Lessons Learned
from Natural Gas STAR Partners
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A
NaturalGasf\
EPA POLLUTION PREVENTER '
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Reducing Emissions When Taking
Compressors Off-Line
Executive Summary
Compressors are used throughout the natural gas industry
to move natural gas from production and processing sites
to customer distribution systems. Compressors must
periodically be taken off-line for maintenance, operational
stand-by, or emergency shut down testing, and as a result,
methane may be released to the atmosphere from a
number of sources. When compressor units are shut down,
typically the high pressure gas remaining within the
compressors and associated piping between isolation
valves is vented to the atmosphere ('blowdown') or to a
flare. In addition to blowdown emissions, a depressurized
system may continue to leak gas from faulty or improperly
sealed unit isolation valves.
Natural Gas STAR Partners have found that simple
changes in operating practices and in the design of
blowdown systems can save money and significantly
reduce methane emissions by keeping systems fully or
partially pressurized during shutdown. Though
pressurized systems may also leak from the closed
blowdown valve and from reciprocating compressor rod
packing, total emissions can be significantly reduced. Four
options for reducing emissions when taking compressors
off-line are discussed in this paper. These include:
* Keeping compressors pressurized when off-line.
* Connecting blowdown vent lines to the fuel gas
system and recovering all, or a portion, of the vented
gas to the fuel gas system.
* Installing static seals on compressor rod packing.
* Installing ejectors on compressor blowdown vent
lines.
Keeping compressors fully pressurized when off-line
achieves immediate payback—there are no capital costs
and emissions are avoided by reducing the net leakage
rate. Routing blowdown vent lines to the fuel gas system
or to a lower pressure gas line reduces fuel costs for the
compressor or other facility equipment, in addition to
Method for Volume of
Reducing Natural Natural Gas
Economic and Environmental Benefits
Value of Natural Gas Savings ($)
Implementation
rnct f*A
Gas Losses Savings (Mcf) ~w" v-lv
Option 1. Keep
1 compressor at pipeline 3,800
pressure2
Option 2. Keep
1 compressor pressurized _ ...
and route gas to fuel '
svstem2
$3 per Mcf $5 per Mcf $7 per Mcf
$11,400 $19,000 $26,600 $0
$15,300 $25,500 $35,700 $2,040
Payback1 (months)
$3 oer Mcf $5 per $7 per
$3 per Met Mcf Mcf
Immediate Immediate Immediate
2 1 1
Option 3. Keep
compressor pressurized
and install static seal2
Option 4. Install
1 Ejector.3
1 10 percent discount rate. 2
blowdown valves.
5,000
780
$15,000
$2,340
Incremental savings for peak load compressors
$25,000
$3,900
$35,000
$5,460
$4,900
$11,644
3 Assumes 1 5 Mcf per blowdown and 52 blowdowns per year
4
60
3
36
2
26
does not include capturing leakage from unit or
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
avoiding blowdown emissions. Static seals installed on
compression rods eliminate gas leaking back through the
rod packing while a compressor is shutdown under
pressure. An ejector uses the discharge of an adjacent
compressor as motive to pump blowdown or leaked gas
from a shut down compressor into the suction of an
operating compressor or a fuel gas system. Benefits of
these practices include fewer bulk gas releases, lower leak
rates, and lower fuel costs, with a payback in most cases of
less than a year.
Technology Background
Compressors used throughout the natural gas system are
cycled on- and off-line to meet fluctuating demand for gas.
Maintenance and emergency shut down are other
occasions when compressors are taken off-line. Standard
practice is to blow down or vent the high pressure gas left
in the compressor when it is taken off-line. While the
compressor is depressurized, leakage can continue from
the unit isolation valves, which are estimated to leak at an
average rate of 1.4 Mcf/hour. When a compressor is fully
pressurized, methane can leak from the closed blowdown
valve and the compressor rod packings. Per Exhibit 1, this
leakage rate from pressurized compressors is estimated to
be smaller, totaling 0.45 Mcf/hour versus 1.4 Mcf/hour for
a depressurized system.
The number of times a compressor is taken off-line for
normal operations depends on its operating mode. Some
compressors are designated as base load; these
compressors are operated most of the time, and might be
taken off-line only a few times per year. Down time for
base load compressors averages 500 hours per year. Other
compressors operate for peak load service, coming on line
as demand increases and additional pipeline volumes are
required. These units drop off the system (shut down) as
market demand decreases. Peak load compressors may be
operated for approximately 4000 hours total (less than 50
percent of the year), but cycling on- and off-line as many as
40 times per year.
The ratio of base load compressors to peak load
compressors varies widely among pipeline operators
because of different operating strategies, system
configurations, and markets. On some pipelines, 40
percent of the compressors might be base loaded; on
others, 75 percent might operate as base load. Regardless
of the operating mode, significant emission savings can be
gained by modifying operating practices and facility
designs to minimize the amount of natural gas emitted
Exhibit 1: Compressor Diagram
Blowdown Scenario
Compressor
Rod Packing
Pressurized Scenario
4
D
\
Blowdown V
L-] (Open)
epressurized
Unit Valve
(closed-leak)
Compressor
Rod Packing
(leak)
Blowdown Valve
(Closed-leak)
Unit Valve
(closed)
Pressurized
Leaks
- Unit Valves
(1.4 Mcf/hour)
- Blowdown
(15 Mcf/event)
Leaks
- Blowdown Valves
(0.15 Mcf/hour)
- Compressor Rod
Packing
(4 rods per
compressor,
0.30 Mcf/hour)
- Blowdown
(will vary according
to control option)
Source: 1999 PRCI Final Re
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
during down periods.
The largest source of methane emissions associated with
taking compressors off-line is from depressurizing the
system by venting the gas that remains within the
compressor and the piping associated with the compressor.
The gas volume released during a compressor blow down
depends on several factors including the size of the
compressor, the pipeline pressure, and the pipe volume
contained between unit isolation valves. On average, a
single blowdown will release approximately 15 thousand
standard cubic feet (Mcf) of gas to the atmosphere.
It should be noted that all options discussed in this paper
require blowdown of a compressor before it can be taken on
-line again. The main difference between the baseline
scenario (blowing the compressor down on shutdown and
maintaining it depressurized) and the options presented is
the timing of the blowdown and the volume of the
blowdown (for example, if blowdown gas is routed to the
fuel system).
Unit isolation valves are another source of methane
emissions from off-line depressurized compressors. Large
unit valves are used to isolate the compressor from the
pipeline and can leak significant amounts of methane.
Unit valves have acceptable ranges of leakage specified by
design tolerances for this type of valve. Unit isolation
valves are periodically maintained to reduce leakage, but
the limited accessibility of such valves can result in
increased leakage between scheduled maintenance. A
typical leak rate for unit valves is 1.4 Mcf per hour.
If the compressor is kept pressurized while off-line,
emissions from compressor rod packings and blowdown
valves can be observed. Seals on compressor piston rods
will leak during normal operations, but this leakage
increases approximately fifty percent (to about 75 scfh per
rod, or 0.3 Mcf/ hour, per four-cylinder compressor) when a
compressor is idle with a fully pressurized suction line.
Leaks occur through gaps between the seal rings and their
support cups, which are closed by the dynamic movement
Methane Content of Natural Gas
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
Production
Processing
Transmission and Distribution
79%
87%
94%
of the piston rod and lubricating oil (see EPA's Lessons
Learned: Reducing Methane Emissions from Compressor
Rod Packing). Vent and flare system valves can also leak
from pressurized systems at a rate of 150 scfh.
Natural Gas STAR Partners have significantly reduced
methane emissions from compressors taken off-line by
implementing changes in maintenance and operating
procedures as well as installing new equipment. Following
are some of the practices recommended by Natural Gas
Star Partners.
1. Maintain pipeline pressure on the compressor
during shutdown. As shown in Exhibit 1, leakage from
the compressor seal and closed blowdown valve will
increase for the pressurized system, but is still less than
anticipated leakage at the unit isolation valve for a
depressurized system. Partners report that total fugitive
gas emissions will be reduced by as much as 68 percent,
compared to leakage that would occur through the unit
valve if the compressor were offline and depressurized, to
approximately 0.45 Mcf/ hour for a pressurized
compressor.
2. Keep the compressor at fuel gas pressure and
connect to the fuel gas system. Connecting the
blowdown vent or flare lines to the fuel gas system allows
the gas that is purged when taking a compressor off-line to
be routed to a useful outlet. The pressure of an off-line
compressor equalizes to fuel line gas pressure (typically
100-150 pounds per square inch, psi). At the lower
pressure, total leakage from the compressor system is
reduced by more than 90 percent, compared to leakage
that would occur through the unit valve if the compressor
were offline and depressurized, to approximately 0.125
Mcf/hour from the compressor rod packing. Leakage
across the unit valves into the compressor continues to
feed the fuel system via the vent connection, rather than
vent to the atmosphere or flare in the fully depressurized
system.
3. Keep the compressor at pipeline pressure and
install a static seal on the compressor rods. A static
seal on the compressor rods can eliminate rod packing
leaks during shutdown periods with the compressor still
pressurized. A static seal is installed on each rod shaft
outside the conventional packing. An automatic controller
activates when the compressor is shutdown to wedge a gas
-tight seal around the shaft; the controller deactivates the
seal on start-up. With this equipment installed, leakage
will only occur from the closed blowdown valve at about
0.15 Mcf/h with the system at high pressure. The new
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
leakage rate would represent a reduction of 89% of the
emissions that would take place if the compressor were to
be kept off-line and depressurized.
4. Install Ejector. An ejector is a venturi nozzle that uses
high-pressure gas as motive fluid to draw suction on a
lower pressure gas source, discharging into an
intermediate pressure gas stream. The ejector can be
installed on vent connections up and down stream of a
partly closed valve, or between the discharge and suction
of a compressor which creates the necessary pressure
differential. The captured gas and the motive gas are then
routed to compressor suction or fuel gas system.
Economic and Environmental Benefits
Natural Gas STAR Partners can achieve substantial
environmental and economic benefits by taking simple
steps to avoid blowing down, or depressurizing,
compressors to the atmosphere when a shut down occurs.
These benefits include:
* Fewer Bulk Gas Releases: by routing compressor
blowdown gas to the fuel gas system, operators can
significantly reduce the volume of emissions while
recovering a useful product. Similar results can be
achieved by installing an ejector to capture the
blowdown gas and route it to a useful outlet.
* Lower Leak Rates: maintaining compressors fully
pressurized can avoid significant leaks across the
unit valves of 475 Mcf per year for base load units
and 3,800 Mcf per year for peak load units (see
Exhibit 2). The installation of ejectors and static
seals on the compressor rods when the unit is off-line
will also reduce the amount of methane leaking to
atmosphere.
* Lower Fuel Costs: routing compressor gas to the
fuel system utilizes methane that would otherwise be
vented or flared. This reduces fuel costs and
increases the volume of gas available for sale or use.
Decision Process
When taking compressors off-line, operators can easily and
cost-effectively reduce methane emissions by following
these steps:
Step 1: Identify blowdown alternatives.
Four options previously described are available for
Decision Steps for Reducing Emissions When Taking Compressors
Off-Line:
1. Identify blowdown alternatives.
2. Calculate quantity and value of methane emissions from the baseline
(depressurized) scenario.
3. Calculate the cost and savings of alternatives.
4. Conduct economic analysis.
reducing methane emissions when taking compressors off-
line. The feasibility and cost of implementing each option,
either singly or in combination, must be considered by
operators when modifications to compressor shut down
procedures are developed.
* Option 1: Maintain pipeline pressure on the
compressor during shutdown.
* Option 2: Route high pressure pipeline gas to
fuel while keeping the compressor at fuel gas
pressure.
* Option 3: Keep compressors pressurized and
install a static seal on compressor rods.
* Option 4: Install Ejector to route gas to
compressor suction or fuel gas system
A prudent operating practice is to avoid fully
depressurizing compressors until they are to be taken on-
line again. Option 3 (installing static seals) provides
added gas savings when used together with Option 1
(maintaining the compressor at pipeline pressure) by
limiting fugitive gas emissions when maintaining a
pressurized system. Option 4, install ejector, will recover
blowdown gas that would otherwise have been vented and
allow the operator to direct it to a useful outlet. In
addition, Option 4 can capture leakage and route it to a
useful outlet, making it possible to be implemented in
combination with any of the other options.
Step 2: Calculate quantity and value of methane
emissions from the baseline (depressurized) scenario.
The total methane emissions from off-line, depressurized
compressors is the sum of the losses from venting the
compressor and associated piping and the losses across the
unit valves for the period of time the compressor is
depressurized. Key inputs for calculating the total losses
per compressor per year include:
* The number of blowdowns per year (B).
* The pressurized compressor's volume between unit
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
isolation valves (V). The volume of gas vented per
blowdown depends on the compressor cavity volume,
the suction and discharge bottles and piping volume
between isolation valves, and the pressure. This can
be calculated directly using Henry's Law (volume is
inversely proportional to pressure, or PiVi = P2V2).
An average of 15 Mcf per blowdown is accepted as a
default emissions factor by the Natural Gas STAR
Program.
* The duration of the shut-down periods (T).
* The leakage rate at the unit valves (U). Unit valve
leaks can be measured at the blowdown vent using
hand-held measuring devices. Leak rates generally
increase since the last maintenance of the valves. A
default value of 1,400 scfh is used in this analysis.
Total emissions (TE) are calculated as: TE = B*V + T*U.
The total value (TV) or cost of these emissions is TE times
the price (P) of gas or TV = TE x P.
Most of this information is easily accessible from operating
records and nameplate specifications, or can be estimated.
Exhibit 2 presents two sample calculations of losses from
the baseline scenario versus Option 1, one for a base load
compressor and one from a peak load compressor.
Step 3: Calculate the cost and savings of alternatives.
The costs of each alternative include the capital
investment, incremental operations and maintenance
(O&M) cost, and the off-line leak rate associated with the
option. Some Partner-reported costs of each option are
summarized below.
* Option 1: Maintain pipeline pressure on the
compressor during shutdown. This option has no
capital or O&M costs. When instituted, leakage
occurs at the compressor rod packing (0.3 Mcf/h per
compressor) and at the blowdown valve (0.15 Mcf/h),
totaling approximately 0.45 Mcf/h when the
compressor is fully pressurized.
* Option 2: Keep the compressor at fuel gas
pressure and connect to the fuel gas system.
This option involves adding piping and valves to
bleed gas from an idle compressor into the
compressor station's fuel gas system or other low
pressure sales line. Facility modification costs range
between $1,470 and $2,600 per compressor. Major
determinants of cost are the size of the compressor,
the number of fittings, valves, and piping supports,
size of piping, length of piping, and whether an
automatic analyzer is installed. After the pressure in
Exhibit 2: Sample Calculations of Savings due to Implementation of Option 1 as Compared to Baseline
Scenario of Maintaining Compressor Fully Depressurized
Assumptions:
Base Load
Peak Load
Hours off-line/year
Unit valve leak rate (Mcf/h)
Blowdown valve leak rate (Mcf/h)
Rod packing leak rate (Mcf/h)
Sample 1: Base Load Compressor
Total Fugitive Emissions Savings = Baseline
Emissions - Option 1 Emissions
Total Value of Saved Gas
Sample 2: Peak Load Compressor
Total Fugitive Emissions Savings = Baseline
Emissions - Option 1 Emissions
Total Value of Saved Gas
500
1.4
.15
.30
: (500 hours x 1.4 Mcf/h) - (500 hours x 0.45 Mcf/h)
: 475 Mcf/year
: 475 Mcf/year x $7.00/Mcf
: $3,325 per year
: (4,000 hours x 1.4 Mcf/h) - (4,000 hours x 0.45 Mcf/h)
: 3,800 Mcf/year
: 3,800 Mcf/year x $7.00/Mcf
: $26,600 per year
4,000
1.4
.15
.30
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
the compressor equilibrates with the fuel line
pressure, leakage from compressor rod packings falls
to about 50 scfh and from the blowdown valve to
about 75 scfh, totaling 0.125 Mcf/h.
* Option 3: Keep pressurized and install a
positive static seal on compressor rods. While
technically feasible and compatible with either
Option 1 or 2, Option 3 may not be cost-effective
when used in conjunction with Option 2 (because leak
rates are significantly lower when floating the
compressor at the lower fuel line pressures). Static
seals cost about $825 per rod, plus $1,600 for an
automatic activation controller for the entire
compressor, totaling $4,900 per four-rod compressor.
With leakage from the compressor rod packing
virtually eliminated, the only remaining leakage is
from the blowdown valves, approximately 150 scfh.
* Option 4: Install Ejector. Similar to Option 3,
Option 4 is technically feasible and compatible with
Options 1 and 2, as the ejector can capture gas that
leaks through valves. Option 4 may not be as cost-
effective when used with Option 2 (because leak rates
are significantly lower when floating the compressor
at lower fuel line pressures). The capital and
installation costs of a typical venturi ejector are
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.
estimated to be $11,644. In addition to the ejector
itself, capital expenditures include ejector block
valves, piping from the blowdown vent line
connections, and engineering design work to size the
nozzle and expander for the site.
Exhibits 3a, 3b, and 3c show sample costs and savings
associated with these options.
Exhibit 3a: Sample Calculations of Savings due to Implementation of Option 2 as Compared to
Baseline Scenario of Maintaining Compressor Fully Depressurized
Assumptions:
Base Load
Peak Load
Hours off-line/year
Unit valve leak rate (Mcf/h)
Blowdown valve leak rate (Mcf/h)
Rod packing leak rate (Mcf/h)
Sample 1: Base Load Compressor
Total Fugitive Emissions Savings = Baseline
Emissions - Option 2 Emissions
Total Value of Saved Gas
Sample 2: Peak Load Compressor
Total Fugitive Emissions Savings = Baseline
Emissions - Option 2 Emissions
Total Value of Saved Gas
500
1.4
.050
.075
(500 hours x 1.4 Mcf/h) - (500 hours x 0.125 Mcf/h)
638 Mcf/year
638 Mcf/year x $7.00/Mcf
$4,466
(4,000 hours x 1.4 Mcf/h) - (4,000 hours x 0.125 Mcf/h)
5,100 Mcf/year
5,100 Mcf/year x $7.00/Mcf
$35,700
4,000
1.4
.050
.075
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
Exhibit 3b: Sample Calculations of Savings due to Implementation of Option 3 as Compared to
Baseline Scenario of Maintaining Compressor Fully Depressurized
Assumptions:
Base Load
Peak Load
Hours off-line/year
Unit valve leak rate (Mcf/h)
Slowdown valve leak rate (Mcf/h)
Rod packing leak rate (Mcf/h)
Sample 1: Base Load Compressor
Total Fugitive Emissions Savings = Baseline
Emissions - Option 3 Emissions
Total Value of Saved Gas
Sample 2: Peak Load Compressor
Total Fugitive Emissions Savings = Baseline
Emissions - Option 3 Emissions
Total Value of Saved Gas
500
1.4
.150
0
(500 hours x 1.4 Mcf/h) - (500 hours x 0.150 Mcf/h)
625 Mcf/year
625 Mcf/year x $7.00/Mcf
$4,375
(4,000 hours x 1.4 Mcf/h) - (4,000 hours x 0.150 Mcf/h)
5,000 Mcf/year
5,000 Mcf/year x $7.00/Mcf
$35,000
4,000
1.4
.150
0
Exhibit 3c: Sample Calculations of Savings due to
Implementation of Option 4
Assumptions:
Slowdowns per year
Emissions per Slowdown
Capital Cost
Operating Costs
52
15 Mcf
$11,644
$1,575
780 Mcf / yr
= 780 Mcf/year x $7.00/Mcf
= $5,460
* Assumes 15 Mcf per blowdown and 52 blowdowns per year and that virtually all of
the gas is captured by the ejector. Does not include capture of leaked emissions from
blowdown or unit valve..
Natural Gas Emissions Savings
Total Value of Gas Saved
Step 4: Conduct economic analysis.
Once the quantity and value of natural gas losses and
methane emissions are determined and the cost of each
alternative is established, an economic analysis of the
emission mitigation options is conducted. Simple payback
is an industry standard economic analysis method in
which the first year costs of each option are compared
against the annual value of gas saved.
When maintaining pipeline pressure on compressor sets
(Option 1), the net emissions savings are the difference
between methane emissions from off-line leakage that
occurs when the compressor is kept fully depressurized
and off-line leakage that occurs when the compressor is
kept fully pressurized (calculated in Exhibit 2.
Exhibit 4 presents the estimated savings of Option 1 and
the incremental savings from implementing Options 2 and/
or 3 in addition to Option 1. Maintaining the system
under pressure while the compressor is shutdown or on
standby (Option 1) demonstrates an immediate payback
with no investment required. Option 2, tying vent lines
into a low pressure gas pipeline while maintaining
pressure on the compressor system during a shut down, is
economic for both base load and peak load compressors,
but significantly more attractive for peak compressors.
For Option 3, the incremental gas savings for base load
compressors require just over one year to recover the
facility investment but payback for peak load compressors
is less than one year.
Option 4 can be implemented in combination with Options
1, 2, and 3 or individually. The cost-effectiveness of Option
4 will depend on the volume of gas vented per blowdown as
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
Exhibit 4: Economic Comparison of Options
_ . 1 Option 2 Option 3
I .. „ . . Keep Pressurized and Tie to Fuel Keep Pressurized and Install
Keep Pressurized _ <-.. ..- <- i
1 Gas Static Seal
1 Net Gas Savings (Mcf/yr)
Dollar Savings/yr1
1 Facilities Investment
Payback
1 IRR2
1 Assuming value of gas $7.00/Mcf
2 5 year life (not including annual O&M costs)
Base Peak Base Peak Base Peak
475 3,800 638 5,100 625 5,000
$3,325 $26,600 $4,466 $35,700 $4,375 $35,000
0 0 $2,040 $2,040 $4,900 $4,900
Immediate Immediate 6 months 1 months 14 months 2 months
>100% >100% 218% 1750% 85% 714%
well as the number of blowdowns per year. The economic
evaluation presented in Exhibit 4a assumes 15 Mcf per
blowdown and 52 blowdowns per year. The economic
evaluation does not account for additional gas that can be
recovered from leakage through the blowdown valve or
unit valve.
Exhibit 4a: Economic Evaluation
of Option 4
Option 4
Install Ejector
Net Gas Savings
(Mcf/yr)1
Dollar Savings/yr2
Facilities Investment
Operating Costs
Payback3
IRR3
780
$5,460
$11,644
$1,575
26 months
37%
1 Assuming 15 Mcf per blowdown and 52 blowdowns per year 2 Assuming value of gas
$7.00/Mcf 3 5 year life (not including annual O&M costs)
Implementation Tips
Listed below are tips that Natural Gas STAR Partners use
to evaluate options and reduce emissions from off-line
compressors:
* Operators generally conduct total station
maintenance turnarounds every 12 to 36 months,
overhauling unit isolation valves and making major
modifications such as fuel gas tie-ins. Unit valves,
blowdown valves, and compressor rod packing likely
experience maximum leakage rates toward the end of
the operating cycle between turnarounds. Therefore,
it is typically more cost-effective to make
replacements during the next scheduled turnaround.
* Safety is a priority when designing and operating
natural gas facilities. Maintaining gas pressure on
idle compressors and valves causes increased leakage
through the equipment inside the compressor station,
and the appropriate precautions must be taken
within the facility for gas detection, the potential
energy hazards of high pressure vessels, and
adequate ventilation to prevent accumulation of
leaked gases. Installing static seals on compressor
rods and maintaining and selecting the appropriate
valves can minimize this leakage, and, by extension,
safety concerns.
* Depressurizing off-line compressors to fuel gas is
effective only where there is sufficient fuel demand to
consume the gas at the rate of unit isolation valve
leakage (estimated 1.4 Mcf/h).
* Appropriate valve selection and maintenance of the
seal integrity of unit isolation valves can eliminate
up to 90 percent of annual emissions from the typical
shutdown and blowdown practice. Repairs on these
valves are expensive in terms of material and labor,
as well as the gas emissions that result from the need
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
to depressurize the entire station to access these
valves.
Although the maintenance and repair cost of gas handling
equipment to eliminate blow down emissions can be
prohibitive in terms of valve materials and labor, when
combined with better operating routines, better facility
and equipment design, and elimination of unnecessary
blow down practices, significant cash flow can be added to
the bottom line of many operations who have economic
incentives to reduce lost and unaccounted-for gas.
When assessing options for reducing emissions when
taking compressors off-line, the expected price of natural
gas influences decision-making. Exhibit 5a shows the
impact of gas price on the economic analysis of Option 2,
keeping the compressor pressurized and routing the
blowdown vent to the fuel gas system.
Exhibit 5c shows the impact of gas price on the economic
analysis of Option 4, install ejectors.
Exhibit 5a: Impact of Gas Price on Option 2: Keep
Compressor Pressurized and
Route Blowdown Gas to Fuel
$3/ Mcf
1 Value of Gas Saved $15,300
1 Payback Period (months) 2
Internal Rate of Return -,,-nn,
1 (IRR) 750%
Net Present Value ~
1 (1=10%) $50'871
$5/ Mcf $7/ Mcf
$25,500 $35,700
1 1
1,250% 1,750%
$86,022 $121,173
Exhibit 5b shows the impact of gas price on the economic
analysis of Option 3, keeping the compressors pressurized
and installing a static seal on the compressor rods.
Exhibit 5b: Impact of Gas Price on Option 3: Keep
Compressor Pressurized and Install Static Seals
Value of Gas Saved
Payback Period
(months)
Internal Rate of
1 Return (IRR)
Net Present Value
| (i=10%)
$3/ Mcf
$15,000
4
306%
$47,238
$5/ Mcf
$25,000
3
510%
$81,700
$7/ Mcf
$35,000
2
714%
$116,161
Exhibit 5c: Impact of Gas Price on Option 4:
Install Ejectors
Value of Gas Saved
Payback Period
(months)
Internal Rate of
1 Return (IRR)
Net Present Value
1 (i=10%)
$3/ Mcf
$2,340
60
0%
- $2,521
$5/ Mcf
$3,900
36
20%
$2,854
$7/ Mcf
$5,460
26
37%
$8,230
The impact of the gas price on the economic analysis of
Option 1 is not shown since no capital investment is
required to implement Option 1, making the payback
immediate regardless of gas price.
Lessons Learned
Partners will find that significant emissions reductions
and cost saving will result from altering routine
compressor blowdown practices, and, where applicable,
from rerouting vented gas. Savings accrue from retained
product or displacement of fuel gas. The principal lessons
learned from Natural Gas STAR Partners are:
* Avoid depressurizing to atmosphere whenever
possible. Large immediate savings can be realized at
no cost by keeping off-line compressors pressurized
Case Study: An EPA Partner's Experience
With growing interest in identifying practical financial savings and
reducing gas losses, Company A investigated several strategies
to reduce leakage from its compressor rod packing. During a
period when compressors were taken out of service, the company
tied the compressor to the fuel gas system. At this lower
compressor cylinder pressure, the leakage through rod packing
cases and blowdown valves was reduced considerably. For
3,022 compressor cylinders (a total of 577 compressor units)
operative 40 percent of the time, the total gas savings amounted
to a significant 1.58 Bcf/year.
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
during the majority of their time off-line.
* Educate field staff about the benefits of delaying or
avoiding blowdowns.
* Determine if individual compressors operate in base
or peak load. Use this information to conduct
economic analyses of Options 2 and 3.
* Measure gas emissions from blowdown valves and
individual unit isolation valves, as well as emissions
from individual compressors to evaluate your actual
economics of the alternatives presented.
* Where economic, develop a schedule for retrofitting
compressors with fuel gas routing systems and
installing compressor rod static seals.
* Record reductions at each compressor.
* Reductions in methane emissions should be included
in annual reports submitted as part of the Natural
Gas STAR Program.
References
Borders, Robert S. C. Lee Cook, personal contact.
Campbell, Alastair J. Bently Nevada Corporation, Houston, Texas. Optical
Alignment of Reciprocating Compressors.
"Compressor Shutdown Leakage." Pipeline & Gas Journal, December
1985.
France Compressor Products. Mechanical Packing - Design and Theory of
Operation, Bulletin 691.
Howard, T., R. Kantamaneni, G. Jones, Indaco Air Quality Services, Inc.
PRCI Final Report. "Cost Effective Leak Mitigation at Natural Gas
Transmission Compressor Stations". August 1999.
Maholic, James. France Compressor Products, personal contact.
Minotti, Marcello. ENRON, personal contact.
Common Leak Detection and Measurement
Devices
* Infrared Camera
- Able to screen inaccessible equipment components
- Displays hydrocarbon emissions in a moving image using
infrared properties of the hydrocarbons
* Electronic Screening
- Equipped with catalytic oxidation and thermal conductivity
sensors designed to detect certain gases
— Typically used on larger openings that cannot be
screened by soaping.
* Acoustic Leak Detection
— High frequency acoustic detectors or ultrasonic leak
detectors are two types of acoustic leak detectors
- Rely on acoustic signals upstream and downstream of a
possible leak to determine if gas is escaping
* OVAs and TVAs
- Organic Vapor Analyzers (OVAs) are flame ionization
detectors which measure the concentration of organic
vapors over a range of 9 to 10,000 parts per million (ppm)
Toxic Vapor Analyzers (TVAs) combine both flame
ionization detectors and photoionization detectors and can
measure organic vapors at concentrations exceeding
10,000 ppm
* Calibrated Bagging
— Used to measure mass emissions from equipment leaks.
— The leaking component is enclosed in a "bag" of known
volume and a timer is used to determine the time to fill the
bag
* Rotameters
- Used to measure extremely large leaks that would
overwhelm other instruments.
— Ideal for open-ended lines and similar components where
the entire flow can be channeled through the meter.
* High Volume Samplers
— Capture all of the emissions from a leaking component
through a vacuum sampling hose to accurately quantify
leak emissions rates.
— Sample measurements are corrected for the ambient
hydrocarbon concentration, and mass leak rate is
calculated by multiplying the flow rate of the measured
sample by the difference between the ambient gas
concentration and the gas concentration in the measured
sample.
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Reducing Emissions When Taking Compressors Off-Line (Cont'd)
§
\
Ul
(3
United States
Environmental Protection Agency
Air and Radiation (6202J)
1200 Pennsylvania Ave., NW
Washington, DC 20460
October 2006
the Greenhouse Gas Reporting Rule, 40 CFR Part 98, Subpart W methods or those in other EPA regulations.
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