LOW FLOW-HIGH PRESSURE CENTRIFUGAL PUMP

Abstract

Low flow centrifugal pump that develops more than 25 percent greater pressure than equivalent standard type spiral impellers with circular casings.

Description

BACKGROUND OF THE INVENTION

There has been a market for very low flow centrifugal pumps used on low viscosity liquids such as water, chemicals or hydrocarbons, but these products have not been available.

The centrifugal pump basically consists of a rotating impeller within a volute as shown in FIG. 1. The majority of the volutes are designed like a logarithmic spiral as shown in FIG. 1a. The shape of the impeller and volute depends on the combination of desired flow, pressure and speed. In the industry of centrifugal pumps, the combination of the three criteria in a specific equation known throughout the industry is called specific speed (Ns). The range of this dimensionless number is 500 to over 4000. Pumps with Ns of 500 have a casing that has a narrow width and spiral shape that for all intents and purpose is a circle. In contrast, a pump with Ns of 4000, has a casing width that is relatively wide and the spiral is similar to Nautilus shell as shown in FIG. 2. To obtain this shaped casing, it is usually made from a casting. Within the spiral, as the impeller rotates, 25% of the pumped liquid is collected in the first 90 degrees of the spiral, 50% is collected by 180 degrees, 75% at 270 degrees and 100% at the 360 degrees or end of the spiral. Where the spiral ends, is called the throat. This is where, in theory, 100% of the liquid goes into the discharge nozzle. The bottom of the throat is called the cutwater. This is also the beginning of the spiral, hence the name. The casing of a 4000 Ns pump has the biggest spiral, but the least circular leakage. At the other end of the range, a pump with 500 Ns speed or less has so little spiral that the inside of the casing is made circular as shown in FIG. 3. By making the volute circular, the inside surface can be machined resulting in a smooth surface, which increases the efficiency.

The intersection of the circular shape casing with the tangential discharge hole has no defined cutwater, just a tangential hole to the circle. This produces a lot of circular leakage; resulting in low hydraulic efficiency. However, even with the low efficiency there is still a market for a low flow pump.

Similarly, the impellers of a specific speed range change shape with a change in number. A 4000 Ns impeller is relatively wide and with a swooping profile from the inlet to the periphery; while one with a 500 or less Ns is very narrow with the passage ways almost radial to the shaft centerline (FIG. 4). Traditionally, the impellers of 4000 Ns may have 7 to 8 spiral vanes that include 100 to 120 degrees of the circumferential profile. 500 or less Ns have only 3 to 4 vanes with long spiral vanes that may encompass 270 degrees of the profile. In the design of a pump there are hydraulic force that are produced that create hydraulic forces that react on the support bearing of the shaft. These are axial forces that are parallel to the shaft, radial forces that act perpendicular to the shaft and coupling forces that acts as a twisting torque on the end of the shaft.

The axial force can be the greatest force. To reduce it, a designer may employ an enclosed impeller, which means that the hydraulic vanes of the impeller are between two disks, called shrouds. As the liquid exits the vane the hydraulic pressure goes down the sides of the shrouds and counter acts the axial thrust one either side, reducing the thrust. This is a good solution for pumps that have Ns above 1000. Below that, the ratio of width to impeller diameter, number of vanes and wrap around of the vanes makes it very difficult to make a casting. Sometimes a two piece impeller can be molded from a polymer and then attached together. The cost of the molds, cleaning techniques, and quality of the attachment cannot always justify the costs. Another option is to design the impeller as an open impeller. This utilizes only one shroud, which has an open thee so the cast or molded vanes are fully exposed. Because there is only one shroud, a high portion of the developed pressure of the vanes only is on back side of the impeller and results in high axial thrust. Designers reduce the pressure acting on the shroud by scalloping (cutting out material of the shroud). This does results in negative features; mechanically it is difficult to maintain the flatness of the impeller, it also weakens the support of the vanes, hydraulically it reduces the efficiency of the pump. The open face of the vanes is normally machined to have a flat surface. This surface is matched to a flat corresponding stationary surface. The clearance between the rotating impeller and stationary surface is about 0.10″. When this clearance is not maintained the hydraulic performance and efficiency is reduced.

This whole process gas more difficult to produce when the Ns is 500 or less.

BRIEF DESCRIPTION OF THE INVENTION

This present invention relates to pumps with a specific speed of less than 500. A pump according to the invention has a circular casing with replaceable cutwater with close clearance to the periphery of the impeller that increases the developed pressure. The inlet for the casing has a singular suction, with a double suction impeller. The double suction impeller reduces axial thrust. However, a singular suction casing is less expensive and easier to install than a casing designed specifically for a double suction impeller.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction with the appended drawing figures wherein like numerals denote like elements.

FIG. 1 shows a volute and spiral vane impeller;

FIG. 1 a shows the basic profile of a spiral volute;

FIG. 2 shows the profile and end shape of volutes for various specific speeds;

FIG. 3 shows the profile of a circular volute;

FIG. 4 shows the change in impeller profile with change in specific speed;

FIG. 5 are left side and front views of the double suction impeller;

FIG. 6 is a composite view of the replaceable cutwater and its various features;

FIG. 7 shows the assembly of the circular volute with a replaceable cutwater and single inlet, double suction, radial slot with recirculation slots impeller;

FIG. 8 a shows an impeller with spiral vanes;

FIG. 8 b shows an impeller using slots;

FIG. 9 is a photograph of a test set-up using an apparatus according to the present invention

FIG. 10 a and FIG. 10b are photographs respectively of the back and the front surface an impeller according to the present invention;

FIG. 11 a and FIG. 11b are respective side views of the replaceable cutwater according to the present invention; and

FIG. 11 c is a top view of a replaceable cut water according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention pertains to pumps in the Ns range of 300 to 400, which is below previous limit of 500, requiring the invention of a new type of volute, discharge nozzle, and impeller.

A single end suction casing, overhung close coupled centrifugal pump with an open impeller was obtained. The 1.25″ suction nozzle was reduced to 0.625″ to accommodate a garden hose. The 1″ discharge was reduced to a 0.25″ tube. The axial position of the tube in the nozzle could be adjusted.

An impeller was made from a disk that had a width to diameter ratio of 0.052. It had four radial slots from the inlet diameter to periphery machined on the front face and then 60 quarter inch deep radial slots put around the periphery. The feature of radial slots to develop head is new, but usually there is a thin membrane in the middle of the slot to guide the liquid from both sides up to the middle of the volute. This is expensive and is accomplished with a gear cutter. This is method of increasing pressure is call regeneration. This developed a head a little greater than the standard impeller with a flow 2 gpm. There was a relatively large gap between the front and back of the disk and the mating surfaces of the casing and casing cover, resulting in internal slippage and leakage of the liquid. These surfaces were built up with disks of polymer so the front and back clearances were in the range of 0.020 to 0.040″. The developed pressure increased. At this point the impeller was an open front with 60 slotted teeth. This was developing substantial axial thrust resulting in the impeller rubbing the casing causing the motor to trip out. Some balance holes were added in the disk, but there was a significant drop in pressure. A new impeller of the same width and diameter was made. This one had four radial holes drilled into the body of the disk. These replace the four radial slots. This made it a closed impeller and eliminated the axial thrust. The small diameter- long holes are difficult to drill without breaking out into the outer surface of the disk. In addition to the holes the 60 slots were replace with 20 special shaped slots that looked like the teeth of a circular saw. This is similar to U.S. Pat. No. 3,746,467 dated Jul. 17, 1973. The developed head was less than the 60 slots, but the flow doubled. A circumferential groove half the width of the impeller was put in the middle of the teeth. The groove was deep as the bottom of the teeth. The teeth now looked somewhat like the teeth in the '467 patent. The pressure did not increase. The groove was refilled with epoxy. The tube in the discharge was pushed forward so it was 0.010″ from the impeller periphery. The pressure increased. The end of the tube was cut on a bias; this too resulted in an increase in head. The circular saw teeth were filled back to look like radial teeth. The pressure increased.

Since the radial holes are difficult to make, open radial slots were selected but with a change. The front face of two opposite holes were machined and then the impeller was turned over and machined to open the other two opposing holes. These last two terminated at the circumference of the back hub. Where they terminated, axial holes were drilled into the suction inlet in the front resulting in a double suction impeller with radial slots that was basically axially balanced. It also put the mechanical seal under suction pressure. The resulting pressure was a little less than with the closed impeller. The flow was greater.

The difference between 20 degree spiral vanes and radial vanes was tested. Four radial slots were filled in with an epoxy resin and four spiral slots were machined into the impeller. The spiral slots ran quieter but produced less pressure resulting in a return to the radial slots. More radial slots were added which increased the pressure. The number of teeth was doubled by vertically band sawing the existing teeth. This resulted in a large increase in pressure.

The resulting impeller is a suction double suction, double sided open impeller with radial slots on the periphery as shown in FIG. 5.

The casing also went through a number of changes and additions. The tube in the discharge nozzle was replaced with a replaceable cutwater that is set screwed into position. The replaceable cutwater can be adjusted to maintain the close clearance between it and the periphery of the impeller as shown in FIG. 6. It is made of a material that resists erosion and corrosion.

The side profile of the volute has dimensions and shape to guide the liquid from the impeller to split to both sides of the volute and guide it back into the impeller to regenerate the pressure of the liquid as shown in FIG. 7.

The impeller according to the present invention has three by features, namely:

a. Slots vs. vanes: Traditionally centrifugal impellers used wide spaced a spiral shaped passage ways with thin vanes along or within the impeller disk to develop pressure from the inlet to periphery of the impeller as shown in FIG. 8. When used in an enclosed impeller it is very difficult to produce. In an open impeller, the thin vanes are can be cast, machined or molded. The open space between the vanes results in a lot of internal slippage. This pump uses radial slots instead of vanes. Therefore, there is less internal slippage (FIG. 8b). Testing of this pump showed that there was little performance difference between radial and spiral grooves. The radial groove produced slightly higher pressure. The pump seemed to be quieter with spiral grooves;

b. Double suction vs. single suction: The impeller has a singular inlet that matches the casing inlet. There are radial pumping slots on both sides of the impeller disk. The front slots are directly connected to the periphery of the inlet. The back slots terminate at the back hub of the impeller. There are axial holes the same width as the slots adjacent to the hub that communicate the back slots to the impeller inlet. This allows liquid to be pumped up the back slots. The amount of liquid pumped of the front are about equal, resulting in a minimum of residual axial thrust.

c Radial grooves vs. spiral vanes: As the impeller rotates it adds energy to the moving liquid. Some of the liquid is re-circulated within the impeller passages. With the discharge valve fully open most of the energy passes through the pump. As the valve is closed to reduce flow, energy is trapped within the casing and impeller. When the valve is fully or almost closed this energy is changes to heat. The rise of temperature of the casing and internal parts can result in catastrophic mechanical failure. An impeller with only radial grooves rather than spiral vanes has much less contained water and absorbed energy, resulting lower internal heat build.

d. Dam vs. no cutwater: The placement of a dam (replaceable cutwater in this design) in the volute passage where the tangential discharge meets the circular volute increases the developed pressure. This principal of a dam is employed in what is referred to as a regenerative pump. These pumps have a circular volute. A regenerative pump has tine teeth along the periphery of the impeller. There may be from 40 to over 80 teeth. There is usually a thin radial web between the teeth to act as a guide for the liquid and a support between the teeth. The present invention does not employ a web. In a regenerative pump the inlet and discharge connections are usually adjacent (located at the same diameter) as the teeth. A close fitting permanent dam separates the two connections. The principal is that the liquid comes into the teeth it is discharged radial into the volute, but is forced back into other passing teeth. This cork screwing of the liquid in and out of the teeth continuously increases the pressure of the liquid as it goes around to the volute and exits out the discharge. These pumps required dose side and dam clearances. The dam wears down from erosion caused by the pressure break down from discharge to suction ports. Since this pump has a circular volute with a dam (replaceable cutwater), radial teeth without a radial web were added to the periphery of the impeller. These teeth are fed by the liquids coming from the radial grooves are both sides of the impeller as shown in FIGS. 5 and 7.

The assembly of the impeller in the casing is shown in FIG. 7. It shows the single inlet in the casing, the liquid going into both sides of the impeller, the radial teeth on the periphery and the replaceable cutwater in the casing.

A pump according to the present invention was constructed and tested as shown in FIG. 9. After all adjustments were made, the increase in pressure was from 10 to 30 percent over a standard design centrifugal impeller.

FIGS. 10 a and 10b are photographs of the back and front surfaces of an impeller according to the invention.

FIGS. 11 a and 11b are side views and FIG. 11e is a top view of a replaceable cut water according to the present invention.

While the principles of the invention have been described above in connection with preferred embodiments, it is to be dearly understood that this description is made only by way of example and not as a limitation of the scope of the invention.

Claims

1. A low flow centrifugal-regenerative pump having an impeller disk, said impeller disk having radial grooves on the face of said impeller disk and radial peripheral teeth on a peripheral edge of said impeller disk to increase pressure of fluid exiting said pump due to corkscrew circulation within a volute of said pump and a dam to push said liquid into a discharge nozzle in said pump.

2. A low flow centrifugal-regenerative pump according to claim 1 wherein said dam is a replaceable cutwater to increase developed pressure.

3. A low flow centrifugal-regenerative pump according to claim 2 wherein said replaceable cutwater Tater has a shape to provide optimal increase in performance of said pump.

4. A low flow centrifugal-regenerative pump according to claim 1 having a single entrance impeller with passages communicating with an entrance to said radial grooves to effect a double suction impeller.

5. A pump impeller disk having radial grooves to reduce internal flow which results in less internal heat build up at low fluid flow through a pump fitted with said impeller.