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Checking Your Apps Before Buying a New Jet Vac or Combination Machine — Know How It Will Be Used By Tom Jody A combination machine — either new or used — is a big investment for even the largest contractor or municipality. In order to get the right machine for your job, there are a number of things to think about and virtually all of them have to do with the application in which the machine is to be used. There are many different applications for these machines and each has its own special set of circumstances to consider. What will be the primary purpose of the machine? Will it be used almost exclusively in one application or on many different types of jobs? 48 TRENCHLESS TECHNOLOGY February 2011 www.trenchlessonline.com
What’s Your App? For example, if the machine is to be used exclusively in sanitary sewer applications, most common sanitary lines are no larger than 8 in. in diameter. Simple hydraulic theory indicates that high-volume water systems (more than 50 gpm) are really not required for this type of operation, so it makes sense to keep the water jet system at or below 50 gpm. Conversely, if the primary application is in larger diameter storm water collection systems, then a higher flow system is desirable, such as 80 gpm or even 120 gpm. If the machine will be used in both applications, then a variable flow water system may be the right choice. In any case, water pressure is the cleaning force, so try to specify the maximum pressure available for the water volume you require for the job. Commercially available combination machines offer water systems with up to 3,000 psi. The other half of the “combination” is vacuum. Again, this depends entirely on the application. If the machine will be used exclusively to recover solids from below grade sanitary or storm lines through a manhole opening, then a less expensive centrifugal compressor (fan) system will be more than enough to do the job. The higher airflow (CFM) generated by centrifugal compressors moves the material effectively and efficiently. If the application requires a lot of liquid pumping or long-distance lateral pulls (more than 100 ft), then a positive displacement (PD) blower may be the better choice. In either case, it is wise to learn about the simple physics of vacuum before making a decision and to understand what will work best in your application. If your application requires a PD blower, it is advisable to seek the highest possible CFM rating to allow flexibility in different applications where air flow may be more important than pure vacuum. Other Factors There are other factors to consider beyond the actual performance of the water and vacuum systems. Ease of operation is an important factor in being able to perform the job efficiently, and the simpler it is to set up and operate the machine, the easier it is to complete one job and move on to the next. Careful consideration should be given to component locations, such as hose reels and booms, since these components are vital to operating efficiency. Certainly greater flexibility and range of movement of these components are some of the most important factors. A vacuum boom with a longer reach is important to allow flexibility in positioning the machine for optimum performance and safety. For example, a telescopic boom allows a longer reach, without requiring additional vacuum hose and/or pipe. This is also true for the hose reel — greater flexibility and reach allows the operator to get the best position for working. Being able to quickly position the machine on the job and having the flexibility to adjust the working position of the reel without moving the machine again saves time on every job. For more information visit www.trenchlessonline.com/info For more information visit www.trenchlessonline.com/info www.trenchlessonline.com February 2011 TRENCHLESS TECHNOLOGY 49
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