually face one of three situations when considering installation of
dry seals: they are replacing an entire compressor; they are replacing a
worn-out wet seal at an existing compressor; or they are replacing a fully
functioning wet seal at an existing compressor. About 90 percent of all new
compressors come with dry seals. When purchasing a new compressor,
partners should be sure that it includes a dry seal.
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The analysis for replacing a wet Four Steps for converting to Dry
seal on an existing compressor Seals:
should consider the methane
emissions savings along with 1 • lde"tify cantdidates for wet seal
replacement.
capital and operational costs and
2. Estimate the savings of a dry seal
benefits. The economics for retrofit.
replacing operating wet seals are 3 Determine the costs for conversion to
compelling, and wherever possi- dry seals.
ble, partners should undertake 4. Compare costs to savings.
such replacements. The decision
process below is a guideline for determining candidates, benefits, and costs
for replacing wet seals with dry seals in compressors.
Step 1: Identify candidates for wet seal replacement. Operators should
make a comprehensive inventory and technical evaluation of their existing
compressors. Factors to consider include compressor type, age, hardware,
and operating conditions. All wet seal compressors should be identified and
evaluated for dry seals. When deciding which compressors are candidates
for replacement of the wet seal, consider the following:
* Dry seals can be used for compressors up to 3,000 psi safely; appli-
cations of 1,500 psi are routine. Dry seals, however, might not be safe
for higher pressures. Further, dry seals are not appropriate for applica-
tions with temperatures above 300 to 400 degrees Fahrenheit (due to
O-ring material limitations)1. Some compressor designs prohibit the
retrofit of dry seals.
* Some older compressors might be at the end of their economic life
and, thus, are candidates for complete replacement rather than a seal
replacement. This is usually determined while planning a major over-
haul, when operating and maintenance (O&M) costs for the old com-
pressor are projected to increase to a level much greater than O&M
costs for a new unit. Some clues that this stage might have been
reached include sudden increases in the frequency and magnitude of
unscheduled maintenance and the unavailability of replacement parts
or lack of technical support.
Centrifugal compressors that meet the Step 1 criteria should be evaluated
further as follows.
Step 2: Estimate the savings of a dry seal retrofit. In general, the majority of
savings from replacing a wet seal with a dry seal are attributable to reduc-
tions in methane gas loss. To estimate these savings, partners can measure
the majority of methane loss from their wet seal compressors at the vent
from the seal oil degassing unit by bagging or using a high flow sampler.
Some gas also escapes at the seal face, but this is more difficult to measure
1John Stahley, Dresser-Rand Co.
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and amounts to less than 10 percent of emissions from the seal oil degassing
unit. Typical wet gas seal leakage ranges from 40 to 200 scfm for a beam
type compressor.
Methane Content of Natural Gas
Pipeline quality natural gas found in the transmission sector contains
approximately 93% methane. Methane emission reductions can be
approximated by applying the methane content of pipeline quality gas to
By comparison, expected losses from dry seals can be seen in Exhibit 3, a
performance chart provided by a dry seal vendor. This chart shows an
example of one type of tandem seal with leak rates ranging between 0.5 to
3 scfm for 1.5 to 10 inch compressor shafts, for compressors operating at
580 to 1,300 psig pressure. Replacing the wet seal with two tandem dry
seals can reduce emissions between 34 to 194 scfm. This is equivalent to
16,320 to 93,120 Mcf per 8,000-hour year, with total annual savings of
$114,240 to $651,840.
This process is applicable to all compressor designs. The less common over-
hung compressors have a single seal, and switching from wet to dry seals
would yield half the savings of doing the same for a beam type compressor.
Exhibit 3: Dry Seal Performance Chart
10" seal
S
CO
CB
03
CO
1K2"seal
10" seal
Ifc'seal
580 1300
Pressure (PSI)
Wote:This graph, utilizing hard vs. hard seal faces, is for reference only. Performance characteristics
may vary depending on equipment and service.
Source: BW/IP International, Inc., Seal Division. Durco International and BW/IP International, Inc.
are now known as FLOWSERVE Corporation.
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Beyond gas savings, dry seals also yield significant operational and mainte-
nance savings compared with wet seals. Annual O&M costs for dry seals
range widely, between $8,400 and $14,000 per year. Wet seal O&M costs
can reach up to $140,000 per year. Detailed calculations of the differences in
O&M costs between dry and wet seals are well documented (see Uptigrove
et al., 1987). Exhibit 4 summarizes these estimates for a compressor with a
7.5-inch shaft diameter, operating 8,000 hours per year.
Exhibit 4: Dry Seal Annual O&M Costs Savings per Compressor1
1. Reduced seal power losses = $19,500
2. Reduced oil pump/fan losses= $5,600
3. Increased pipeline flow efficiency = $37,300
4. Reduced oil losses = $4,900
5. Reduced O&M, downtime = $21,000
TOTAL SAVINGS = $88,300
1S.O. Uptigrove et al., adjusted to 2006 operating and maintenance costs.
Site specific factors used in the calculations include: (1) wet and dry seal
drag losses, (2) seal oil pump and cooling fan horsepower, (3) compressor
horsepower, (4) seal oil consumption, and (5) annual emergency and sched-
uled maintenance costs.
Step 3: Determine the costs for conversion to dry seals. The cost for a dry
seal system will depend on compressor operating pressure, shaft size, rota-
tion speed, and other installation-specific factors. Costs for the seal typically
range between $6,750 and $8,100 per inch of shaft diameter for wet seals
and $10,800 to $13,500 per inch for tandem dry seals. These costs will
double for beam type compressors (two seals).
Nelson Price Indexes
In order to account for inflation in equipment and operating & maintenance
costs, Nelson-Farrar Quarterly Cost Indexes (available in the first issue of each
quarter in the Oil and Gas Journal) are used to update costs in the Lessons
Learned documents.
The "Refinery Operation Index" is used to revise operating costs while the
"Machinery: Oilfield Itemized Refining Cost Index" is used to update equipment
costs.
To use these indexes in the future, simply look up the most current Nelson-
Farrar index number, divide that by the February 2006 Nelson-Farrar index num-
ber, and, finally, multiply by the appropriate costs in the Lessons Learned.
Other costs include engineering, installation and ancillary equipment. Dry
seals require a gas console, filtration unit, controls, and monitoring instru-
ments, while wet seals require the seal oil pumps, fan coolers, degassing unit,
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and controls. Depending on location, type of equipment, number of controls,
and availability of components, costs range from $40,500 to $135,000 for
dry seals, and up to $270,000 for wet seals. These ancillary facility costs are
the same for both the single and dual seal compressor types.
Step 4: Compare costs to savings. A simple cost comparison between con-
verting a compressor to dry seals and replacing existing wet seals with new
components will show substantial savings over a five-year period. Exhibit 5
shows an example for a beam type compressor with a 6-inch shaft operat-
ing for 8,000 hours per year using the costs from Steps 2 and 3.
In this example, implementation costs for a conversion to dry seals include
the cost of both the seals and the dry gas conditioning, monitoring, and
control console. For wet seals, the seal oil circulation, degassing, and cool-
ing ancillary facilities are reused, so only seal replacement costs are incurred.
Exhibit 5: Cost Comparison for 6-Inch Shaft Beam Type
Compressor Seal Replacement
Cost Category
Dry Seal
($)
Wet Seal
($)
Implementation Costs1
Seal costs (2 dry @ $13,000/shaft-inch, w/testing)
Seal costs (2 wet @ $6,750/shaft-inch)
Other costs (engineering, equipment installation)
Total Implementation Costs
Annual O&M3
Annual methane emissions4 (@ $7.00/Mcf; 8,000 hrs/yr)
2 dry seals at a total of 6 scfm
2 wet seals at total 1 00 scfm
Total Costs Over 5-Year Period ($):
Total Dry Seal Savings Over 5 Years:
Savings ($)
Methane Emissions Reductions (Met) (at 45,120 Mcf/yr)
1 Flowserve Corporation, adjusted to 2006 equipment costs.
2 Re-use existing seal oil circulation, degassing, and control
3 From Exhibit 4; assumes same O&M cost as 7.5-inch shaft
4 Based on typical vent rates.
162,000
162,000
324,000
14,100
20,160
495,300
1,777,700
225,600
equipment.
81,000
O2
81,000
102,400
336,000
2,273,000
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Estimated
Savings
Another way of illustrating the economics of this practice is through a five
year cash flow table. This analysis considers capital costs, methane emis-
sions savings, operating and maintenance costs, and assigns a salvage
value to the wet seal system. It is important to note that all analyses will be
highly site-specific, but the economics of a dry seal retrofit are so attractive
that companies should consider replacing all wet seals, regardless of age.
Exhibit 6A presents the economics of replacing a fully funtional wet seal sys-
tem with a dry seal system.
Exhibit 6A: Economics of Replacing a Fully Functional Wet Seal
System with a New Dry Seal System
Retrofit of dry gas seals on a "beam" type compressor, 6-inch shaft, operating
hours per year, with fully functional wet seals.
Costs and
Savings ($) Year 0 Year 1 Year 2 Year 3 Year 4
8,000
Years
Dry seal capital &
installation costs (324,000)
Annual natural
gas savings1 315,840 315,840 315,840 315,840
Dry seal annual O&M Costs (14,100) (14,100) (14,100) (14,100)
315,840
(14,100)
Wet seal salvage value 20,000
Avoided wet seal O&M Cost 102,400 102,400 102,400 102,400
Annual Totals (304,000) 404,140 404,140 404,140 404,140
102,400
404,140
NPV (Net Present Value)2 = $1,228,009
IRR (Internal Rate of Return) = 131%
Payback Period3 = 10 months
'Annual savings represent the difference of natural gas loss between new dry seals and replaced wet seals,
at $7.00/Mcf.
2 Net present value based on 10% discount over five years.
3Payback period ranges between 3 and 12 months for wet seal leakage rates between 200 and 40 scfm.
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Exhibit 6B shows the economics for replacing an old wet seal nearing the
end of its useful life: salvage value is zero and annual O&M costs for the wet
seal system increase (in this example, to $140,000 per year). These two
examples demonstrate that replacing a wet seal with a dry seal can be cost
effective regardless of the age or condition of the wet seal system.
Exhibit 6B: Economics of Replacing an Aging Wet Seal System with a
New Dry Seal System
Retrofit of dry gas seals on a "beam" type compressor, 6-inch shaft, operating 8,000
hours per year, with wet seals needing replacement.
Costs and
Savings ($)
Year 0 Year 1 Year 2 Year 3 Year 4 Year 5
Dry seal capital &
installation costs (324,000)
Annual natural
gas savings1 315,840 315,840 315,840
Dry seal annual O&M Costs (14,100) (14,100) (14,100)
315,840 315,840
(14,100) (14,100)
Wet seal salvage value 0
Avoided wet seal O&M Cost 140,000 140,000 140,000
Annual Totals (324,000) 441,740 441,740 441,740
140,000 140,000
441,740 441,740
NPV (Net Present Value)2 = $1 ,350,542
IRR (Internal Rate of Return) = 134%
Payback Period3 = 9 months
1. Annual savings represent the difference of natural gas loss between new dry seals and
$7.00/Mcf.
2. Net present value based on 10% discount over five years.
replaced wet seals, at
When assessing options for replacing wet seals with dry seals in centrifugal
compressors, natural gas price may influence the decision making process.
Exhibit 7 shows an economic analysis of early replacement of wet seals in a
centrifugal compressor with dry seals at different natural gas prices.
Exhibit 7: Gas Price Impact on Economic Analysis
Value of Gas
Saved
Payback Period
(Months)
Internal Rate
of Return (IRR)
Net Present
Value (1=10%)
$3/Mcf
$135,360
17
68%
$543,847
$5/Mcf
$225,600
12
100%
$885,928
$7/Mcf
$315,840
10
131%
$1,228,009
$8/Mcf
$360,960
9
146%
$1,399,049
$10/Mcf
$451,200
7
176%
$1,741,129
10
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Lessons
Learned
Partners can achieve significant cost savings and emissions reductions by
converting to dry seal technology. Partners offer the following lessons
learned when changing to dry seals:
* Dry seals are considered safer to operate than wet seals, because they
eliminate the need for a high pressure oil system.
* To make the switch to dry seals most efficiently, schedule the conversion
for a normal downtime period to avoid disrupting operations.
* When determining the benefits of a seal replacement, partners should
take into account that properly installed and maintained dry seals can
last more than twice as long as wet seals.
* If the wet seal is near the end of its useful life, a straightforward cost
analysis between new seal systems will favor the dry seal. Even if the
existing wet seal has substantial remaining useful life, the operational
characteristics of dry seals will provide significant savings and could jus-
tify early replacement.
* Given the clear economic advantages of dry seals, they should be
installed wherever it is technically feasible.
* Ninety percent of all new compressors now have dry gas seal systems.
Dry seals should be the technology of choice for all new compressors.
* After replacing wet seals with dry seals, record emissions reductions in
annual reports submitted as part of the Natural Gas STAR Program.
11
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Canadian Association of Petroleum Producers. Options for Reducing
Methane and VOC Emissions from Upstream Oil and Gas Operations.
December 1993.
Henderson, Carolyn. U.S. EPA Natural Gas STAR Program. Personal con-
tact.
Hesje, R.C. and R.A. Peterson. Mechanical Dry Seal Applied to Pipeline
(Natural Gas) Centrifugai Compressors. American Society of Mechanical
Engineering. Gas Turbine Conference and Exhibition. June 1984.
Kennedy, J.L. Oil and Gas Pipeline Fundamentals, Second Edition. PennWell
Books. 1993.
Klosek, Marty. Flowserve Corporation. Bridgeport, New Jersey. Personal
contact.
Ronsky, N. Daryl; Harris, T.A.; Conquergood, C. Peter; and Davies, I. An
Effective System for Sealing Toxic Gases in Centrifugal Compressors.
American Society of Mechanical Engineers Gas Turbine Conference and
Exhibition. June 1987.
Sears, John. Personal contact.
Stahley, John. Dresser-Rand Company. Clean, New York. Personal contact.
Tingley, Kevin. U.S. EPA Natural Gas STAR Program. Personal contact.
Uptigrove, S.O.; Harris, T.A.; and Holzner, D.O. Economic Justification of
Magnetic Bearings and Mechanical Dry Seals for Centrifugal Compressors.
American Society of Mechanical Engineers Gas Turbine Conference and
Exhibition. June 1987.
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United States
Environmental Protection Agency
Air and Radiation (6202J)
1200 Pennsylvania Ave., NW
Washington, DC 20460
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