US3241295A

US3241295A - Drilling mud degassers for oil wells

Info

Publication number: US3241295A
Application number: US308314A
Authority: US
Inventors: Iii Phil H Griffin; Jr John W Melton
Original assignee: Bass Brothers Enterprises Inc
Current assignee: Bass Brothers Enterprises Inc
Priority date: 1963-09-11
Filing date: 1963-09-11
Publication date: 1966-03-22
Anticipated expiration: 1983-03-22

Description

' March 22, 1966 P. H. GRIFFIN Ill, ETAL DRILLING MUD DEGASSERS FOR OIL WELLS Filed Sept. 11, 1963 4 Sheets-Sheet 1 PHIL H. GRIFFIN,||I JOHN w. MELTON,JR..

z W y March 22, 1966 P. H. GRIFFIN 1 11, ETAL 3,241,295

DRILLING MUD DEGASSERS FOR OIL WELLS Filed Sept. 11, 1963 4 Sheets-Sheet 2 PHIL H. GRIFFIN, III

JOHN W. MELTON, UR.

P. H. GRIFFIN m, EI'AL 3,241,295

DRILLING MUD DEGASSERS FOR OIL WELLS 4 Sheets-Sheet 5 //1Ve/7/b/1s.-

PHIL H GRIFFIN m JOHN W. MELTON, JR.

March 22, 1966 Filed Sept. 11 1963 March 22, 1966 P. H. GRIFFIN m, ETAL 3,241,295

DRILLING MUD DEGASSERS FOR OIL WELLS Filed Sept. 11 1963 4 Sheets-Sheet 4 INVENTORS:

PHIL H. GRIFFIN, III

JAKE MELTON, JR. By: W F%' W Km AT'T'Y.

IOO

United States Patent 3,241,295 DRILLING MUD DEGASSERS FOR OIL WELLS Phil H. Griflin III, Fort Worth, Tex., and John W. Melton, Jr., Lafayette, La., assignors to Bass Brothers Enterprises, Inc., Fort Worth, Tex.

Filed Sept. 11, 1963, Ser. No. 308,314 8 Claims. (Cl. 55-165) This application is a continuation-in-part of our earlier application Serial No. 37,685, filed June 21, 1960, and application Serial No. 299,317, filed August 1, 1963, both now abandoned.

This invention relates to new and useful improvements in drilling mud degassers for oil wells of the general type disclosed, for example, in US. Patents No. 2,748,884 dated June 5, 1956, and No. 2,869,673 dated January 20, 1959, to R. W. Erwin.

The degassers disclosed in those patents are mounted above the level of mud in the mud trough, and an object of the present invention is to facilitate positioning of the degasser either beside or below as well as above the mud level, as is best suited for particular operating conditions.

A feature of the invention resides in the provision of means comprising an automatically actuated valve for controlling the level of mud in the degasser tank, so that even if the tank is disposed at or below the level of mud in the trough, it is positively safeguarded against flooding.

Another feature of the invention involves a novel structural arrangement of the degasser tank per se, the same being vertically rather than horizontally disposed and containing a set of vertically spaced baflles over which the mud flows in a casading manner to facilitate and expedite degassing thereof.

Another feature of the invention, contemplated by a modified embodiment thereof, resides in the provision of a vacuum actuated pump of a novel construction, forcausing the flow of mud through the degasser tank, as distinguished from the jet type propelling means disclosed in the aforementioned patents.

Another feature of the invention, contemplated by another modified embodiment, facilitates the use of a centrifugal pump for causing the flow of mud through the degasser tank, with valve means for effectively controlling the mud flow in relation to the mud level in the tank.

Wiih the foregoing object and features in view and such other objects and features as may become apparent as this specification proceeds, the invention will be understood from the following description taken in conjunction with the accompanying drawings, wherein like characters of reference are used to, designate life parts, and wherein:

FIGURE 1 is an elevational view of the degasser apparatus, with certain portions thereof shown in section;

FIGURE 2 is a vertical sectional view of the degasser tank;

FIGURE 3 is an elevational view, similar to that shown in FIGURE 1 but on a reduced scale and showing a modified embodiment of the invention;

FIGURE 4 is an elevational view, partly in section, of the pump used in the embodiment of FIGURE 3;

FIGURE 5 is a bottom plan view of the pump;

FIGURE 6 is a fragmentary sectional view on an enlarged scale, taken substantially in the plane of the line 6-6 in FIGURE 4;

FIGURE 7 is a view, partly in vertical section and partly in elevation, showing another modified embodiment; and

FIGURE 8 is a diagramatic view showing connections of a valve used in the embodiment of FIGURES 4-6.

Referring now to the accompanying drawings in detail, more particularly to FIGURES l and 2, the mud deice gasser apparatus in accordance with the invention is designated generally by the reference numeral 10 and, principally, embodies in its construction a de-gasser tank 11 which may be positioned above, below, or, as shown, at the level 12 of mud in a tank 13 into which the mud is discharged from the well through a pipe 14, the same mud level 12 also existing in a tank 15 from which degassed mud is subsequently pumped or otherwise delivered through a pipe 16 back to the well.

The degasser tank 11 comprises a cylindrical, vertically disposed body mounted by suitable supporting brackets 17 on a platform or skid 18 and provided with a removable cover 11a to facilitate access to the interior thereof for purposes of inspection or repair. A tubular adapter 19 extends into the tank through the cover 11a and is connected to an inlet pipe 20 which communicates with the tank 13 as shown. A butterfly valve 21 is provided in the adapter 19 for actuation by an arm 22 as will be hereinafter described, the purpose of this valve being to regulate the volume of mud passing into the degasser tank 11 from the tank 13, whereby to prevent. the degasser tank from flooding.

The lower end of the tank is provided with an adapter 23 connected to a discharge pipe 24 which communicates with the tank 15, the pipe 24 being shown as provided, as in the aforementioned Erwin patents, with a jet 25 for propelling the degassed rnud therethrough and simultaneously causingmud to be drawn into the trough and from the tank 13 through the inlet pipe 20 into the tank 11, as set forth in the aforesaid patents. The lower end of the pipe 24 may be equipped with a wear plate 26.

The interior of the degasser tank 11 is shown as provided with a set of vertically spaced bafiles, namely, a conical upper baflle 27, an inverted frusto-conical inter mediate baflle 28, and a conical lower baflle 29. The upper baffle 27 is equipped with an annular rim 30 which is spaced outwardly from the marginal edge ofthe baflle by a set of supporting straps 31, so that mud flowing downwardly through the adapter 19 may, after becoming deposited on and evenly distributed over the b-aflle 27, cascade downwardly as indicated at 32 onto the marginal edge portion of the baflle 28.

The marginal edge of the baffle 28 is provided with an upstanding flange or rim 33, within which there is provided an apertured pipe or conduit 34 whereby water under pressure may be discharged onto the baflle 28 for purposes of washing the interior of the tank, or whereby suitable chemicals may be introduced for mixing with the mud, if so desire-d. The water or chemicals are delivered to the conduit 34 from a suitable source exteriorly of the tank by a suitable pipe (not shown). The battle 28 is provided with a central opening 35 whereby mud may flow onto the lower bafile 29, for subsequent accumulation in the bottom of the tank.

It is to be noted that the slope of the intermediate baffle 28 is greater than that of the upper baflle 27 and that the slope of the lower baffle 29 is also greater than that of the baflle 28. Thus, in flowing downwardly from the adapter 19, the mud first is evenly spread over the upper baffle 27, then re-directed toward the center of the tank by the intermediate baflle 28, and then again spread evenly over the lower baffle 29, until it drops to the bottom of the tank. The tank bottom, also of inverted conical form, safeguards against sediments dropping out of the mud, and the different slopes of the several baflles, as above noted, accelerate the flow of mud during its cascading over the baflies, whereby a self-cleaning action of the baffies is produced.

The pressure within the degasser tank 11 is normally maintained at less than atmospheric pressure in order that gas bubbles entrained in the mud passing through the tank will expand and break away from the mud. The subatmospheric pressure within the tank is produced not only by the jet discharge 25 of the mud from the tank, but also and, mainly by a suitable pump 36, mounted on the platform or skid 18, and driven by a suitable motor 37. The gases released from the mud in the tank 11 enter the open, lower end of a tube 38 which extends from the underside of the baffle 29, through the

baflles

29, 28, 27 and through the tank cover 11a to a conventional threeway valve 39 mounted by a bracket 40 on top of the cover 11a. The valve 39 also has a port 41 communieating with the atmosphere and a suction line or pipe 42 extending to a conventional float-actuated liquid trap 43 mounted on the skid 18. The trap 43 communicates through a pipe 44 with the pump 36, while the exhaust of the.pump to the atmosphere is indicated at 45. If de-. sired, a shut-off valve 46, a sight glass 47 and a pressure gauge 48 may be provided in the line 42, as shown.

Sub-atmospheric pressure within the tank 11 is regulated by the valve 39 in response to raising and falling of a float 49 on the mud level 50 in the bottom of the tank 11. The float 49 is secured to the lower end of a rod 50 which extends slidably through the

batfles

29, 28, 27 and through a packing gland 51 in the tank cover 11a, being operatively connected at its upper end as at 52 to an actuating arm 53 of the valve 39. As the mud level 50 in the tank rises, the float 49 causes the three-way valve 39 to open the port 41 to the atmosphere while at the same time cutting off communication through the line 42 to the pump 36, thus allowing atmospheric pressure to enter the tank and reduce the vacuum therein. On the other hand, when the mud level falls, the valve port 41 is closed, and a greater sub-atmospheric pressure is produced in the tank by suction of the pump 36 through the

lines

44, 42 and 38.

The aforementioned butterfly valve 21 in the adapter 19 is opened and closed automatically in response to air pressure within the tank 11, that is, in response to variations in pressure of air within the tank, this being effected by the provision of a vacuum cylinder 54 which is pivotally mounted as at 55 on the tank cover 11a and includes a piston 56 with a piston rod 57 pivoted as at 58 to the valve actuating arm 22. The cylinder 54 is connected by a vacuum line 59 to the tube 38 and as the pressure within the tube 38 and tank 11 decreases, the piston 56 is retracted into the cylinder 54 and causes the valve 21 to open, thus permitting more mud to enter the tank. On the other hand, when pressure within the tank increases, outward sliding of the piston 56 in the cylinder 54 causes the valve 21 to close, thus restricting the flow of mud into the tank accordingly.

Means are provided for biasing the valve 21 to its closed position, such means comprising a cylinder 59', pivotally mounted as at 60 on the tank cover 11a adjacent the cylinder 54, and having a reciprooable piston 61, springbiased by a compression spring 62, with a piston rod 63 extending from the piston 61 and pivoted to the arm 22 as at 64.

In order to prevent degassed mud from flowing back to the tank 11, a suitable check valve 65 is provided in the discharge pipe 24 between the adapter 23 and the jet 25.

When the apparatus is in operation, the rate of flow of mud therethrough is controlled by the action of the jet 25, regulated by a suitable valve 66, whereby the rate of mud flow out of the tank and the mud level in the bottom of the tank are maintained substantially constant. How ever, if the inflow of mud should become greater than the outflow, rising of the mud level in the tank will cause the float 49 to vent the valve 39 to the atmosphere, and the resultingly increased pressure of air in the tank and in the

lines

38, 59 will cause the piston 56 in the cylinder 54 to close the valve 21, thus reducing or shutting off the inflow of mud through the adapter 19. Conversely, with the inlet of mud reduced or stopped, lowering of the mud 4 level 50 in the tank will produce a greater vacuum in the tank, thus causing the cylinder 54 to open the valve 21 to permit a greater inflow of mud into the tank. As a combined result of these automatic operations, both the mud level and sub-atmospheric pressure within the tank will be maintained substantially constant.

The purpose of the trap 43 is to prevent liquids from entering the vacuum pump 36, the trap fulfilling this function by collecting and storing the undesired liquid which may be occasionally drained from the trap through a drain valve 99. The latter is manually operated and instances may arise when draining of the trap is overlooked, in which event an excessive amount of liquid accumulated in the trap will cause the float in the trap to close a valve on the line 42, thus preventing further vacuum to form in the degasser tank 11. In order to eliminate this difficulty and to provide for automatic draining of the trap 43, a check valve 100 is connected to the trap 43 so that it communicates with the trap even when the drain valve 99 is closed, and a line 101 extends from the check valve 100 to the interior of the degasser tank, as shown. The valve 100 permits liquid to flow from the trap 43 to the tank 11 only. When the pump 36 is stopped, the vacuum in the line 42 will leak off more rapidly than the vacuum in the tank 11 and when this condition arises, the liquid accumulated in the trap will flow through the check valve 100 and line 101 into the tank 11, thus automatically draining the trap.

Referring now to FIGURES 3-6 which show a modified embodiment of the invention denoted generally by the reference numeral 70, the same is adapted to be mounted above the level of mud in a mud trough 71 having inlet and outlet tanks or sumps at opposite ends thereof corresponding to the

tanks

13 and 15 of FIGURE 1, with the degasser tank 11 being arranged substantially as described in connection with the embodiment 11).

However, in place of the jet 25 for propelling the mud through the degasser tank, a vacuum pump designated generally by the numeral 72 is employed. This pump is mounted above the mud trough 71 adjacent the tank 11 and comprises a suitable housing 73 having a partition 74 therein which separates the interior of the housing into a pair of

chambers

75, 76. The bottom of the housing 73 is provided with a pair of door openings having a pair of

doors

77, 78 in communication with the

respective chambers

75, 76, the

doors

77, 78 being hingedly mounted as indicated at 79 so that they are normally urged by gravity to their closed position. A manifold 80 is provided at the underside of the housing 73 in communication with the

doors

77, 78 and is connected by a duct or pipe 24a to the mud outlet of the degasser tank 11.

The housing 73 also has a front wall provided with openings for a pair of

doors

81, 82 in communication with the

respective chambers

75, 76, these doors being hingedly mounted as at 83 so that they are normally urged by gravity to their closed position.

In this embodiment, the aforementioned motor 37, the vacuum pump 36 and the liquid trap 43 are mounted on top of the housing 73 and the suction line 42 which extends between the valve 39 on the tank 11 and the liquid trap 43 is provided with a T 84 having a branch suction line 85 connected thereto. The suction line 85 communicates with one port of a four-way valve 86, also mounted on top of the housing 73, while the exhaust line 87 of the pump 36 communicates with another port of this valve. The two remaining ports of the four-way valve 86 have

conduits

88, 89 connected thereto for communication with the

respective chambers

75, 76, it being understood that in one position of the valve 86, air may be withdrawn from the chamber 75 through the conduit 88 and

lines

85, 42, 44 while air under pressure from the pump 36 is delivered into the chamber 76 through the

lines

87 and 89. Conversely, in a second position of the valve 86, air may be withdrawn from the chamber 76 and delivered under pressure into the chamber 75.

The valve 86 is actuated by an arm 90 which has operatively connected thereto as at 91 the upper end of a rod 92 which extends into a cylinder 93. The latter is provided at the outside of the housing 73 and communicates with the chamber 75 by a pair of connecting passages. 94, 95. A mud level responsive float 96 is slidable in the cylinder 93 on the rod 92 and when the float rises sufficiently high in the cylinder, it comes in abutment with an adjustable stop 97 on the rod 92, whereupon further upward movement of the float causes the rod 92 to slide upwardly to actuate the valve 86 through the medium of the arm 91 In operation, mud under sub-atmospheric pressure discharged from the degasser tank 11 stands in the pipe 24a and manifold 80, while the pump 36 exhausts air from the chamber 75 through the valve 86 and conduit 88. When the vacuum in the chamber 75 is greater than that in the manifold 80, the door 77 is caused to open, with the result that the mud enters through the open door into the chamber 75. With the rising of the mud in the chamber 75 and in the cylinder 93 communicating therewith, upward movement of the float 96 eventually reverses the valve 86, so that air under pressure from the pump 36 is delivered into that chamber, As this occurs, the door 77 is automatically closed and the mud in the chamber 75 flows by gravity through the door 81 into the discharge sump of the mud trough 71. With the discharge of mud from the chamber 75 and cylinder 93, lowering of the float 96 causes the same to engage another stop 98 on the lower portion of the rod 92, thus sliding the rod downwardly and returning the valve 86 to its position where suction is again applied to the chamber 75 for the next cycle of operation.

The pumping operation in the chamber 76 is reverse to that in the chamber 75, that is, while mud is being discharged from the chamber 75 through the door 31, mud is being drawn into the chamber 76 through the door 78, and conversely, when mud is drawn into the chamber 75 through the door 77, mud is discharged from the chamber 76 through the door 82. As a result, the flow of mud through the pump 72 is substantially continuous by virtue of the two, reversely acting pump chambers.

Referring now to FIGURE 7, the same illustrates another modified embodiment of the invention including a degasser tank 111 through which mud is propelled by a centrifugal pump 112 connected to the mud outlet 24 of the tank, as by a pipe or conduit 113. To the extent that construction and arrangement of the tank 111 is similar to the degasser tank 11, corresponding reference numerals are used in the illustration of FIGURE 7.

The float 49 on the mud level 50 in the tank 11 is carried by a vertically swingable arm 114 which operates the three-way valve 115 mounted exteriorly on the side of the tank. The arm 114 extends from the valve into the tank through an opening 116 in the tank Wall, such opening being covered by the valve 115 to prevent leakage of air therethrough. The valve 115 is connected to the suction pump by a suction line 42, has a port 41 vented to the atmosphere, and a suction pipe 38 which communicates with the interior of the degasser tank. The operation of the float-controlled valve 115 is the same as that of the aforementioned valve 39, but it is to be noted that since the valve 115 is mounted at the side of the tank rather than on top of the tank cover 11a, it is not necessary to pass the float arm 114 and the suction pipe 38 through the

various baflles

27, 28, 29, as in the embodiment of the tank 11.

A pressure-responsive governor 117 of a conventional, commercial type is mounted as by a bracket 118 on the tank cover 11a and is operatively connected as by linkage 119 to the arm 22 of the mud inlet valve 21 at the top of the tank. The governor 117 has a pilot control 120 mounted at the side of the tank 111 and connected by a line 121 to the vacuum pump or to the line 42, as desired. The connection between the governor 117 and the control is by a line 122, and a vacuum line 123 extends from the control 120 to the interior of the tank.

When the mud level in the tank falls and the air pressure in the tank is decreased by the valve 115, the air pressure change is sensed by the pilot control 120 through the line 123 and the control responds by communicating the line 122 with the suction line 121 so that the governor 117 opens the valve 21 and admits more mud into the tank. Conversely, when the mud level in the tank rises, the governor closes the valve 21 to decrease or stop the inflow of mud into the tank.

A similar governor 124, connected by a line 125 to the control 120, may be used to operate a valve 126 in the discharge 127 of the pump 112 by which the mud is drawn through the tank 111 and delivered to the mud sump. The arrangement is such that when the mud level in the degasser tank falls, the regulator 124 closes the discharge valve 126, thus permitting the mud in the tank to rise to a higher level while the mud inlet valve 21 is opened by the regulator 117. On the other hand, if the mud level rises, the regulator 124 will open the discharge valve 126, thus permitting excess mud to be Withdrawn from the tank by the pump 112 while the inflow of mud into the tank is decreased or shut off by closing of the valve 21 by the regulator 117.

The mud valve 126 may be located downstream from the pump 112 as shown, or between the pump 112 and the tank outlet 24, or in a by-pass around the pump, as dis closed in the aforementioned application filed August 1, 1963.

While in the foregoing there have been shown and described the preferred embodiments of the invention, various modifications may become apparent to those skilled in the art to Which the invention relates. Accordingly, it is not desired to limit the invention to this disclosure and various modifications and equivalents may be resorted to, falling within the spirit and scope of the invention as claimed.

What is claimed as new is:

1. In a mud degasser apparatus, the combination of a mud degasser vacuum tank having .a mud inlet and a mud outlet for continuous flow of mud through the tank, means for maintaining the interior of said tank at sub-atmospheric pressure, valve means provided in said mud inlet for controlling the rate of flow of mud into the tank, means responsive to variations of level of mud in said tank for varying the sub-atmospheric pressure in the tank, and means responsive to variations of pressure in the tank for opening and closing said valve means.

2. In a mud degasser apparatus, the combination of a mud degasser vacuum tank having a mud inlet and a mud outlet for continuous flow of mud through the tank, means for maintaining the interior of said tank at sub-atmospheric pressure, valve means provided in said mud inlet for controlling the rate of flow of mud into the tank, means responsive to variations of level of mud in said tank for varying the sub-atmospheric pressure in the tank, and means responsive to variation of pressure in the tank for opening and closing said valve means, said pressure responsive means comprising a cylinder having piston means operable therein for actuating said valve means and having a conduit at one end thereof connecting with the interior of said tank for actuating said piston means.

3. In a mud degasser apparatus, a mud degasser vacuum tank having a mud inlet at the top and a mud outlet at the bottom thereof, a plurality of vertically spaced cascade baflles provided in said tank in the path of mud flowing from said inlet to said outlet, the slope of the baffles being progressively steep in a downwardly direction to render the same self-cleaning, a vacuum pump, a suction line extending from said pump to the interior of said tank, a valve provided in said suction line for selectively communicating the interior of the tank with said pump and with the atmosphere, float means responsive to variations of mud level in the bottom of the tank for actuating said valve, valve means provided in the inlet of said tank for controlling the rate of flow of mud thereinto, and means responsive to actuation of said valve for actuating said valve means.

4. In a mud degasser apparatus, the combination of a mud degasser vacuum tank disposed in a vertical position and having a mud inlet and a mud outlet respectively at the top and bottom thereof for continuous flow of mud through the tank, a vacuum pump, a suction line extending from said pump to the interior of said tank for maintaining the same at sub-atmospheric pressure, a valve provided in said suction line for selectively communicating the interior of the tank with said pump and with the atmosphere, float means responsive to variations of mud level in the bottom of the tank for actuating said valve whereby the sub-atmospheric pressure in the tank may be maintained substantially constant, valve means provided in the inlet of the tank for controlling the rate of flow of mud thereinto, and means responsive to variations in the sub-atmospheric pressure in the tank for actuating said valve means, whereby the level of mud in the bottom of the tank may be maintained substantially constant.

5. The device as defined in claim 4 wherein said last mentioned means include a vacuum cylinder in communication with the interior of said tank, said cylinder having a reciprocable piston rod operatively connected to said valve means.

6. In a mud degasser apparatus, the combination of a vacuum tank having a mud inlet and a mud outlet, for continuous flow of mud through the tank means for maintaining air pressure in said tank below that of the atmosphere, valve means provided in said mud inlet for controlling the rate of flow of mud into the tank, means responsive to raising of the mud level in the tank for increasing the air pressure in the tank and conversely for de creasing the air pressure in response to lowering of the mud level, and means responsive to increasing and decreasing of the air pressure in the tank for respectively closing and opening said valve means.

7. In a mud degasser aparatus, the combination of a vertically disposed vacuum tank having a mud inlet and a mud outlet respectively at the top and bottom thereof for continuous flow of mud through the tank, a gas suction line extending outwardly from said tank, a vacuum pump, a three-way valve for communicating said suction line selectively with said vacuum pump and with the atmosphere to respectively decrease and increase the air pressure in said tank, valve means provided in said mud inlet for controlling the rate of flow of mud into the tank, a float provided in the tank and operatively connected to said three-way valve whereby to respectively increase and decrease the air pressure in the tank when the mud level in the tank rises and falls, and a fluid operator connected to said suction line and to said valve means for respectively closing and opening the valve means in response to an increase and decrease of air pressure in said tank.

8. A mud degasser vacuum tank having a mud inlet and a mud outlet for continuous flow of mud through the tank, means for maintaining the interior of said tank at sub-atmospheric pressure, valve means provided in said mud inlet for controlling the rate of flow of mud into the tank, means responsive to variations of level of mud in the tank for varying the subatmospheric pressure therein and means responsive to variations in subatmospheric pressure for actuating said valve means.

References Cited by the Examiner UNITED STATES PATENTS 1,268,594 6/1918 MacKenzie 103-236 1,518,890 12/1924 Aikman 103-236 1,556,098 10/1925 Gibson 55-39 1,708,471 4/1929 Farnsworth 103-246 1,710,178 4/1929 McMurray 55-206 2,015,127 9/1935 Rieger 103-236 2,126,596 8/1938 Wood 55-193 2,195,898 4/1940 Newton 55-190 2,748,884 6/1956 Erwin 55-193 2,815,765 12/1957 Adelson 137-205 2,869,673 1/1959 Erwin 55-192 REUBEN FRIEDMAN, Primary Examiner.

Claims

1. IN A MUD DEGASSER APPARATUS, THE COMBINATION OF A MUD DEGASSER VACUUM TANK HAVING A MUD INLET AND A MUD OUTLET FOR CONTINUOUS FLOW OF MUD THROUGH THE TANK, MEANS FOR MAINTAINING THE INTERIOR OF SAID TANK AT SUB- ATMOSPHERIC PRESSURE, VALVE MEANS PROVIDED IN SAID MUD INLET FOR CONTROLLING THE RATE OF FLOW OF MUD INTO THE TANK,