CA2311049C - Method for cleaning and renovating pipelines - Google Patents

Abstract

A pipeline cleaning and renovating method of the present invention comprises introduction of abrasive particulates by pressurized air through the pipelines to remove the incrustation adhering to the inner surface of the pipelines. The air flow through the pipelines is controlled to induce a substantially helical flow pattern such that a substantial amount of abrasive particulates are driven by the air flow to move along the inner surface of the pipeline in order to efficiently remove the incrustation. After the pipelines are cleaned, a coating material is added into the pipelines and driven by the air flow in the substantially helical flow pattern to provide an even coating film on the cleaned inner surface of the pipelines. This method can be broadly applied to various types of pipelines and various diameters of the pipelines from 13mm to 300mm. The length of the pipeline being cleaned at one time can be from several hundred meters to more than one kilometer, depending on the type of pipeline.

Description

OR File No.9-16016-1CA

METHOD FOR CLEANING AND
RENOVATING PIPELINES
FIELD OF THE INVENTION
The present invention relates to a method for cleaning and renovating pipelines, in particular for cleaning and renovating pipelines which are difficult to access, such as pipelines buried underground, installed in walls, and the like.
BACKGROUND OF THE INVENTION
As is well known, after pipelines have been in use for some time encrustation accumulates therein, which arises partly from salts in the fluids passing therethrough and partly from corrosion of the pipelines. The encrustation usually further includes thick and tough deposits of micro-organisms such as algae, bacteria, etc., which steadily increase over time. The pipelines can be cleaned by chemical cleaning, knocking and mechanical cleaning. These cleaning methods have their disadvantages and negative affects on the pipelines, or have limited applicabilities. A popularly used method is to clean the inner surface of the pipelines by supplying a fluid stream containing abrasive agents such as sand, through the pipelines so that the abrasive agents impinge upon the inner surfaces of the pipelines to remove the deposits thereon.
Examples of applying this method for pipeline cleaning are described in United States Patent 1,890,164, entitled SAND BLASTING METHOD AND APPARATUS and issued to Rosenberger on December 6, 1932, United States Patent 2,087,694, entitled CLEANING PIPE and issued to OR File No.9-16016-1CA

Malmros on July 20, 1937, United States Patent 3,073,687, entitled METHOD FOR THE CLEANING OF PIPELINES and issued to McCune on January 15, 1963 and United States Patent 5,924,913, entitled PROCESS FOR RENOVATING PIPES and issued to Reimelt on July 20, 1999.
More especially, in United Kingdom patent application 2,140,337, which is entitled CLEANING AND
LINING A PIPE and published on November 28, 1984, Shinno describes a method for renovating a pipe which comprises cleaning an internal surface of the pipe by supplying a fluid stream containing at least entrained abrasive agents and thereafter lining the pipe by introducing into the pipe a mixed stream of paint and air to deposit paint on the cleaned internal surface of the pipe. The mixture stream used in Shinno's cleaning and lining process is subjected to pulsations in a predetermined cycle and at a predetermined pulsation rate. Shinno states that in a conventional system, when a flow speed of the mixture stream reaches 30-100m/sec, the air current is turbulent, and the flow speed near the inner surface of the pipe is slower than the flow speed at the center portion of the pipe. Shinno therefore suggests using a rotating vane as a pulsation-generating apparatus installed within the pipeline to be cleaned in order to reduce the fluid speed difference between the flow through the center of the pipe and the flow near the inner surface of the pipe. The suggested flow speed is 20-100m/sec. Shinno's system is relatively complicated and it is not convenient to install the pulsation-generating apparatus in pipelines. The pulsation-generating apparatus installed in the pipelines can cause fluid flow resistance which is not desirable.
The length of the pipelines which can be cleaned according to Shinno is about 30-100 meters, therefore more than 10 OR File No.9-16016-1CA

vertical holes would have to be dug in streets when, for example a one kilometer length of pipeline buried underground is to be cleaned. This is not satisfactory.
Therefore a simple and efficient method for cleaning and renovating pipelines is desirable.
SU1~IARY OF THE INVENTION
One object of the present invention is to provide a pipeline cleaning and renovating method in order to clean and renovate pipelines effectively.
In accordance with one aspect of the present invention, a method of cleaning a pipeline having open entry and exit ends, comprises steps of introducing a plurality of abrasive particulates into the pipeline from the entry end thereof by means of pressurized fluid; and controlling a flow of the pressurized fluid at the entry end of the pipeline to induce a substantially helical flow pattern along an inner surface of the pipeline, thereby causing a substantial amount of the abrasive particulates to move along the inner surface of the pipeline.
The method preferably comprises a further step after the pipeline is cleaned, of introducing a coating material into the pipeline by, for example an air flow in a substantially helical flow pattern along the inner surface of the pipeline, thereby moving a substantial amount of the coating material in the substantially helical flow pattern on and along the inner surface of the pipeline.
The substantially helical flow pattern is preferably obtained by controlling the speed of the fluid flow. In accordance with Applicant's test results, the flow speed through a pipeline reaches about OR File No.9-16016-1CA

40m/sec-100m/sec the fluid flow is rotated to create a substantially helical flow pattern, although other flow patterns may jointly exist. The substantially helical flow pattern in the pipeline causes a substantial amount of the abrasive particulates or the coating material to move along the inner surface of the pipeline such that the inner surface of the pipeline is effectively cleaned or coated.
The pipeline cleaning and renovating method of the present invention advantageously provides very effective cleaning results for a relatively long length of pipelines at one time and uniform coating to the inner surface of the cleaned pipelines in order to protect the surface from encrustation for a relatively long period of time. The pipeline cleaning and renovating process is relatively easy to conduct which results in less labour and time consumption and thereby, lowers costs.
Other advantages and features of the present invention will be better understood with reference to the preferred embodiment of the present invention described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus generally described the nature of the present invention, reference will now be made to the accompanying drawings, showing by way of illustration the preferred embodiments thereof, in which:
Figs. 1-3 are schematic illustrations showing fluid flow patterns within a section of a pipeline when the speed of the fluid flow is controlled within various predetermined speed ranges; and OR File No.9-16016-1CA

Fig. 4 is a schematic illustration showing an apparatus used in a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The pipeline cleaning and renovating method will now be described in detail, as the embodiment of the present invention. The pipelines to be cleaned can be, for example water pipes buried underground, installed in the walls of a high-rise building, or in other locations.
Therefore, preliminary planning is needed prior to pipeline cleaning and renovating on site. During the preliminary planning stage, the layout of the pipelines, including switches, branches, etc. should be defined according to the construction drawings. The sections of the pipelines to be cleaned at one time are to be divided to have open ends according to the layout of the pipelines. For a straight pipeline buried underground, the length of the pipeline to be cleaned at one time can be several hundred meters to more than one kilometer depending on the diameter of the pipeline, which can vary from l3mm to 300mm. The entry end and exit end as well as the flow direction through the pipeline section are also determined together with the determination of pipeline sections. The type of pressurized fluid used for cleaning and renovating pipelines can vary. Nevertheless, the most convenient and inexpensive pressurized fluid is pressurized air. A final step of the preliminary planning stage is to select pressure air volume, air flow speed and the air pressure to be used in the pipeline cleaning and renovating process.
The air flow speed is selected from a range of between 40m/sec and 100m/sec, preferably 70m/sec-80m/sec and the air pressure is selected from a pressure range of OR File No.9-16016-1CA

between 2-8kg/cm2 depending on the length and diameter of the selected section of the pipeline and on other considerations, to ensure that the air flow reaches the selected flow speed. For example, the air pressure should be increased 0.5kg per 10 meters of rise when the pipeline is substantially in a vertical condition, such as in a high-rise building. The determined volume of the pressurized air depends on the diameter of the selected section of the pipeline to be cleaned.
The type of abrasive particulates is also selected during this planning step, for example the abrasive particulates from sands including quartz, iron or steel particulates. When a coating of the inner surface of the section of the pipeline after cleaning is required, the type of coating material is also selected. The coating material must be harmless to health, water resistant, preferably wear resistant and quick-drying material such as rosin.
The next stage is site preparation. The water supply to the pipelines is shut down and a first section of the pipeline to be cleaned according to the preliminary planning, is located and cut from the pipeline. The terminal equipment, if any, is removed such that the section of the pipeline has two open ends and is ready to be cleaned. The pipeline section is connected at its predetermined entry end to an apparatus for controllably supplying the pressurized air and the abrasive particulates to generate an air flow mixed with the abrasive particulates in the desired substantially helical flow pattern, through the section of the pipeline. The apparatus will be described below with reference to Fig. 4.
A flexible pipe is preferably connected to the exit end of OR File No.9-16016-1CA
the section of the pipeline in order to direct the exhaust flow of the mixture for the waste collection. A safety check is conducted for a safe operation of the cleaning and renovating process. For example, all valves installed in the section of the pipeline to be cleaned must be open, or removed if the valve cannot be fully opened, such as butterfly valves.
The cleaning and renovating process begins with introducing the pressurized air supply into the entry end of the section of the pipeline to be cleaned. This is a preliminary air cleaning step without the abrasive particulates to be used later in this step. The section of the pipeline to be cleaned usually contains some water remaining therein. The pressurized air flow in this step will blow the remaining water out of the secion of the pipeline. Furthermore, the inner surface of the section of the pipeline to be cleaned may be heavily encrusted at some points and will clog easily when a certain amount of abrasive particulates or the removed deposits pass through those heavily encrusted points. Therefore, the air flow under pressure without the mixed abrasive particulates first removes those deposits which are relatively easy to dislodge, and increases the passage cross-section, particularly at those heavily encrusted points. Thus, the risk of clogging the section of the pipeline to be cleaned when the abrasive particulates are introduced is significantly reduced.
After the preliminary air cleaning step, the air flow introduced into the section of the pipeline being cleaned is adjusted to reach the selected flow speed, between 40-100m/sec, preferably 70-80m/sec, and the abrasive particulates are controllably introduced into the OR File No.9-16016-1CA
_ g _ entry end of the section of the pipeline being cleaned, in order to ensure that the air flow with the mixed abrasive particulates is generated in the desired substantially helical flow pattern. Tests show that different flow patterns through a pipe can be achieved when the flow speed is varied, as illustrated in Figs. 1-3. The flow pattern under a relatively low air pressure and at a low air flow speed produces a moderate turbulence in a wave-shaped pattern, as illustrated in Fig. 1. When the air pressure is increased and the air flow speed reaches the range of between 40-100m/sec, the air flow through the pipe produces a substantially helical flow pattern as illustrated in Fig. 2, although other flow patterns may jointly exist.
When the air pressure is further increased and the air flow speed exceeds 100m/sec, the substantially helical flow pattern through the pipe no longer occurs and a smog-shaped straight flow pattern is produced, as illustrated in Fig. 3. These flow patterns can be observed when the exhaust flow is discharged from the exit end of the pipe, particularly if the flow is mixed with a coloured coating material such as rosin. The cleaning and renovating method of the present invention uses the substantially helical flow pattern to cause a substantial amount of the abrasive particulates to move along the inner surface of the pipeline section, thereby impinging upon the deposits adhering to the inner surface and removing the same therefrom. The removed deposits are then blown out of the exit end of the pipeline section by the air flow.
It is optional to heat the section of the pipeline being cleaned prior to the abrasive particulates cleaning step in order to dry the incrustation on the inner surface of the section of the pipeline being cleaned. The heating step is conducted by introducing hot airflow through the _ g _ OR File No.9-16016-1CA
section of the pipeline. The temperature of the hot air can be selected from a temperature range of between 30°C
and 40°C. After the abrasive particulates cleaning step is completed, a quality check is conducted to ensure the fineness and cleanness of the inner surface of the section of the pipeline which has been cleaned.
A coating step is similar to the abrasive particulate cleaning step. The coating material, for example rosin, is introduced into the entry end of the section of the pipeline which has been cleaned by an air flow under the predetermined pressure flowing through the section of the pipeline. The air flow under the predetermined pressure is controlled to reach a predetermined flow speed of between 40-100m/sec, preferably 70-80m/sec in order to induce a substantially helical flow pattern through the section of the pipeline. Thus, the semi-liquid rosin is blown into the section of the pipeline and a substantial amount of rosin is brought by the substantially helical flow pattern to move on and along the inner surface of the section of the pipeline, thereby forming a rosin film of 0.2-0.3mm thick, covering the entire inner surface of the section of the pipeline. The rosin coating can be allowed to dry naturally or can be dried by introducing a hot air flow through the section of the pipeline. After the rosin coating is completely dried, the apparatus which is used to introduce the pressurized air flow and the abrasive particulates as well as the coating material, is then disconnected from the entry end of the section of the pipeline. The flexible tube connected to the exit end of the section of the pipeline is also disconnected, This cleaned and renovated section of the pipeline is then ready to be re-coupled to the OR File No.9-16016-1CA

pipelines when adjacent sections of the pipelines have been cleaned and renovated.
The substantially helical flow pattern is imprinted on the inner coating of the cleaned and renovated section of the pipeline, which can be observed from one end of a section of the pipeline if this section is straight.
When the section of the pipeline to be cleaned has small cracks or tiny holes, those defects can be automatically repaired during cleaning and renovating procedures. The abrasive particulates driven by the air flow in the substantially helical flow pattern through the section will move into the fine cracks and tiny holes in the pipe wall and will lodge there. The rosin coating will adhere those abrasive particulates to the pipe wall and further cover the same. The rosin coating becomes very solid after drying and will also protect the mended defects.
As illustrated in Fig. 4, the apparatus used in the above described embodiment of the present invention, generally indicated by numeral 10 includes a section of a connection pipe 12 having a first end 14 for connection to a pressurized air source, such as an air compressor or pressurized air container (not shown), and a second end 16 for connection to an entry end of the section of a pipeline to be cleaned (not shown). The diameter of the connection pipe 12 is preferably equal to the diameter of the section of pipeline to be cleaned. The connection pipe 12 is provided with a first main control valve 18, which is a ball valve, positioned near the first end 14 of the connection pipe 12, and a second main control valve 20, which is also a ball valve, downstream of the first main control valve 18. Air pressure indicators 22 and 24 are OR File No.9-16016-1CA
- 10a -installed in the connection pipe 12 for measuring the air pressure OR File No.9-16016-1CA

inside of the connection pipe 12. The air pressure indicator 22 is positioned between the main control valves 18 and 20, and the air pressure indicator 24 is positioned downstream of the second main control valve 20.
The apparatus 10 further includes a first container 26 containing for example, sand, which will be referred to below as the sand container. The sand container 26 is preferably positioned above the connection pipe 12 and is removeably attached to the connection pipe 12 via an output pipe 28 with a control valve 30 for controllably delivering the sand from the sand container 26 into the connection pipe 12. The output pipe 28 is connected to the connection pipe 12 downstream of the second main control valve 20. The sand container 26 has an input pipe 32 with a control valve 34. A back pressure pipe 36 with a valve 38 is provided to interconnect the connection pipe 12 and the sand container 26. The back pressure pipe 36 at its one end is detachably connected to the input pipe 32 and at its other end is connected to the connection pipe 12 at a position between the two main control valves 18 and 20.
Optionally, a second container 40 is provided, preferably positioned above the connection pipe 12. The second container 40 is removeably attached to the connection pipe 12 via an output pipe 42 with a control valve 44. The output pipe 42 is connected to the connection pipe 12 close to the second end 16 thereof. The second container 40 further includes an input pipe 46 with a valve 48. A back pressure pipe 50 with valve 52 is provided to interconnect at one end thereof the input pipe 46 and at the other end thereof to the connection pipe 12 at a position between the main control valves 18 OR File No.9-16016-1CA

and 20. The connection of the back pressure pipe 50 to the input pipe 46 is detachable.
In operation, the apparatus 10 is connected at its second end 16, perhaps via an extension pipe, to the entry end of the section of the pipeline to be cleaned and at its first end 14 to the air compressor. The air compressor is enabled to provide 2m3/min to 30m3/min under a pressure range of from 2-8kg/cm2, depending on the preliminary planning of the cleaning and renovating process. If the pressurized air supply is not sufficient, an additional pressurized air container can be used. It is preferable to remove the second container 40 prior to the cleaning step.
Before starting the cleaning and renovating process, all valves of the apparatus 10 are tightly closed.
After the air compressor is turned on and begins to supply pressurized air, the first main control valve 18 is opened.
The air pressure indicator 22 indicates the desired air pressure. The second main control valve 20 is then gradually opened, until preferably being one-third open for the first step. The air flow under pressure passes through the section of the pipeline being cleaned at increasing speeds so that the deposits which are relatively easy to remove from the encrustation are removed and, together with the water remaining therein are blown out gradually, in order to avoid clogging. The operation of the second main control valve 20 is carefully manually conducted to correspond with the exhaust flow discharged from the exit end of the section of the pipeline being cleaned. When no significant amount of waste material mixed with the exhaust flow is observed, the second main control valve 20 is gradually further opened to a position about its two-thirds opening and then still further to be fully open. During OR File No.9-16016-1CA

this step, the first main control 18 is closed and reopened several times and this close and reopen operating pattern causes the pressurized air to suddenly surge which helps clear the removed deposits away from the sections of the pipeline being cleaned.
After this preliminary air cleaning step, the remaining deposits adhering to the inner surface of the section of the pipeline being cleaned are those which are relatively difficult to remove. Nevertheless, the internal passage of the section of the pipeline being cleaned now has a larger cross-section than it had prior to the preliminary cleaning step, which facilitates the subsequent sand cleaning step.
After the preliminary air cleaning step, the valve 34 is opened and sand particles sized from lmm to 5mm, depending on the sizes of the pipeline and the clog conditions, is added through the input pipe 32 into the sand container 26. Preferably, sand particles sized less than 2mm and are used for cleaning pipelines with a diameter of up to 2 inches, and sand particles sized from 3.5 to 4.5mm are used for cleaning pipelines with a diameter about and greater than 6 inches . The valve 34 is closed immediately after the sand loading process is completed. The second main control valve 20 is closed during the sand loading process. Before the second main control valve 20 is opened again to begin the sand cleaning step, the valve 38 is opened first to provide a back pressure to the sand container 26. The second main control valve 20 is then fully opened to induce a substantially helical flow pattern through the section of the pipeline being cleaned. The air pressure is selected to generate an air flow having a preferable flow speed of, OR File No.9-16016-1CA
- 13a -for example 70-80m/sec. The valve 30 is controlled to gradually open such that the sand is released from the sand container 26 gradually in order to avoid clogging in the section of the pipeline being cleaned. Optionally, valve 30 and the main control valves 18 or 20 may be closed and reopened several times during the sand cleaning step.

OR File No.9-16016-1CA

Such an operation can be repeated until no waste material is observed in the exhaust flow discharged from the exit end of the section of the pipeline. The pressure indicators 22, 24 are observed. The readings of both indicators 22, 24 are significantly below the predetermined air pressure and indicate a pressure differential therebetween when the air flow passes the section of the pipeline being cleaned. Otherwise, the section of the pipeline being cleaned is clogged and the sand cleaning step must be stopped immediately. The sand cleaning step begins again after actions have been taken to unclog the section of pipeline being cleaned.
When the sand cleaning step is completed the valves 30 and 38, as well as the second main control valve 20 are closed and then the sand container 26 is removed. The second main control valve 20 is then fully reopened to provide the pressurized air through the section of the pipeline to thoroughly clean the inner surface of the section of the pipeline.
In order to conduct the coating step, the second container 40 is installed and the valve 48 is opened for addition of rosin through the input pipe 46 into the container 40. After the valve 48 is closed, the valve 52 is opened to provide a backpressure to the second container 40. The valve 44 is then fully opened to allow the rosin to be delivered in its total amount at one time into the connection pipe 12, after which the valve 44 is immediately closed. The second main control valve 20 is then fully opened to apply the selected air pressure which was observed on the pressure indicator 22 prior to the opening of the second main control valve 20. Thus, the rosin is blown into the section of the pipeline and is OR File No.9-16016-1CA

mixed with the air flow in a substantially helical flow pattern, and thus the rosin moves on and along the inner surface of the section of the pipeline, thereby forming a thin film of rosin evenly covering the entire inner surface of the section of the pipeline.
After the coating step is completed, the second main control valve 20 is partially closed to maintain an air flow at a relatively low flow speed passing through the section of the pipeline until the rosin coating on the inner surface thereof is completely dry. At this stage, the air compressor is switched off and all valves are closed. The apparatus 10 can be disconnected and removed, and then the cleaned and renovated section of the pipeline is ready to be reconnected to the pipeline.
It is optional in the coating step to add the rosin material directly into the connection pipe 12. Therefore, the second container 40 need not be used. Thus, the apparatus 10 in this embodiment may only include the sand container 26. The rosin material can be added through the input pipe 42 and the valve 44 into the connection pipe 12 or the rosin material can simply be added into the entry end of the section of the pipeline, or into the extension pipe connected to the connection pipe 12, which requires disconnection of the entry end of the section of the pipeline from the connection pipe 12 or from its extension pipe for the addition of the rosin material. Therefore, the apparatus 10 is further simplified in this embodiment.
If hot air is needed to dry the section of the pipeline prior to the preliminary air cleaning step or the sand cleaning step, and to dry the rosin coating after the coating step, an electric heating device (not shown) may be additionally connected either between the apparatus 10 and OR File No.9-16016-1CA

the entry end of the section of the pipeline, or between the air compressor and the apparatus 10. The heating device has connection pipes which have a diameter preferably similar to the diameter of the section of the pipeline being cleaned.
The pipeline cleaning and renovating method of the present invention can be broadly applied for various pipelines such as water, gas, oil pipelines.
Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Claims (15)

1. A method of cleaning a pipeline having open entry and exit ends, comprising steps of:

introducing a plurality of abrasive particulates in a controlled and gradually increasing amount into the pipeline from the entry end thereof by means of pressurized fluid; and controlling a flow of the pressurized fluid at the entry end of the pipeline to have a speed of between 40m/sec and 100m/sec in order to induce a substantially helical flow pattern along an inner surface of the pipeline, thereby causing a substantial amount of the abrasive particulates to move along the inner surface of the pipeline.

2. A method as claimed in claim 1 wherein the pressurized fluid is pressurized air.

3. A method as claimed in claim 2 wherein the air flow is controlled to have a speed substantially between 70-80m/sec in order to induce the substantially helical flow pattern.

4. A method as claimed in claim 3 further comprising a step after the pipeline is cleaned, of introducing a coating material into the pipeline by an air flow in a substantially helical flow pattern along the inner surface of the pipeline from the entry end to the exit end thereof, thereby moving a substantial amount of the coating material in the substantially helical flow pattern on and along the inner surface of the pipeline.

5. A method as claimed in claim 2 wherein the pressure of the pressurized air ranges from 2kg/cm2 to 8kg/cm2.

6. A method as claimed in claim 4 wherein the coating material is rosin.

7. A method as claimed in claim 1 wherein the abrasive particulates comprise sand particles sized from 1mm to 5mm.

8. A method as claimed in claim 2 wherein the introduction of the abrasive particulates is conducted by using an apparatus including a connection pipe having a first end connected to a source of pressurized fluid, a second end connected to the entry end of the pipeline and a container positioned above the connection pipe for containing the abrasive particulates, the container including a first pipe with a first valve connecting the connection pipe for delivering the abrasive particulates into the connection pipe, and a second pipe with a second valve connecting the connection pipe at a point upstream of the first pipe for providing back pressure to the container.

9. A method as claimed in claim 8 further comprising steps of firstly opening the second valve in order to provide back pressure to the container, and then gradually opening the first valve for introduction of the abrasive particulates into the pipeline while the pressurized fluid passes through the pipeline.

10. A method as claimed in claim 9 wherein the controlling of the flow of pressurized fluid is conducted by operating a first main control valve installed in the connection pipe upstream of the second pipe and a second main control valve installed in the connection pipe downstream of the second pipe but upstream of the first pipe.

11. A method as claimed in claim 10 further comprising a step of repeatedly opening and closing the first valve and the main control valve when the second main control valve is open.

12. A method as claimed in claim 11 comprising an additional preliminary air cleaning step prior to the abrasive cleaning process, the preliminary air cleaning step comprising manipulating the first and second main control valves to gradually introduce the pressurized air into the pipeline to remove an amount of deposits therefrom.

13. A method as claimed in claim 7 wherein the sand particles are sized less than 2mm when used for cleaning pipelines up to 2 inches in diameter.

14. A method as claimed in claim 7 wherein the sand particles are sized 3.5mm to 4.5mm when used for cleaning pipelines equal to and larger than 6 inches in diameter.

15. A method as claimed in claim 11 further comprising a step of repeatedly opening and closing the first valve for controllably delivering the abrasive particulates into the connection pipe corresponding to the repeatedly opening and closing operation of the main control valve.