US2187972A - Pumping apparatus - Google Patents

Description

. 3an- 23, 1940- I A. HQLLANDER 2,187,972

PUMP ING APPARATUS Filed'uarch 21, 19:58 Sweets-sheet 1 Jan. 23, 1940. A. HOLLANDER 2,187,972

l PUMPING APPARATUS Filed March 21, 1958 3 Sheets-Sheet 2 Lauda/MM @Hanna/ga Jan. 23, 1940. A.- HQLLANDER PJlP ING APPARATUS Filed uarcn 21, 195s s sneetssneet s Patented Jan.

PATENT orsi- CE '2,187,972 i PUMPING APPARATUS Aladar Hollander, Los Angeles, Calif., assigner to Byron Jackson Co., Huntington Park. Calif.. a corporation of Delaware yrippiicatmn March 21, 1938, serial No. 197,139'

11 claims. (o1. 10s- 52) This invention relates generally to hydraulically actuated plston pumps, and more particularly to pumps especially adapted to handle drilling mud `used in--drillingsoil wells by the rotary method.

In drilling wells by the rotary method, the bit is rotated by a string of hollow drill pipe suspended from the surface and through which drilling mud is circulated downwardly to the bit vand 'upwardly-outside the drill pipe to the surface. The upward circulation carries the cuttings to the surface where they are separated from the mud prior to recirculation of the mud in the well. Settling' pits, screens, and separa--- tors of various types are employed to remove the cuttings, but complete removal thereof is never effected. `Consequently thesmudvpumps are re` quired to handle iiuid containing `a considerable amount of abrasive material, resulting in rapid abrasion of pump parts.

It is well established that thejetting action of the mud issuing from the bit passages has a pronounced effect on the rate of penetration of the bit into the formation, and, inasmuch as the force of the jets is a function of the mud pressure at the `bitkthe drilling speed can be materially augmented by maintaining a relatively high mud pressure'. 'Ihe pressure required at the. pump `discharge in order to obtain the desired jetting action depends on the length of the drill pipe, the internal diameter of the drill pipe, and the size of the mud passages in the bit. Since the size of thedrill pipe necessarily decreases with greater depth, the mud pressure at the pump mustbe increased greatly for deep Wells.l Pressures ranging from 300 lbs./sq. in. for shallow depths up to 1600 lbs`./sq. in. for depths of around 8000 feet are typical. In the event the drill pipe becomes stuck or the bit openings become clogged itis sometimes necessary to boost the pressure to as high as 5000 lbs/sq. in. to overcome this condition. The volume requirements are from 200 to 800 gal/min. It is thus seen that the mud pumps -are required to handle a wide range of volumes of abrasive-laden uid at a Wide range of high pressures.

The only available type of pump capable of handling mud in the volume and pressure ranges required is the duplex double acting reciprocating pump. This type of pump is, however, far from ideal for this purpose, and is used only because nothing more suitable is available. Of the many defects inherent in such pumps, the following are among the most troublesome:

1. The pressure diierent-ial across the pistons practice.

1 the pistons, the size of cylinders is definitely limited at the prevailing pressures because of the piston drag, which' increases with the pressure and'in proportion to the piston diameter. Small pistons necessitate high piston speeds in order to obtain the required volume, and theresulting high velocity ofabrasive fluid causes rapid abrasion 'pf cylinder liners. With the full pressure differential across the pistons, any leakage rapidly develops into high velocity jets which cut the rubber pistons as though a knife were used.

2J Even with duplex pumps the discharge is pulsating, and at the high `pressures prevailing this pulsation causes destructive vibration of the pumps, piping connections, and drill pipe.

3l The piston rods are necessarily under .compression when moving 'in one direction, and the length of stroke is therefore limited due to the tendency of long rods to`buckle.

4. Pum-pfrod stuing bdxesareexposed on one side to the drilling mud, requiring frequent servicing of stuing boxes and replacement of piston rods. r

5. Because of the. short stroke and small diameter pistons, the number'of valve actuations per unit of time is necessarily high, and the rapidity of change from extremely high pressure tosuc-` tion at each piston reversal causes pounding o'f the valves, resulting in relatively short life of valves and valve seats.

. 6. When changing to vsmaller size drill pipe after reaching considerable depth, it is customary to replace cylinder liners and pistons-with pressures. This is a troublesome and expensive Because' of the excessive Wear and tear on pump parts as the lresult of thev above-mentioned causes, the maintenance cost is very high. For example, the time required to drill an 8000- foot well is, on an average, about 120 days; during this time the mud pumps are operating about 90 days. It is customary to replace cylinder 1iners, pistons, cylinder heads, valves, and valve seats as many as l5 to 20 times in this 90. day period, resulting in a replacement cost per pump for pump parts alone on a single SOOO-foot Vones 'of smaller size, in order to obtain higher well, in the neighborhood of $3000, which is practically the initial cost of a pump.

The most common sources of power for mud pumps are steam, electricity, and internal combustion engines. Various combinations of the latter two have been tried in recent years, none of which are entirely satisfactory. 'Ihey require expensive and cumbersome equipment, and unless additional speed reduction and speed control mechanism is provided they lack the required range of flexibility. Furthermore they lack the feature of automatic adjustment to load, and in the event of sudden stoppage of ow of mud the pressure is likely to rise to the bursting point before lit can be relieved. To the best' of my knowledge, there is not available at present a pressure relief valve capable of handling such a situation atthe prevailing pressures.

rotary table and is readily available. It also is more flexible than the so-called "power drives. The mud pumping units almost universally used at the present time comprise direct-acting steam-- driven duplex pumps, in which the engine piston rods are connected directly to the pump piston rods. These units, however, have several outstanding defects in addition to the defects inherent in the pumps per se, such as:

1. A large majority of boilers used with drilling rigs are designed to operate at a pressure of about 225 lbs., and it is obvious that with a directacting steam engine operating at this pressure it is impossible to obtain the high mud pressures mentioned above while also maintaining the required volume of mud. Consequently it is cstomary to operate 225-lb. boilers at pressures up to 350 lbs., creatinga highly dangerous and otherwise unsatisfactory condition.

2. In order to increase the mud pressure it is the usual practice to admit steam to the engine cylinders for the full-stroke of the pistons, thus failing to utilize the adiabatic expansion of the steam. This.obviously results in low fuel emciency, and requires a boiler plant of higher steam capacity than would otherwise be necessary.

3. It is customary toI mount the mud pumps in close proximity to the well, whereas the boilers are required to be set back a distance suillcient to remove the fire hazard. Consequently, long steam lines are required from the boilers to the mud pumps, resulting in considerable heat loss. With the aforementioned defects in view, I have invented a novel mud pump free from all such defects and having other advantages which will be set forth more in detail hereinafter.

Inasmuch as one of the chief causes of trouble prior pumps is the existence of full pressure differential across the pistons, a principal object of this invention is to provide a pump in which the pressure differential across the pistons is re-V duced to a minimum. This I have accomplished by the provision of a single cylinder divided centrally by a transverse partition, forming in effect two coaxial cylindersf A piston in each cylinder is connected to opposite ends of a common piston rod extending through the partition. One cylinder contains a driving or motive liquid under pressure and drilling mud under pressure, the two liquids being separatedby the piston and the motive liquid being under a pressure only slightly greater than the mud pressure. The other cylinder contains spent motive liquid and suction mud on opposite sides ofits-pisto'q, both of these liquids obviously being under subi;3 stantially equal pressure with the motive .liquid pressure slightly higher than the mud pressure. The motive liquid is alternately admitted tothe two cylinders, causing reciprocation of the pistons and the pumping of mud from one cylinder while the other draws in mud.

A hydraulic pump of this type functions similarly to a single cylinder, double-acting pump direct-connected to a single cylinder, doubleacting hydraulic motor, and is therefore properly characterizedyas a simplex,lor single cylinder pump, as distinguished from a duplex, doubleacting pump. With the substantial equalization of presures on opposite sides ofthe pistons, the sealing effect of the piston need not be so nearly perfect, and the drag exerted by the pistons on the cylinder walls is reduced in proportion to-l the reduction in pressure differential across the pistons. The wear resulting from this piston drag -is thus eliminated. With' a slightly higher pressure on the motive liquid side of the pistons, any leakage across the pistons will be of clean motive liquid rather than of drilling mud, and the peripheries of the pistons will be kept washed clean. Contamination of the clean motive liquid by' drilling imposed by the piston drag as discussed above, is

removed and relatively large pistons and cylin- `ders may be employed. With increased piston area the piston speed, and consequently the fluid velocity, may be reduced while still maintaining the same output volume of mud. Reduction in fluid velocity results in additional reduction of wear on the cylinder walls.

By the present arrangement, the piston rod is l always under tension, and never under compression, making it possible to provide a long stroke such as would be impossible if the piston rod were under compression, because of the tendency of the rod to buckle. Of equal importance is the fact that there is no stufng box exposed to mud-the only stuffing box is exposed to clear motive liquid on both sides.

An actual embodiment of a pump according to my invention has two coaxial cylinders each 6 feet long and 10 inches in diameter. It will be apparent that by the provision of cylinders of this length and diameter a long, slow stroke is possible, resulting in relatively few reversals per minute. The number of valve actuations is proportionately reduced, and the pounding of valves on their seats as the result of rapid valve actuation is eliminated entirely.

A principal object 'of this invention is to provide a. hydraulic pump for pumping liquids at' high pressure, in which the pressure dilerential across the pistons is reduced to a negligible amount.

A further object of the invention is to provide a hydraulic pump having a long stroke and large diameter pistons, whereby the piston speed and the rapidity of valve actuations is greatly reduced.

In a simplex hydraulic pump, there is presented the problem of providing continuous iiow of motive liquid and of-pump discharge, when operating at high pressure. In the absence of means for accomplishing this, the vibration resulting from a pulsating discharge would destroy the pump and piping connections in a short tons.

A still further object-of this invention is to provide a mud pumping system wherein the highly desirable performance characteristics of a centrifugal pump may be availed of, without subjecting the centrifugal pump to the harmful ei'- fects of the abrasive mud. The performance characteristics of a centrifugal pump are ideally suited to pumping-drilling mud. When driven-l at constant speed it has no definite head or capacity, but is dependent on the resistance to iiow of the pumped liquid for the establishment of any fixed head or capacity. It has, however, la definite shut-ofi head for a given speed, and regardless of flow resistance it will not develop a greater head. It also has a definite maximum capacity at zero head for any given speed. By merely varying the pump speed or throttling the discharge, the pressure and volume can be varied through a wide range. There is no necessityv of changing the size of the mud pump liners and pistons to obtain varying pressures, as in pumps now in use. These characteristics make the centrifugal pump ideal for pumping drilling mud, so far ashydraulic performance isconcerned.'

However, because of the abrasive nature of A drilling mud, it would not be practicable to handle the mud directly by a centrifugal pump. Replacement of worn parts would be required even more frequently than with pumps now' inl use. In a system in accordance with this invention, however, the volume and pressure characteristics of a centrifugal pump may be transferred to the drilling mud indirectly through the novel balanced-piston hydraulic pump, wherein the displacement of mud equals that of the clear motive liquid, and the mud pressure is always only a few pounds less than that of the motive liquid.

A still further object of this invention is to provide a hydraulic mud pumping system comprising a prime mover, a centrifugal pump driven thereby and delivering clear motive liquid, and a hydraulic mud pump actuated by the motive liquid. t

Another characteristic of centrifugal pumps which makes them especially well adapted to supply motive liquid 4for hydraulic pumps handling drilling mud is their capability of being compounded either in series or in parallel, providing unusually wide pressure and volume ranges. When drilling at relatively shallow depths, a large volume of mud at pressures ranging from 600 to 800 1bs./sq. in. is required while at great depths a decreased volume at pressures from 1000 to 1500 lbs/sq. in. is required. These pressure and volume requirements are obtainable with a seriesparallel multi-stage centrifugal pump, or a plurality of pumps connectible in series or in parallel", whilel being driven at constant speed.

A still further object of this invention is to provide a hydraulic mud-pumping system including a prime mover, a series-parallel centrifugal pump driven thereby, and a hydraulic mud pump operated by motive liquid supplied by the centrifugal pump.

v As stated previously, the steam-driven mud lpumps now in use are very ineiiicient -from a standpoint of fuel economy, chiefly because steam is admitted to the cylinders during the full stroke of the pistons and the adiabatic expansion of the steam. is not utilized. In the system of this invention, however, the prime mover may be a steam turbine, the most efficient of any steamdriven prime mover. Or, if desired, any other type of prime mover may be used, such as a Dieselengine or an electric motor.

Another object of this invention is tov provide a hydraulic mud pumping system including a centrifugal pump and a hydraulicmud pump actuated by motive liquid supplied by the centrifugal pump, the latter being driven by a steam turbine, a Diesel engine, an electric motor, or

I any other prime mover of high emciency.

In present steam-driven mud pump installations the steam engine is a built-in part of the l pump, and is necessarily located with the pump close to the well. On the other hand, the boiles are set back from the well a;distance suiilcient to reduce the iire hazard to a minimum. Consequently, long steam lines are required to extend from the boilers to the mud pumps, resulting in considerable heat loss. In a steam-driven hydraulic mud pumping system according to this i close -to the boilers and remote froml the well,

While the mud pump is located clo'se to the Well. Referring once more to the novel mud pump, continuous ow of pump discharge and'of motive liquid is aorded in vthe present instance by providing a limited sliding connection between the pistons and the piston rod, whereby the reversal of the strokes oi.' the piston may be slightly out of phase. -In conjunction -with the floating pistons, a special valve arrangement is provided for admitting motive liquid to act on one piston before it is cut oif fromthe other piston.

A- still further object of this invention is to provide a hydraulically actuated piston pump including a plurality of pistons slidably mountedy on a common piston rod or other interconnecting Ameans, the extent of relativemovement between the pistons and the rod being limited but suiiicient to provide continuous flow of motive liquid and of pumped liquid.

In reciprocating piston motors of the doubleactng type the alternate admission of motive fluid to opposite ends of the-cylinder is usually controlled by a reversing valve actuated directlyy `or indirectlyv by the piston or p`1ston. rod at or If the motive fluid center. yfl'or this reason, it is customary to actuate the reversing valve indirectly by a pilot valve which is actuated mechanically by the piston or piston rod. K

' parentfrom the following description, in conjunction with the accompanying drawings, in which Y 1 Fig'. 1 is a central vertical longitudinal section through one form of the invention, illustrating conditions near the end of piston travel to the left; A

Fig. 2 is a view similar to Fig. 1, with the reversing valve inl mid-position;

Fig. 3 is a view similar to Figs. l and 2, with the reversing valve completely reversed and the stroke of the pistons to the right begun;

Fig. 4 is a central horizontal longitudinal section through a slightly modified form of the invention;

Fig. 5 is a transverse section taken substantially on line V-V of Fig. 4; and

.Fig. 6 is a plan view of the hydraulic mud pumping system installed at a well being' drilled.

Referring to Figs. 1 to 3 of the drawings, a hollow base plate I serves the dual purpose of supporting the pump and of providingI a suction reservoir for uid to be pumped. Inasmuch as the pump is particularly adapted to handle drilling mud, the pumped fluid will for convenience be referred to hereinafter as mud. A centrally disposed mud suction inlet 2 connects the interior of the hollow base with the source of mud, such as the mud pit located adjacent a well being drilled. Mud suction connections 3 and 3 at the ends of the base I lead from the reservoir to the outer ends of the pump cylinders.

Secured in coaxial relation on opposite sides of a central body 4 isa pair of cylinders 5 and 5. Mud valve heads 6 and 6 on the outer ends of the cylinders contain valves controlling the inflow of suction mud and the outflow of discharge mud, the valve heads also serying as supporting connections between the oiter ends of the cylinders and the hollow base. sl'ieciprocable within the cylinders are pistons I .and 1', slidably mounted on the opposite ends of a co on piston rod 8. The extentofo utward movement of the pistons relative to the piston rod is limited by collars 9 and 9' formedon the ends of the piston rod and engageable by the piston. lnward movement of the pistons on the rod is limited by engagement of the pistons with collars I and l0' secured to the piston rod. Springs II and I I' interposed between the pistons and collars 9 and 9 yieldablynrge the pistons inwardly. The collars 9 and'9 are slidable in bores in the pistons 1, 'l' in uid-tight relation thereto, and are in eect pistons, preventing leakage of fluidi from one side to the other of the pistons 1, 1'.

The central body 4 constitutes a valve chamber for motive liquid, hereinafter referred to as water for convenience, although it will be understood that any other suitable liquid may be employed. Water under suitable high oper ting pressure is admitted from a suitable source t the valve chamber through a water inlet I2, the spent water returning to the source through an outlet I3. The inlet l2 communicates with acentral passage between the two valves of a-balanced piston valve assembly I4. 'I'he valves are operated by a. pivoted lever I operably connected to a plunger 20 reciprocable in a longitudinally extending bore in the central body and disposed in the path of the collars I0 and IIlto be engaged by one of the latter at theend of each stroke.

The valve I4 alternately admits high pressure water from the inlet I2 to passages I6 and I6 leading respectively to cylinders 5 and 5'. When one of the passages I6 or I6 is connected to inlet I2, the other passage is in communication with outlet I3 to vent its respective cylinder, except for a brief interval during reversal of the stroke, as will be explained hereinafter.

The mud valve head 6 at the outer end of cyl? inder 5 contains an inwardly opening mud suction valve Il controlling communication between suction connection 3 and cylinder 5, and an outwardly opening mud discharge valve I8` communicating with a discharge nozzle I9. Valve head 6'; on cylinder 5' is equipped with similar valves I'I' and I 8 and discharge nozzle I9. The discharge nozzles are adapted to be connected to a common discharge pipe for delivery of mud to the point of use. f

The mode of operation of the pump is as follows: Water or other liquid under pressureis supplied tothe inlet I2 from a suitable source, preferably a centrifugal pump. When the valve It is in the position shown in Fig. 1, the pressure water enters cylinder 5 through passage I6,

moving` piston 'I to the left and forcing mud out through the discharge valve I8 and nozzle I9.

.The pressure of the water will exceed by only a flow, and the slower the piston speed the higher the pressure developed, up to the shut-off head of the particular pump. K

At the same time that piston 'I is pumping mud out of cylinder 5, piston 1 is also moved to the left, forcing spent motive liquid out of cylinder 5' through passage I 6 an'd outlet I3, and drawi 'piston 5 and rod 8. The spring II', however.

exerts initial force greater than that necessary. to force spent water out of cylinder 5', and spring II' will remain extended as shown in Fig. l.

As the pistons approach the left ends of the cylinders, collar I0' engages the plunger 20 and gradually moves it to the left, thus shifting valves I4 to the right. The spacing of the valves I4 is so related to the distance between passages I6 andkIS that passage I6' is opened to admit high pressure water to cylinder 5 before passage I6 is closed to high pressure water. In the central position of the valve, as shown in Fig. 2, high pressure water is admitted to both cylinders momentarily, thus moving both pistons outwardly and building up pressure on the mud in both cylinders. y

'Ihe equalization of the pressures acting on the opposite ends of the piston rod 8 causes the latter to move further to the left until the compression of the springs II. and II' is equalized.

2,187,972 lThis movement of the pistony rod causes the valve I4 to be shifted'to its extreme right-hand position, fully opening the passage I6' to high presv sure water and closing passageIB to high pressure water and establishing communication between the latter passage and the outlet I3. Both pistons then move to the right, forcing mud under pressure from cylinder 5' and drawing mud ting on dead center, by providing a positive force which acts on the valve to complete the reversal thereof.

Referring now to Figs. 4 and 5, a modied reversing valve assembly is disclosed. It will be observed that with the pivoted lever valve-actuating means shown in Figs. 1 to 3 it is necessary that the reversing valve be reciprocable longitudinally of the main cylinders. In the form shown in Figs. 4 and 5, however, thevalve is reciprocable transversely of the main cylinders. This is made possible byutilizing a compound rack and pinion arrangement. The central body, main cylinders, pistons and mud valves of this form of the invention are substantially identical with those shown in Figs. 1 to 3, and hence will not be described in connection with this form.

A plunger 3|, corresponding to the plunger 20 of Figs. 1 to 3, is provided with rack teeth 32 along the central portion of one side thereof.

Teeth 32 mesh with teeth on an elongated pinion 33 journaled on a vertical axis in the central body. As shown in Fig. 5, the plunger-3I engages the pinion 33 adjacent its lower end, and the toothed portion ofthe pinion extends a substantial distance along the pinion shaft. Spaced above the plunger 3I and extending. at right angles thereto and also at right angles to `the pinion 33 is a valve stem 35 having rackteeth 35 along the central portion of one side thereof meshing with the teeth on pinion 33. It will be apparent that reciprocation of the plunger 3| will cause oscillation of pion 33 *about its axis, which 'in turn will cause reciprocation of valve stem 35. i*

Formed on the valve stem 35 is a pair 'of spaced valves '36, 36', corresponding to valves I4 of Figs. 1 to 3, the valves .being spaced relative to the parts so as to provide for admission of high pressure water to both cylinders simultaneously for a brief interval during reversal of the valve.

The valve arrangement of Figs. 4 and 5 has one advantage over that of Figs. 1 to 3, in that byl From the foregoing description, it win be apparent that a simple and ingenious arrangement has been provided for continuing the ow of motive liquid and of pumped liquid during'reversal of the stroke of the pistons of a simplex hythe necessity of providing auxiliary equipment such as accumulators or air chambers. Furthermore, in addition to the continuous ilow referred to above, the oating piston arrangement makes it possible to actuate the reversing valve directly by the piston rod without any danger of the valve stalling in dead center-a defect inherent in directly actuated reversing valves for hydraulically operated devices.

The substantial balancing of the pressures on opposite sides of the pistons is an outstanding characteristic of this pump, reducing to a minimum the piston drag and the consequent wear` on cylinder walls. The piston rod is always under tension, andnever under compression,mak

ing possible 'a long stroke without the danger of the piston rod buckling. The piston rod stufdng box is exposed only to clear water and never to abrasive-laden mud, thus eliminating the necessity of frequent servicing of the stuffing box and replacement of the pistonrod. Other advantages will be apparent to those familiar with the problems encountered in the pumping of drilling mud. f

In Fig. 6 is shown a schematic layout of the hydraulic` mud pumping system at a well beingA drilled. The well derrick oor is shown at 4I, the rotary table at 42, the draw-works -at 43, and a conventional steam drilling'engine at 44. The battery of boilers .45 are shown set back from the Well in a customary manner, and connected by a steam line 46 to the drilling engine vd4; a second steam line 41 supplies steam to the boiler feed water pumps 48. Thus far the layout follows conventionalpractice.

A steam turbine 5I, located adjacent the boilers, is supplied with steam -through an extension 52 of the steam line 41, and is direct-connected to a centrifugal pump 53. This pump is a multistage centrifugal pump having a series-parallel hook-up, whereby the pump is divided into two halves, the stages of the two halves being capable of being compounded either in series, for high pressure, or in parallel, for lower pressure and greater volume. A water supply line 54 leading from a tank 55 is connected to the inlets 56 and 5l of the two halves of the pump, the inlet 5'! ,in parallel, each having an inlet and an outlet,

and delivering a relatively large amount of water at relatively low pressure. When valves 58 and 65 are closed and valve 64 is opened, the discharge from the first half enters the intake of the second half, and the two halves are connected in series, delivering a relativelysmall amount of water at relatively high pressure.

The common discharge line 62 extends to the hydraulic mud pump "I0, and a return line 'II leads from the mud pump to the tank 55. A water line 'I2 extends from the tank to the boiler feed water pumps. A mud suction line 'I3 extends from the usual mud pit 'I4 to the suction inlet of the mud pump, and a mud discharge line "l5 leads from the pump to the usual connection tion, adapted for use at a steam-operated rig. It f will be understood that if preferred any other desired type of prime mover may be employed, such as a Diesel engine or an electric motor.

Although the invention has been explained by describing in detail a particular embodiment thereof, it is to be understood that many departures can b e made from the specic construction shown without departing from the invention which is to belimited only to the extent set forth in the appended claims.

I claim:

1. A pump comprising in combination: cylinder means and a pair of pistons reciprocable therein, intake and discharge lines, valve means for connecting said intake line to said cylinder means on the suction side of each piston when the piston is moving in one direction and connecting said discharge line to said cylinder means on the same side of each piston when the piston is moving in thel opposite direction, and means for moving said pistons simultaneously at equal speeds in opposite phase relation throughout the mid-portions of their stroke and moving said pistons in in-phase relation to each other adjacent the end of each stroke whereby the pumping strokes of the pistons overlap.

2. A pump comprising in combination: cylinder means and a pair of pistons reciprocable therein, intake and discharge lines, valve means for connecting said intake line to said cylinder means on the suction side of each piston when the piston is moving in one direction, and connecting said discharge line to said cylinder means on the same side of each piston when the piston is moving in the opposite direction, means for independently applying moving force to each piston during its pumping stroke, and means constraining said pistons to move in opposite phase relationat equal speed throughout the mid-portions of their strokes while permitting said pistons to move in in-phase relation to each other adjavvcent the end of each stroke, whereby the pistons can move to pump simultaneously at the end of each stroke. f

3. A pump as described in claim 2,'in which said means for constraining the movements of said pistons comprises a lost motion mechanism mechanically interconnecting said two pistons.

4. A pump as described in claim 2, in which said means for constraining the movements of said pistons comprises: a connecting means, means providing lost motion connection between said connecting means and each piston, and spring means for resiliently resisting lost motion of each piston with respect to said connecting means.

5. A pump as described in claim 2, in which said means for moving said pistons is constituted by means for applying pressure uid from a common source to one piston only during the midportion of each stroke and to both pistons simul^ taneously adjacent the end of each stroke.

6. A pumpas described in claim 2, in which said means for applying moving force to each piston comprises a motive liquid source, and valve means for applying said motive liquid to one piston only during the mid-portion of each stroke and to both pistons simultaneously near the end of each stroke, valve-actuating means, and means for connecting each piston to said valveactuatng means with a lost motion connection.

7. A pump comprising in combination: cylinder means and a pair of pistons reciprocable therein, intake and discharge lines, valve means for connecting said intake line to said cylinder means on the suction side of each piston when the piston is moving in one direction and connecting said discharge line to said cylinder means on the same side of each piston when the piston is moving in the opposite direction, a common piston rod, and means mounting each piston on said rod with limited sliding movement axially of said rod, and means for moving said pistons simultaneously in unison with said rod throughout the mid portions of their stroke and moving one, at least, of said pistons relative to said rod and the other piston adjacent the end of each stroke, whereby the pumping strokes of the two pistons overlap.

8. A pump as described in claim 7, including spring means associated with said pistons, which is movable with respect tb said rod for yieldingly resisting movement of said pistons axially of said rod.

9. Apump as described in claimv '7, in which each piston is slidable axially 'on said rod, two pairs of stop means on said rod, one pair for each piston, an abutting surface on each piston disposed between adjacent stop means on said rod, and spring means between said abutting surface and one of said stop means for yieldingly resisting movementv of said piston axially of said rod.

10. A pump as described in claim 7, in which said pistons are slidable on said piston rod in substantially uid-tight relation thereto.

1l. A hydraulic pump comprising a pair of cylinders each having a piston reciprocable therein, pump liquid suction and discharge connections to each cylinder on one side of the piston therein, motive liquid inlet and exhaust connections to each cylinder-on the other side of the piston therein, lost motion means interconnecting said pistons for forcing movement thereof in unison during intermediate portions of their strokes while permitting relative move- Y ment therebetween during end portions of their strokes, and valve mechanism arranged to apply.l

ALADAR HOLLANDER.

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