EP0486556B1

EP0486556B1 - Pulseless piston pump

Info

Publication number: EP0486556B1
Application number: EP90912068A
Authority: EP
Inventors: Kenneth E. Lehrke; Bruce A. Mcfadden
Original assignee: Graco Inc
Current assignee: Graco Inc
Priority date: 1989-08-08
Filing date: 1990-07-05
Publication date: 1996-05-08
Anticipated expiration: 2010-07-05
Also published as: EP0486556A4; EP0486556A1; DE69026945T2; JPH05501138A; WO1991002158A1; DE69026945D1

Abstract

A multiple piston cylinder reciprocating pump (10) is provided with a cam drive (18) such that the sum of the velocities during the pumping strokes off all of the cylinders is generally constant. The leak free design is provided by utilizing a diaphragm (36) attached to the piston (34) between the main seal assembly and the cam (18). A flow through intake design is provided which flows incoming material around the piston (34) between the diaphragm (36) and the main seal to prevent the build-up and hardening of material on the piston (34) and in the seal area. The intake (40) and exhaust (56) passages are arranged such that air pockets cannot be formed and any air bubbles which find their way into the pump will rise upwardly out of the pump without restriction.

Description

BACKGROUND OF THE INVENTION

A myriad of different types of pumps are known for use in pumping various materials. When it is desired to pump difficult materials, i.e., those that are highly viscous and/or abrasive, the number of choices of pumps suitable for such applications drops substantially, particularly when it is desired to pump such materials at relatively elevated pressures and/or at predetermined flow rates. While reciprocating piston pumps have been widely used in such applications, such pumps suffer from having pulses in the pressure output of the pumps during piston reversal. Such pumps also suffer to a certain extent from leakage and seepage of pumped material past the seals which is particularly critical when the material is air-sensitive such as isocyanates. This leakage is in both directions and can cause environmental contamination, pumped fluid contamination and regenerative abrasive wear damage to the pump. The reduction and/or elimination of pulses in the output is particularly important for circulating systems, fine spray applications and proportional metering to produce constant output.
Centrifugal pumps are capable of pumping abrasive materials without pressure pulses but suffer from the problems of not being positive displacement type (flow rate is not directly related to speed), inefficiency, shaft seal leakage and impose a high degree of shear on materials which may be shear-sensitive.
Gear pumps are commonly used for metering and proportioning apparatus due their ease in synchronising with other pumps. Such products, however, are ill-suited for pumping of abrasive materials which cause unacceptable wear.
US-A-4,453,898 discloses a dual-piston reciprocating pump having a rotating cam driving pistons spring-urged against it and reciprocable within a chamber having inlet and outlet check valves, and being sealed thereinto. The profile of the cam is chosen so as to synchronise the pistons for pumping and filling to minimise pressure pulsations and produce a substantially constant flow of fluid. The cam profile, which is engaged by rollers attached to the pistons on opposite faces, provides a parabolic rise, during rotation, from 0° to 30° to create an hydraulic pulse and a dwell from 345° to 360° to ensure complete cylinder refill. It has been found that this does not give pulseless output, which it is the object of the present invention to provide.
US-A-3,680,985 discloses a pump having a single piston which is reciprocated into and out of a cylinder to force fluid from the cylinder into a conduit. The piston is in a chamber to which inlet and outlet check valves have access and is sealingly connected to the reciprocating means by a diaphragm.
US-A-2,711,137 discloses a chemical feed pump with a piston reciprocated by a piston rod, the piston and rod carried by a diaphragm sealing the pumping chamber from the environment.
DE-A-3113737 discloses a multi-piston pump with a rotating cam whose profile is smoothly curved and engaged by each of the pistons through rollers.
DE-A-2608664 discloses a two piston pump driven by a cam whose profile is such as to give a dwell at the crossover points.
DE-A-437298 discloses a multi-piston pump driven by a cam whose profile is smoothly curved in Archimedean spirals and a dwell.
It has been found by the applicants that whilst a cam profile might be designed for theoretically pulseless output, this would not be achieved in practice.
It is therefore an object of this invention to provide a pump capable of handling such materials while providing substantially pulseless operation. It is further an object of this invention to provide such a pump which is easily manufactured and which is capable of being operated at varying speeds, flow rates and pressures in an efficient manner. It is yet a further object of this invention to provide such a pump which has leak-proof operation to avoid contamination of the environment in which the pump is located or contamination of the pumped fluid by the environment.
According to the invention, there is provided a fluid pump for providing substantially pulseless output, comprising a plurality of piston-cylinder combinations , cam means for driving each piston in its cylinder in a reciprocating motion alternating between intake strokes and pumping strokes, so that at least one piston is in a pumping stroke at all times and the sum of the velocities of the pistons in the pumping strokes is substantially constant at any given speed of the cam means, a pressure seal between each piston and its cylinder for sealing material to be pumped, and inlet check valves, and a sealing diaphragm attached to a housing and to each piston intermediate the high pressure seal and the cam means to form a chamber to contain any material that might leak past the high pressure seal and as a barrier between the material to be pumped and the environment, the cam means having a blip for increasing the velocity sum slightly near the point of check valve seating to compensate for the non-linearity of pump output during seating of the check valves.
In an embodiment of the invention described hereinafter, a multi-piston pump is driven by a cam. The use of pistons in conjunction with diaphragms allows a much higher pressure output capability that a simple diaphragm pump and a more positive displacement action than diaphragm pumps. The cam is powered by a DC motor or other type of conventional variable speed rotary driving mechanism (electric, hydraulic or the like). When used with these drives, the pump can be stalled against pressure just like a typical air-operated reciprocating piston pump. This mode allows adjustable constant flow. A constant speed motor driving the pump would use a pressure switch to turn the motor on and off. Because the motion input to the pump is rotary, it can be easily synchronized with another pump(s) to provide a plural component material proportioning system or with a conveyor to more fully automate production.
The cam profile is designed so that the reciprocating pistons (which alternate between pumping and intake strokes) have a net velocity sum of their pumping strokes which is generally constant. By doing so, one essentially can eliminate pressure losses that create pulses which result from the piston reversal of a conventional piston pump. In the preferred embodiment, two pistons are used although it can be appreciated that more pistons may be used if desired.
As shown in this application, intake flow is controlled by check valves which typically take a discreet amount of time to seat. Fluid can flow backwards during this time causing small pump output pressure variations during the valve seating but such can be compensated for by shaping the cam profile to provide a nearly totally pulseless operation.
Each piston is sealed in its respective cylinder by a relatively conventional type seal mechanism. Attached to the piston on the low pressure intake side of the seal is a diaphragm which serves to isolate the fluid from the environment and assure a leak proof device. As used in this application, the term "diaphragm" is understood to include membranes, bellows or other such structures performing a similar function. An intake passage provides flow directly over the piston between the main seal and the diaphragm to prevent the build-up and hardening of material in the intake section and on the piston. The intake flow then passes through the intake check and into the pumping chamber and then exits through an outlet passage which also has a check valve. This flow path minimises stagnant areas of non-flowing fluid where fluids may settle out and/or harden. The passage is oriented to minimise air entrapment and continually replenish the fluid in the intake area.
The cam can either be of a push-pull type, that is, where the roller rides in a track or can be a conventional outer profile cam wherein the piston assembly roller is spring loaded against the cam to maintain it in position.
The scope of the invention is defined by the appended claims; and how it can be carried into effect is hereinafter particularly described with reference to the accompanying drawings wherein:-

Figure 1 is an elevation of a pump according to the present invention, partly broken away and partly in section;
Figure 2 is a plan from below in the direction of the arrows 2-2 of Figure 1, showing part of the pump including the cam;
Figure 3 shows an alternative embodiment of cam; and
Figure 3A is a chart showing the velocities and outputs of a two piston pump.

A pump 10 (Figure 1) according to the present invention, comprises a main housing 12 in which runs a shaft 14 having a gear 16 mounted thereon. A motor (not shown) which may be a DC brushless type motor, drives gear 16 and shaft 14 to turn cam 18 mounted on the end thereof. A cam follower assembly 20 rides on cam 18 and comprises a follower housing 22 having a follower 24 mounted thereto via shaft 26. Follower housing 22 has guide rollers 28 mounted on the outside thereof which run in slots 30 in housing 12. Follower assembly 20 is spring loaded against cam 18 by means of a spring 32.
Follower assembly 20 is attached to a piston 34 and located in between follower 22 and piston 34 is a diaphragm 36. Those three parts are fastened together by a bolt 38 which passes consecutively therethrough. An initial inlet passage 40 leads into a flushing chamber 42 located about piston 34 between diaphragm 36 and main pressure seal 44 in cylinder 46. Flushing chamber 42 runs circumferentially around piston 34 thus inlet flow therethrough serves to flush material through which might potentially harden off the surface of piston 34. Inlet flow thence passes through passage 48 in to main inlet passage 50 which has located in series therein a check valve 52 of a conventional nature.
Pumping chamber 54 is located in the end of cylinder 46 over piston 34 and also has connected thereto outlet passage 56 having an outlet check 58 of conventional design therein. When the device is positioned as oriented in Figure 1, that is with the inlet and outlet ports 40 and 56 respectively facing upwardly, the product is designed so as to prevent the accumulation of air or other gas within pockets of the pump, that is, all such bubbles and gas may freely flow upwardly and out of the pump thereby reducing problems of priming and assuring full volumetric flow without air entrapment. It can be seen as piston 34 moves upwardly into pumping chamber 54, diaphragm 36 flexes upwardly to the point of nearly touching the upper surface 42a of flushing chamber 42 thereby continually assuring a fresh flow of material through the pump and the prevention of stagnant flow zones therein.
While the embodiment shown in the drawing figures utilizes a spring loaded follower and cam, it can also be appreciated that the cam drive may be of a different type wherein no such spring is necessary. Such a type of cam is often referred to as a desmodromic type cam, and an example of such a cam is shown in Figure 3 wherein the roller is guided in a track 60 and is driven in both its pumping and intake strokes. It can also be appreciated that seal 44 may be of any conventional type which is capable of performing a proper sealing function, however, it can be appreciated that because diaphragm 36 is subjected to relatively low pressures, its service life will be dramatically increased to maintain the pump in a substantially leak-free state. It can also be seen that if seal 44 should leak, its leakage is from the high pressure side back into the inlet rather than into the environment.
Up to this point, the description has been of a theoretically perfect pump. In reality, check valve physics (closing time, etc.), fluid compressibility and viscosity preclude perfect pulseless output. Satisfactory pulseless output may be obtained by modifying the cam profile to compensate for the above factors. By increasing the velocity of the opposite piston during check valve closing time by putting a "blip" in the cam to change the velocity profile, the pumping action can be slightly increased near the point of check valve seating to compensate for the decreased output during the seating time. The required net velocity profile for pulseless output may be different for any material which is pumped. Using a representative fluid such as oil for the purposes of optimizing the velocity profile of the pump results in a solution which is satisfactory for most other fluids.
Additionally, it can be appreciated that such a pump is easily adaptable to power operated valving, that is, valving which could be operated electrically and/or through a mechanical linkage not unlike an automotive engine such that the valve opening and closing time can be selected as desired.
It is contemplated that various changes and modifications may be made to the pump without departing from the scope of the invention as defined by the following claims.

Claims

A fluid pump for providing substantially pulseless output, comprising a plurality of piston-cylinder combinations (34,46), cam means (18,24) for driving each piston (34) in its cylinder (46) in a reciprocating motion alternating between intake strokes and pumping strokes, so that at least one piston (34) is in a pumping stroke at all times and the sum of the velocities of the pistons in the pumping strokes is substantially constant at any given speed of the cam means, a pressure seal (44) between each piston (34) and its cylinder (46) for sealing material to be pumped, and inlet check valves (52), and a sealing diaphragm (36) attached to a housing (12) and to each piston (34) intermediate the high pressure seal (44) and the cam means (18,24) to form a chamber to contain any material that might leak past the high pressure seal (44) and as a barrier between the material to be pumped and the environment, the cam means having a blip for increasing the velocity sum slightly near the point of check valve seating to compensate for the non-linearity of pump output during seating of the check valves (52).
A pump as claimed in claim 1, including a flushing inlet passage (40) leading from a source of material to be pumped around each piston (34) intermediate the diaphragm (36) and the high pressure seal (44) to minimise stagnation and prevent build-up or solidification of pumped material on said piston.
A pump as claimed in claim 2, wherein each cylinder (46), piston (34) and high pressure seal (36) form a pumping chamber (54) and the pump includes a main inlet passage (50) connecting the flushing inlet passage and the pumping chamber (54).
A pump as claimed in claim 3, wherein the main inlet passage (50) contains the inlet check valve (52).
A pump as claimed in any preceding claim, in which each piston (34) remains in contact with its seal (44) during reciprocation, characterised by a sealing diaphragm (36) attached to a housing (12) and to each piston (34) intermediate the high pressure seal (44) and the cam means (18,24) to form a chamber to contain any material that might leak past the high pressure seal (44) and as a barrier between the material to be pumped and the environment, a flushing passage (40) leading from a source of material to be pumped around each piston (34) intermediate the diaphragm (36) and the high pressure seal (44) to minimise stagnation and prevent build-up or solidification of pumped material on the piston (34) and a main inlet passage (50) connecting the flushing passage (40) and a pumping chamber (54) formed by each cylinder (46), piston (34) and high pressure seal (44).
A pump as claimed in claim 4 or 5, wherein the inlet passage (50) is located to run in a generally vertical direction and is configured to prevent the trapping of gases in the pumping chamber (54) and in the passage (50), whereby any gases will rise through the passage out of the pump.
A pump as claimed in claim 6, including an outlet passage (56) leading from the pumping chamber (54), the inlet and outlet passages (50,56) being located to run in a generally vertical direction and configured to prevent the trapping of gases in the pumping chamber (54) and the passages (50,56) whereby any gases will rise through the passages out of the pump.
A pump as claimed in any preceding claim, wherein the cam means (18) is driven by a variable speed motor.
A pump as claimed in any preceding claim, including power operated valving.