11 MARCH 2024 WorldWide Drilling Resource® Tips for Choosing the Right Size Reamer Adapted from Information by Melfred Borzall It can be a little tricky determining the correct size for a back reamer. You can’t just hold up a ruler to such an oddly-shaped tool and expect to get an accurate measurement. However, HDD (horizontal directional drilling) reamers need to be the right size, or you run the risk of leaving too large a void around your pipe, or drilling a hole too small which stresses the product pipe during pullback. How to Measure Your Reamer and Select the Right Size - To obtain an accurate measurement of the diameter a reamer will cut, place a straightedge or flat board parallel to the shaft, across the top cutter. Be sure to keep it level and measure up from the middle of the shaft, perpendicular to the straightedge. Double this measurement to calculate the reamer’s true diameter. Don’t try to cut corners and take measurements by following the angle of the blade or measuring the body of the reamer. The measurement should always include the furthermost edge of the widest cutter. Measuring only the body of the reamer will leave you with a hole much bigger than anticipated. Once you’ve got an accurate reading on your reamer’s measurements, it’s time to determine what size you need, based on the job conditions. The general rule of thumb is to select a back reamer 1½ times larger than the outside diameter of the pipe you’re pulling in. Of course, there are exceptions; here’s a few things to consider: Product Pipe Diameter - Think about the diameter of the pipe being pulled, including the ends where two pieces slip together. Those areas might be up to two inches wider than the diameter of the main pipe. Make sure you base the 1½ measurement on the widest part at any point in the pipe. Solid Rock - It’s not always necessary to use a 1½ overcut, especially when drilling into rock. When drilling into solid rock, you know you’ll have a sturdy borehole not in danger of collapsing or ground swelling. You also won’t have a lot of slurry in the hole. In these conditions, you can come down to a hole diameter a bit closer to the diameter of your pipe. On-Grade Bores - Drilling to match a grading specification, for example with sewer lines, usually means there’s a tighter tolerance for bore size. Bore paths must hold a grade within a fraction of a percentage of the slope. In this case, you’ll want to ream a hole as close to the diameter of the product pipe as possible. This may require a custom-built reamer and slowing down your pullback speed to allow more time for the cuttings to move out of the hole. Clay - In this soil type, it’s important to strictly adhere to the 1½ times diameter rule - or even add a little extra. Clay swells when liquid (like your drilling fluid) is introduced, then shrinks back down as it dries. Drilling an insufficient hole might seem permissible when the hole is wet, but your pipe could feel the pressure once the clay solidifies. Unstable Conditions - In cobble, sand, or other unstable material, extra space makes it easier to pull product through. Make sure to allow sufficient clearance in these situations by drilling a hole 1½ times the size of the pipe diameter. Length of Bore - If you’re drilling a short bore (under 150 feet), you might be able to get away with lesser clearance around the pipe. Shorter bores make it easier to remove cuttings from the hole, so you don’t have to worry about material filling up the hole and cramping your pipe. On the other hand, when drilling longer bores, it is important to drill a hole with enough space surrounding the pipe to make up for unreachable cuttings left behind. You may be wondering why you need to be concerned about the borehole being the right size. Drilling the right size hole can have a major impact on your job. After all, if the hole is too small, the job takes forever, plus, you risk humping the road during pullback, or your product pipe can stretch and break. If the hole is too big, more expensive tooling and drilling fluid is required, progress goes slower, you risk encroaching on other lines, and your pipe can sink with time. When measuring your reamer and choosing the right tool for the job, it’s always a good idea to double-check everything. Remember - measure twice, and ream once. DIR
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