How to Spec a DPE System for a Small Gas-Station Release

A typical gas-station release sits in the middle of the remediation work most consultants run: small enough to be funded under a state UST program, big enough that the wrong equipment will burn the budget. This guide walks through how we size a dual-phase extraction (DPE) system at Fliteway when the call comes in, and the questions we ask before recommending a specific blower / vacuum / liquid-ring configuration.

What you need before sizing anything

Before we recommend a DPE skid, we need five inputs from the consultant. Without them every number below is a guess.

• Site lithology and vadose-zone thickness
• Depth to groundwater and observed recovery rate (gallons/min/well)
• Number and spacing of recovery wells, plus screened intervals
• Vapor concentrations at the wellhead, ideally a recent 24-hour pilot test
• Discharge requirements — air permit, POTW limits, NPDES limits

Vacuum target — start at the wellhead

TODO: walk through the tradeoff between high-vacuum (liquid-ring) and moderate-vacuum (regenerative blower) DPE for a typical 15–30 ft vadose zone with a ~1 gpm/well recovery rate. Include the rule of thumb on slug flow vs. continuous flow and why it changes blower selection.

When liquid-ring is the right answer

TODO: 12–25 in Hg, when groundwater drawdown matters, when the well field has cold pockets that flood the line. Cite a real spec from the GHD Durango or Bandera Road systems.

When regenerative is plenty

TODO: 6–12 in Hg, dry vadose zones, soil that breathes. Note the cost delta — typically 40–60% less than liquid-ring for the same wellhead suction at low flow.

Sizing the air-water separator

TODO: rule of thumb on residence time, why over-sizing is cheaper than under-sizing, and what changes if the site has free product.

Treatment train downstream of the blower

• Vapor-phase GAC — sizing rules and lead-lag arrangement
• Particulate filtration — when to add it, when it's a maintenance burden
• Liquid handling — oil/water separator, transfer pumps, water-phase GAC if discharge requires it

Common mistakes we see in DPE specs

1. Sizing the blower for the highest flow ever measured during the pilot, not the steady-state flow
2. Forgetting heat — vacuum work is hot, and downstream GAC vessels do not like 180°F vapor
3. Specifying a dust filter where a moisture knockout was the actual need
4. Picking 480V because the budget said so, when 230V single-phase would have shipped two weeks faster

What a sized system looks like on a project

TODO: short walkthrough of the GHD Durango biosparge / DPE rental — show the bill of materials, the Modified Enclosure footprint, and the lead time. Link to the case study below.

When to call us

If you have pilot test data and need a sized DPE system on paper before the next state submittal, send the data over. We'll mark up a system diagram and a budgetary cost the same week — usually faster.