1. Methane Emission Reductions from Reciprocating Compressors
Lessons Learned
from Natural Gas STAR
EPA’s Natural Gas STAR Program,
Pioneer Natural Resources USA, Inc., and
The Gas Processors Association
June 17, 2003
Compressors are the workhorse of the Natural Gas Industry, providing the motive power to move gas throughout the production, processing, transmission and distribution systems.
Wherever there is a compressor, there are seals designed to minimize the leakage of compressed gas out of the high pressure compressor cylinder or case, into the environment.
Because natural gas has been viewed as low cost, abundant, and of no environmental detriment, the Natural Gas Industry has used compressor seal designs and practices that release more gas to the atmosphere than is economically practical by today’s standards.
This presentation presents methods and technologies that economically reduce the natural gas, and methane emissions from compressors.

2. Methane Emission Reductions from Reciprocating Compressors
Introduction to Compressors
Methane Losses
Steps to Reduce Methane Losses
Cost Effectiveness
Future Trends
This presentation:
Describes common compressor seal technologies;
Explains methane losses from these devices;
Discusses some methods for reducing those methane losses;
Shows how these methods can be cost effective; and
identifies some directions that compressor seals can take in the future when the economics favor higher technology and practices.

3. Location and Types of Compressors
Compressors are found throughout the natural gas industry.
At the well-head of gas and crude oil with associated gas production where the well-head flowing pressure has declined
There are about 25,000 production compressors in the U.S.
Gathering/booster stations pick-up the gas from several wells and boost it into gas processing plants.
Most of the gas processing operations are carried out at elevated pressures to induce condensation of gas liquids at ambient temperatures
Residue gas, primarily methane with some ethane, is compressed for delivery to the transmission cross-country transmission systems
There are about 5,000 methane compressors in the U.S. processing sector
Transmission pipelines re-compress the gas every few hundred miles as it is economically transported from remote production regions to industrial, commercial and residential centers for consumption
Gas is sometimes stored in underground caverns or depleted reservoirs near the consumer locations, for re-production during peak demands
There are about 8,500 transmission and storage compressors
The distribution systems do not usually operate compressors, but rather control the pressure reduction and delivery up to the consumer’s meter.

4. Reciprocating Compressor Rod Packing Systems
There are two basic designs for large, high pressure compressors: reciprocating and centrifugal. This diagram shows a reciprocal, or “piston” compressor.
The piston’s reciprocal motion alternatively draws in, compresses and discharges gas on both sides of the piston.
On the inboard side, the piston rod passes outside the cylinder through a packing box, which seals against excessive gas leakage without binding the rod movement
The packing box contains the seal, source of methane leakage
The lower diagram shows a sectional view of the packing box, and an exploded view of one of the packing cups containing two seal rings.
Rings are in sections, and held tight against the rod by a spring
Seals wear in use from rubbing on the piston rod
Lubrication slows wear and also improves the seal by filling spaces around the rings with viscous fluid
Depending on the smoothness of the piston rod and compressor operating hours, the rings will wear enough to require replacing in one to five years.
A longer time between ring replacement results in more gas leakage
Also, more wear on the seals results in more abrasion on the rod shaft, which also increases gas leakage and hastens ring wear.

5. Methane Losses
Compressor seals are fourth largest emission source at 16 Bcf/yr
Leakage typically occurs
Around the packing case nose gasket
Between the packing cups
Around rings from slight movement in the cup
Between the rings and shaft
All packing systems normally leak
New systems lose ~ 60 scfh
Badly worn systems lose ~ 900 scfh
Recalling the major sources of methane leakage from the natural gas industry:
Pneumatic controllers
Compressor seals
Vents, and
Valves and piping connections;
Compressor seals are the fourth largest emission source at 16 Bcf/yr
Compressor blowdown vents are a major portion of the second largest emission source: venting.
Reciprocating compressor seal leakage typically occurs:
Around the seal case
Between the seal cups stacked in the packing box
Around the rings from their movement in the cup, and
Between the rings and the piston rod shaft.
All compressor seals are designed to leak some gas
Reciprocating seals leak up to 60 scfh when newly installed, and have been measured as high as 900 scfh by Gas STAR Partners.

6. Steps to Reduce Methane Losses
Leakage can be reduced though monitoring and economic replacement
Conventional packing rings need to be replaced every 3 to 5 years
An economic leak rate can be determined based on costs and gas savings
Replacing rings when it is economical
Saves gas and money
Extends the life of the compressor rod
While there are many different types and materials used for reciprocating compressor rings and piston rod coatings to economically extend the life of the seal, all systems eventually need to have the rings replaced.
Conventional bronze metallic rings need to be replaced every 3 to 5 years, assuming normal wear on the piston rod shaft and proper alignment of the shaft.
Newer graphite filled TeflonTM rings may last a year longer with a very good piston rod shaft, or much shorter with a warn shaft.
The best practice among Gas STAR Partner operators is to establish an economic leak rate to replace the seals at the individual company’s economic hurdle rate.
Seal leakage can be monitored with a “Directed Inspection and Maintenance” program, using a device like a “High Flow Sampler” to measure seal leakage.
The High Flow Sampler was developed by the U.S. Gas Research Institute in conjunction with the U.S. EPA and Indaco, Inc.
It is a man-portable mass flow metering instrument.
Replacing seal rings when economic saves money, saves methane emissions and extends the life of the much more expensive piston rod.

7. Economic Replacement Threshold (scfh) =
Economic Analysis
Partners should develop an “economic replacement threshold” that defines the point when it is cost-effective to replace rings and rods
Economic Replacement Threshold (scfh) =
(CR * DF) / [(H * GP) /1,000]
where:
CR = cost of replacement ($)
DF = capital recovery
H = hours of compressor operation
GP = gas price ($/Mcf)
DF = i (1+ i)n /[(1+ i)n –1]
For economic replacement of reciprocating compressor rod packing rings:
An economic replacement threshold can be calculated with the equation shown.
Economic is defined by the individual company in terms of a discount rate for the current time value of money, and the desired payout period.
The economic replacement threshold, in scfh, is equal to:
The cost of equipment and labor to replace the rings, CR
Multiplied by the capital recovery factor at the discount rate and desired payout period, DF
Divided by the hours per year that the compressor typically operates, H
And divided by the price of natural gas, GP (adjusted for units: $/Mcf divided by 1,000 cf per Mcf.
The economic replacement leakage threshold calculated is the INCREASE in leakage over that leakage experience when the last seals were newly installed.
It is necessary to measure leakage of newly installed seals to properly determine when the economic increase has been reached.
Over time, as the shaft wears, the initial leak rate with new seal rings will also increase.

8. Economic Analysis Compressor Rod Packing System
Economic Replacement
Threshold
for Packing Rings
Economic Replacement
Threshold
for Rod and Rings
These tables show results of the economic replacement threshold leak rates for replacing just the rings on the left, and both the rings and piston rod on the right.
These tables are constructed using a ring replacement cost of $1,200,
Using a natural gas value of $3.00 per Mcf,
For a compressor operating 8,000 hours per year,
And with a discount rate of 10%.
The second table further uses a piston rod replacement cost of $7,000.
The tables are read as follows:
With the above costs and factors, the rings for this compressor will have a two year economic payout when the leakage rate has increased by 43 scfh.
In other words, if the initial leak rate when the current rings were last installed was 60 scfh, a two year payout can be achieved when the leakage increases to 103 scfh.
This leakage may occur over a shorter or longer time, depending on the condition and alignment of the piston rod, and the lubrication of the seal.
Both the rings and rod can be replaced with a two year payout when the the leakage rate increases 295 scfh over the initial rate when the rod was last installed.

9. Partner Experience
One partner conducted semi-annual inspections of compressor rod packing
Replaced packing cases at eight stations costing $1,050 per case, installed
Saved 55 MMcf/yr valued at $165,000
This slide shows Natural Gas STAR Partner companies who have reported these practices.
The first company conducted a semi-annual inspection of compressor rod packings, measuring the leakage rates.
They replace packing rings at eight stations, costing an average of $1,050 per packing case.
This saved 55 million cubic feet per year, valued at $165,000.

10. Future Trends
Install axially loaded rings in reciprocating compressors
Install combination rings that serve as a static seal when compressors are shut down and kept pressurized
Vented and purged seals
The first measure installs a different design of the rings in reciprocating compressors.
Axially loaded rings have been used in the refining and petrochemical industries for years.
These rings do not have smooth sides like the current gas industry rings, but rather a shoulder that, presses the rings against each other in the cup as well as against the piston rod.
This reduces leakage around the rings.
Each cup has five rings, and must be a deeper cup, so, retrofitting a compressor involves either fewer cups or a deeper packing case.
The second measure is a design for rings that function as normal when in service, but can be pressure loaded with nitrogen when the compressor is shut down, and form a static seal around the piston rod, reducing leakage off-line.
The third measure is to adopt packing designs again used today in oil refineries and petrochemical plants on both reciprocating and centrifugal compressors.
Vented packing routes the gas leakage between the seals, in the middle of the packing, to a control device (flare) or vapor recovery system.
Purged packing goes one step further by putting a purge gas, like nitrogen, into the packing system preventing any gas leakage.

11. Axially Loaded Rings
Source: Compressor Engineering Corporation

12. Three Ring Rod Packing
Three ring rod packing is becoming more wide spread
The rings are typically installed in one of the last two cups
This design could be installed without any replacement or modification on the packing case cup

13. Discussion Questions
To what extent are you implementing this Lesson Learned?
How can this Lesson Learned be improved upon or altered for use in your operation(s)?
What are the barriers (technological, economic, lack of information, regulatory, etc.) that are preventing you from implementing this technology?