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 highpressure 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 for a two seal system), 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

11months. 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. 

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 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. 

Method for Reducing 

Natural Gas Losses 

Volume of 

Natural Gas 

Savings (Mcf) 

Value of Natural Gas Savings ($) Implementation 

Cost ($) 

Payback (Months) 

$3 per Mcf $5 per Mcf $7 per Mcf $3 per 

Mcf 

$5 per 

Mcf 

$7 per 

Mcf 

Replacing wet oil 

seals with dry seals 

in centrifugal 

compressors 

45,120a per 

year 

$135,360 

per year 

$225,600 

per year 

$315,840 

per year $324,000b 29 18 

General Assumptions: 

a 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 (2 seals) operating 8,000 hrs/yr. b Based on replacement of a fully functioning wet seal with additional $102,400 in O&M cost reductions. 

13 

Economic and Environmental Benefits 

1 
Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

Exhibit 1: Wet Seals 

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 

Exhibit 2a: Dry Seal 

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. 

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. 

Exhibit 2b: Dry Seal 

2 
Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

Economic and Environmental Benefits 

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 (1 to 6 

scfm for a two seal system), 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 seals 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 to 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 to 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. 

Decision Process 

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. 

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

indentified 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 or 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

included 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 

3 
 

 

Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

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. 

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 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. 

Exhibit 3: Dry Seal Performance Chart 

This process is applicable to all compressor designs. The 

less common overhung compressors have a single seal, and 

The average methane content of natural gas varies by natural gas 

industry sector. The Natural Gas STAR Program assumes the 

following methane content of natural gas when estimating 

methane savings for Partner Reported Opportunities. 

Production 79 % 

Processing 87 % 

Transmission and Distribution 94 % 

Methane Content of Natural Gas 

switching from wet to dry seals would yield half the 

savings of doing the same for a beam type compressor. 

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 a7.5-

inch shaft diameter, operating 8,000 hours per year. 

Exhibit 4: Dry Seal Annual O&M Costs Savings 

per Compressora 

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 

a

S.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 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 

4 
Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

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, 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. 

Estimated Savings 

Another way of illustrating the economics of this practice

is through a five year cash flow table. This analysis 

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 by 

the February 2006 Nelson-Farrar index number, and, 

finally multiply by the appropriate costs in the Lessons 

Learned. 

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

functional wet seal system with a dry seal system. 

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 

Exhibit 5: Cost Comparison for 6-Inch Shaft Beam 

Type Compressor Seal Replacement 

Cost Category Dry Seal ($) Wet Seal ($) 

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 0b 

Total Implementation Costs 324,000 81,000 

Annual O&Mc 14,100 102,400 

Annual methane emissionsd

 (@$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 

Implementation Costs a 

a Flowserve Corporation, adjusted to 2006 equipment costs. b Re-use existing seal oil circulation, degassing, and control equipment. 

c From Exhibit 4; assumes same O&M costs as 7.5-inch shaft. 

d Based on typical vent rates. 

5 
 

 

 

 

Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

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. 

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 

Exhibit 6A: Economics of Replacing a Fully Functional Wet Seal System with a New Dry Seal System 

Costs and Savings ($) Year 0 Year 1 Year 2 Year 3 

Dry seal capitol & installation costs (324,000) 

Annual natural gas savingsa 315,840 315,840 315,840 

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

Wet seal salvage value 20,000 

NPV (Net Present Value)b

 = $1,228,009 

IRR (Internal Rate of Return) = 131% 

Payback Periodc

 = 10 months 

Retrofit of dry gas seals on a “beam” type compressor, 6-inch shaft, operating 8,000 hours per year, with fully functional wet seals. 

Year 4 

315,840 

(14,100) 

Year 5 

315,840 

(14,100) 

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 

a Annual savings represent the difference of natural gas loss between new dry seals and replaced wet seals, at $7.00/Mcf. b

 NPV based 

on 10% discount over five years. C

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

scfm. 

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

Costs and Savings ($) Year 0 Year 1 Year 2 Year 3 

Dry seal capitol & installation costs (324,000) 

Annual natural gas savings 315,840 315,840 315,840 

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

Wet seal salvage value 0 

NPV (Net Present Value) = $1,350,542 

IRR (Internal Rate of Return) = 134% 

Payback Period = 9 months 

Retrofit of dry gas seals on a “beam” type compressor, 6-inch shaft, operating 8,000 hours per year, with fully functional wet seals. 

Year 4 

315,840 

(14,100) 

Year 5 

315,840 

(14,100) 

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 

a Annual savings represent the difference of natural gas loss between new dry seals and replaced wet seals, at $7.00/Mcf. b

 NPV based 

on 10% discount over five years. 

6 
 

Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

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 yearly

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. 

References 

American Society of Mechanical Engineers Gas Turbine Conference and 

Exhibition. June 1987. 

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. 

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. 

Exhibit 7: Gas Price Impact on Economic Analysis 

Value of Gas Saved 

Payback Period 

(Months) 

Internal Rate of Return (IRR) 

NPV (i=10%) 

$3/Mcf $5/Mcf $7/Mcf $8/Mcf 

$135,360 $225,600 $315,840 $360,960 

17 12 10 9 

68% 100% 131% 146% 

$543,847 $885,928 $1,288,009 $1,399,049 

$10/Mcf 

$451,200 

7 

176% 

$1,741,129 

7 
Replacing Wet Seals with Dry Seals In Centrifugal Compressors 

(Cont’d) 

United States 

Environmental Protection Agency 

Air and Radiation (6202J) 

1200 Pennsylvania Ave., NW 

Washington, DC 20460 

October 2006 

EPA provides the suggested methane emissions estimating methods contained in this document as a tool to develop basic methane emissions estimates only. As 

regulatory reporting demands a higher-level of accuracy, the methane emission estimating methods and terminology contained in this document may not conform to 

the Greenhouse Gas Reporting Rule, 40 CFR Part 98, Subpart W methods or those in other EPA regulations. 

8