Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)

Damage Caused by High pressure Water Jetting Processes

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Image 3.3.4.2.1-1:  Examples of damage due to unprofessionally applied HP water jetting process [Prüfattest]
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Image 3.3.4.2.1-2: 
Examples of damage due to unprofessionally applied HP water jetting process [Prüfattest]

Unprofessional application of the high pressure water jetting process can cause pipe damage in the form of grooves, chipping, cracks or holes in pipe walling and linings (Abschnitt 2.1) (Image 3.3.4.2.1-1) (Image 3.3.4.2.1-2).

Especially dangerous moments occur due to the knocking of the cleaning nozzle, the turbulence of sediment or stones as well as in the remaining of the working cleaning nozzle in one place.

The loading of the pipe surface due to impinging jets of water is dependent on:

  • Water pressure at the cleaning nozzle;
  • Quantity of water;
  • The distance of the cleaning nozzle from the pipe wall; and
  • The number, the size and the exit angle of the nozzle inserts.

As already mentioned, general guidelines for the planning and execution of the cleaning work are not available [Störn96a] [Störn96b]. In the choice of the operating parameters mentioned above, besides those laid down in the task of the cleaning work, there must also be considered the pipe materials, the wall thickness and especially the structural condition of the sewer section.

In this regard, investigations of the HP water jetting process by the Tunnelling Department of the City of Zurich [Prüfattest] [Stein92e] led to the conclusion that for a water pressure at the cleaning nozzle of up to 120 bar and a water volume flow of 300 l/min, sewers of asbestos cement, concrete, PVC and HD-PE will not be damaged.

Similar results in the cleaning with the HP water jetting process were obtained in investigations of the behaviour of ductile cast iron pipes with spray lining coated high alumina cement. For pipes with this type of coating and with a coating layer of ≥ 6 mm and the usual cleaning intervals, and water jetting pressures of < 170 bar (measured at the HP pump), over a time period of 50 years, there was no damage to be expected from the HP cleaning apparatus [Brune90].

Spurred on by a definite case of damage, attempts have been made by the City Drainage Department of Hamburg since 1988 to solve this problem. In a first step, the operational limiting conditions were investigated at site and, depending on the cross sectional shape and dimensions of the sewer and the maximum height of the deposits, to define the operating parameters of:

  • Type of the cleaning nozzle;
  • Water volume flow;
  • Quantity and diameters of the jets;
  • Exit angle
  • Water pressure at the cleaning nozzle, etc.

for the HP water jetting process (Table 3.3.4.2.1-1).

In individual cases, the cleaning sections that may not be cleaned with the HP water jetting process were defined.

Table 3.3.4.2.1-1: 

Parameters for high-pressure water jetting in Hamburg sewers [Hambu]

Con−secutive
Number
Sewer
cross
sectional
shape and
sizes
Height of
deposits
(max)
[m]
Nozzle
head
Water
pressure at
the nozzle
[bar]
Water
volume
flow
[l⁄min]
Exit angle
[Degrees]
Pull−
through
velocity
[m/s]
No. of
nozzle
inserts
[No.⁄mm]
1 DN 250 0.10 Grenade
KM−HD
06
100 320 30 0−1 3x2.9
3x3.3
2 DN 300 0.10 Round
head
KM−HD
04
100 320 30 0−1 10x2.8
3 DN 400 0.15 Bomb
KM−HD
08
100 320 30 0−1 6x3.1
4 DN 500 0.20 Swing
KM−Typ
2
100 320 3 − 15 0−1 5x3.3
5 Kl. VI 0.30 Bomb
KM−HD
08
100 640 30 0−1 6x4.4
6 Kl. V 0.40 Swing
KM−Typ
2
100 640 3 − 15 0−1 7x4.0
7 Kl. IV 0.50
8 Kl. III 0.50 Flat
wedge
KM−Gr.
II
100 640 10 0−1 6x4.4
9 Kl. II 0.10 Aqua
Bull sled
MA−Sch
TA
100 640 No
information
0−1 20x2.0
10 Kl. I 0.70
Annotation: Consecutive number 5 to 10 see (Table 3.3.4.2.1-1)

At present, there are no general guidelines and criteria for the testing of pipes of sections of the sewer with reference to their reaction to the HP water jetting process. For corresponding investigations, special investigative programmes with limited queries are usually carried out. An example of the above-mentioned investigations of ductile cast iron pipes is given in the following [Brune90]. These investigations will be carried out under stationary as well as moving conditions using a normal high pressure water jetting vehicle with a maximum water volume flow of about 350 l/min. This investigation can also be transferred to other materials [Stein95b].

Rehabilitation and Maintenance of Drains and Sewers / Prof. Dr.-Ing. D. Stein, Dipl.-Ing. R. Stein (2004)