Construction and maintenance challenges demand fast and reliable solutions. Polywater’s pull-planning software combines the best cable installation practices with robust testing methods. Coefficient of Friction (COF) testing supports the friction values in the software’s integrated database. Polywater pioneered coefficient of friction testing and measurement with the development of the Polywater Friction Table. For nearly five decades, Polywater has profiled the friction coefficients of thousands of cable, conduit and lubricant combinations using this device. While Polywater uses several friction measurement methods, the friction table is the fastest and most convenient method to derive accurate COF values. These values are the primary source for the Pull-Planner friction database and on-going testing has allowed Polywater to optimise the performance of pulling lubricants.
Field realities often dictate how underground cable is deployed. The placement, grouping and magnitude of bends in the conduit layout affect cable tension and sidewall bearing pressures. Exceeding these limits set by the cable manufacturer, can result in cable damage due to excessive tension, bending and/or crushing forces. The Pull-Planner can be used to determine such areas of risk so that adjustments can be made to bend radii, pulling distance or to pulling accessories to be used.
As an example, we will look at a common issue related to sidewall pressure. The normal force generated on a cable when pulled through a straight length of conduit is equal to the weight of the cable. If the straight section is sloped, the normal force is the vectored gravitational force or weight. However, if there is a conduit bend in the duct configuration, the normal force, sometimes called “sidewall pressure” is greater than just the weight of the cable. Sidewall Pressure creates a radial force on the cable in a bend. This additional force is a function of the pulling tension and increases exponentially with higher incoming tension. Sidewall pressure is a crushing force on the cable. It can damage the cable if it is too high. Small differences in the friction coefficient have a dramatic impact on incoming tension and sidewall pressures in bends. Excess sidewall pressure on medium and high voltage cable can tear cable jackets, generate insulation treeing or deform foil shields. All of these problems can shorten cable life and reduce overall grid performance. Polywater’s Pull-Planner Tension and Sidewall pressure estimation software can help to pinpoint areas in the cable installation where excessive sidewall pressures might be high enough to cause these problems.
Sidewall pressure is directly proportional to the tension coming out of the bend and inversely proportional to radius of the bend. It also depends on the number of cables being pulled, the configuration of those cables, and the resulting weight correction factor. Once these factors have been estimated by the Pull-Planner and the risks of excess sidewall pressure have been determined, the engineer or contractor can make adjustments before the cable installation is started to protect the cable from damage. Measures such as lengthening the bend or incorporating a larger diameter elbow can be used to reduce sidewall pressures below the cable’s maximum limit.