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AOC 65 Soil Vapor Extraction System - Operation and Maintenance Plan

Section 3 - System Description

An overall schematic of the SVE systems showing the VEWs and the associated equipment is provided in Figure 3.1. Specifications of the major equipment for both the subslab and exterior SVE systems and other pertinent information are provided in Table 3.1.

Table 3.1 - SVE Equipment Specifications

3.1 - Vacuum System and Appurtenances

3.1.1   Vacuum Blowers

The main component of the SVE system is the device producing the vacuum. The two SVE systems at AOC-65 use regenerative blowers mounted on square steel tubing anchored to the loading dock of Building 90. Rubber grommets underneath the blower dampen vibrations. A plan view of the blower area is shown on Figure 3.2.

Two blowers were installed, one existing blower salvaged from the former SWMU B-3 SVE treatability study and one new blower. The existing blower, a GAST R6 Series Regenair^® blower, which was used periodically at the SWMU B-3 site for approximately three years, was installed at AOC-65 to vent the subslab VEWs. A new regenerative blower, a GAST Regenair^® R6 Series unit was procured to produce the vacuum for the seven VEWs installed outside or exterior to Building 90. The two R6 Series blowers share the same electrical requirements, and are similar in size and components (gauges, filters, plumbing, etc.). The R6 Series blowers can maintain a vacuum of about 55 in. to 70 in. H₂O at the blower inlet depending on the flowrate.

Motor disconnects, which are used for stopping and starting the units, are mounted on west wall of building. Blowers are installed on individual circuits so they operate independently of one another.

The blowers are relatively maintenance free and should not require any mechanical maintenance during the operational period. Both the blower and motor have sealed bearings that do not require periodic lubrication.

Table 3.1 shows the blowers’ rated flow rates in actual cfm and vacuum in inches of water column (in. H₂O). The blower system includes an inlet air filter and several valves and monitoring gauges, which will be described later in this section. Blower performance curves and other blower information are provided in Appendix A, Manufacturers’ Equipment Information.

3.1.2   Moisture Separators

Two 40-gallon moisture separating knockout tanks were installed between the VEWs and each of the blower inlets. Each knockout tank separates any condensate from the vapor recovered from the VEWs. One separator is installed on each of the two systems. The tanks are piped in parallel with suction provided by the manifolded VEWs and the discharge leading directly into the SVE blowers. Tanks have a floating ball valve that ties into a vacuum relief valve (VRV), which automatically stops VEW evacuation by providing fresh air to the blower on high liquid level in the moisture separator. A high-level float switch shuts down the vacuum blower associated with that knockout tank in the event excess liquid accumulates in the separator. This seemingly redundant instrumentation protects the blowers, minimizes power consumption and alerts operators to a high liquid level in the moisture separators.

Condensate accumulation in the moisture separators should not normally be of concern. However, during the cooler months of the year, weather systems with cooler ambient air, can result in the ambient temperature being considerably below that of the soil vapor. Since the soil vapor has a relative humidity of essentially 100 percent, condensate can readily fall out of the vapor and collect in the separator. This could occur whenever the ambient air temperature is lower than the dew point of the soil vapor.

3.1.3   Blower Inlet Filter

To prevent damage caused by particles entering the blowers, an 8” diameter inlet filter with 2.5” diameter pipe connections is installed in-line upstream of each blower. The pressure (or vacuum) drop across a clean filter is approximately 2 in. H₂O at 200 cfm. The filters will slowly accumulate particles. Once the pressure drop across a filter is greater than 6 in. H₂O, the filter element will need to be replaced with a new element.

3.2 - Emissions Control

3.2.1   Recovered Vapor Treatment (Granular Activated Carbon Adsorption Unit)

Air emissions are controlled by a Waterlink/Barnebey Sutcliffe V-1M Vapor Phase GAC Adsorber with 16” manway on top for removal and refill of GAC. The adsorber vessel captures any VOCs discharging from both blowers. The vessel is movable by forklift but can be emptied and recharged in place. The vessel was installed on the loading dock directly adjacent to the two blowers.

Sampling ports on the inlet and the discharge piping of the GAC vessel allow sampling of the air stream into and out of the GAC unit to assure VOCs do not break through. The interval between GAC recharge is estimated at more than one year, depending on quantity of VOC recovered and the operational continuity and usage time of the SVE systems. Recharge intervals will be estimated based on vapor concentrations and flowrates over the operating life of the system.

3.2.2   Recovered Liquid Treatment

Routine recovery of free liquid is not expected from the moisture separators. Only on the rare occasions when the ambient air temperature is below the dew point of the VEW vapor is there a chance of liquid accumulation. During these periods, if free liquid does accumulate, it will be collected, transported and treated at the GAC unit for Well 16.

Testing of accumulated liquid is not required since sampling and analysis was already done during initial startup at which time the VOC concentrations measured would not have resulted in a classification as hazardous waste.

3.3 - Monitoring and Control Equipment

3.3.1   Vacuum Gauges

The SVE system is equipped with gauges and a flow velocity measurement port at each well. Gauges were also installed on the blower units to allow monitoring of operational conditions. Monitoring will be done in accordance with the schedules and checklists provided in Appendix B.

3.3.2   Flow Control Equipment

Manually operated ball or gate valves were installed in the piping to each VEW to serve as flow control valves (FCVs). This allows the individual flow rate from each VEW to be manually balanced. Initially, the FCVs were set in the fully open position to maximize air flow out of the VEWs. Air flows for the subslab VEWs are set in open position for all VEWs. Balancing flows from the subslab can only be accomplished by taking selective VEWs off-line, which must be done inside the building.

Attainable flowrates were lower than expected for the exterior SVE system due to excessive friction losses in the SVE piping network. To balance the flowrates, some of the FCVs to the higher flow VEWs were partially closed. The optimal flow settings were established during the final site visit on July 30, 2003 for the exterior VEW system. FCVs should probably remain 100 percent open until piping restrictions are reduced or some of the VEWs are selectively taken off line. However, if adjustment is desired to balance the flows from the exterior VEWs, then the FCVs are the proper tool to achieve that objective.

Combination flow measurement and sample collection ports, which consist of brass bushings threaded into the galvanized piping, were installed in line to allow direct measurement of flows and sampling of soil gases. Flow ports were installed at each wellhead and at the blowers. These ports allow the insertion of a thermal anemometer for the measurement of vapor velocity, which can then be converted to estimate the flow of vapor out of each individual VEW. However, the bushings should be plugged during normal system operation when measurements are not being taken. These ports can also be used to take soil gas samples to obtain contaminant concentrations for estimating mass removal rates.

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