US3690732A

US3690732A - Cap formation in slurry pipelines

Info

Publication number: US3690732A
Application number: US116678A
Authority: US
Inventors: Edward J Wasp
Original assignee: BESHTEL INTERN CORP
Current assignee: Bechtel Group Inc
Priority date: 1971-02-18
Filing date: 1971-02-18
Publication date: 1972-09-12
Anticipated expiration: 1989-09-12

Abstract

My prior U.S. Pat. No. 2,920,923 of Jan. 12, 1960 teaches the desirability of including a quantity of a ''''pseudo-fluid'''' between a slug of water and a slug of slurry in a pipeline used for the transportation of the slurry. The pseudo fluid is also known as a ''''cap.'''' The utility of the pseudo fluid or cap is to exert a suspending influence on the relatively coarse particles of the solids in the slurry which is substantially greater than the suspending force exerted by the liquid alone. Preparation of the pseudo fluid or cap is carried on as a separate operation and one must be present between each slug of water and slurry. In accordance with this invention, the pseudo fluid or cap is created from the slurry itself and in a very simple manner, thus facilitating the pumping of the slurry and decreasing the overall cost of operation.

Description

United States Patent Wasp [54] CAP FORMATION IN SLURRY PIPELINES 72 Inventor: Edward J. Wasp, San Rafael, Calif.

[73] Assignee: Beshtel International Corporation [22] Filed: Feb. 18, 1971 [21] Appl. No.: 116,678

[52] U.S. Cl ..302/66, 302/14 [51] Int. Cl ..B65g 53/30 [58] Field of Search ..302ll4, 66

[56] References Cited UNITED STATES PATENTS 2,920,923 1/1960 Wasp et al. ..302/l4 X Primary Examiner-Richard E. Aegerter Assistant Examiner-H. S. Lane Attorney-Eckhoff & Hoppe [451 Sept. 12, 1972 [5 7] ABSTRACT My prior US. Pat. No. 2,920,923 of Jan. 12, 1960 teaches the desirability of including a quantity of a pseudo-fluid" between a slug of water and a slug of slurry in a pipeline used for the transportation of the slurry. The pseudo fluid is also known as a cap." The utility of the pseudo fluid or cap is to exert a suspending influence on the relatively coarse particles of the solids in the slurry which is substantially greater than the suspending force exerted by the liquid alone. Preparation of the pseudo fluid or cap is carried on as a separate operation and one must be present between each slug of water and slurry. In accordance with this invention, the pseudo fluid or cap is created from the slurry itself and in a very simple manner, thus facilitating the pumping of the slurry and decreasing the overall cost of operation.

CAP FORMATION IN SLURRY PIPELINES BACKGROUND OF THE INVENTION A slurry containing comminuted solids from an entire particle size spectrum and including coarse solids having a high settling rate and fine solids having a low settling rate, can be considered as a suspension of the relatively coarser particles in a pseudo fluid which comprises the liquid and the relatively finer particles of the solid. This pseudo fluid has a much greater suspending property for the relatively coarse particles than does the liquid alone. It is this interaction of solid particles throughout the entire spectrum which facilitates the transportation of slurries comprising comminuted solids in a liquid for the interaction hinders particle settling resulting from gravity. The nominal top size of the comminuted solid particles determines a minimum pipeline velocity for each particular type of solid which is required to prevent settling of the moving solids to the bottom of the pipeline under the influence of gravity. This determined velocity is referred to as the minimum non-settling velocity. I

Expressed in different language, the transportation of slurries is made possible by the existence of turbulent eddy currents which continually resuspend the coarser particles of the slurry. Hence, the interaction of solid particles throughout the entire spectrum of particle sizes depends upon a turbulent flow condition.

Once the slurry pipeline is constructed, it is initially filled with water from one end to the other. Thereupon a normal slug of slurry, such as a coal slurry, is introduced into the pipeline. It will be found that immediately following the introduction of the slurry slug, the boundary between the slug of clear water and the slug of slurry is sharply defined.

However, as the slurry moves through the pipeline, the boundary between the slurry slug and the clear water slug becomes less sharply defined as intermixing of the two fluids occurs. The coarser particles within the slurry do not move forward through the pipeline at exactly the same velocity as the liquid and finer particles which comprises the slurry. Instead, the solid particles experience a certain slip" and consequently lag behind the water portion. This slip factor combined with the turbulence of the moving fluids causes the zone of intermixing to increase in length as the slurry slug moves over a greater distances through the pipeline. For example, the zone of intermixing may be several thousand feet long when the slurry has moved a distance of thirty miles through a pipeline.

In the zone of intermixing, the coarser particles are no longer supported in a fluid having the suspending power of fresh slurry, i.e. the pseudo fluid, but instead are supported almost entirely by the water alone. Accordingly, the coarser particles in the zone of intermixing have a positive tendency to settle downwardly to the bottom of the pipe under the influence of gravity. Since the zone of intermixing may extend for distances of several thousand feet along the pipeline, a substantial quantity of solid particles will settle to the bottom of the pipeline.

As the slurry slug continues in its passage through the pipeline, it is continuously following the zone of intermixing. The slurry slug contains the pseudo fluid which has a substantial suspending power. The slurry slug accordingly will suspend the particles of settled coal which have already been deposited on the bottom of the pipeline from the zone of intermixing. The settled solids which thus are resuspended by the slurry slug increase the concentration at the head of the slurry slug. Thus, a zone of increased solids concentration is formed at the head of the slurry slug.

This zone may be too concentrated to move in a turbulent manner. Instead the highly concentrated slurries exhibit almost no relative motion between the particles. These highly concentrated slurries move through a pipeline in what has been called plug flow. Plug flow results in an extremely high pressure drop per unit of length of travel. Moreover, if the factors which promote plug formation are allowed to continue, they will eventually result in formation of a highly concentrated solids bridge across the pipeline and will prevent further movement of the slurry through the pipeline.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view of a first form of the invention, wherein a slurry is pumped directly into a pipeline to form a slug of pseudo fluid.

FIG. 2 is a schematic view of a second form of the invention wherein a slurry is first pumped into a pipeline loop to form a slug of pseudo fluid, and then pumped into a main pipeline.

SUMMARY OF THE INVENTION According to the principles of the present invention, plug formation conditions can be avoided during the introduction of coal slurry into a transportation pipeline of substantial length. To effect this, a slug of clear water is maintained initially in the pipeline as before. Thereafter a slug of pseudo fluid having a greater suspending power than water is provided in the pipeline.

In accordance with one form of the present invention, as shown in FIG. 1, a slug of pseudo fluid or cap C is formed in situ in a pipeline from the slurry S. Once the pipeline has been filled with water, formation of the cap is achieved by introducing the slurry into the pipeline at a sub-critical velocity. The coarser solid particles settle out at this velocity. Thus, the interfacial mixing with the water takes place and relatively little coarse material remains in the forward part of the slurry to create a problem. The length of the slug of pseudo fluid should be sufficient to accommodate all intermixing between the clear water slug and the following slurry slug. This can usually be accomplished by pumping at the lowered velocity for 10 to about 30 minutes. As the pseudo fluid slug progresses through the pipeline, it will intermix at its forward end with the clear water slug to form a zone of intermixing. As the slurry slug is introduced into the pipeline at a normal velocity, there will be initially a sharp dividing boundary between the pseudo fluid slug and the slurry slug just as if the pseudo fluid or cap had been formed outside of the pipeline and then introduced as is taught in my prior patent.

DESCRIPTION OF THE PREFERRED EMBODIMENT The invention has been successfully applied in the operation of the pipeline which is utilized to convey coal from the Black Mesa reserves in northeastern Arizona to the new 1.5 million kilowatt Mohave Generating Station in Nevada, 273 miles away. This pipeline delivers 6 million tons of coal annually at the rate of 600 tons of coal hourly, 24 hours daily. The operation of the pipeline is characterized particularly in that only one operator monitors the slurry preparation plant and controls the flow of coal through the four pumping stations.

As seen in FIG. 2, to start the operation, a batch of the coal slurry S was passed through a 24-inch O.D. pipe loop L,- l000 feet in length, at the rate of 1 foot per second to form the slurry cap C. It was found that in the first minutes or 300 feet of slurry, the concentration of coarse particles in the slurry decreased. In other words, the first part of the slurry had deposited out the coarser fractions, thus forming the required cap of finer material. The cap so formed was then introduced into the main 18-inch O.D. pipeline P after which the pumping of the coal slurry was commenced.

I claim:

1. In the method of transporting through a pipeline of substantial length, a normal slug of slurry of comminuted solids in a liquid in which said solids have a tendency to settle downwardly under the influence of gravity and including solids having a high settling rate and solids having a low settling rate, the improvement for avoiding plug formation in the pipeline comprising, filling the pipeline with water, introducing said slurry into said pipeline for a period of time and at a sub-critical velocity, as determined by the size of the pipeline, sufficient for the solids having a high settling rate to settle and thus form in situ in said pipeline a slug of pseudo fluid from said slurry having only said solids with a low settling rate suspended therein, and thereafter introducing a further quantity of said slurry at a velocity greater than the sub-critical velocity to form a normal slugof slurry in the pipeline, said period of time being at least equal to that required to produce a length of slug of pseudo fluid sufficient to accommodate all intermixing between the water and the following normal slug of slurry to thus maintain the normal concentration of solids in the slug of slurry throughout said length of pipeline.

2. A method as in claim 1, wherein the pipeline is at least 10 miles long.

3. A method as in claim 1, wherein the length of time is from about 10 minutes to about 30 minutes.

4. A method of avoiding plug formation while transporting through a pipeline of substantial length, a normal slug of slurry of comminuted solids in a liquid including coarse solids having a high settling rate and fine solids having a low settling rate, comprising filling a pipeline loop with water, introducing said slurry into said pipeline loop for a period of time and at a velocity, as determined by the size of the pipeline loop, sufficient for the coarse solid to settle and thus form in situ in said pipeline loop a body of liquid and suspended fine solids having a suspending power greater than the liquid, filling said pipeline with water, introducing said body of liquid and suspended fine solids into said pipeline, and thereafter introducing said slurry into said plpelme at a greater velocity sufficient to prevent settling of said coarse solids and thus forming a normal flow of said slurry in the pipeline, said body of liquid and suspended fine solids having a length sufficient to accommodate all intermixing between said water and the following normal slug of slurry and thus to maintain the normal concentration of solids in said slug of slurry throughout the length of pipeline.

5. A method as in claim 3, wherein the pipeline is at least 10 miles long.

6. A method as in claim 5, wherein the length of time is from about 10 minutes to about 30 minutes.

7. A method as in claim 5, wherein the pipeline loop is about 1,000 feet long.

Claims

1. In the method of transporting through a pipeline of substantial length, a normal slug of slurry of comminuted solids in a liquid in which said solids have a tendency to settle downwardly under the influence of gravity and including solids having a high settling rate and solids having a low settling rate, the improvement for avoiding plug formation in the pipeline comprising, filling the pipeline with water, introducing said slurry into said pipeline for a period of time and at a subcritical velocity, as determined by the size of the pipeline, sufficient for the solids having a high settling rate to settle and thus form in situ in said pipeline a slug of pseudo fluid from said slurry having only said solids with a low settling rate suspended therein, and thereafter introducing a further quantity of said slurry at a velocity greater than the sub-critical velocity to form a normal slug of slurry in the pipeline, said period of time being at least equal to that required to produce a length of slug of pseudo fluid sufficient to accommodate all intermixing between the water and the following normal slug of slurry to thus maintain the normal concentration of solids in the slug of slurry throughout said length of pipeline.

2. A method as in claim 1, wherein the pipeline is at least 10 miles long.

4. A method of avoiding plug formation while transporting through a pipeline of substantial length, a normal slug of slurry of comminuted solids in a liquid including coarse solids having a high settling rate and fine solids having a low settling rate, comprising filling a pipeline loop with water, introducing said slurry into said pipeline loop for a period of time and at a velocity, as determined by the size of the pipeline loop, sufficient for the coarse solid to settle and thus form in situ in said pipeline loop a body of liquid and suspended fine solids having a suspending power greater than the liquid, filling said pipeline with water, introducing said body of liquid and suspended fine solids into said pipeline, and thereafter introducing said slurry into said pipeline at a greater velocity sufficient to prevent settling of said coarse solids and thus forming a normal flow of said slurry in the pipeline, said body of liquid and suspended fine solids having a length sufficient to accommodate all intermixing between said water and the following normal slug of slurry and thus to maintain the normal concentration of solids in said slug of slurry throughout the length of pipeline.

5. A method as in claim 3, wherein the pipeline is at least 10 miles long.

7. A method as in claim 5, wherein the pipeline loop is about 1, 000 feet long.