WO2012070945A1

WO2012070945A1 - System and method for removing sediment from sand traps

Info

Publication number: WO2012070945A1
Application number: PCT/NO2011/000281
Authority: WO
Inventors: Tom Jacobsen
Original assignee: Jarala As
Priority date: 2010-11-25
Filing date: 2011-10-03
Publication date: 2012-05-31
Also published as: CL2012002050A1; CN103221612A; MX2012013169A; MX335947B; NO20101657A1; NO336317B1; PE20140461A1; ECSP12012336A; CN103221612B

Abstract

Method and System for hydraulic removal of sediment in sand traps, where sediment are sucked into slotted pipes (15) arranged near the bottom of the sand trap. The slotted pipes (15) are branched out from connecting pieces (16) where at least two slotted pipes are connected to a common discharge pipe (17) in a direction independent of the orientation of the slotted pipes (15), the slotted pipes (15) being such separated from one another that they can operate mutually independent and still contribute to a mutual balancing of a common under pressure.

Description

System and method for removing sediment from sand traps

The present invention concerns a system and a method for hydraulic removal of sediment in a sand trap.

Background In a number of situations it is desirable to be able to remove sediment settled under water without drain or otherwise lowering the water level of the covering water. Sediment in this connection is particulate materials such as gravel, sand, "silt", organic particles or other particles. Water flowing in rivers, canals, tunnels or pipes can transport sediment. If the velocity of the water is reduces, sediment heavier than water will sink to the bottom and settle. This may be in buffer basins, intake or sedimentation basins in connection with power plants or irrigation plants, sand traps in tunnels or tanks or basins related to process plants. It is often desirable to empty storage tanks of particulate materials by means of water or air. Below all such sediment basins are denoted as sand traps irrespective of their intended purpose. Sediment will lead to excessive wear if it follows the water through turbines of a power plant. It is well known that turbines must be replaced or repaired several times a year due to wear caused by sediment. Furthermore canals and tunnels may be filled with sediment. For this reason a number of sand traps have been established in relation to hydropower plants, primarily in countries in which rivers have a high concentration of sediment. In addition it is many places desirable in relation to water inlets to irrigation plants, to separate out at least the coarser fraction of particles so that irrigation canals do not become blocked or plants flushed with saline water.

Sediment settled in sand traps can comprise materials from rocks and gravel to very fine grain material. In hydro power plants a typical criteria for particles to be separated can be 0.15 - 0.5 mm.

Known techniques for removal of sediment comprise drainage and mechanical excavation or flushing of sediment while the water level is lowered, so that the velocity over the sediment increases. Both these methods have the disadvantage that the water supply to e.g. a power plant is interrupted. Thus the production is interrupted and in addition there are costs related to shutting off and turning on a power plant. Furthermore the conflict between desired production and removal of sediment will typically imply that removal is postponed which leads to unnecessary turbine wearage.

Other known techniques comprise flushing through holes at the bottom of the sand trap down to a flushing canal. These holes can optionally be equipped with closing mechanisms (Bieri patented system). A disadvantage with this system is that it depends upon motors, transmissions and movable parts which may cause stops in its operation.

WO publication No 02088472 (Jacobsen) teaches use of slotted pipes in sand traps for continuous removal of sediment from sand traps in a simple manner. The slotted pipes are laid in parallel at certain intervals in recesses at the bottom of the sand trap, each one connected to a discharge pipe at or near the (lower) end of the sand trap. The system I s well functioning in sand traps up to a certain size, but requires a preparation of the bottom of the sand trap in the form of ditches in order to avoid the risk of liquefaction at the slotted pipe and failure of the above sediment (hereinafter just "liquefaction") becoming to high. In large sand traps it is a problem to obtain the intended function in the entire extension from the uppermost to the lowermost part of the san trap due to its length dimension; i.e. it is difficult to maintain substantially even conditions in the slotted pipes when they are long.

Objects

It is an object of the present invention to provide a method for removing sediment from sand traps in a simple, inexpensive, and reliable manner requiring a minimum of maintenance.

It is a further objective that said method shall be suitable for use also in large sand traps, that it periodically shall be able to handle large amounts of sediment and preferably that the removal shall take place continuously. It is an object to be able to remove sediment particles and objects with a largest possible diameter.

The present invention

The above mentioned objects are achieved through the present method which is defined by claim 1. According to another aspect the present invention comprises a system of slotted pipes as defined by claim 10. Preferred embodiments are disclosed by the dependent claims.

By means of the present invention it becomes significantly simpler to cover large sand traps with a system of slotted pipes that are interconnected at least in pairs to a common discharge pipe. This implies a.o. that the length of the slotted pipes in a certain field can be reduced to one half by allowing the discharge pipe to leave at "the middle" of the long side of the field rather than at the end of the field. With the embodiment described one will also achieve a hydraulic balance between the two pipes such that the suction force at any point in time is smoothed and a more even suction is obtained, compared to if the slotted pipes were connected individually to discharge pipes such as (Al). It should be emphasized, as later to be documented, that it certainly is not an obvious measure to connect two slotted pipes in this manner, since such a combination might disturb the delicate hydraulic conditions that must be present in order for the slotted pipes to perform their intended task.

It is thus important that this is taken into consideration and that each slotted pipe is sufficiently separated from other slotted pipes to be able to operate independent of other slotted pipes connected to same discharge pipe with regard to the general hydraulic conditions, but still allowing the slotted pipes to positively interact when required. This is explained below.

There needs to be sufficient distance between the lower end of the slots for each slotted pipe in order for the pipes to function mutually independent. It is thus ensured that;

• The discharge pipe can have the same or a larger diameter than the slotted pipes to thereby reduce the risk of blockage.

• Each discharge pipe serves a larger extension, i.e. the individual slotted pipes is allowed to be shorter, which again reduces the risk of liquefaction. · The risk of liquefaction is further reduces due to the levelling of the underpressure in the slotted pipes and the discharge pipe. If a slide or the like temporarily blocks or prevents flow into one slotted pipe the flow into the other will increase.

• In continuous operation only the downstream end of the slots will be active, but sediment will still be removed over a larger extension. Below the invention is described in further detail with reference to the accompanying drawings where:

Figure 1 shows a system comprising two slotted pipes connected to a common discharge pipe in a sand trap.

Figure 2 shows a variant different from that of Figure 1.

Figure 3 shows a system comprising eight slotted pipes connected impairs to discharge pipes in a sand trap.

Figure 4 shows a in perspective a part of a slotted pipe suitable for use according to the present invention.

Figure 5 shows a T-connection with a partition wall.

Figure 6 shows two pairs of slotted pipes connected to one common discharge pipe. Figure 7 shows a particular embodiment of the present invention.

Figure 1 provides a top view of two slotted pipes 15 arranged mainly aligned with the length direction L of a sand trap. The slotted pipes generally comprises a mainly horizontal part 15a (see Fig. 2) and near upstream end both slotted pipes are arranged with an inclined part 15b that ensures that upstream end 15c of the slotted pipes are free from sediment at all times, which is important for the functioning of the slotted pipes. Sand traps typically are several times as long as they are wide. The two slotted pipes 15 are connected to a common discharge pipe 17 via a connecting piece 16. The connecting piece shown in Figure 1 is commonly called a T-connection. The sand trap has inclined side walls 12 and inclined end walls 13 which contribute to early sediment coverage of the horizontal part 15a of each pipe and the fact that there is no need for additional slotted pipes acrosswise.

Figure 2 shows principally the same as Figure 1, but as a further element, a partition wall 21 is arranged across the sand trap where the slotted pipes are interconnected to ensure that they do not influence each other undersirably (as earlier described). Figure 2 also shows the possibility of arranging a choke valve 22 to each slotted pipe 15 near the connecting piece, to allow a gradually reduced flow from one or both pipes, e.g. in connection with liquefaction or the risk thereof in at least one of the slotted pipes of the pair in question. Such a valve must naturally be remotely controlled. The person skilled in the art will understand that such valves are at least equally relevant in the embodiment of Figure 1 where no partition wall separates the slotted pipes.

Figure 3 shows a larger sand traps comprising a right hand section R and a left hand section L separate from each other by means of partition wall 31 which can, but need not be, as high as the outer walls of the sand trap. In the length section R two pairs of slotted pipes are arranged with a common discharge pipe for each pair. Correspondingly the length section L comprises two pairs of slotted pipes with a common discharge pipe for each pair. In total there are four pairs of slotted pipes, i.e. eight slotted pipes. In this way the advantage of present invention is achieved also in sand traps too long to use one pair of slotted pipes cover the entire length of the sand trap, not to mention one single slotted pipe alone. Correspondingly is illustrated how the advantage of the present invention can be achieved in sand traps too wide for one slotted pipe to effectively draw sediment from its entire width.

In Figure 3 there is only a barrier shown in the length direction of the sand trap, but it is also feasible to arrange also at least one barrier across the sand trap. The purpose of such barrier is to reduce the risk of liquefaction of sediment, implying that one or several of the slotted pipes become(s) inactive over whole or part of it/ their length(s) until new layers of sediment have been accumulated.

Also if liquefaction occurs the system according to the present invention includes a possibility of remedying this situation by temporarily interrupting all fluid flow through the slotted pipe in question, preferably with a valve on each slotted pipe, thereby avoiding interruption of stop in both the slotted pipes of a pair.

Figure 4 shows a particularly preferred variant of the slotted pipe where the slot 41 is surrounded by a skirt 42 at both sides, said skirt extending downwards to make the transportation route Tr for water and sediment into the slot longer, which also contributes to reduced risk of liquefaction at any arbitrary point along the slotted pipe. The height of the skirt can vary along the length of the slotted pipe and should in case be highest (deepest) at the downstream end. If liquefaction occurs it will imply that the pair of slotted pipes in question temporarily will be unable to suck in particles from any point upstream of the point of the liquefaction. This situation will remain until sediment again covers the slotted pipe continuously along its length. The slot along the slotted pipe has the form of discontinuous slot openings which is convenient for obtaining a slotted pipe of sufficient strength based on convenient dimensions and materials. The individual slot openings can, however, have a completely different form from that shown in Figure 4. As an example the slot openings can be longer than their width, but should generally be dimensioned such that the T-connection and the discharge pipe do not risk blockage of particles that the slot openings allow to enter the slotted pipe.

The slots are typically smaller than the diameter of the discharge pipe but larger than the largest particles settling in the sand trap, to thereby avoid blockage.

Figure 5 shows a preferred embodiment of a T-connection 16 suitable for connecting two slotted pipes to a common discharge pipe (connected to the vertical leg of the T- connection shown in the drawing). An internal partition wall 52 (dotted) extends downstream to a desired point in the discharge part of the T-connection. The internal partition wall 52 may preferably be controllably movable for when needed to control the flow of water from each of the slotted pipes differently. Figure 6 is included for the purpose of illustrating that also the discharge pipes (17) may be combined, typically in pairs, to save meters of pipe length. In this way a further acceleration of the water flow in the discharge pipe can be obtained, but same effect can also be accomplished by using pipes with gradually reduced diameter.

Figure 7 shows a particular embodiment of the present invention for draining from a mainly circular basin 71 (or silo), in which six slotted pipes 15 are arranged in a star configuration towards a connecting piece having six inlet ports and a discharge port mainly perpendicular to the inlet ports (downwards into the plane of the paper), said discharge port directing the flow of water into a common discharge pipe 17. The number of slotted pipes can naturally be varied. The central issue of the present invention is the dynamic interaction between pairs of slotted pipes being joined to a common discharge pipe. Normally one would assume that this is a disadvantage and that it would interfere with the hydraulic conditions that need to be present if the slotted pipes consistently and reliably shall suck in sediment along the entire length of the slotted pipes and transport them out of the sand trap. That would e.g. be the case if one of the pipes is finished sucking long before the other one and short circuits this before the covering sediment has been removed. It has been proven, however, that with the right precautions taken, the connection of at least two slotted pipes to a common discharge pipe, ensures a more stable operation with respect to removing sediment. An important issue is mentioned already, being the fact that the length of each individual slotted pipe can be reduced. A further benefit consists in the ability of one slotted pipe to temporarily relieve the other and thereby prevent pressure pulses which else might lead to liquefaction.

To ensure that the slotted pipes can be operated substantially independent of each other (so that one continues to suck sediment even when the other one is done sucking, the following conditions must be fulfilled: 1) There must be a sufficient distance between the two slotted pipes, or precautions must be taken to ensure that the sediment covering the still operating slotted pipe has a sufficient barrier against liquefaction. 2) The flow of water (in the discharge part) in the connection to the common discharge pipe should be accelerated sufficiently so that the suction force in each of the slotted pipes is maintained. This is obtainable by allowing the flow of water from each slotted pipe to be held separately a certain distance, e.g. by a partition wall (52). This partition wall can also be extended into the discharge pipe and it can be made thicker so that the water velocity along the partition wall increases and the flows in the slotted pipes are further separated from each other. This should however, not be exaggerated since this will reduce the desired interaction between the two slotted pipes, i.e. the ability for one to relieve the other if that is partially blocked.

One might also mount monitoring equipment to monitor the sediment coverage and either manually or automatically controlling the operation of the slotted pipes according to the monitored sediment level. This can be further refined by measuring the rate of the change of sediment level and when the sediment level increases rapidly (e.g. during flooding) change to a more rapid operation or even continuous operation.

Flushing nozzles may be arranged along the slotted pipes for making cohesive sediment or agglutinated organic material to be loosened. Such nozzles can be permanently or temporarily connected to a high pressure pump to be operated when the need therefore occurs. The slot width can also be optimized for optimizing the concentration while obtaining an effective suction extension consistent with the conditions. The slotted pipes are constructed sufficiently rigid and their fundaments have a sufficient strength to prevent that the pipe is compressed by the covering sediment.

The velocity in the discharge pipe depends on the diameter of the slotted pipe, diameter of the discharge pipe, length and the driving pressure differential. When discharge length and pressure difference is defined, the diameter of the slotted pipe and discharge pipe must be adapted accordingly so that the velocity in the discharge pipe is sufficiently high and so that large particles do not settle in and block the pipes, and in manner so that the sediment concentration is adapted to the capacity of the discharge pipe.

In order to control the sediment concentration so that it corresponds with the capacity of the discharge pipe 17, there may in some cases be convenient to provide the slotted pipes with water supply at their downstream end. Dependent upon the rate of water added the sediment concentration will be changes in the discharge water.

Claims

1. Method for hydraulic removal of sediment from sand trap where the sediment is sucked into slotted pipes (15) arranged near the bottom of the sand trap, characterized in that the slotted pipes (15) are branched out from connecting pieces (16) of which at least two slotted pipes are joined in a common discharge pipe (17) in a direction independent of the orientation of the slotted pipes (15) and where the slotted pipes (15) are separated from one another to operate mutually independent of each other and still contribute to mutually balancing a common underpressure.

2. Method as claimed in claim 1, characterized in that the slotted pipes (15) are separated from one another by means of physical distance and by terminating the slots at a distance from the connecting piece (16).

3. Method as claimed in claim 1, characterized in that the slotted pipes (15) are separated from one another by means of a physical barrier (21, 31) between them.

4. Method as claimed in claim 3, characterized in that the physical barrier consists of a separating wall (21) between two joined slotted pipes (15) so that a flow of water and sediment from the two slotted pipes (15) are held mutually separate towards the connecting piece (16).

5. Method as claimed in claim 3, characterized in that the flows of water and sediment from to adjoined slotted pipes (15) are separated by an optionally controlled, movable separating wall (52) in the connecting piece (16).

6. Method as claimed in claim 3, characterized in that the slotted pipes (15) are arranged in pairs, the slotted pipes (15) of each pair being arranged along a common line and via a t-shaped connecting piece being connected to a common discharge pipe (17).

7. Method as claimed in claim 3, characterized in that the slotted pipes (15) are used in a pressurized tunnel sand traps.

8. Method as claimed in claim 3, characterized in that the slotted pipes (15) are operated continuously.

9. Method as claimed in claim 3, characterized in that the slotted pipes (15) are used when the sediment coverage has reached a predetermined level or at certain time intervals.

10. System of slotted pipes for hydraulic removal of sediment in sand traps where the sediment is sucked into slotted pipes arranged near the bottom of the sand trap,

characterized in that the slotted pipes (15) are branched out from connecting pieces (16) where at least two slotted pipes (15) are joined to a common discharge pipe (17) in a direction independent of the orientation of the slotted pipes (15) and where the slotted pipes (15) are separated from one another to operate mutually independent of each other and still contribute to mutually balancing a common (under)pressure.

11. System as claimed in claim 10, characterized in that the slotted pipes (15) are provided with skirts (42) at each side of the slot (41) to reduce the risk of liquefaction.

12. System as claimed in claim 9, characterized in that jet nozzles are arranged along the slotted pipes to loosen cohesive sediment.

13. System as claimed in claim 10, characterized in that the system comprises several pairs of slotted pipes (15) arranged in a row, one behind the other.

14. System as claimed in claim 19, characterized in that the system comprises several pairs of slotted pipes (15) arranged in parallel with one another, preferably separated by a physical barrier (31).

15. System as claimed in claim 10, characterized in that the size of the slots (41) varies along the length of the slotted pipes, with a larger slot opening upstream in the sand trap than downstream, to compensate for how the size distribution of the particles also varies in the sand trap.

16. System as claimed in claim 10, characterized in that the connecting pieces (16) includes an optionally controllable, moveably arranged separating wall (52) which holds the flow of water from the connected slotted pipes (15) separate from one another immediately downstream of the connecting piece (16).

17. System as claimed in claim 10, characterized in that the system comprises equipment adapted to monitor the sediment level and optionally equipment to automatically start and pause the operation in dependence of registered sediment level.

18. System as claimed in claim 10, characterized in that one or both slotted pipes (15) of each pair is/ are provided with a choke valve (22) near the connecting piece (16) to enable gradually closing of the opening to the connecting piece (16).