GB2092228A - Centrifugal pumps - Google Patents

Abstract

The pump comprises an impeller 11 provided with a polyurethane layer 22 to resist corrosion due to the abrasive mud which is pumped. The layer may be moulded or cast on a metal core 14, or alternatively the impeller may be cast entirely from polyurethane. The pump housing has a polyurethane or rubber liner 13 and the rear face of the impeller is provided with blades which draw in air through a packing gland 19 to prevent the pumped mud reaching the shaft 15. <IMAGE>

Description

SPECIFICATION Horizontal centrifugal mud pump Background of the Invention Field: This invention relates to mud pumps for surface mud handling systems in the oil drilling industry. It is specifically directed to an improved impeller for a horizontal centrifugal pump of this type.

State oftheArt: Horizontal centrifugal pumps are well known. They are widely used in the mining, chemical and petroleum industries. Each of these industries imposes unique operating conditions on pumping equipment. As a consequence, somewhat different physical types have evolved for special applications. For example, the horizontal centrifugal pumps used in the mining industry characteristically have large impellers which are driven at relatively slow rates of speed. These pumps are necessarily large.

Accordingly, it is practical to use such pumps only when adequate space is available, as is normally the case in mining and chemical plant applications. Drilling rigs, however, include surface mud handling systems which are not well suited for the use of large pump equipment. Accordingly, there presently exist in the field several thousand drilling rigs constructed to accommodate horizontal centrifugal mud pumps of relatively small size. The relatively small size of the impellers permitted by the space restrictions imposed upon such pumps necessitates their operation at relatively high speeds (typically in excess of 1 500 rpm). These high speeds are required to maintain the hydrostatic head (typically 90 feet) demanded by other equipment (e.g., desander cqnes and desilter cones) within the system.Accordingly, the dynamic conditions within the wet end, and particularly those influencing the impeller, of mud pumps for surface mud handling systems of the oil drilling industry are quite dissimilar from those encountered with the larger pumps prevalent in the mining and chemical industries.

The pumping of drilling muds is itself an operation which imposes severe wear conditions on the impeller of a centrifugal pump. The nature of the material being pumped, as well as the high pressure heads which must be produced are dissimilar from those encountered in other industries.

Horizontal centrifugal pumps are often used to pump corrosive or abrasive materials. In the mining and chemical industries, various elastomeric (e.g., Buna elastomer) materials have been used to line the wear surface of the "wet side" of such pumps. These linings have been applied to both the interior surface of the pump chamber and to the exterior of the impeller. They have been effective to substantially increase the useful life of the large, slow speed impellers prevalent in large scale pumps.

Unfortunately, even though elastomer-covered impellers have found wide application in the mining industry, they have not performed satisfactorily when scaled down to fit within the space limitations imposed by surface mud handling systems. Prior art elastomer-coated impellers have not been able to withstandWthe dynamic conditions created when they are sized to fit within the surface mud handling pumps, and operated at the speeds required to maintain the hydrostatic head specified for desilter and desander cores.

Heretofore, the horizontal centrifugal mud pumps used for surface mud handling systems in the drilling industry have included metal impellers.

These impellers have been connected directly to motors operating above 1 500 rpm, typically 1 750 rpm, and have worn very rapidly. Accordingly, it has been an accepted practice on oil drilling rigs to maintain such pumps on a 30 to 60 day scheduie.

Such maintenance sometimes requires the shutdown of the drilling rig, and is extremely costly. Efforts have been made to replace the metal impellers with scaled down versions of the elastomer-covered impellers used in horizontal centrifugal prevalent in the mining industry. Such efforts have resulted in increased wear life, but have resulted in a mode of failure not acceptable to mud handling systems. The elastomer materials customarily used in such equipment to coat jump impellers have a tendency to disintegrate upon failure, thereby introducing foreign matter into the drilling mud. The individual particles resulting from such disintegration are often too small to be efficiently separated from the mud stream. Yet, they are sufficiently large to foul the down hold apparatus. This result cannot be tolerated, even at the gain of extended wear life.

The static wear surfaces of oil field equipment have been lined with elastomeric materials. For such applications, materials such as Buna rubber and certain polyurethanes have been satisfactory.

There remains a need in the oil drilling industry for a mud pump of extended life without the disadvantages associated with the scale down of conventional elastomer-coated impellers.

Summary of the Invention The present invention provides a horizontal centrifugal mud pump constructed within an envelope which permits its ready installation on mud handling systems of conventional drilling rigs.

For example, a typical "5 x 6" pump of this type fits within an envelope approximately 3 feet long by approximately 2 feet wide by less than 2 feet high (typically 20 inches). Such a pump can ordinarily be configurated to replace existing metal impeller pumps without modifications to either the piping or motor coupling available at the rig.

The pump of this invention is capable of handling the abrasive materials associated with the mud handling systems of oil well drilling rigs for relatively long periods without maintenance.

Accordingly, it is a superior pump for desanding, degassing, desilting, charging and mixing applications. It has been found that the mud pumps of this invention will function for five to ten times longer than conventional horizontal centrifugal mud pumps with metal impellers before maintenance is necessary.

The horizontal centrifugal mud pump of this invention is constructed at least superficially similar to the horizontal centrifugal mud pumps most commonly used in the drilling industry. The internal mechanism, especially at the wet end, is somewhat different, however, to provide for a superior seal. For example, it is highly preferred to incorporate a semi-open impeller with pumping blades on its suction side and expelling vanes of longer radius on its drive side. The larger effective diameter of the expelling vanes, as compared to the effective diameter of the pumping blades creates sufficient centrifugal force on the drive side of the impeller to more than offset the discharge pressure and normal suction heads.This pressure differential forces the pumped material away from the stuffing box of the pump, and forms an effective seal, as will be explained in more detail in a later portion of this disclosure. The most notable feature of the improved pump of this invention is its reliance upon carefully selected polyurethane material for the wear surfaces of the impeller. Although it is contemplated that the entire impeller may be case from polyurethane material, it is presently preferred to include within the impeller a rigid metallic core coated or clad with a polyurethane liner or wear surface. In either event, the polyurethane material is selected for resistance to wear and corrosion appropriate for the specific intended application.When drilling muds inhibited by hydrochloric acid, hydrogen sulfide or gels are being pumped in a mud handling system, suitable polyurethane materials include polyether based or polyester based liquid polymers reacted to produce elastomers in the hardness range of approximately 80 to approximately 90 Shore A with a B ashore resilience of at least 50, typically between about 55 and about 60. Examples of suitable prepolymer materials are the polyether based polymers sold under the trademark "VIBRATHANE" B-601 and B-602 by Uniroyal Chemical of Naugatuck, Connecticut and under the trademark "ANDUR" 80-5AP by Anderson Development Company of Adrian, Michigan.

These prepolymers are cured in accordance with the supplier's published literature to produce elastomers within the hardness and rebound ranges selected. A useful polyester based prepolymer is that sold under the trademark "CYANAP REN E" by American Cyanamide of Bound Brook, New Jersey.

Polyurethane materials selected from within the useful range disclosed herein are uniquely suitable materials for impellers subjected to the dynamic conditions encountered in a horizontal centrifugal mud pump operated at high speeds specifically to move drilling muds. While other elastomeric materials provide enhanced wear resistance, experience has shown that they do not produce an impeller which is stable under the conditions encountered in pumps of this type under the dynamic conditions produced by running relatively small impellers at relatively high speeds. Although other elastomeric materials may be used to line the interior of the wet side of the pump, it is convenient and often preferable to rely upon polyurethanes of compositions similar to those selected for the impellers of this invention to line these interior surfaces as well.

Brief Description of the Drawings In the drawings, which illustrate what is presently regarded as the best mode for carrying out the invention, FIG. 1 is a view in cross-section of a typical horizontal centrifugal mud pump of this invention; FIG. 2 is a view in elevation of the drive side of a typical impeller of this invention; FIG. 3 is a view in elevation of the suction side of the impeller of Figure 2; and FIG. 4 is a view in perspective of an impeller similar to that of Figures 2 and 3.

Description of the Illustrated Embodiment Referring to the drawings, a pump 10 of this invention includes an impeller 11 mounted to turn within the wet side 12 of the pump. As illustrated, the wet side 12 is lined with a removable liner 13 to improve its wear resistance. For many oil rig applications, a suitable liner 1 3 may be fashioned from natural or synthetic rubber materials, such as Buna 51-MA. (A Buna elastomer available from The Galigher Company, Salt Lake City, Utah).

The impeller 11 includes a metallic core element 14 mounted to turn on a shaft 1 5 which is journaled through heavy-duty bearings 16, and is sealed from the wet side 12 by a stuffing box 17. This assembly 17 includes a shaft sleeve 18, packing gland 19 and gland bushing 20.

A polyurethane layer 22 is molded or cast about the metal core 14 so that all of the wear surfaces within the wet side 12 of the pump are of an elastomeric material. The liner 1 3 may, but need not be, of similar polyurethane material. An exemplary polyurethane for this purpose is produced from VIBRATHANE B-601.

In the illustrated instance, as may be seen more clearly from FIGS. 2 through 4, the impeller has special pumping blades 25 on its suction side 26 and special expelling blades 27 on its drive side 28. The distal ends 27a of the vanes 27 terminate closer to the perimeter 1 a of the impeller 11 than do the distal ends 25a of the blades 25. The effective diameter of the expelling vanes (the distance through the hub 29 between two distal ends 27a) is thus larger than the effective diameter of the pumping blades. Preferably, the ratio of these diameters is approximately 1.2 + 10%.

Impellers of differing diameters may be interchanged depending upon performance requirements. A typical 5 x 6 pump of this invention may have impellers with shroud diameters ranging from about 9 to about 11 inches. Larger pumps, such as 6 x 8, may receive impellers with shroud diameters up to about 1 5 inches; but such large sizes will ordinarily be spun at lower speeds, e.g., below about 1200 rpm. The illustrated impeller structure, together with the previously noted stuffing box assembly 1 7, creates a particularly advantageous seal not previously available on the mud pumps in customary use on oil rigs. The semi-open impeller structure illustrated (see FIG. 4) creates centrifugal force on the drive side of the impeller which offsets opposing pressures. The material being pumped is thus forced away from the stuffing box behind the hub 29.As mud enters the inlet 30 and exits through the discharge 31, air is drawn through the gland 1 9 to fill the void created by the expelling veins 27. Once this void is filled, no more air can enter the pump. The material, because of its additional weight and stronger response to centrifugal force, acts as a wall which cannot be penetrated by air. Accordingly, the gland is exposed only to atmospheric pressure after the seal is formed, regardless of impeller speed. The seal prevents the intrusion of solids into the vicinity of the shaft, a particularly beneficial feature for pumps handling drilling muds.

From the foregoing, it is clear that the pump cannot leak as long as material is being pumped away from the gland. Separation of material from the gland 19 (i.e., the annular void behind the hub 29 around the shaft 15) will increase as the impeller speed increases. When impeller speed decreases, the outer circumference of the annular seal decreases, thereby forcing air out of the pump through the packing in the gland, The primary purpose of the packing 1 9 is thus to restrict leakage when the pump is not in operation.

Ordinarily, under those conditions the pumped material will drain from the pump through the inlet 30. This seal arrangement is belieVed to be novel in drilling mud applications.

The size, arrangement and spacing of the impeller vanes 27 are important considerations in the impellers of this invention. It has been found that the optimum spacing 40 between the back 41 of the wet side 12 and the back 42 of the vanes 27 is about 1/1 6 inch. (A typical specification is .060 + .020 inches) The depth of the vanes is preferably about 1/4 + 1/1 6 inch.

Although the shape and number of vanes 27 is not criticai, eight radial vanes (FIG. 3) is the presently preferred arrangement. The vanes are usually evenly spaced from adjacent vanes.

Reference herein to certain details of the illustrated embodiments is not intended to restrict the scope of the appended Claims, which themselves recite those details regarded as essential to the invention. It should be recognised that the specific configuration of the impeller may be varied, although the general configuration illustrated by the drawings is presently regarded as preferred. Although it is contemplated that impellers of the type herein disclosed may be useful in centrifugal pumps generally, such impellers are not essential in such applications as they are presently considered to be in the case of small diameter impellers. The use of polyurethane materials is regarded as essential in impellers of diameters below about 1 5 inches required to develop high pressure heads while pumping drilling muds. Such impellers larger in diameter than about 12 inches should ordinarily bedriven at speeds below about 1 200 rpm, while those of smaller diameters may be driven at speeds of 1 500 rpm or more. Direct drive by conventional motors running at 1 150 rpm or 1 750 rpm is thus practical.

Claims (12)

1. In a mud pump for moving drilling muds in the surface mud handling system of a drilling rig of the type including an impeller less than about 1 5 inches in diameter mounted on a shaft to rotate at a speed in excess of about 1200 rpm within the wet side of said pump to pull drilling mud from an intake through the wet side and out an exhaust, the improvement which comprises constructing said impeller such that at least its wear surfaces are of polyurethane elastomer having a Shore A hardness of between about 80 and about 95 and a B ashore resilience of between about 55 and about 60.

2. An improvement according to Claim 1, wherein the impeller is semi-open with extended expelling vanes mounted on its drive side and pumping blades on its suction side, the effective diameters of said vanes being larger than the corresponding effective diameter of said blades so that the suction created by said vanes offsets opposing internal pump pressures, thereby forcing material outward away from said shaft behind the hub of said impeller.

3. An improvement according to Claim 1, wherein said impeller comprises a central rigid core clad in polyurethane material.

4. An improvement according to Claim 1, wherein the internal surfaces of said pumping chamber are lined with elastomeric lining material.

5. An improvement according to Claim 4, wherein said liner is of polyurethane material similar in composition to that of the impeller.

6. An improvement according to Claim 1, wherein said impeller is cast substantially entirely from said polyurethane material.

7. An impeller for a horizontal centrifugal mud pump for moving drilling muds, comprising: a cylindrical shroud structure between about 9 and about 15 inches in diameter having a suction side and a drive side; a plurality of pumping blades extending from said suction side, with distal ends terminating short of the perimeter of said shroud; and a plurality of expelling vanes extending approximately 1/4 inch from said drive side; the wear surfaces of said impeller being of polyurethane elastomer having a Shore A hardness of between about 80 and about 95 and a B ashore resilience of between about 55 and about 60.

8. An impeller according to Claim 7 wherein said vanes are radial, and extend to the perimeter of said shroud.

9. An impeller according to Claim 8 including eight said vanes evenly spaced from adjacent said vanes.

10. An impeller according to Claim 7 wherein said shroud is between about 9 and about 11 inches in diameter, and said impeller is adapted for installation on a shaft driven at speeds above about 1 500 rpm.

11. A mud pump substantially as herein described and shown in the accompanying drawings.

12. An impeller substantially as herein described and shown in the accompanying drawings.