£V United States
Environmental Protection
Agency
Office of Enforcement
and Compliance Assurance
Publicatio
Enforcement Alert
n no. EPA 325-F-19-001
SEPTEMBER 2019
EPA Observes Air Emissions from Natural Gas Gathering
Operations in Violation of the Clean Air Act
Purpose
EPA and state investigations have identified Clean Air
Act (CAA) non compliance caused by unauthorized
and/or excess emissions from depressurizing pig
launchers and receivers in natural gas gathering
operations. For example, EPA and Pennsylvania resolved
CAA violations by MarkWest (described at the end of
this Alert) in a consent decree in April 2018, The
settlement provides an example of potential compliance
issues that operators may experience if Volatile
Organic Compound (VOC) emissions from
depressurizing pig launchers and receivers in natural
gas gathering operations are not properly controlled.
This Alert discusses engineering, design, operations, and
maintenance practices that EPA and state inspectors have
found that can cause non-compliance and summarizes
engineering solutions to reduce emissions and help
alleviate potential safety issues. While this Alert provides
information intended to help operators and state
regulators identify and address compliance concerns,
the solutions discussed in this Alert do not guarantee
compliance with federal and state regulations, including,
but not limited to requirements to obtain an air permit,
keep records, and/or control emissions.
Non-Compliance Concerns
State and federal laws require owners and operators of
certain facilities to register air emission sources, obtain
Infrared Image of Compressor Station Pig Receiver
Depressurization Event
authorization to emit from a registered source, including
natural gas gathering operations, and design, install,
operate, and maintain effective emission control
measures. Such laws include state permitting and air
pollution regulations—many of which are federally-
enforceable and collectively referred to as the State Im-
plementation Plan or "SIP"—and the federal New Source
Review (NSR) air permitting regulations including the
Prevention of Significant Deterioration (PSD)
requirements.
EPA and state inspectors have observed numerous
instances where depressurizing pig launchers and
receivers in natural gas gathering operations emit
unauthorized or excess VOC emissions, due to the
company's failure to obtain an air permit for the pigging
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equipment, to address deficiencies in
the design of the pigging equipment, or
to operate the pigging equipment in
accordance with an air permit, air permit
application or air permit registration.
In most jurisdictions, a company must
determine the mass emissions of pollutants when
registering a source, applying for an air permit, and/or
claiming an air permit exemption; which are
prerequisites to constructing a source. Failing to
calculate the potential for VOC or HAP emissions from
pigging operations or otherwise underestimating
the mass of the pollutants released may result in a
misrepresentation to the permit authority when the
company applies for a permit or permit exemption.
Depending on the state and local air permitting
rules	and regulations,	a company's
misrepresentation to the permit authority may mean
that the emissions from the pigging operations are
considered unauthorized or excess. In this situation, the
non-compliance is caused by a company's failure to
comply with federal and state permitting requirements,
including, but not limited to: failure to register an air
emission source, failure to obtain an air permit or obtain
the correct air permit, or failure to adhere to
representations in the air permit registration, failure to
maintain records, and/or failure to control emissions.
See, e.g., 40 C.F.R. §§ 51.165 and 70.7(b); 25 Pa. Code
§§ 127.11 and 127.402(a); Ohio Admin. Code
3745-3l-02(A)(l)(a)-(b) and 3745-15-05(B), (D) and (E).
Emissions from Pigging Operations
Raw natural gas is transported from production wells to
processing plants through networks of gathering
pipelines. Although liquid separation may occur at the
well pad, much of the raw natural gas passing through
Main Isolation Valve
^-Vent Balance Line	£J*
A „
Hatch
Barrel or Pig Trap
Nominal Pipe
Gas Flow
Bypass Isolation
Valve
Bypass Line
Main Pipeline
Figure 1. Conventional pig receiver.
the gathering pipelines is saturated with hydrocarbons
other than methane and may contain other components
such as water, carbon dioxide, and hydrogen sulfide.
During the transportation of this gas through gathering
pipeline systems, the gas often experiences a
temperature drop and pressure change that causes the
hydrocarbons and other components to condense to a
liquid phase. These natural gas condensates can
accumulate in low elevation segments of the gathering
pipelines, impeding the flow of natural gas. To maintain
gas flow and operational integrity of the gathering
pipelines, operators mechanically push these
condensates out of the low elevations and down the
pipeline by an operation called "pigging," which involves
first inserting a device called a pig into a pig launcher
upstream of the pipeline segment where condensates
have accumulated. The gas flowing through the pipeline
then pushes the pig through the pipeline, allowing the
pig to sweep along the accumulated condensates. The
pig is removed from the pipeline segment when it is
caught in a pig receiver.
A conventional pig receiver is shown in Figure 1. Pig
launchers are very similar in design. The pipeline at the
rear of the barrel is called a "kicker line" on pig launchers
and a "bypass line" on pig receivers; these lines direct the
main gas flow through the barrel to launch or receive a
pig, respectively.
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Before a pig can be inserted or removed through the
hatch of a pig launcher or a pig receiver, the pipeline
gas in the launcher or receiver barrel must be
removed, leading to potential emissions. This gas is
under the same pressure as the pipeline and contains
methane, ethane, and VOCs including benzene,
toluene, ethylbenzene, and xylene. Pig receivers
can also contain collected condensate liquid that had
accumulated in the pipeline.
When the launcher and receiver barrels are
depressurized (via a vent as shown in Figure 1) prior to
opening the hatch for pig insertion or removal, the
depressurization gases contain mixtures of methane,
ethane, VOCs and hazardous air pollutants (HAPs). The
barrel depressurization gases are often vented to the
atmosphere but can be controlled or recovered to
reduce emissions. When the hatch is opened, additional
gas is released, and any liquids collected in a pig receiver
that did not volatilize in the depressurization process
may be spilled from the open hatch as the pig is
withdrawn. These spilled liquids can fall to the ground or
be collected in an open tub, either of which allow for
further evaporation that contributes to emissions.
The amount of potential emissions from pigging
operations depends on several factors including the
launcher or receiver volume, pipeline pressure (higher
pressure correlating with higher potential emissions),
amount of liquid trapped in the pig receiver barrel prior
to depressurization, frequency of pigging, and gas
composition.
Depending on these system characteristics, the pigging
operation may emit VOCs or HAPs in excess of federal or
state limits.
Engineering Solutions
EPA inspectors have identified general characteristics of
a pig launcher and receiver system that control and
reduce emissions of air pollutants and may require less
management. These systems: (1) minimize the total
volume between the kicker and/or bypass isolation
valves and the main isolation valve, so that less gas is
trapped in the barrel; and (2) minimize the potential for
liquid accumulation or hold-up in the receiver, and
include features such as sloped receiver barrels and
piping so that liquids collected in the barrel are gravity-
drained through the bypass piping into the main pipeline
for collection downstream. However, depending on the
quantity of emissions released, a company may be
required to implement additional design measures, and
emissions capture and control approaches to comply
with federal and state regulations.
The following sections discuss design alternatives to the
conventional launcher/receiver system (i.e., pig ball
valves and multi-pig launcher systems), considerations
for receiver barrel design (i.e., condensate drains and pig
ramps), and additional site modifications to capture or
control emissions (i.e., jumper lines, combustion devices,
rerouting low pressure systems, barrel pump-down
systems, and enhanced liquid containment). In addition
to reducing potential VOC emissions to alleviate
potential non-compliance, application of these solutions
reduces methane (a potent greenhouse gas) and
generally improves safety of the pigging operation. The
applicability and effectiveness of these engineering
solutions are site-specific; solutions can generally be
combined to effectively reduce emissions.
Install Pig Ball Valves
Pig ball valves are a design alternative to conventional
launcher/receiver systems that result in fewer emissions
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from pigging operations due to a smaller sized barrel (or
chamber) that launches and receives the pig. A
conventional pig launcher or receiver system can be
retrofitted by replacing the conventional launcher and
receiver barrels with special ball valves used to insert and
remove the pig directly from the main pipeline. Figure 2
shows a pig ball valve installed in a pipeline, with a
bypass loop around the pig ball valve for use when the
pig ball valve is closed. By replacing the large volume
barrel with the much smaller volume ball valve, the
volume of gas vented during each pigging operation can
be reduced by as much as 80% to 95% with a
corresponding reduction in emissions and other risks
associated with pipeline pigging operations.
Install Multi-Pig Launcher Systems
Multi-pig launcher systems are a design alternative to
conventional launcher/receiver systems and reduce
pigging emissions by reducing the frequency that
launchers and receivers must be vented prior to pig
insertion and removal. An example manufacturer
design for a multi-pig launcher system is shown in
Figure 3. The launcher barrel is designed to hold
multiple spherical pigs, which are each held in place
by gates or pins prior to release. Emission reductions
are approximately proportional to the reduction in
frequency of opening the launcher and receiver
hatch.
Bypass Loop
Pig Ball Valve
flow
Main Pipeline
Figure 2. Pig Ball Valve Installation
depicts a conventional receiver design, but additional
drains can be used. These drains can be retrofitted to
existing systems.
The effectiveness of condensate drains in controlling
pigging emissions is based on the quantity of condensate
trapped in the receiver system, and the subsequent
ability of the drain to recover the full quantity of this
condensate during each pigging operation, thereby
preventing its volatilization during pig receiver
depressurization or hatch opening.
Figure 3. Multi-Pig Launcher System
Receiver Barrel Design: Condensate Drains
Drains can be installed in the bottom of receiver
barrels and pig ball valves to ensure that all
condensate is drained from the system prior to
depressurization. These drains generally route the
condensate back into the main pipelines, to onsite
storage tanks, or to onsite processes via enclosed
piping. Two drains are shown in Figure 4 which
Figure 4. Conventional Pig Receiver with Condensate
and Vapor Recovery
Pig Control Gates
Pipeline
Flow
Access Hatch
Spherical Pigs ' Main Isolation Valve
Gas Flow
Kicker Line-
Kicker
Isolation Valve
Pipeline
Balance Line
JJj Bypass Isolation
Valve
gk Bypass Line
Gas Flow
Pipeline
Depressurization
Main Isolation Valve
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Receiver Barrel Design: Pig Ramps
A pig ramp is a device that can be installed inside a
receiver barrel to minimize the accumulation of
condensate, or VOCs in the condensed liquid phase, in
the receiver by directing the movement of the pig and
liquid in the barrel. The pig ramp promotes the flow of
liquid through the barrel and into the bypass line by
preventing the pig from creating blockages in the
receiver. By promoting the flow of liquid back into the
pipeline system while the receiver is under pressure, pig
ramps reduce the amount of condensed VOCs trapped in
the barrel priorto depressurization which reduces the air
emission that occur during depressurization and removal
of the pig from the receiver. As part of MarkWest's
recent settlement with EPA, pig ramps were installed at
hundreds of barrels in its wet gas midstream pipeline
systems. As discussed below, MarkWest will provide a
royalty-free license to others interested in using their
proprietary pig ramp design.
Route High Pressure Systems to Lower Pressure
Lines (Install Jumper Lines)
The depressurization emissions from high pressure
launchers and receivers can be reduced by routing the
gases to a lower pressure system before venting the
remaining gases to the atmosphere or to control
equipment. Routing to a lower pressure system is
achieved with a depressurization line (or, "jumper line")
exiting the top of the barrel, as shown in Figure 4, or
exiting the top of the pig ball valve. Compressor stations
and gas plants have low pressure lines on the site that
can receive these depressurization gases and recycle
them through the process. Similarly, launchers and
receivers along high pressure pipelines are occasionally
located near low pressure pipelines that can receive
depressurization gases exiting the barrel or pig ball valve.
Route Depressurization Gases to Combustion
Devices
Depressurization gases from barrels and pig ball valves
can be routed through the depressurization line (see
Figure 4) to onsite combustion devices. Well-designed
and operated combustion devices can achieve vapor
destruction efficiencies as high as 95% to 98%.
Combustion devices can be used in conjunction with
engineering solutions discussed above that first reduce
accumulation of or recover as much natural gas and
condensate as possible before destroying the remaining
vapors in the combustion device. An example would be
to route high pressure systems to low pressure lines and
drain barrel condensate, then route the remaining
vapors to a combustion device. Large, high capacity
combustion devices are typically available at compressor
stations and processing plants and can be used to control
pigging gases while meeting the other flaring needs of
the facility. There are also numerous low capacity
combustion devices available for serving remote
launcher/receiver sites.
Route Low-Pressure Systems into Fuel Gas Systems
or Vapor Recovery Unit (VRU)
Gases that remain in high pressure barrels after venting
to low pressure systems and gases in low pressure
barrels can be recovered during depressurization by
discharging the gases to very low-pressure systems at
the site (e.g., 10-15 psig). Two examples of very low-
pressure systems at compressor stations are a fuel gas
system and a condensate tank vapor recovery unit
(VRU). Applying such an approach can reduce the gas
pressure in the barrels to the pressure of the very low-
pressure system (e.g., 10 psig), with a
corresponding reduction in depressurization
emissions.
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Barrel Pump-down Systems
In barrel pump-down systems, small fixed or portable
compressors are used to pump vapors in the receiver or
a launcher barrel back into the main pipeline prior to
venting and opening the barrel hatch. In barrel pump-
down systems, the inlet of a gas compressor is connected
to the receiver or launcher depressurization line, and the
compressor discharge is connected into the main
pipeline. Vapors exiting the depressurization line are
pulled into the compression system and recovered back
into the pipeline at system pressure. These control
systems can recover greater than 99% of the
depressurization vapors from pig launchers and
receivers.
Enhanced Liquid Containment
Covering containers that collect liquids remaining in a
receiver barrel after depressurization with a fitted
impermeable material will reduce emissions from
evaporation. Use of grounded steel receptacles
additionally helps ensure safety.
Estimating Potential Emissions from
Pigging Activities in Natural Gas
Gathering Operations
The Real Gas Law has been used to estimate the
emissions from depressurizing launcher and receiver
barrels, where a gas compressibility factor is applied to
account for non-ideal gas initial conditions (e.g., high
pressure).
However, the fundamental assumption of the
methodology guiding this approach is that the average
pipeline gas profile accurately characterizes the contents
of the barrel prior to depressurization. This assumption
neglects changes in physical conditions along the
pipeline which lead to compositional changes and
potential formation of condensate liquid that can
accumulate in and/or is swept into the pig receiver
during pigging.
Therefore, when considering emissions from pig
receivers, the Real Gas Law methodology accounts for
depressurizing the natural gas mixture from pressurized
conditions to atmospheric; however, the equation does
not address the potential flash emissions from
volatilization of condensates during depressurization.
Using the Real Gas Law alone in circumstances with liquid
accumulation can result in an underestimation of the
calculated VOC emissions from the receiver.
A complete emissions estimation methodology for
receivers would account for the impact of any liquid
accumulation in receivers, as a supplement to estimates
based on the Real Gas Law.
Enforcement Settlement with
MarkWest
On April 24, 2018, the U.S. EPA, the U.S. Department of
Justice, and the Pennsylvania Department of
Environmental Protection announced a settlement with
MarkWest that requires innovative solutions designed to
evaluate and address VOC emissions from pigging
operations at gathering compressor stations and stand-
alone pigging stations. This settlement resulted from
joint inspections conducted by the EPA and the
Commonwealth of Pennsylvania. MarkWest failed to
apply for, and comply with, the required permits and/or
recordkeeping requirements under the Nonattainment
New Source Review, Prevention of Significant
Deterioration and Title V programs, and the Pennsylvania
and Ohio State Implementation Plans, for natural gas
pigging and venting activities that released excess VOC
emissions. MarkWest is undertaking measures to help
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ensure compliance with federal and state regulations
and reduce emissions including:
•	Operating jumper lines to depressurize launchers
and receivers.
•	Installing pig ramps in receivers.
•	Using a mobile flare to control emissions during
depressurization.
•	Using shorter (smaller volume) barrels than those
presently used in the system.
•	Using liquid containers with grounded steel
receptacles that are covered at all times when not in
use.
•	Using an estimation methodology that accounts for
the flash emissions from condensates that
accumulate in the pig barrel.
MarkWest additionally is performing the following
environmental projects to further EPA's goal of
protecting and enhancing the public health and the
environment:
•	Innovation transfer and emission control education:
MarkWest will provide a royalty-free license and
public educational sessions to promote rapid
adoption of its proprietary pig ramp design. This
project is aimed at increasing awareness of the
emissions from pigging operations and sharing cost-
effective methods to reduce these emissions. The
adoption of this technology by other midstream
operators will further benefit the environment by
reducing emissions from pigging operations.
• Ambient air monitoring: MarkWest will install and
operate air sampling stations to analyze VOCs
downstream of two compressor stations over a
period of at least two years. Air monitoring at these
facilities is not required by law, but MarkWest
agreed to conduct these supplemental
environmental projects as part of the settlement to
provide local communities, industry, regulators, and
other stakeholders valuable information about air
emissions generated during midstream operation
and maintenance activities, and their impacts on
local air quality.
For more information, visit
https://www.epa.gov/enforcement/markwest-clean-
air-act-settlement-information-sheet
DISCLAIMER: This document aims to explain the application of certain EPA regulatory provisions using plain language. Nothing in
this Alert revises or replaces any regulatory provisions, any other part of the Code of Federal Regulations, the Federal Register, or
the Clean Air Act. Following the approaches as discussed in this Alert do not equate to or guarantee compliance with the Clean Air
Act, its implementing regulations, and associated state/local requirements. For more information, visit:
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