A roots blower for multi-phase extraction should be evaluated only for vapor service after gas-liquid separation is defined. Multi-Phase Extraction (MPE) removes contaminated soil vapor, groundwater and potentially free-phase product, combining elements of SVE and groundwater recovery.
MPE Process and Extraction Configurations
MPE applies vacuum through wells installed in or near contaminated soil and groundwater. Depending on well construction and the extraction arrangement, the system may recover soil gas, groundwater containing dissolved contaminants, light non-aqueous-phase liquid (LNAPL), and vapor released from the liquid stream. Entrained sediment may also be present and should be considered in the process design. Above ground, the extracted stream is separated into gas and liquid phases, which are then treated or managed according to their composition and the project requirements.
MPE can address contamination in both the unsaturated zone and the saturated zone. Groundwater recovery may lower the local water table and expose previously saturated soil to vapor extraction. The technology can also support free-product recovery and control contaminant migration, but its suitability depends on site geology, groundwater conditions, contaminant properties, well configuration, and the rate of liquid production. MPE does not guarantee a particular cleanup rate or outcome; pilot testing and site data are needed to establish expected performance.
Two broad extraction arrangements are common:
- Separate-phase pumping: Groundwater or free product is recovered through a pump or lift arrangement, while a separate connection extracts soil vapor. Separate lines and controls allow the liquid and gas duties to be adjusted and measured independently. A roots blower may be suitable for the vapor branch if its required airflow and vacuum fall within the selected model’s approved operating range.
- Combined-stream extraction: Vacuum draws vapor and liquid through the same well connection or pipework. The mixed stream passes to an above-ground separator, where gas is routed toward vapor treatment and water or product is transferred to liquid treatment or recovery. The vacuum source must be compatible with the conditions downstream of the separator, and the separator must prevent liquid carryover.
Some MPE configurations use relatively high vacuum to lift groundwater through a tube while extracting soil vapor. The achievable liquid lift depends on the applied vacuum, groundwater depth, liquid density and vapor pressure, pipe and well losses, air leakage, elevation, and well configuration. A conventional roots blower may not provide the vacuum required for a particular lift. Selection should use the actual duty point and the manufacturer’s pressure, vacuum, temperature, and speed limits; if the required vacuum exceeds those limits, another vacuum technology or process arrangement is needed.
Groundwater drawdown can expose contaminated soil and alter groundwater movement around an extraction well. Excessive or poorly controlled drawdown can increase water-treatment demand, move contamination toward the well, contribute to settlement in sensitive soils, consume more energy, or reduce effectiveness farther from the well. Set liquid-extraction rates to the remediation objective and site conditions rather than maximizing them by default.
Separation and Treatment of Extracted Phases
Reliable gas-liquid separation is essential when a roots blower is used on the vapor side of an MPE system. The separator should be designed for expected liquid flow and credible surges, including changes caused by startup, rainfall, groundwater levels, well operation, or free-product production. It should account for condensate, foam, entrained droplets, and sediment as well as the normal liquid stream.
A separator may include an inlet arrangement that limits turbulence, gas-disengagement volume, a demister, a liquid reservoir, sediment collection, level measurement, a high-level alarm, and an independent high-high-level shutdown. Liquid-transfer equipment, inspection access, drains, cleanouts, and pressure or vacuum ratings should also be defined. The high-high-level shutdown should stop or isolate the blower before liquid can reach its inlet. Critical or unattended installations may warrant independent alarm and shutdown devices; the protective design should be confirmed against the project’s risk assessment and equipment requirements.
A demister removes droplets that remain in the gas after bulk separation. Its selection should consider gas velocity, droplet size, liquid loading and properties, contaminant compatibility, fouling potential, and pressure loss. Oily deposits, sediment, or biological material can increase demister resistance, so provide inspection access and a defined cleaning or replacement interval. Foam can pass through a demister more readily than bulk liquid. Possible controls include lower inlet turbulence, additional separator volume, a mechanical foam-control device, a controlled extraction rate, or compatible antifoam treatment where permitted. Check the effect of any chemical addition on downstream water and vapor treatment before use.
Liquid collected in the separator must be transferred without destabilizing the vacuum system or creating uncontrolled air leakage. Options can include an automatic or duplex transfer pump, a vacuum-compatible discharge vessel, a batch transfer chamber, or gravity drainage where the site elevation allows it. If the separator operates under significant vacuum, the transfer system must be designed for that condition. A centrifugal pump also needs adequate inlet conditions to avoid cavitation.
Recovered groundwater may contain dissolved hydrocarbons, solvents, metals, suspended solids, emulsified product, and treatment chemicals. Depending on the contaminants, concentrations, and discharge or disposal requirements, treatment may include oil-water separation, sediment removal, filtration, air stripping, activated-carbon adsorption, biological treatment, or chemical oxidation. The water-treatment system should be sized for credible peak liquid flow and contaminant loading, not only long-term averages.
If light non-aqueous-phase liquid is recovered, it may require separate collection, oil-water separation, storage, level monitoring, and controlled recovery or disposal. Free product can increase flammability and material-compatibility concerns. Separator and storage design should address vapor release, overflow prevention, and applicable site handling requirements.
After gas-liquid separation, the vapor stream proceeds to the blower and vapor-treatment equipment in an arrangement selected for the process. The vapor may contain soil gas, infiltrated air, water vapor, volatilized groundwater contaminants, free-product vapors, and gas released inside the separator. Concentrations may change during startup, when wells are opened or adjusted, or when liquid production changes. Treatment options can include activated carbon, oxidation, biological treatment, scrubbing, or another approved process, depending on contaminant chemistry, concentration, flow, and applicable requirements. The blower and treatment system must be compatible with the maximum credible vapor composition and operating conditions.
Sediment and fine particles can cause wear, imbalance, filter blockage, valve problems, or contamination of downstream equipment. Depending on the site, protection may include a sediment section in the separator, accessible pre-filters, a replaceable fine filter, and differential-pressure monitoring. Filter and demister pressure loss should be included in the vacuum calculation, and components should remain accessible because MPE service can produce frequent loading.
roots blower selection, position and protection
A roots blower is a positive-displacement machine that transfers gas from inlet to discharge. System resistance and flow establish the operating pressure or vacuum, subject to the blower’s limits and the protection provided by the package. In MPE service, it is normally considered for the separated vapor stream, downstream of the primary gas-liquid separator, demister, and suitable particle protection. It should not receive the untreated mixed gas-liquid stream.
Airflow depends on the number of active wells, soil permeability, applied vacuum, well-seal condition, surface-air entry, groundwater level, separator vacuum, and any air leakage associated with liquid lifting. Gas flow can change when liquid extraction starts or stops. Use pilot-test measurements where available and define minimum, normal, and peak gas flow on a consistent basis, with the inlet pressure, temperature, moisture, and gas composition stated. Groundwater flow alone is not a basis for sizing the gas blower.
Required blower-inlet vacuum includes the vacuum needed at the wells and, where applicable, for liquid lift, plus losses through wells, piping, manifolds, separator, demister, filters, and treatment equipment on the suction side. Evaluate credible operating combinations such as high groundwater, a blocked branch, or a liquid surge; do not simply add unrelated maximum values that cannot occur at the same time. Confirm that the selected blower remains inside its approved operating envelope across startup, normal operation, and credible upset conditions.
Provide layered protection against liquid and solids carryover. Depending on the installation, this can include an adequately sized knockout vessel, demister, level alarm, independent high-high-level shutdown, automatic liquid transfer, inlet filtration, separator-pressure monitoring, and a controlled shutdown sequence. A small moisture trap intended for routine condensate is not protection against an MPE liquid surge. Any protective devices and shutdown logic should be selected for the site hazard assessment and coordinated with the blower manufacturer’s operating instructions.
A roots blower may be unsuitable if liquid cannot be reliably separated, the process requires the vacuum source to handle mixed gas and liquid directly, required vacuum or pressure exceeds the model’s limits, gas chemistry is incompatible with wetted materials, solids cannot be controlled, or the duty varies beyond a stable operating range. A hazardous vapor service may also require equipment and installation measures that the selected package cannot provide. In these cases, evaluate another vacuum technology or a different process configuration using the actual MPE conditions.
System Design, Controls and Safety
An MPE manifold must either transport the mixed gas-liquid stream or provide separate paths for each phase. For mixed-phase piping, assess slope, pipe diameter, gas velocity, liquid loading, low-point collection, pressure loss, cleanout access, corrosion resistance, settlement, and freeze protection. A low point can collect liquid, restrict the gas path, and create unstable vacuum. Slug flow—alternating movement of gas and liquid—can cause pressure pulsation, separator surges, vibration, noise, and irregular well vacuum. Its likelihood depends on pipe geometry, gas velocity, liquid rate, and operating conditions. If mixed-phase transport remains unstable, separate gas and liquid lines may provide more reliable operation.
Controls should coordinate the blower, liquid-transfer pumps, extraction wells, and treatment equipment. Monitoring may include wellhead vacuum, gas and liquid flow, separator pressure and level, free-product thickness, blower-inlet vacuum, discharge pressure, filter differential pressure, gas temperature, motor current, vapor concentration, treatment performance, and groundwater level. Review process and mechanical data together: a change in blower vacuum may reflect soil or groundwater conditions, liquid production, a blocked line, or separator resistance rather than a blower fault.
A variable-frequency drive can adjust blower capacity as gas flow changes, for example to maintain separator vacuum, limit wellhead vacuum, match the number of active wells, or reduce energy use at lower demand. Speed control should operate within the manufacturer’s limits and should not replace independent protection against excessive vacuum, discharge pressure, temperature, or liquid level. Set maximum speed and vacuum limits, and provide pressure-relief and shutdown protection as required by the blower package and system design.
Shutdown and restart sequences should be engineered for the specific wells, separator, blower, and vapor-treatment equipment. Depending on the design, a controlled shutdown may isolate extraction wells, continue vapor removal from the separator, transfer collected liquid, purge selected gas sections, and stop equipment in a defined order. Emergency logic should move the system to a safe state if a transfer pump, separator, blower, or treatment component fails. Follow the approved process-safety procedure and blower instructions rather than treating any generic sequence as universal.
MPE can release volatile contaminants at concentrations that create toxic-exposure or flammability hazards. The site safety review should consider maximum credible vapor concentration, lower explosive limit, oxygen content, exposure limits, electrical-area classification, grounding, leak detection, ventilation, treatment-unit temperature, and emergency isolation. Conditions can vary during startup or when a well begins producing free product. Select materials for the complete expected contaminant mixture, moisture, and temperature, including separator internals, demister, instruments, pumps, valves, gaskets, flexible connectors, blower seals, and liquid-storage equipment.
Information Required for Blower Selection
A technical MPE enquiry should describe the process and the complete gas-side duty. Include:
- Number of extraction wells, active-well combinations, well construction, and expected operating schedule
- Minimum, normal, and peak gas flow, with the flow reference conditions and expected gas composition, temperature, and moisture
- Required wellhead vacuum and blower-inlet vacuum, including liquid-lift requirements and losses through wells, manifolds, separator, demister, filters, and suction-side treatment equipment
- Maximum liquid flow, credible liquid surges, groundwater depth, expected free-product quantity, and separator operating conditions
- Soil permeability, pilot-test results, air-leakage assumptions, and anticipated changes in groundwater level
- Separator, demister, filter, and manifold design, including clean and fouled pressure losses and solids or foaming potential
- Vapor-treatment process and its location relative to the blower, expected contaminant concentrations, and hazardous-area requirements
- Ambient range, site access, unattended duration, materials compatibility, control range, and required instruments and alarms
- Duty/standby philosophy, shutdown sequence, and the service consequence of a blower or transfer-pump failure
State gas and liquid duties separately even when they share an extraction line. Final selection should be based on site data, pilot testing where available, and the selected blower’s approved performance and operating limits.
Glossary
Demister: An internal separator element that removes fine liquid droplets from a moving gas stream.
Free Product: A separate non-aqueous contaminant phase that can move or accumulate independently of groundwater.
Gas-Liquid Separator: A vessel that removes entrained or collected liquid from a gas stream before downstream equipment.
Multi-Phase Extraction: Simultaneous or coordinated recovery of soil gas, groundwater and potentially separate-phase product under vacuum.
NAPL: A non-aqueous-phase liquid that exists as a separate contaminant phase rather than dissolving completely in water.
Slug Flow: Intermittent movement of large liquid plugs through gas piping, producing unstable pressure and liquid carryover.
Vapor Capture: Controlled collection of volatilized contaminants before they migrate to uncontrolled receptors or the atmosphere.