It is known to introduce elongated elements into ducts with the help of compressed air. This method, called “jetting”, is described in the document U.S. Pat. No. 5,197,715 as well as the use of a “sonic head”, where “jetting” is a synergy of pushing and blowing and where at the “sonic head” a sonic flow is created, generating a pulling force at the foremost end of the cable. Blowing creates a distributed forward propelling force onto the cable, compensating locally the friction forces, limiting build-up of axial forces in the cable, in turn limiting extra exponential friction build-up by the well known “capstan effect” (See e.g. http://en.wikipedia.org/wiki/Capstan_equation). As air is a compressible fluid, it expands when the pressure decreases towards the end of the duct, resulting the air propelling forces to being relatively small in the first section of the duct (injection side) and relatively large at the last section of the duct (exhaust side). During jetting, typically there is a deficiency of air propelling forces to compensate the friction in the first section, where pushing assists the cable installation, and an excess of air propelling forces in said last section. The so-called “critical point” is the location in the duct where the air propelling forces have just become equal to the friction forces. In a situation where the cable weight is dominant for the friction (as is the case in jetting), the friction forces are mainly constant, and it is easily understood that the “critical point” is the most difficult point to reach: upstream of this point there is less length for the pushing force to build up and downstream of this point a pulling force will build up first. It shall be noted that when the cable diameter becomes close to that of the internal diameter of the duct, this reasoning does not hold anymore, because the pressure drop over the part of duct that is filled with cable will be higher because of the increased hydraulic resistance, as the hydraulic diameter is significantly reduced by the presence of the cable. For this jetting method the “sonic head” can be advantageously used. Calculation in the document U.S. Pat. No. 5,197,715 showed that the improvement is enhanced because the “sonic head” is as it were “pulling” the cable over the “critical point”.
When the duct is bended with a continuous radius over a great length, the aforementioned synergy between pushing and blowing disappears: the action of axial forces in the cable is much shorter here because the afore mentioned capstan effect is much larger. In this case, the air has to do the job all by itself, and installation will only go for duct lengths short enough such that the blowing generates sufficient air propelling forces from the very beginning, which will cause the installation length to decrease by about 50%. However, for today's larger relative cable diameters the increased hydraulic resistance of the part of the duct filled with cable, helps to “pull” the cable through the most difficult first section as in this first section, the hydraulic diameter is (significantly) reduced by the presence of the cable.
Another method to introduce an elongated element is known, with the help of water, where the liquid at least partly compensates the weight of the elongated element. This method is called floating, and the operating conditions are quite different from the jetting method, especially regarding the flow and speed of the liquid injected into the duct. Indeed, the speed of the liquid is quite low compared to compressed air, and there is no expansion along the pipe. In many cases, much longer cable installation lengths can be reached in straight ducts by floating than by jetting.
However, it is still difficult to operate (i.e. to introduce or remove) elongated elements in a duct when the latter is bended with a continuous radius over a great length. A typical case is when the duct is coiled over a drum for a length, up to more than one thousand meters for some cases. Another typical case is when the duct is helically stranded with another duct or cable over its full length, for example for longer than one thousand meter. Jetting or floating always end up in a length being limited by the friction between the duct walls and the elongated element, created by the continuous bending. Calculations demonstrate that the jetting method often achieves a higher installation length than the floating method in such bended ducts, but some improvements are still needed to achieve higher and higher target installation lengths.