Specialized equipment is available to clean the pipes. This should meet a number of conditions:
• the dirt should be loosened and removed as quickly as possible;
• drainpipe and casing should not be damaged;
• the soil around the drain should not be seriously disturbed;
• to clean the pipes should be affordable;
• the operation should be simple and user-friendly.
A certain amount of pressure is required to loosen up the dirt. If the pressure is too high, there is a chance of damage to the field drain and the casing. The structure of the soil around the field drain might also be disturbed. This is affected by the volume of water which the nozzle of the jetter produces. A large amount of water means better removal of dirt. A compromise has to be found between the amount of pressure and the volume of water in a specific period. Usually the pressure at the nozzle is 10 to 15 bar, at 70 litres of water per minute at least. The desired volume of water at this pressure is also related to the diameter of the field drain. In a narrow field drain the input of the same volume of water in the same amount of time, will cause a swifter flow than in a wide drain. Pressure will be lost in the hose due to friction. The amount of this loss depends on the diameter and length of the hose.
In the nozzle of the hose there are a limited number of holes pointed forwards and a significantly greater number of holes pointed backwards. The speed of insertion into the field drain should be higher than the speed with which it is retracted. If the input speed is too low, a large volume of water will collect in front of the nozzle and counter-pressure will occur. If the speed of retraction is too fast, a vacuum may occur in front of the nozzle because of the water column being drawn out. This vacuum can cause soil particles to be sucked into the drain. If the pressure is too high, the soil around the drain may also be loosened thereby sucking more soil into the drain. If the speed of retraction is too fast, there is a risk that the nozzle will be pulled through the column of dirty water with the possibility that dirt can settle down behind the nozzle. The speed of retraction can be greater when there is little dirt in the drain. A constant speed should be maintained over the entire length of the drain. On average the speed of insertion should be 25 to 30 metres per minute while that of retraction is usually about 20% slower which would mean 20 to 25 metres per minute in this example. Certain drain jetters take care of this automatically. Nowadays many drain jetters are easily operated from the cabin of a tractor. A fully hydraulic system takes care of automatic insertion into the outlet. Wheels driven by hydraulic motors ensure a continuously adjustable speed of retraction. To prevent the hose from being damaged, it should not be allowed to slip. Distance counters determine the distance to a possible problem. A reel with a large diameter is preferred.