LBG METHOD ELIMINATES GUESSWORK FROM MPE SYSTEM DESIGN

 By Matthew Peramaki, P.E., Associate,
St. Paul office

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Multi-phase extraction (MPE) has become a frequently used technology for source area remediation at contaminated sites. While there have been many successful applications of the technology, there is a lack of readily available standard design methods for calculating the pressure drop in multi-phase flow (soil vapor and ground water) applications. This has left remediation engineers without essential information needed for the proper design of MPE systems.

Designers have resorted to methodologies that are based on previous experience or, at worst, guesswork. As a result, an MPE system may have to be run far longer than necessary to achieve clean-up goals – significantly increasing the owner’s costs. Recently, LBG engineers have devised a method that accurately predicts pressure drop in an MPE system, effectively eliminating this guesswork.

MPE systems are typically ideal for application in sites with low permeability, such as silty, clayey soils, and in geologic formations that do not produce enough groundwater to allow effective use of a down-hole, submersible pump.

In addition to allowing effective groundwater extraction under these conditions, MPE systems offer the additional benefit of extracting soil gases, including very volatile contaminants such as petroleum hydrocarbons and chlorinated solvents. These are often concentrated in the "smear zone," that is, the area of the aquifer where the water table fluctuates. MPE can be effectively used to target and quickly remediate this smear zone, even if this area contains large amounts of free-phase product.

Why pressure drop is significant

Where designers can get away with a sloppy design in air sparging by oversizing the air compressor, no such luxury exists for the MPE system designer. When an inefficient MPE piping network is designed the penalty is a reduced well casing vacuum and reduced water and vapor extraction rates. These reductions can often be so significant that the system fails or must run years longer than anticipated.

The deeper the water table at a site, the more crucial it is to properly design the system because a good deal of the pressure drop occurs in the vertical uplift from the base of the well to the start of the horizontal pipe.

In an MPE system, two-phase flow consists of a mixture of ground water existing in a liquid state, or "phase," and soil gas, which exists in a gaseous phase. Two-phase flow is the simultaneous flow of these two fluids within the piping of the remediation system. Single-phase flow prediction methods can not accurately be applied in an MPE system –that is, a designer cannot simply use the sum of the single-phase prediction for air plus that for water. In fact, in testing this approach, LBG engineers found that the actual pressure drop was ten times greater than what was predicted using single-phase models.

Finding an answer

LBG compared these methods to data collected from operating remediation systems and found them to be suitable for MPE system design. Based on discussions with nationally recognized remediation experts, this appears to be the first time engineers in the environmental remediation field have found a method that accurately predicts pressure drop in an MPE system.

Using this method, an MPE designer can engineer a system that -- simply put -- does what it is supposed to do.

In their search for an accurate method to predict pressure drops in two-phase flow systems, LBG engineers examined empirical methods developed decades ago by researchers from other industries -- petroleum refining, oil production, chemical and refrigeration engineering -- which can predict the conditions common in MPE systems. These methods account for changes in vapor flow, liquid flow, pipe diameter, pipe length, pipe geometry, fitting losses and pressure drop in a closed conduit. Knowing the pressure drop in a two-phase flow system is of primary importance to the MPE system designer to select vacuum pumps and minimize pressure losses in the system piping. Minimizing pressure drop is of supreme importance because vacuum pumps are limited in the vacuum that they can practically produce. Most readily available vacuum pumps cannot produce more than 25 inches of mercury.