WO1998034030A1

WO1998034030A1 - Centrifugal process pump with auxiliary impeller

Info

Publication number: WO1998034030A1
Application number: PCT/US1998/001900
Authority: WO
Inventors: William E. Murray
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-02-04
Filing date: 1998-02-03
Publication date: 1998-08-06
Anticipated expiration: 1999-08-04
Also published as: AU6260198A

Abstract

A single-stage, overhung centrifugal process pump (10) incorporates a low-flow auxiliary impeller (36) which receives intake liquid from the discharge (16) of the primary impeller (20). This feature saves energy by significantly reducing the pump horsepower requirements when the pump is sized for two or more process conditions, with one discharge flowstream at a higher-head and lower-flow than the other. This feature allows the pump to separate the inlet flow into a primary high-flow, low-head discharge and a secondary lower-flow, higher-head discharge (48). The pump is effectively two pumps in one with each impeller selected for the discrete process requirements of the different flowstreams.

Description

CENTRIFUGAL PROCESS PUMP WITH AUXILIARY IMPELLER

Background of the Invention The present invention relates to a centrifugal process pump. For specific applications, the invention relates to a single stage, overhung type centrifugal process pump incorporating a discharge booster auxiliary impeller.

Centrifugal process pumps are widely used in the petroleum industry, particularly in refining and petrochemical plants. Process pumps operate at temperatures varying from cryogenic to about 800°F, with pressures up to approximately 600 pounds per square inch (psi) and flow rates ranging between ten and 8,000 gallons per minute. The American Petroleum Institute (API) has published pump specification API Standard 610 which outlines basic mechanical, hydraulic and testing requirements for various pump classifications used by the petroleum refining industry. API Standard 610 standardizes safety factors related to pumps, and is generally referenced. In industries outside of the petroleum industry, the American National Standards Institute (ANSI) pump specifications are frequently referenced. The term "booster pump" may refer to a separate pump on the suction / inlet side of a primary pump to increase the net positive suction head (NPSH) to the primary pump. A "suction booster device" can also comprise an inducer incorporated as part of the primary pump to improve the NPSH characteristic. A secondary pump or another impeller, downstream of and in series with the primary pump or primary impeller to increase discharge pressure, may also be termed a booster. One example is disclosed in U.S. Patent No. 4,209,282 to Eberhart. A pump may be classified as a single-stage, with one impeller pumping all the liquid, or as a multi-stage where two or more impellers, in series, pump the same flow of liquid.

Certain multi-stage pumps incorporate booster impellers, in series with the last stage of between bearing pumps, produce a low flow discharge at higher than primary discharge pressure. These pumps are typically very large (3,000 to 50,000 horsepower), high pressure (2,500 to 4,500 psi) double case type. These pumps are typically used for main boiler feed service in electric generating plants. The incorporation of booster impellers in double case pumps requires more horsepower. The booster impeller is incorporated not as an energy saving feature but rather as a means of avoiding the use of an expensive auxiliary pump.

Additionally, centrifugal pump design configurations are classified as "overhung" and "between bearing." "Overhung" pumps are characterized by impellers mounted on a shaft cantilevered from a bearing bracket or mounted on an extended motor shaft where the motor bearings also serve as pump bearings. Overhung pumps may be single or two-stage, may use single or double-suction impellers, and may be mounted with shafts horizontal or vertical. A single-suction impeller has an inlet on one side and a double-suction impeller has inlets on both sides of the impeller. Due to the cantilevered shaft, overhung pumps are typically small (5 to 800 horsepower), the discharge pressure ranges between 30 and 600 psi, and such pumps operate between 1200 and 3600 rpm. Typically, the maximum impeller diameter for an overhung pump operating at 3600 rpm is approximately thirteen to fifteen inches and for an overhung pump operating at 1800 rpm is approximately twenty-three inches. "Between bearing" pumps are characterized by impellers mounted on a shaft supported by bearings on both sides of the impellers. They may be single-stage or multi-stage. This type of pump is typically used where flows, pressures or speeds exceed accepted limits for the less expensive overhung types.

Where suitable, overhung pumps are preferred over the between bearing types because they are less expensive and require only one mechanical seal compared with two mechanical seals for between bearing pumps. In refinery applications complying with API 610, between bearing pumps may cost up to twice that of comparable overhung types.

A primary concern with overhung pumps is minimizing the shaft deflection at the seal and impeller. Pump shaft deflection is affected by various factors including the length and diameter of the cantilevered shaft and the diameter of the pump impeller.

Some pumping systems require pumping a liquid from a single source to two or more different destinations, each having different head-capacity requirements. Such pumping systems require compromises when one head-capacity requirement is a high-pressure (head) at low-flow (capacity) and the second head-capacity requirement is a lower-pressure (head) at a higher-flow (capacity). To accomplish this requirement, two pumps could pump liquids to primary and secondary destinations. Options may be to have separate pumps for each destination, or a separate booster pump driven by a common motor. US Patent No. 4,209,282 to Eberhardt discloses a pump assembly having a primary pump and a booster pump driven from a common rotating shaft. The primary pump delivers low head at high flow and the separate booster pump delivers a higher head at lower flow.

Alternatively, an oversized pump can pump liquids from a single source to two destinations. In this case, the pump must have a head-capacity characteristic enveloping the requirements of both destinations. The pump cannot envelop the high head / low flow requirement without producing excess head at the low head / high flow requirement.

Common practice is to select pumps with head-capacity characteristics enveloping all the various process conditions. This practice results in inefficient pumping systems due to the different head-capacity requirements of the dual destinations. When the pump's head- capacity characteristic must envelop such different rating points, the pump selected will be larger and more costly. This larger pump requires more horsepower delivered to the pump shaft which in turn requires a larger horsepower, more expensive motor. The required throttling control valve also results in additional cost and maintenance.

A single pump assembly which can satisfy the head-capacity requirements of two different flow streams would save energy and reduce operating cost. Preferably, a single pump assembly would produce the primary flow stream and would also produce the separate higher-head, lower-capacity flow stream.

Summary of the Invention The present invention discloses a centrifugal overhung type pump for pumping liquids. The invention comprises a housing having a primary inlet, a primary discharge, a secondary inlet, and a secondary discharge. A rotatable shaft within is engaged with a single primary impeller rotating with the shaft, for receiving the liquid from a primary inlet, and for discharging the fluid through a primary discharge to generate a first liquid stream. A conduit transfers liquid from the primary discharge to a secondary inlet, and an auxiliary impeller proximate to said primary impeller receives liquid from the secondary inlet and discharges the liquid through the secondary discharge to generate a second liquid stream having a pressure greater than the first liquid stream pressure.

In various embodiments of the invention, the auxiliary impeller can have the same or different diameter as the primary impeller, all liquid to the auxiliary impeller is received from the primary discharge, the auxiliary impeller can have an entrance facing opposite or in the direction of the primary impeller, and the auxiliary impeller can comprise a drilled hole type impeller.

Brief Description of the Drawings

Figure 1 is a sectional view in elevation of the centrifugal pump assembly according to a first embodiment of the present invention. The assembly is shown as an overhung, single- stage, end suction pump with an auxiliary impeller.

Figure 2 is a sectional view in elevation of the centrifugal pump assembly according to a second embodiment of the present invention. The assembly is shown as an overhung, inline, single-stage pump with an auxiliary impeller.

Detailed Description of the Preferred Embodiments Figure 1 discloses a centrifugal pump assembly 10 in which the centrifugal pump assembly is shown as an overhung, horizontal, single-stage, end suction, top discharge pump with an auxiliary impeller. Although not illustrated, the pump assembly in Figure 1 could comprise a top suction pump. Figure 2 discloses a centrifugal pump assembly 100 where the pump assembly is illustrated as an overhung, vertical in-line, single-stage pump with an auxiliary impeller. As shown in Figures 1 and 2, the centrifugal pump assemblies 10 and 100 each include a pump housing assembly 11 compromising a pump case 12, a pump case cover 13 and a bearing bracket 32. The pump housing assembly 11 has a primary inlet 14 and a primary discharge 16. The primary inlet 14 includes an inlet passageway 18 which communicates at its downstream end with the entrance 22 of the single-stage, single suction primary impeller 20 as shown in Figures 1 and 2. The primary impeller 20 has an exit 24 communicating with the primary discharge 16.

Referring to Figures 1 and 2, the primary impeller 20 is mounted on a cantilevered portion 28 of a shaft 26. Typically, the shaft 26 in the horizontal single-stage pump with auxiliary impeller in Figure 1 is in a horizontal position whereas the shaft 26 in the in-line single-stage pump with auxiliary impeller of Figure 2 is in a vertical position.

The shaft 26 in Figure 1 includes a second end 30 opposite the cantilevered portion 28 which is adapted to be connected to a prime mover such as a motor for rotating the shaft 26. Alternatively, the shaft 26 could be the motor shaft with the primary impeller 20 mounted thereon as shown in Figure 2. Typically, the primary impeller 20 is secured on the shaft 26 with a primary impeller key received in an elongated slot (not shown) in the shaft 26.

Auxiliary impeller 36 is also mounted on the cantilevered portion 28 of the shaft 26. The auxiliary impeller 36 is secured on the shaft 26 with an auxiliary impeller key received in the elongated slot (not shown) in the shaft 26. Preferably, the auxiliary impeller 36 is positioned on the shaft 26 between the primary impeller 20 and the bearings 34. As shown in Figures 1 and 2, the auxiliary impeller 36 is preferably located proximate to the primary impeller 20 to minimize the overall length of the cantilevered portion 28 of the shaft 26. The shaft 26 is supported by the bearing bracket 32. Preferably, the bearing bracket 32 includes an outboard bearing 34 and an inboard bearing 34' to rotatably support the shaft 26. A seal means 52, typically a mechanical seal, surrounds the shaft 26 between the inboard bearing 34' and the auxiliary impeller 36. Mechanical seals are well known in the art, and the type of mechanical seal 52 may vary. Mechanical seal 52 prevents undesirable leakage of liquid to the atmosphere. In overhung-type pumps only one mechanical seal is required. In between bearing pumps two mechanical seals are required.

In the preferred embodiment of the invention, conduit means 42 are provided for diverting some of the liquid flow from the primary discharge 16 to a secondary inlet or auxiliary inlet 38. Typically, the conduit means 42 comprises tubing or piping. Alternatively, if the auxiliary impeller were installed with the entrance facing toward the primary impeller, primary impeller balance holes and back wear rings would not be used, the flow path to the auxiliary impeller entrance would be internal to the pump housing, and conduit 42 and inlet 38 with passageway 40 would not be used. Liquid could alternatively be introduced to the secondary or auxiliary inlet 38 without passing through the primary impeller 20, as for example, by diverting some of the liquid flow upstream of the centrifugal process pump or the primary impeller 20 directly to the auxiliary inlet 38.

The auxiliary inlet 38 includes an auxiliary inlet passageway 40 which communicates at its downstream end with the auxiliary impeller entrance 44 of the single suction auxiliary impeller 36 as shown in Figure 1. The auxiliary impeller 36 has an exit 46 communicating with a secondary discharge or auxiliary discharge 48.

In the preferred embodiment of the invention, the auxiliary impeller is a drilled hole type impeller. Other types of impellers may be used. The auxiliary impeller 36 in Figures 1 and 2 includes peripheral shoulder 62 extending from the circular disc 54. A first spacer sleeve 63 is mounted on the shaft 26 between the auxiliary impeller 36 and the mechanical seal 52. The auxiliary impeller entrance 44 is defined by the annular spacing between the peripheral shoulder 62 and the first spacer sleeve 63. Opposite the peripheral shoulder 62 is an impeller hub 64 which is secured to the shaft 26.

Referring to Figures 1 and 2, an intermediate cover 66 is positioned between the primary impeller 20 and the auxiliary impeller 36. The intermediate cover 66 is nonrotatably secured to the pump case assembly 12. The intermediate cover 66 includes a first interior annular recess 68 and a second interior annular recess 70. The annular recesses 68 and 70 receive stationary wear rings 69 and 71 , respectfully. A mating rotating wear ring 69' is installed on a neck 72 of the primary impeller 20 and a mating rotating wear ring 71' is installed on the impeller hub 64 of the auxiliary impeller 36.

As shown in Figures 1 and 2, the pair of wear rings 69 and 69' rotatably mate with each other and the pair of wear rings 71 and 71 ' rotatably mate with each other. Each pair of wear rings permit controlled leakage of liquid therebetween.

Referring to Figures 1 and 2, a second spacer sleeve 74 is mounted on the shaft 26 between the primary impeller 20 and the auxiliary impeller 36. Balance holes extend through a rear shroud 78 of the primary impeller to the entrance 22. The balance hole or holes 76 can permit liquid communication between the entrance or suction of the primary impeller 20 and the fluid passing between the pairs of wear rings 69, 69' and 71 , 71'. If the auxiliary impeller 36 is deadheaded to prevent flow therethrough, such as by closing off the auxiliary discharge 48, the controlled leakage through the pair of mating wear rings 71 , 71 ' and the balance holes 76 to the suction of the primary impeller 20 prevents deadheading damage caused by overheating. Balance holes 76 and primary impeller back wear rings 69, 69' are not essential to the practice of the invention and can be eliminated for various embodiments of the invention, such as where the auxiliary impeller entrance faces the primary impeller.

The auxiliary impeller entrance 44 faces opposite the primary impeller as shown in Figures 1 and 2. This reduces the sealing pressure requirement of the mechanical seal 52 to that of the primary discharge pressure. The primary impeller balance holes and back wear rings allow the auxiliary impeller discharge 48 to be run dead-headed providing the primary impeller is protected in accordance with desired minimum flow criteria.

Referring to Figures 1 and 2, the diameter of the auxiliary impeller 36 is approximately the same diameter as the primary impeller 20. The diameter of an impeller has a direct effect on the head produced. Typically, the auxiliary impeller 36 will have a diameter approximately the same as the primary impeller 20 to optimize the design. However, the auxiliary impeller 36 may have a larger or smaller diameter than the primary impeller 20.

Primary discharge 16 is a high-flow, low-head discharge and the auxiliary discharge 48 is a lower-flow, higher-head discharge. The centrifugal pump assembly with auxiliary impeller 10, 100 is effectively two pumps in one case cover assembly 12, 13 with the pump assembly 10, 100 having a common suction 18 and two separate discharges 16 and 48. Such pump assemblies do not comprise conventional two stage pumps because the primary and auxiliary impellers 20 and 36 do not pump the same flow.

Because certain pump requirements do not allow impellers larger than thirteen inch diameter impellers for overhung, 3600 rpm pumps, the present invention uniquely provides an alternative pump which reduces operating costs and is less expensive than between bearing pumps. Although the invention has been described in terms of certain preferred embodiments, it will be apparent to those of ordinary skill in the art that modifications and improvements can be made to the inventive concepts herein without departing from the scope of the invention. The embodiments shown herein are merely illustrative of the inventive concepts and should not be interpreted as limiting the scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A centrifugal pump for pumping a liquid, comprising: a housing having a primary inlet, a primary discharge, a secondary inlet, and a secondary discharge; a rotatable shaft within said housing; an overhung, single primary impeller rotatable with said shaft for receiving the liquid from said primary inlet and for discharging fluid through said primary discharge to generate a first liquid stream; a passage for transferring liquid from said primary discharge to said secondary inlet; and an auxiliary impeller proximate to said primary impeller and rotatable with said shaft for receiving liquid from said secondary inlet and for discharging liquid through said secondary discharge to generate a second liquid stream having a pressure greater than said first liquid stream pressure.

2. A pump as recited in Claim 1 , wherein said auxiliary impeller receives liquid through an auxiliary impeller entrance.

3. A pump as recited in Claim 1 , wherein said auxiliary impeller is substantially the same diameter as said primary impeller.

4. A pump as recited in Claim 1 , wherein all liquid to said secondary inlet is received from said primary discharge.

5. A pump as recited in Claim 1 , wherein said passage for transferring liquid from said primary discharge to said secondary inlet comprises an opening through a conduit.

6. A pump as recited in Claim 1 , wherein said auxiliary impeller has an entrance at said secondary inlet facing toward said primary impeller.

7. A pump as recited in Claim 1 , wherein said auxiliary impeller has an entrance at said secondary inlet facing opposite said primary impeller.

8. A pump as recited in Claim 1 , wherein said auxiliary impeller comprises a drilled hole type impeller.

9. An auxiliary impeller as recited in Claim 8, wherein said auxiliary impeller comprises a cylindrical disc having a plurality of radial bore extending from an auxiliary impeller entrance to an outer peripheral surface of said cylindrical disk.

10. An overhung, centrifugal pump for converting a liquid into two liquid streams at different pressures, comprising: a housing having a primary inlet, a primary discharge, a secondary inlet, and a secondary discharge; a rotatable shaft having an overhung shaft section within said housing; a bearing bracket for supporting said overhung shaft section; an overhung, single primary impeller engaged with said overhung shaft section, and rotatable with said shaft, for receiving the liquid from said primary inlet and for discharging fluid through said primary discharge to generate a first liquid stream; a passage for transferring liquid from said primary discharge to said secondary inlet; and an auxiliary impeller proximate to said primary impeller and rotatable with said shaft for receiving a portion of the liquid at said secondary inlet and for discharging a second liquid stream from said secondary discharge at a pressure greater than said first liquid stream pressure.

11. A pump as recited in Claim 10, wherein said auxiliary impeller is substantially the same diameter as said primary impeller.

12. A pump as recited in Claim 10, wherein all liquid to said secondary inlet is received from said primary discharge.

13. A pump as recited in Claim 10, wherein said auxiliary impeller has an entrance at said secondary inlet facing opposite said primary impeller.

14. A pump as recited in Claim 10, wherein said auxiliary impeller has an entrance at said secondary inlet facing toward said primary impeller.

15. A pump as recited in Claim 10, wherein said auxiliary impeller comprises a drilled hole type impeller.