A roots blower for soil vapor extraction (SVE) must deliver the required gas flow at operating vacuum while handling site moisture and contaminant vapor. SVE removes volatile contaminants from the unsaturated zone by drawing soil gas through extraction wells and routing it to collection and treatment.
Blower selection cannot be based only on the number of extraction wells. Soil permeability, moisture, contaminant properties, well construction, manifold resistance and vapor-treatment equipment all affect the required duty.
SVE Process and Subsurface Conditions
The unsaturated soil zone contains air and moisture in the spaces between soil particles. Applying a vacuum at an extraction well lowers pressure around the well and draws air through the surrounding soil. Air may also enter through controlled surface-inlet points.
As this air passes through contaminated soil, volatile compounds transfer into the gas phase. The soil gas moves through extraction piping; moisture and entrained liquid are separated before the blower moves the gas to treatment. Treated gas is discharged in accordance with applicable environmental requirements.
SVE removes contaminants from soil by physical mass transfer and vapor extraction. It does not automatically destroy them. Unless a destructive treatment method is used, the collected vapor or adsorbed contaminant must still be managed appropriately.
SVE is generally most effective for contaminants that have sufficient volatility to transfer into the soil-gas phase.
Potentially suitable contaminant groups include gasoline-related hydrocarbons; benzene, toluene, ethylbenzene and xylenes; selected chlorinated solvents; volatile organic compounds; certain fuel components; and some lighter semi-volatile compounds under suitable conditions.
SVE is generally less effective for heavy metals, strongly adsorbed non-volatile chemicals, heavy oils, high-molecular-weight hydrocarbons, contaminants located entirely below the groundwater table, and compounds with very low vapor pressure.
Temperature, soil composition and contaminant aging can also affect volatility. Material that has remained in the soil for many years may be more difficult to remove than a recent spill because it can diffuse into small pores or become strongly adsorbed onto soil particles.
A roots blower displaces a nominal volume of gas per rotor revolution. At a fixed speed, it can provide relatively steady flow over part of its permitted operating range, but actual flow and power vary with differential pressure, inlet conditions, speed and internal leakage.
SVE system resistance can change with soil moisture, seasonal groundwater movement, individual well adjustments, condensate accumulation, treatment-filter loading, and changes to active extraction branches. These factors can alter airflow distribution and the blower operating point. Roots blowers may suit SVE duties in the medium-vacuum range when the required airflow and inlet vacuum fall within the selected model’s verified operating envelope.
A properly selected roots blower can support continuous extraction from multiple wells when the required duty falls within its verified performance range. Variable-speed control can be used where the duty and blower limits allow it. A roots blower is not suitable for every SVE duty: applications requiring very high vacuum, direct liquid handling or extremely low airflow may need another vacuum technology.
High vacuum and high airflow are not the same thing.
Vacuum creates the pressure difference that causes air to move through the soil. Airflow represents the quantity of soil gas actually extracted.
A system can show high vacuum with very little airflow when soil permeability is low, the well screen is blocked, branch valves are excessively throttled, moisture occupies soil pores, or the extraction pipe, filter or separator is restricted. The opposite condition can also occur. High airflow with relatively low vacuum may indicate permeable soil, a surface leak or a short airflow path near the extraction well. The blower must be selected for the required combination of airflow and inlet vacuum. Selecting from only one of these values can produce an unsuitable machine.
Soil permeability determines how readily gas can travel through the subsurface.
Gravel and coarse sand generally permit comparatively high airflow at modest vacuum. Fine sand, silt and clay offer greater resistance.
Layered soil can create more complicated behaviour. Air may travel easily through a permeable sand layer while bypassing contamination located within a less permeable clay or silt layer.
Increasing the blower vacuum may extend airflow into some resistant areas, but it may also draw more atmospheric air through surface cracks or utility trenches.
Pilot testing can measure airflow at different applied vacuum levels, pressure response at surrounding monitoring points, moisture carryover and contaminant concentration, and show how extraction wells interact. It can also help estimate the radius of influence and identify surface-air leakage. Pilot-test data provide a more reliable basis for blower selection than estimates based only on the contaminated land area.
The radius of influence describes the area around an extraction well in which a measurable pressure response occurs.
It is useful for preliminary well-spacing decisions, but it should not be treated as a sharply defined circular treatment boundary.
Subsurface airflow is affected by soil layering, moisture distribution, surface sealing, buildings and pavement, utility corridors, and fractures. Performance also depends on groundwater position and nearby extraction wells. The area showing a pressure response is not necessarily identical to the area receiving effective contaminant removal. Pressure, airflow and contaminant concentration must be evaluated together.
Wellfield and Extraction System Design
SVE systems may use vertical or horizontal extraction wells.
Vertical wells are commonly used where contamination extends through a significant soil depth. Horizontal wells can be beneficial beneath buildings, tanks or shallow contaminated zones where vertical access is limited.
Well design should consider screened depth, screen length, well diameter, filter-pack material, surface seal, and groundwater level. The design should also consider contaminant distribution, soil permeability, and required airflow. The well screen should be positioned within the contaminated unsaturated zone. A screen that extends below the groundwater table may increase water extraction and reduce useful vapor flow. The upper part of the well should be sealed to prevent atmospheric air from travelling directly down the outside of the casing.
The extraction manifold connects the wells to the moisture separator and blower.
Each well branch should normally include an isolation valve, a flow-control or balancing valve, a vacuum measurement point, an airflow measurement point, a vapor-sampling connection, and a condensate-drainage provision where required. The main header must be sized for the combined airflow from all simultaneously operating wells.
An undersized header can cause excessive pressure loss, weak extraction from remote wells, excessive vacuum at nearby wells, difficult branch balancing, increased blower power, and reduced remediation performance. The pipeline should slope toward controlled condensate-collection points. Unplanned low points can fill with water and partially or completely block the gas path.
Air naturally follows the path of least resistance. Highly permeable wells may take most of the available airflow, leaving little capacity for more resistant parts of the site. Each well should therefore be balanced using measured airflow, vacuum and contaminant concentration.
Closing a high-flow well can sometimes redirect vacuum toward another area. However, excessive throttling increases system resistance and can raise blower vacuum and temperature.
Balancing decisions should consider contaminant mass removal rather than airflow alone. A well with moderate airflow and high vapor concentration may remove more contaminant than a well with very high airflow but a low concentration.
Gas Handling and Blower Inlet Conditions
Soil gas commonly contains water vapor. Liquid water may also enter the extraction system because of groundwater movement, rainfall or incorrect well configuration. When warm, moisture-laden soil gas cools inside the above-ground pipeline, condensate can form.
Liquid entry can cause reduced pipeline area, unstable airflow, corrosion, damage to instruments, contamination of treatment equipment, and blower damage. Possible consequences include sudden changes in inlet vacuum. A standard roots blower should not ingest liquid slugs.
The system should normally include a gas-liquid separator or knockout vessel, a demister, a high-liquid-level alarm, an automatic or controlled drain, and a high-level blower shutdown. It should also include a condensate-storage facility and secondary containment. The separator should be located upstream of the blower and sized for both normal condensate production and credible liquid surges.
Soil-gas extraction can carry dust, sediment, rust and other particles into the vacuum system.
An inlet filter can protect the blower, but it must be selected carefully. A filter with insufficient area can become a major source of vacuum loss.
Filter selection should consider maximum airflow, particle size, moisture content, initial pressure loss, dust-holding capacity, and chemical compatibility. The design should also consider maintenance access and required filtration efficiency. A differential-pressure indicator allows operators to monitor loading. Replacing the filter only according to a calendar schedule may be unreliable because dust conditions can change significantly between wells and operating periods.
The extracted gas must normally be treated before discharge.
Potential treatment methods include activated-carbon adsorption, thermal oxidation, catalytic oxidation, condensation, biological vapor treatment, and chemical scrubbing. Treatment equipment can be installed on the vacuum or pressure side of the blower, depending on the process design.
Every treatment component creates resistance. This can include carbon vessels, flame arresters, heat exchangers, scrubbers, filters, and silencers, as well as exhaust stacks and control valves. The blower selection must include the maximum expected resistance of the treatment train. Activated carbon can become wet or contaminated and develop greater pressure loss. Selecting the blower using only the clean-carbon condition may underestimate the maximum operating load.
Blower capacity should be evaluated at its actual inlet conditions.
The extracted gas may enter the blower at sub-atmospheric pressure, elevated or reduced temperature, high humidity, changing contaminant concentration, and site-specific altitude. Because gas density changes with absolute pressure and temperature, a standard airflow value is incomplete unless its reference condition is stated.
The project should state both the measurement location and the flow reference conditions. Actual flow at the blower inlet or wellfield differs from standard, normal or free-air delivery; the blower manufacturer and remediation designer should use consistent pressure and temperature references when comparing these values.
Leaks on the vacuum side allow atmospheric air to enter the system without passing through the contaminated soil.
Common leakage points include loose hose connections, damaged flexible pipes, valve stems, separator covers, drain valves, and instrument fittings, as well as cracked underground piping and poorly sealed wellheads. Vacuum leakage reduces the capacity available for soil extraction and can dilute the contaminant concentration reaching the treatment system. Dilution may make the vapor appear cleaner even though contaminant removal from the soil has not improved.
Atmospheric air entering directly through the ground surface can create a short flow path to the extraction well.
Surface sealing may be used to encourage airflow through deeper contaminated soil. Possible surface covers include existing concrete floors, asphalt pavement, low-permeability membranes, compacted soil covers, and temporary geomembranes. Surface sealing can increase the effective subsurface pressure response, but it can also influence rainwater drainage and soil-gas accumulation. The design should consider the complete site condition.
Blower Operation and Safety
A roots blower working under vacuum can produce a substantial increase in discharge-gas temperature. The temperature depends on inlet vacuum, pressure ratio, blower speed, inlet-gas temperature, internal leakage, and gas composition. Performance also depends on cooling conditions. Excessive temperature can damage seals, reduce lubricant life and affect downstream treatment equipment.
High discharge temperature may indicate excessive vacuum, closed well branches, blocked inlet filter, restricted vapor-treatment equipment, incorrect blower speed, and insufficient cooling. Temperature monitoring and high-temperature shutdown protection should be included.
SVE airflow demand can change during the remediation project.
Variable-frequency control can adjust blower capacity according to number of active wells, target manifold vacuum, seasonal soil conditions, vapor-treatment capacity, contaminant concentration, and different operating phases. The blower must remain within its permitted minimum and maximum speeds. Branch valves are still needed for wellfield balancing, but using heavy throttling as the main capacity-control method wastes energy and increases blower loading. A vacuum-relief valve may protect the blower if inlet vacuum becomes excessive. It should not be used as the primary method of normal airflow control.
Petroleum hydrocarbons and solvents can create a flammable gas mixture.
Safety evaluation should consider expected contaminant concentration, lower explosive limit, oxygen concentration, startup conditions, changes in vapor concentration, and air leakage. The design should also consider treatment-unit temperature, static electricity, electrical equipment classification, and possible ignition sources. Vapor concentration may be highest during initial operation or after a long shutdown. Safety design should not be based only on concentrations measured after the system has been running for several months. The complete blower package, motor, instruments and controls must be suitable for the defined area classification where a hazardous atmosphere can occur.
Some extracted vapors may be toxic or corrosive even when their concentration is below the flammable range.
The system should evaluate blower material compatibility, seal compatibility, pipeline corrosion resistance, worker exposure, ventilation, and sampling procedures. The system should also account for exhaust treatment, emergency isolation, and maintenance contamination. Personnel should not open separators, filters or blower connections before the system has been isolated, purged and assessed for residual contamination.
A roots blower normally does not inject lubricant into the gas stream. This can reduce the risk of adding oil to the extracted vapor or treatment equipment.
However, the complete blower package still contains lubricated bearings and timing gears. Proper seals separate these compartments from the gas chamber.
Maintenance should include checking oil level and condition, inspecting seals, monitoring oil consumption, checking gearbox ventilation, inspecting for internal contamination, and recording vibration and temperature. The term oil-free refers to the gas path and should not be interpreted as meaning that the entire blower package contains no lubricant.
Monitoring, Troubleshooting, and Selection Inputs
SVE performance should be evaluated using both mechanical and environmental measurements.
Useful operating data include total blower airflow, blower inlet vacuum, discharge pressure, motor current and speed, inlet and discharge temperatures, separator liquid level, filter differential pressure, airflow and vacuum at each well, vapor concentration and treatment efficiency.
Environmental monitoring may also include subsurface pressure response, soil-gas concentration, groundwater level, soil contaminant concentration, and contaminant rebound after shutdown. Mechanical data confirms whether the equipment is operating correctly. Environmental data determines whether remediation is progressing.
Weak airflow from all wells may be caused by low blower speed, blocked inlet filter, water-filled manifold, major vacuum leak, restricted separator, and insufficient blower capacity.
Weak airflow from one well may indicate blocked well screen, closed branch valve, water in the branch pipe, low soil permeability, and incorrect well construction. High vacuum with low total airflow may indicate a restriction. Low vacuum with unexpectedly high airflow may indicate a major leak or excessive surface-air entry. Increasing blower speed should not be the first response to every performance problem. The cause should be identified through pressure, airflow and concentration measurements.
A complete SVE blower enquiry should provide required inlet airflow and operating vacuum, minimum and maximum duty points, airflow reference conditions, inlet-gas temperature and contaminant composition. It should also quantify vapor concentration, moisture and condensate, particles, extraction-well count, main-header dimensions, separator and filter losses, vapor-treatment resistance, site altitude, control range, electrical supply, hazardous-area requirements and operating hours. Base selection on verified vacuum performance rather than positive-pressure capacity alone.
Glossary
Activated Carbon: A porous adsorbent commonly used to capture organic vapors from an off-gas stream.
Condensate: Liquid formed when vapor cools below its dew point or when groundwater enters an extraction system.
Gas-Liquid Separator: A vessel that removes entrained or collected liquid from a gas stream before downstream equipment.
Hazardous Location: An area classified because flammable gas, vapor or combustible dust may be present in an ignitable concentration.
Radius Of Influence: The observed distance over which a test well produces a measurable pressure or vacuum response.
Soil Vapor Extraction: A remediation process that applies vacuum to remove volatile contaminants as soil gas from the unsaturated zone.
Vacuum Manifold: The piping header that connects multiple extraction wells to separators, treatment equipment and the vacuum source.
Wellfield Balancing: Adjustment and verification of branch flows and wellhead pressures across a network of remediation wells.