Vacuum First, Gas Second: A Gas Utility's Approach to Zero-Emission Service Line Commissioning

Gas utilities routinely replace residential service lines as part of infrastructure modernization and maintenance programs. Before a new service line can be placed into service, it must be purged of air. The standard method is to leave the customer connection end of the line open to atmosphere, introduce gas, and allow the air and gas mixture to blow out until only gas remains. The vent is then closed and the line is pressurized.

The problem is structural: every traditional purge releases an unknown volume of gas during the air-to-gas transition. In highly branched networks or systems with dead-legs, achieving 100% gas readings can take significant time, increasing both the volume of emissions and the duration of crew exposure to a live air-gas mixture. These operations also generate noise and odorant release that can trigger complaints from nearby residents.

 "Operators across the globe have come under pressure to reduce greenhouse gas (GHG) and volatile organic compound (VOC) emissions. Traditional purging results in the release of an unknown quantity of gas during the process while an air and gas mixture is being vented."

Uncontrolled Methane and VOC Emissions

Every traditional purge releases a measurable but unquantified volume of gas and mixed air-gas to atmosphere, contributing to GHG and VOC emission totals.

Ignition Risk During Purging

An open air-gas mixture flowing through PE pipe creates ignition risk. PE pipe can also build up a static charge during purging, adding to the hazard for crews on site.

Community and Noise Impacts

Purging in residential neighborhoods generates noise and odorant release, drawing public attention and triggering gas odor calls from nearby residents.

Slow Verification in Complex Networks

In branched systems or lines with dead-legs, achieving 100% gas concentration using a traditional open purge can take considerable time, extending crew exposure and job duration.

Nicor Gas, in partnership with Nelson Technologies, deployed the ZEVAC LP Mini to perform a ventless vacuum purge on a newly replaced 100-foot residential service line in Elk Grove Village, Illinois. The line was a half-inch PE replacement of an existing 55 psi service running from the steel main to the meter riser.

The ventless purging method adapts a technique established in the propane industry, where vacuum is applied to remove air from tanks before filling without an open vent. Per NPGA #133-89(a), a vacuum of -12.77 psig at sea level (26 inches Hg, 86% vacuum) is sufficient to introduce LP-gas vapor without an open purge. ZEVAC applies the same principle to natural gas distribution service lines, using the LP Mini to evacuate air before gas is ever introduced to the line.

The ventless purge was completed faster and with greater certainty than a traditional open purge would have allowed. By evacuating air before gas was introduced, Nicor Gas eliminated the waiting period normally associated with achieving 100% gas concentration and removed all uncertainty from the verification process.

The result also compared favorably on the verification standard. Nicor's procedure requires only one gas purity sample for residential services. Three samples were taken as an additional quality check. All three returned 100% gas on the first draw, confirming the ventless method produces a result at least as reliable as the traditional approach, with none of the associated emissions, hazards, or public impact..

All methane and VOC emissions eliminated during the service line commissioning process

Ignition risk from open air-gas mixture eliminated entirely

No noise, no odorant release, and no public-facing notifications required

Moisture removed from the new service line as part of the vacuum process

Formal ventless purging methodology now in development for LDC operators seeking to standardize this approach across their service programs

The ZEVAC LP Mini is the most portable unit in the ZEVAC lineup, purpose-built for residential and distribution pipeline operations. Weighing 185 pounds and fitted with wheels, it can be moved around a jobsite by one person and fits in the bed of a pickup truck or built directly into a service vehicle. It is available in both low-pressure (LP) and high-pressure (HP) configurations to match specific operating envelopes across a utility's service territory.

On a 200-foot run of 2-inch PE, the LP Mini completes the vacuum process in less than 2 minutes. It operates on a standard 90 CFM air compressor or a truck's PTO compressor, requiring no external power source or combustion. The unit handles 100% gas, 100% liquid, and any mixture in between, making it equally suited to ventless purging, service line drawdowns, and liquid-bearing applications.

Ventless purging with ZEVAC requires minimal additional planning compared to a standard service line replacement. The following questions should be addressed during pre-job setup to ensure a smooth operation.

Meter Connection and Isolation

Identify what ports are available on the meter to allow the ZEVAC vacuum connection to the new service line. Confirm the port can be isolated after the vacuum operation is complete so the line remains sealed until gas is introduced.

Verification Port

Determine whether the same port used for the vacuum connection can also be used to verify the successful purge with a gas LEL meter, or whether a separate sampling port is needed.

Operation Duration

For most residential services, the vacuum process takes under 30 seconds with the LP Mini. On longer runs, the LP Mini completes a 200-foot, 2-inch PE drawdown in under 2 minutes. Incorporate this into the overall job timeline in place of the standard purge waiting period.

Traffic Control and Site Safety

Standard traffic control procedures apply. Although the ventless method greatly reduces the risk of drawing public attention by eliminating noise and odorant release, best practice is to keep unauthorized personnel away from the equipment and gas lines throughout the operation.