Lessons

Learned

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 compressor reliability, and require significantly less maintenance.

Although dry seal conversions might not be possible on some compressors because of housing design or operational 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. 

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). 

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. 2 Value of gas = $7.00/Mcf.

3 Based on replacement of a fully functioning wet seal with additional $102,400 in O&M cost reductions.

Method for Reducing

Methane Emmissons 

Volume of 

Natural Gas 

Savings (Mcf/yr)

Value of 

Natural Gas

Savings ($/yr)

$324,000 Replacing wet oil seals with dry 2

seals in centrifugal compressors 45,1201 $315,840

Cost of

Implementation 

($)

103

Payback

(months)
2

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 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 absorbed by the oil under the high pressures at the “inboard” (compressor side) seal oil/gas interface, thus contaminating 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. 

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. 

Technology

Background

Exhibit 1: Wet Seals

Seal Housing Seal Oil Inlet

Motor and

Shaft Bearing Side

“Outboard”

“Outboard”

Labyrinth

Seal Oil

(Uncontaminated) Seal Oil

(Contaminated with Gas)

Spinning Shaft

Compressor Side

“Inboard”

Process Gas

Leaks Through “Inboard”

Labyrinth Seal
3

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 compressor 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

Spinning Shaft

Stationary Ring

Process Gas Tries

to Leak Between Rings Rotating

Ring

Compressor End

Exhibit 2b: Dry Seal

Tandem Rotating Rings

with Grooves Very Little

Process

Gas Leakage

(Fugitive)

Motor

End

Spinning

Shaft

Spring Pushes Stationary

Ring Against Rotating Ring

Gas Pressure Between

Rings Prevents Process Gas

From Leaking

Process Gas

Leaks Through

Labyrinth
4

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 leakage. 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 circulation 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 compressor 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.

Economic and

Environmental

Benefits

Decision

Process
5

The analysis for replacing a wet

seal on an existing compressor

should consider the methane

emissions savings along with

capital and operational costs and

benefits. The economics for

replacing operating wet seals are

compelling, and wherever possible, partners should undertake

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; applications of 1,500 psi are routine. Dry seals, however, might not be safe

for higher pressures. Further, dry seals are not appropriate for applications 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 overhaul, when operating and maintenance (O&M) costs for the old compressor 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 reductions 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

Four Steps for Converting to Dry

Seals:

1. Identify candidates for wet seal

replacement.

2. Estimate the savings of a dry seal

retrofit.

3. Determine the costs for conversion to

dry seals.

4. Compare costs to savings.

1

John Stahley, Dresser-Rand Co.
6

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.

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 overhung 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

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 
7

Beyond gas savings, dry seals also yield significant operational and maintenance 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. 

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 scheduled 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, rotation 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). 

Other costs include engineering, installation and ancillary equipment. Dry

seals require a gas console, filtration unit, controls, and monitoring instruments, while wet seals require the seal oil pumps, fan coolers, degassing unit,

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

1

S.O. Uptigrove et al., adjusted to 2006 operating and maintenance costs.

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 NelsonFarrar index number, divide that by the February 2006 Nelson-Farrar index number, and, finally, multiply by the appropriate costs in the Lessons Learned.
8

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 converting 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 operating 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 cooling ancillary facilities are reused, so only seal replacement costs are incurred.

Exhibit 5: Cost Comparison for 6-Inch Shaft Beam Type

Compressor Seal Replacement

Dry Seal Wet Seal 

Cost Category ($) ($)

Implementation Costs1

Seal costs (2 dry @ $13,000/shaft-inch, w/testing) 162,000

Seal costs (2 wet @ $6,750/shaft-inch) 81,000

Other costs (engineering, equipment installation) 162,000 02

Total Implementation Costs 324,000 81,000

Annual O&M3 14,100 102,400

Annual methane emissions4 (@ $7.00/Mcf; 8,000 hrs/yr) 

2 dry seals at a total of 6 scfm 20,160

2 wet seals at total 100 scfm 336,000

Total Costs Over 5-Year Period ($): 495,300 2,273,000

Total Dry Seal Savings Over 5 Years:

Savings ($) 1,777,700

Methane Emissions Reductions (Mcf) (at 45,120 Mcf/yr) 225,600

1 Flowserve Corporation, adjusted to 2006 equipment costs.

2 Re-use existing seal oil circulation, degassing, and control equipment.

3 From Exhibit 4; assumes same O&M cost as 7.5-inch shaft.

4 Based on typical vent rates.
Another way of illustrating the economics of this practice is through a five

year cash flow table. This analysis considers capital costs, methane emissions 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 system with a dry seal system.

Estimated

Savings

Retrofit of dry gas seals on a “beam” type compressor, 6-inch shaft, operating 8,000

hours per year, with fully functional wet seals.

Dry seal capital &

installation costs (324,000)

Annual natural

gas savings1 315,840 315,840 315,840 315,840 315,840

Dry seal annual O&M Costs (14,100) (14,100) (14,100) (14,100) (14,100)

Wet seal salvage value 20,000

Avoided wet seal O&M Cost 102,400 102,400 102,400 102,400 102,400

Annual Totals (304,000) 404,140 404,140 404,140 404,140 404,140

NPV (Net Present Value)2 = $1,228,009

IRR (Internal Rate of Return) = 131%

Payback Period3 = 10 months

1 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.

3 Payback period ranges between 3 and 12 months for wet seal leakage rates between 200 and 40 scfm.

Exhibit 6A: Economics of Replacing a Fully Functional Wet Seal

System with a New Dry Seal System

9

Costs and

Savings ($) Year 0 Year 1 Year 2 Year 3 Year 4 Year 5
10

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.

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.

Retrofit of dry gas seals on a “beam” type compressor, 6-inch shaft, operating 8,000

hours per year, with wet seals needing replacement.

Dry seal capital &

installation costs (324,000)

Annual natural

gas savings1 315,840 315,840 315,840 315,840 315,840

Dry seal annual O&M Costs (14,100) (14,100) (14,100) (14,100) (14,100)

Wet seal salvage value 0

Avoided wet seal O&M Cost 140,000 140,000 140,000 140,000 140,000

Annual Totals (324,000) 441,740 441,740 441,740 441,740 441,740

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 replaced wet seals, at

$7.00/Mcf.

2. Net present value based on 10% discount over five years.

Exhibit 7: Gas Price Impact on Economic Analysis

Internal Rate 

of Return (IRR)

Value of Gas

Saved

Net Present 

Value (i=10%)

Payback Period

(Months)

$3/Mcf

17

$135,360

68%

$543,847

$5/Mcf

$225,600

12

100%

$885,928

$7/Mcf

131%

10

$315,840

$1,228,009 $1,399,049

146%

9

$360,960

$8/Mcf $10/Mcf

$451,200

7

176%

$1,741,129

Costs and

Savings ($) Year 0 Year 1 Year 2 Year 3 Year 4 Year 5

Exhibit 6B: Economics of Replacing an Aging Wet Seal System with a

New Dry Seal System
11

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 justify 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. 

Lessons

Learned
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 contact.

Hesje, R.C. and R.A. Peterson. Mechanical Dry Seal Applied to Pipeline

(Natural Gas) Centrifugal 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. Olean, 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.

References
13

United States

Environmental Protection Agency

Air and Radiation (6202J)

1200 Pennsylvania Ave., NW

Washington, DC 20460

October 2006