US 3895885 A
A system for emptying a cargo oil tank or the like comprising a centrifugal pump and a separator tank, which is provided for separation of gas and is interconnected in the suction pipe of the pump. Gas in the separator tank is mixed in the form of smell bubbles into the oil on the suction side of the pump and is evacuated distributed in the oil together therewith by the centrifugal pump.
Description (OCR text may contain errors)
United States Patent Liberg 1451 July 22, 1975  EMPTYING SYSTEM FOR FLUID TANKS 2,452,421 10/1948 Amcs 417/77 2,651,259 9/1953 Brush 1 1 417/77 [751 lnvemo LaTS'OM berg, Gmbenburg/ 5,490,376 1/1970 Valespino 417/477 Sweden 3,619,084 11/1971 Gordon 417/40  Assignee: Jonkopings Meitariiska Werltstads FOREIGN PATENTS OR APPLICATIONS V 1 JOYkOPmE Swede 664.512 4/1949 France 415/53  Filed: Jan. 3, 1973 Primary Examiner-William L. Freeh  Appl' 320828 Assistant ExaminerRichard E. Gluck Attorney, Agent, or Firm-Cushman. Darby &  Foreign Application Priority Data h an Jan. 12, 1972 Sweden 1. 303/72  ABSTRACT  US. Cl. 417/54; 417/77; 417/80 A system for emptying a cargo oil tank or the like  Int. Cl. F04b 23/04; F0410 23/08 comprising a centrifugal pump and a separator tank,  Field of Search 417/77, 199 A, 80, 89, which is provided for separation of gas and is inter- 417/430, 431, 53, 54, 40,4; 415/53 R; connectedin the suction pipe of the pump. Gas in the 261/20, 36 R, 70, 76 separator tank is mixed in the form of smell bubbles into the oil on the suction side of the pump and is  References Cited evacuated distributed in the oil together therewith by UNITED STATES PATENTS the cenmfugal P 2,223,112 11/1940 Lear 417/77 X 13 Claims, 2 Drawing Figures EMPTYING SYSTEM FOR FLUID TANKS This invention relates to a method of emptying a cargo oil tank or another container for oil or another liquid by means of a centrifugal pump, the suction pipe of which is connected to the container through a separator tank or the like, which is provided for separation of gas (air) and/or vapor, which follows the liquid into the separator tank.
The invention further relates to an apparatus for carrying out this method which comprises a cargo oil tank or another liquid container and a centrifugal pump which is provided for emptying said container and has its suction pipe connected to the container through a separator tank which is provided for the separation of gas (air) and/or vapor from the liquid on its way from the container. to the pump.
Formerly there were utilized for the discharge of tankers at the initial stage of the pumping, big primary pumps (centrifugal pumps) which carried out the whole discharge operation up till the final stage, when only a little oil was left, after which the concluding emptying (the so-called stripping pumping) was continued by means of special pumps which did not lose their suction ability when gas and vapor were sucked into the pump together with oil, and on account hereof cease pumping, as centrifugal pumps do. These so called stripping pumps have a low capacity which entails a comparatively long discharge time.
As the dimension of tankers increased, the time for the stripping pumping also increased correspondingly and gradually exceeded acceptable values.
On account hereof different systems have been developed for making possible stripping pumping by means of the big primary pumps which generally are of the centrifugal type. Common to at least some of these systems is the fact that they comprise a separator tank which constitutes a liquid reservoir for the cargo oil pump and in which gas (air) and/or vapor (in the following jointly called gas for the sake of brevity) is separated from the oil which flows into the suction pipe of the pump. In the system in question the quantity of oil which is transported by the pump per unit of time is reduced during the stripping pumping depending upon the liquid level in the separator tank. To make this possible there is required, however, among other things a vacuum pump, for maintaining a vacuum or underpressure in the separator tank and for evacuating gas which is separated therein, and extensive controlling and checking equipment for the centrifugal pump, as well as for the vacuum pump.
On account of the above, the principal object of the invention is to simplify the stripping pumping and the equipment necessary therefor. This object is achieved according to the method of the present invention by the fact that the gas (air) and/or vapor in the separator tank is mixed in the form of bubbles into the liquid on the suction side of the centrifugal pump and is evacuated distributed in the liquid together therewith by the centrifugal pump. In concert herewith the apparatus according to the invention is characterized by the provision of means for recirculating liquid from the pressure pipe of the pump to a device, which is suitably located in the separator tank, for mixing gas (air) and/or vapor which is present in the separator tank, in a more or less finely diffused form into the recirculated liquid,
and means for introducing the gas-liquid mixture into the inlet of the pump.
The basic principle on which the invention is based resides in the concept of finely diffusing gas, which has penetrated into the suction pipe of the pump, in such a way that the gas-liquid mixture becomes pumpable. By transporting the gas through the pump in this way the pressure created by the pump drops on account of the decrease in density of the pump fluid due to the gas introduced therein. The decrease in pressure has the consequence that the quantity of fluid pumped will decrease. Furthermore, part of the gas may be separated out in the pump wheel and thereby block part of the flow space, which also leads to a decrease in the flow of fluid. This drop in the flow of fluid will, in its turn, have the consequence that the leakage of gas into the suction pipe is reduced. Thus, gas which penetrates into the suction pipe is utilized for controlling the quantity of liquid pumped, so that a balance is reached between the leakage of gas into the suction pipe and the capability of the pump to pump air.
To prevent the pump from dropping on account of gas separation in the pump wheel, e.g. at a small flow of liquid in the suction pipe of the pump (before the separator tank), part of the liquid being pumped is recirculated from the pressure pipe of the pump to the separator tank, in a preferred embodiment of the invention. The energy of the recirculated liquid is utilized to finely diffuse the air separated in the separator tank, in the liquid being pumped. Preferably, the liquid level in the separator tank governs the amount of gas which is mixed with the liquid, and accordingly also the volume of the flow through the pump.
On account of the principle of operation of the system, regulation is obtained only when necessary, i.e. when there is gas in the separator tank. The evacuation of gas does, therefore, not require separate air or vacuum pumps, as in prior systems, since according to the invention, the gas follows the pumped liquid through the centrifugal pump.
Further features and advantages of the method and the apparatus according to the invention will become apparent from the following detailed description and the annexed drawings which diagrammatically, and as non-limiting example, illustrate some preferred embodiments of the invention.
FIG. 1 illustrates in side view and in a very simplified way the principal parts of a pump system which is designed according to the invention.
FIG. 2 illustrates in the same way a modification of the system according to FIG. 1.
In the drawings 1 designates a suction pipe which comes from a cargo oil tank 14 or from another arbitrary container, and 2 a separator tank which in its lower portion is connected to the inlet piece 4 of a cargo oil pump 3 or the like. For safety reasons, particularly to ensure that the pump always shall get liquid, the inlet of the suction pipe 1 in the separator tank 2 is located in the upper portion thereof according to FIG. 1. In the alternative embodiment according to FIG. 2, in which same reference characters have been utilized to designate the same or similar details, the separator tank is, by means of a vertical intermediate wall 5, divided into an inlet portion 2a and an outlet portion 2b which have a connection with each other only above the upper edge of the intermediate wall 5. The cargo oil pump 3 has a pressure pipe 6 which, in a conventional manner is provided with a non-return valve 7. A return or feed back-conduit 8 is branched off from the pressure pipe 6 from the lower side of a suitably substantially horizontal portion of the pressure pipe, which preferably is located comparatively high above the pump 3, where gas and oil have had time to separate so that only liquid is returned through feed back conduit 8. The return conduit opens into a gas introduction or admixing device which is suitably located in the separator tank 2, or in its outlet portion 2b respectively. and is preferably of the jet pump or ejector type. This gas introduction device, which is generally designated 9,
' opens into or near the outlet of the separator tank or in the inlet piece 4 of the pump. The return conduit 8 is provided with an adjustment or shut off valve 10 which is operated automatically by a liquid level switch 11 or in another suitable manner depending upon the liquid level in the separator tank.
To the gas introduction device there also leads a substantially vertical gas tube 12 which is located in the separator tank and has small orifices or nozzles 13 which are provided in the tube wall and have a suitable area and a suitable mutual spacing. On account of the velocity of the recirculated fluid in the gas introduction device 9, the liquid absorbs or sucks off gas in finely diffused form, i.e. in the form of bigger or smaller bubbles, from the separator tank 2 into the pump 3. The recirculated liquid has a greater velocity than the liquid not recirculated in the inlet piece 4 of the pump. The device is, to the greatest extent possible, designed in such a way that the gas-liquid mixture becomes so uniformly small bubbly that the mixture becomes pumpable. If necessary the gas-liquid mixture recirculated to the pump 3 may be introduced directed directly towards the inlet of the pump wheel for breaking down ,possible accumulations of gas there. Instead, or as a supplement hereof, the mixture may be introduced into the inlet of the pump in such a way that a suitable prerotation is created therein. Furthermore, the tube 12 may be provided with a slidable nad/or rotatable sleeve or the like which is so arranged that it covers a desired number of orifices by beeing displaced or rotated. Thanks to this arrangement the advantage is gained that the total area of uncovered orifices may be adapted to the capability of the pump to pump air, at every particular instance of operation.
The liquid level in the separator tank controls the amount of gas to the tube 12 by permitting gas to reach the gas introduction or admixing unit 9 solely through those orifices in the tube 12 which are situated above the liquid surface.
The mode of operation may be summarized as follows:
During normal discharge of oil from the cargo oil tank through the suction pipe 1 by means of the centrifugal pump 3, i.e., before said tank starts to get empty, the separator tank 2 is full of oil and the pump 3 thus just pumps plain oil. When most of the oil in the cargo oil tank has been evacuated, i.e. during the final stage or the so called stripping pumping, air is sucked into the suction pipe 1 from the cargo oil tank. The liquid level in the separator tank 2 drops and the air sucked in separates from the oil in the separator tank 2 and accumulates above the oil surface therein, as is known in the pump art.
When the liquid surface in the separator tank has dropped to a predetermined level, switch 11 opens valve 10 for recirculating oil from the pressure pipe 6 to the centrifugal pump inlet 4 through the return conduit 8 and the gas introduction device 9. Since the recirculated liquid hasa comparatively great velocity it sucks off air or gas from the separator tank 2 through those of the orifices 13 which are not covered by the oil in the separator tank 2, and introduces this air in finely diffused form into the pump 3. By thus evacuating the gas, diffused into or mixed with the oil by the centrifugal pump, the self-adjusting effect referred to in the opening paragraphs of this specification is attained.
At a low liquid level a greater number of orifices are uncovered and a great quantity of gas is mixed with the fluid being pumped. Thus, a great reduction of the volume of flow of the centrifugal pump is obtained on account of the influence of the gas quantity upon the lifting height of the pump, and at the same time the pump evacuates much gas. A great admixtion of gas to the fluid being pumped reduces to a comparatively great extent the volume of the flow in the suction pipe to the separator tank and in so doing also reduces the quantity of gas to the tank 2. When the liquid level is high, the condition presumed above is reversed. In this way the system if self-adjusting, so that it maintains the highest possible pumping capability.
When the oil or liquid tank 2 is empty or when the flow of liquid in the suction pipe 1 has ceased for another reason, the centrifugal pump 3 still receives a flow of liquid from the liquid recirculated in the conduit 8. Thanks to the recirculation the pump wheel gets such flow of liquid as is necessary to prevent the pump from dropping, whereby the system can maintain the pumping (or stripping) when the suction pipe is anew supplied with liquid. Such an intermittent flow can arise, e.g. when pumping from a tank with a long suction pipe aboard a tanker. The return pipe 8 is suitably so dimensioned and the flow of liquid through the same so adjusted that it can conduct from zero (before the stripping pumping, when the separator tank is full of liquid) up till the quantity of recirculating liquid which is necessary for the functioning at the current load condition (at empty cargo oil tank).
The advantages which are attained through the invention above include the fact that the pump system becomes simpler and has a reduced number of components and movable parts. On account hereof it becomes less expensive to purchase, install, start up, operate, service and maintain. The control and regulation equipment will be less extensive and the automatic control devices become simpler, which makes the system more reliable in operation. Another advantage resides in the fact that an explosion risk has been eliminated through the disposal of special vacuum pumps for the evacuation of gas.
The embodiments descirbed above and shown in the drawings are, of course, to be regarded merely as nonlimiting examples and may as to their details be modified in several wayswithin the scope of the following claims. In particular, the system may be utilized for other purposes than the pumping of oil, e.g. from tankers. Furthermore the liquid level switch 11 for instance may be replaced by a control device, by means of which the proportions gas/liquid in the gasliquid mix- ;ure supplied to the pump is controlled in direct dependence upon the proportions of gas/liquid in the separator tank.
What I claim is:
1. Method of emptying a container for liquid, such as a cargo oil tank, by means of a centrifugal pump having a pressure pipe, a pump impeller, and an inlet which is connected by a suction pipe to said container through a separator tank provided for separation of gas combined with the liquid as they are drawn from said container into the separator tank, comprising the steps of:
drawing said liquid through said suction pipe into said separator tank by means of a partial vacuum created in said separator tank above the surface level of said liquid,
allowing said gas to separate from said liquid so as to permit said gas to temporarily accumulate above said liquid in said separator tank,
removing said gas from above said liquid in said separator tank and injecting said gas into a mixing device separated from and not forming a part of and disposed below the normal operating surface of said liquid in said separator tank,
removing a portion of said liquid from said pressure P p injecting said portion of said liquid into said mixing device,
mixing said gas with said liquid in said mixing device,
so as to form small bubbles in said liquid, injecting the mixture of gas and liquid into said centrifugal pump, and
evacuating said gas bubbles distributed in said liquid together therewith by means of said centrifugal pump.
2. Method according to claim 1, comprising the steps of recirculating liquid from the pressure pipe of the pump to said separator tank, finely diffusing said gas in said recirculated liquid, and evacuating a mixture of diffused gas and liquid by said centrifugal pump.
3. Method according to claim 2, in which said recirculated liquid is mixed with the gas immediately before the inlet of the pump.
4. Method according to claim 1, in which said quantity of liquid which is recirculated to the pump is regulated in direct dependence upon the liquid level in the separator tank.
5. Method according to claim 1, in which the gasliquid mixture is supplied to the pump in such a proportion that the flow of liquid through the pump is adjusted with respect to the liquid level in the separator tank.
6. Method according to claim 1, in which the proportions of gas/liquid in the gas-liquid mixture supplied to the pump are adjusted in direct dependence upon the liquid level in the separator tank.
7. Method according to claim 1, in which at least part of said mixture of recirculated liquid and gas is introduced into the pump directed directly towards the pump wheel inlet to break down possible accumulations of gas therein.
9. Method according to claim 2, in which said recirculated liquid is taken from a point in the pressure pipe of the pump, where the liquid has been caused to separate from the gas. caused 10. Apparatus for emptying a container for liquid, such as a cargo oil tank, comprising a liquid container, a centrifugal pump which has an inlet, a pump impeller and a pressure pipe, for emptying said container, a suction pipe connecting said container to said pump, a separator tank which is interconnected in said suction pipe between said container and said pump for separating gas from the liquid on its way from said container to said pump,
means for mixing gas which is present in an upper portion of the separator tank, in a diffused form into recirculated liquid, said means being separate from and not forming a part of said separator and disposed in a bottom portion of said tank below the normal operating surface of said liquid,
means for recirculating liquid from the pressure pipe of the pump to said mixing means,
pipe means connected to said mixing means and extending into said upper portion of said separator tank wherein said gas is contained for conducting said gas from said separator tank and introducing it into said mixing means, and
means attached to said mixing means, for introducing the gas-liquid mixture thus created into the inlet of the pump.
11. Apparatus according to claim 10, in which said means for mixing gas into the recirculated liquid and for introducing the mixture into the inlet of the pump comprises an ejector means.
12. Apparatus according to claim 10, in which said means for mixing gas into the recirculated liquid comprises a substantially vertical tube which is provided in the separator tank and has a slightly shorter vertical extension than the separator tank, has a plurality of orifices, which are provided in the tube wall above and below each other, respectively, and has its lower end connected through an ejector means to the pressure pipe of the pump by a return duct.
13. Apparatus according to claim 10, comprising a return duct, a control valve in said return duct, a level gauge associated with said separator tank and said valve and operating in dependence of the level upon the liquid surface in said separator tank for opening said valve in the return duct when said liquid surface drops below a predetermined level.