High pressure Water Jetting Process (HP Water Jetting Process)
The high pressure water jetting process (HP water jetting process) [Lenz96] is the almost universal process, used in about 90 % of all sewer cleaning processes for removing deposits within the scope of regular maintenance as well as for cleaning as a preparatory measure for sewer inspection or rehabilitation.
It can usually not be used when hardened deposits are present or for removal of flow obstacles, e.g. internally projecting laterals, artificial obstacles or roots or for achieving a very high degree of cleanliness on the inner pipe surface. In these cases additional use must be made of mechanical cleaning methods or apparatus (Abschnitt 3.2.4), which can partly also be driven by the high pressure water jetting vehicles.
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In the HP water jetting process, flushing water is pumped from a water tank by means of a high pressure pump through a hose with a cleaning nozzle at the end. The cleaning nozzle is equipped with holes into which are nozzle inserts, which beam the fast flowing water jets and direct them at the pipe walling. This causes a reaction force at the nozzle, which, in the first phase, force them and the hose in the sewer section, to move against the direction of the flow from the starting manhole to the target manhole. After arrival of the nozzle at the target manhole, it is slowly pulled back in the second phase in the direction of flow. The jets of water leaving the nozzle increase the velocity of flow, loosen the deposits, whirl them up and convey them as a suspension towards the target manhole where they are usually vacuumed up by a hose (Animation 22.214.171.124-1) [NASSCO89a] [SIA190] [Führb80] [DIN30702-5:1987] [DIN30705:1991] [Veltr83].
The number and alignment of the nozzle inserts, as well as the number of cleaning passes per section of the sewer and the target velocity of the cleaning nozzle depend on the type of fouling, the amount of cleaning waste to be transported and the purpose of the cleaning.
The technology employed in high pressure cleaning is installed in vehicles.
There are various types of vehicles:
- High pressure water jetting vehicle;
- Vacuuming vehicle with or without water filtration;
- Combined high pressure water jetting and vacuuming vehicle with (Image 126.96.36.199-1) or without water recovery.
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The water required for cleaning is drawn from the water supply via standpipes and sand filters. A direct connection to a hydrant is not permitted [DIN1988-1]. Drawing water off from open waters and springs requires special equipment on the high pressure water jetting vehicle. In the combined high pressure water jetting and vacuuming vehicle with water recovery, the wastewater that is sucked up is filtered and led back to the HP water jetting apparatus as flushing water (Image 188.8.131.52-1) (Animation 184.108.40.206-2). This reduces the number of interruptions of the cleaning process for the purpose of taking up more water, the effective cleaning time is reduced and the use of drinking water is minimised.
The pumps installed in the vehicles must fulfil the following performance requirements [ATVAG17397] :
High pressure pump:
Pumping quantity depending on the type, height and consistency of the deposits as well as water flow
- Approx. 320 l/min for DN 200 to DN 800.
- Approx. 390 - 450 l/min for DN 800 to DN 1200.
- Approx. 640 - 800 l/min for DN > 1200.
Pressure: 100 - 150 bar at the HP pump, 80 - 100 bar at the cleaning nozzle.
Volume flow 750 - 1500 m3/h at 60 % vacuum A vacuum of 60 to 90 % should be achieved, special vehicles have a pumping capacity of 3000 to 5000 m3/h at 90 % vacuum.
"The high pressure water jetting hoses must be able to cope with the safety requirements and be certified for the maximum operating pressure of the high pressure installation" (Table 220.127.116.11-1). " During the transport of water in the high pressure hose there arises the frictional resistance that reduces the water pressure. A high flow velocity creates a high pressure loss. The lengths of the hose in the common vehicles are 120 to 200 m. For special cleaning measures there are special vehicles on the market with hose lengths of up to 800 m. The loss of pressure in the water jetting hoses can be reduced by means of a patented flow accelerator" [ATVAG17397].
|Performance parameters||Guiding values|
|Hose diameter||Up to 325 l⁄min − DN 25|
|Up to 650 l⁄min − DN 32|
|Up to 800 l⁄min − DN 40|
|Weight||Plastics:||DN 25 ∼ 0.5 kg⁄m|
|DN 32 ∼ 0,9 kg⁄m|
|Rubber:||DN 25 ∼ 1,0 kg⁄m|
|DN 32 ∼ 1,1 kg⁄m|
|DN 40 ∼ 1,4 kg⁄m|
|Basically: as light as possible|
|Length||Dependent on the capacity of the pump and the area of application
> 120 m
|Internal ⁄ external
coefficient of friction
|Plastics hoses have better coefficients of friction compared to
|Pressure rating||The permissible operating pressure of the hose should be
approximately 50 bar greater than the maximum operating pressure
of the high pressure installation; the bursting pressure must be a
minimum of 2.5 times the permissible operating pressure of the hose.
|Bending radius||As small as possible (150 − 200 mm)|
|Pressure loss||Plastics and rubber:|
|DN 25 at V = 300 l⁄min ∼ 0.37 bar⁄m|
|DN 32 at V = 400 l⁄min ∼ 0.20 bar⁄m|
|DN 40 at V = 650 l⁄min ∼ 0.17 bar⁄m|
Special cleaning nozzles are available for different types of fouling and sewer cross sections [McCar84] [WPCF85] (Image 18.104.22.168-2) (Image 22.214.171.124-3) (Image 126.96.36.199-4). According to [ATVAG17397], they are divided into:
- Radial nozzles (water outlets distributed radially out of the jet circumference);
- Floor cleaner (water outlet directed at the invert);
- Rotation nozzles (water outlets distributed radially on the jet circumference, nozzle rotates) (Image 188.8.131.52-5) (Image 184.108.40.206-6) (Image 220.127.116.11-7) ; as well as
- Cleaning nozzles for removal of blockages (water sprays directed in forward and backward directions).
The important performance parameters and the associated guiding values are contained in (Table 18.104.22.168-2).
|External ⁄ internal
|Externally round, internally a concave shape providing a good water
|Weight||Dependent on the sewer diameter and profile; "buoyancy" must be
|Jet angle (angle
between water jets and
|Jet angle approx. 15 − 30°.|
|Small jet angle; good advancing performance, poorer cleaning
|Large jet angle; smaller advancing performance, good cleaning
|Number of nozzle
|Few nozzle inserts with large diameters cause a large drive
performance; numerous nozzle inserts with smaller diameters cause a
surface cleaning but a smaller advance performance; thin water jets
atomise quicker than thick water jets.
The following was found in the course of experiments:
"With a small exit angle (e.g. 15 °) the pressurised water energy is transferred to 97 % into a drive force but there remains sufficient energy for dissolving light deposits that are flushed to the sewer manhole with the flowing water quantity. For cleaning strongly fouled sewers and piping, nozzles with a larger exit angle (preferably 30 °) should be utilised (Image 22.214.171.124-8). This exit angle guarantees good dissolving of the deposits and incrustations at the pipe wall and ensures sufficient energy potential to drive the cleaning nozzle along the sewer" [FI-Müller].
According to [Störn90], cleaning efficiency and drive force can be additionally enhanced by the use of a sled in order to prevent the immediate contact between nozzle and pipe walling (Image 126.96.36.199-9).
Cleaning nozzles and high pressure water jetting apparatus must be used such that turning around in the section is prevented. This requirement is satisfied when:
- A cleaning nozzle that is correctly dimensioned in relationship to the sewer is utilised and the twisting of the hose is prevented by means of a turning linkage between nozzle and hose;
- A stiff extension is inserted between nozzle and hose.
A new development in the field of rotating jets is the pressure- and volume-independent controllable rotation nozzle, the ROTO JET (Image 188.8.131.52-10) (Image 184.108.40.206-11) [FI-IBG]. With a speed of revolution of 50 to 600 rpm and a working pressure from 300 to a maximum of 1000 bar, it achieves, according to the claims of the manufacturer, a fast removal of solids and deposits from sewers in the nominal width range of DN 150 to DN 4000 (depending on type) as well as from ovoid cross sections.
When working with high pressure water jetting apparatus, the formation of aerosols - very fine sewage droplets distributed in the air - in the region of the manhole must be prevented. This can be achieved, among others [ATVAG17397] :
- By the use of swinging nozzles for large manhole cross sections;
- By reduction of the pump pressure about 10 m before reaching the starting manhole (this sewer section should be gone over again with the correct pressure once the vehicle has moved to another manhole in order to transport heaped up material further); and
- By covering the manhole opening with thin sheetmetal or plastics with openings for high pressure and vacuuming hoses.
A new development is presented by the floor cleaner shown in (Image 220.127.116.11-12), (Image 18.104.22.168-13) and (Image 22.214.171.124-14) with integrated, self-lighting TV camera. The transfer of the figure from the TV camera equipped with transmitter and the receiver is achieved by means of a wireless connection .
Because of the continuous monitoring of the cleaning process, the cleaning efficiency can be adapted to the degree of fouling, according to the manufacturer's information, so that a reduction of the water use as well as a minimising of time is achieved. Furthermore, a targeted inspection can be carried out when damage in the sewer is recognised and localised during the cleaning process.
This system can be applied in the nominal size region of DN 200 to DN 1200 at temperatures between -5° C and +40° C. According to the information by the manufacturer, a very high performance can be achieved by the HP water jetting process with little manpower input (2 persons) (without water preparation up to 2000 m per day, with water preparation up to 3000 m per day). With the use of flushing water heating, work at temperatures down to -15° C is possible. The polymers added to the flushing water cause a reduction of the friction losses in the hose so that for large sections (e.g. dump drainage), hose lengths of up to 800 m can be achieved [FI-Müller].
Cleaning with the HP water jetting process is economically justified in drains and sewers up to approximately DN 2500. Blockages can be cleared in many cases by means of the selection of a nozzle with additionally placed forward facing nozzle inserts.In the variations shown in (Image 126.96.36.199-15) with boring point, the point digs into the blockage and flushes the deposits away backwards .
In the accessible region, cleaning can be carried out for special purposes with manually operated high pressure water jetting device.
The corresponding safety measures must be adhered to in the application of high pressure water jetting processes.