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Pipeline Pump-down Technique
Operators of natural gas pipeline systems routinely reduce line pressure and discharge gas from pipeline sections to ensure safe working conditions during maintenance and repair activities. Typically, operators block the smallest possible linear section of the pipeline and depressurize it by venting gas to the atmosphere. In 2004, an estimated 12 billion cubic feet (Bcf) of methane was vented to the atmosphere during routine maintenance and pipeline upsets.
Using pump-down techniques to lower gas line pressure before performing maintenance and repair activities is an effective way to reduce emissions and yield significant economic savings. Pipeline pump-down techniques involve using in-line compressors either alone or in sequence with portable compressors. Using in-line compressors is almost always justifiable because there are no capital costs, and payback is immediate. The cost-effectiveness of also using a portable compressor to increase gas recovery, however, depends greatly on site-specific factors and operating costs.
Another alternative is to install an ejector. An ejector is a venturi nozzle that uses high-pressure gas as the 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.
Regardless of the pump-down technique selected, emission reductions are directly proportional to how much pipeline pressure is reduced before venting occurs. On average, up to 90 percent of the gas in the pipeline can be recovered for sale instead of being emitted. Pipeline pump-down techniques are most economical for larger volume, higher pressure gas lines and work most effectively for planned maintenance activities and cases in which sufficient manifolding exists to connect a portable compressor.
Many Natural Gas STAR Partners have realized significant economic savings by using pump-down techniques. In 2004, Natural Gas STAR transmission Partners saved a total of 4.1 Bcf of gas using pump-down techniques. Based on a value of gas saved of $7.00/Mcf, that equals more than $28 million saved.
Natural gas transmission, distribution, and production companies transport methane and other light hydrocarbons via pressurized gas pipelines. These pipelines can require repairs or maintenance throughout their lifetime as a result of interior and exterior corrosion, gasket and weld leaks, failures of defective materials, and damage caused by external factors. Pipeline repairs fall into four general categories:
Class 1 non-emergency repairs that do not involve complete service interruption.
Class 2 non-emergency repairs that require complete service interruption.
Class 3 emergency repairs that require complete service interruption.
Class 4 large-scale projects where the new pipe is being run parallel to the existing pipe and require service interruption.
Pipeline repairs and maintenance activities typically require depressurizing the pipeline to remove gas from the affected section of pipe and ensure safe working conditions. One approach to depressurization is to block off the impacted pipeline segment and vent the gas in that segment to the atmosphere. Alternatively, operators can use pump-down techniques to lower the gas-line pressure before venting. Pump-down techniques are the preferable alternative because they make more gas available for sale and reduce methane emissions.
In implementing pipeline pump-down techniques, operators can use two types of compressors to reduce pipeline pressure: in-line compressors and portable compressors. Depending on the situation, operators can use in-line compressors alone or with portable compressors.
Typically, in-line pipeline compressors have compression ratios of up to 2 to 1. By blocking the upstream valve of the targeted line segment while continuing to run the downstream compressor, the pipeline pressure can be reduced to approximately 50 percent of the working line pressure. The compressor can then be shut down and the line segment fully blocked. Lowering the line pressure by one-half is often sufficient to safely install sleeves over Using in-line pipeline compressors to draw down the pressure within their compression ratio limits. damaged line. This type of line pump-down process should be done only in a manner consistent with safety management policies.
Using a portable compressor to further lower the line pressure. Operators can also consider using portable compressors to achieve additional reduction beyond what in-line compressors can provide. Portable compressors can have up to a 5 to 1 compression ratio. When used in combination with an in-line compressor, portable compressors can lower line pressure by up to 90 percent of its original value without venting. Portable compressors can be used safely only when the downstream block valve is sufficiently manifold. Again, safety policies should be strictly followed when using a portable compressor.
Although a portable compressor can recover an additional 40 percent of the original pipeline gas for sale, it is most appropriately used during planned maintenance, such as Class 1 and 2 repairs. This is because of the difficulty of renting or leasing a unit, mobilizing it, and depressurizing the line in a timely and cost-effective manner during an emergency. Portable compressors also are more easily justified when multiple sections of pipeline are taken out of service, either as a single project or as a set of serial repairs.
Economic and Environmental Benefits
Companies can realize significant environmental and economic benefits by using downstream in-line and portable compressors to lower gas-line pressure before performing maintenance and repair activities. Potential savings include:
Recovery and sale of natural gas that would have been vented to the atmosphere. In the case of production pipelines, the gas stream might also contain valuable heavy hydrocarbons.
Reduction of methane emissions.
Reduction of nuisance odor and noise.
Elimination or significant reduction of hazardous air pollutant (HAP) emissions, primarily benzene, toluene, ethyl benzene, and xylene (BTEX).
Exhibit 1 summarizes which pump-down techniques are applicable for the different classes of pipeline repair.
Exhibit 2 illustrates the basic sequence of events for depressurizing a pipeline segment.
When gas pipelines require maintenance or repair, companies can either:
Vent gas in the damaged section of pipeline to the atmosphere.
Recover as much of the pipeline gas as possible.
Step 1: Estimate the quantity and value of gas that in-line compressors can recover.
Depending on the compression ratio of the downstream in-line compressor(s), up to 50 percent of gas in the line can be recovered at minimal or no cost to the operator. Exhibit 3 provides calculations that operators can use to determine the total amount of gas in the pipeline segment and the amount and value of gas that can be recovered using the in-line compressor(s).
Step 2: Verify technical feasibility of using a portable compressor.
After calculating the potential volume of pipeline gas recoverable by an inline compressor, the operator should determine if the mechanical capability exists to use a portable compressor.
A portable compressor can further reduce the line pressure by moving up to 40 percent of the remaining original gas volume to the pressurized side of the block valve; however, using a portable compressor is only possible if the compressor can physically connect to the pipeline. Exhibit 4 illustrates a typical gas pipeline manifold. At a minimum, proper portable compressor connections should include bleeder valves upstream and downstream of a mainline block valve. The minimum size of bleeder valves depends on the size of the portable compressor. A technical representative from the portable compressor leasing or manufacturing company can specify manifolding requirements for specific units.
Step 3: Determine the appropriate-sized portable compressor for the project.
Selecting an appropriately sized portable compressor is best done with the assistance of a leasing company or manufacturer's technical representative who can recommend a portable compressor that satisfies the project requirements (e.g., amount of gas, discharge pressure requirements, schedule).
Step 4: Check the availability and cost of purchasing or leasing a portable compressor.
Companies considering a portable compressor are often faced with the question of whether to rent or purchase the unit. A limited number of portable gas compressors are available for rent, and rental companies typically prefer long-term leases. If the continual use of portable compressors for line pump-down activities is planned, companies might want to consider purchasing a portable gas compressor. Even then, availability and internal cost remain important considerations. Exhibit 5 shows the wide cost ranges for several operating scenarios.
Other purchasing considerations. In addition to the purchase price, other capital expenditures include taxes and administrative costs, installation costs, and freight costs. Installation costs are often site specific. One vendor indicated these costs could be as low as $3,886 or as high as $19,430 for a small compressor (i.e., less than 100 horsepower), and can range from $19,430 to $77,718 for a large unit (i.e., more than 2,000 horsepower). Freig ht costs are also site-specific, ranging from $7,900 to $13,170 for small units and $26,300 to $39,500 for larger units. All these cost factors should be included in the total purchase price of the compressor and when calculating the annualized cost of a compressor. Vendors indicated the lifetime of compressor units ranged between 15 to 20 years if properly maintained.
Other leasing considerations. Leased compressors also have similar installation and freight costs. Leasing prices are usually on a monthly basis. One vendor indicated that monthly rental expenses were approximately 3 percent of the purchase price. Another vendor provided a rental price based on the horsepower of the compressor. These rental prices ranged from $15 per horsepower per month for large compressors to $20 per horsepower per month for small compressors.
Maximizing the benefits of this investment requires coordinating planned maintenance activities to lower the compressor's mobilization or demobilization costs. Such coordination is especially important for maintenance conducted on smaller, lower pressure gas lines because margins diminish as the volume of potentially recoverable gas is reduced.
Step 5: Estimate the operating costs associated with using a portable compressor.
Operating costs include fuel/energy, maintenance, and labor costs. Natural gas is the fuel most frequently used to operate compressors. Vendors indicated that fuel usage ranged from 7,000 to 8,400 Btu per brake horsepower per hour. Maintenance costs range from $5 to $12 per horsepower per month depending on the compressor's size. In most cases, however, the maintenance costs are included in the rental price.
Step 6: Calculate the volume and value of the gas recovered by a portable compressor.
The gas available for recovery using the portable compressor is a function of the amount of gas remaining in the pipeline section being repaired. Since the in-line compressor already has reduced the gas volume, the portable compressor works with the reduced volume.
The recovery of gas is governed by the compression ratio. The volume of portable compressor-recovered gas is equal to the volume of gas in place minus the volume of gas divided by the compression ratio. The gross value of the recoverable gas using the portable compressor is the amount of gas in Mcf multiplied by the gas price in $/Mcf. These calculations are shown in Exhibit 6.
Step 7: Evaluate the economics of using a portable compressor in sequence with an in-line compressor.
The net value of recovering gas from the pipeline section being repaired can be determined by subtracting the cost (i.e., operating costs, leasing costs, or annualized costs) from the value of gas recovered using the unit. Operators can effectively reduce the cost of using a portable compressor by planning and executing multiple projects in succession. The total value of gas recovered by the in-line compressor and the portable compressor is the sum of the two valuations. The total economic evaluation includes subtracting the costs of this procedure. Exhibit 7 shows this valuation procedure.