US3286640A

US3286640A - Automatic control for hydraulically driven diaphragm-operated compressors and pumps subjected to very high pressures

Info

Publication number: US3286640A
Application number: US479220A
Authority: US
Inventors: Bauer Helmut
Original assignee: Maschinenfabrik Burckhardt AG
Current assignee: Burckhardt Compression AG
Priority date: 1962-04-24
Filing date: 1965-08-12
Publication date: 1966-11-22
Anticipated expiration: 1983-11-22
Also published as: GB1003596A; CH405583A; DE1428001A1

Description

Nov. 22,. 1966 H. BAUER 3,286,640

AUTOMATIC CONTROL FOR HYDRAULICALLY DRIVEN DIAPHRAGM-OPERATED COMPRESSORS AND PUMPS SUBJECTED TO VERY HIGH PRESSURES Filed Aug. 12, 1965 2 Sheets-Sheet 1 48 Helmut Bo uer INVENTOR BY @tdi ATTORN EYS Nov'. 22,1966 UER 3,286,640

. H. BA AUTOMATIC CONTROL FOR HYDRAULICALLY DRIVEN DIAPHRAGM-OPERATED COMPRESSORS'AND PUMPS SUBJECTED TO VERY HIGH PRESSURES Filed Aug, 12, 1965 2 Sheets-Sheet z INVENTOR Helm ut Bauer ATTORNEYS United States Patent This application is a continuation-in-part of my application Serial No. 272,643, filed April 12, 1963, entitled, Automatic Control for Hydraulically Driven Diaphragm- Operated Compressors and Pumps Subjected to Very High Pressures.

Diaphragm-operated compressors and pumps are known which allow feeding liquid and .g-asiform fluids without any lubricant soiling them and without any lack of fluid tightness arising.

An object of the present invention is to utilize such compressors and pumps for use under extremely high pressures of several thousand atmospheres.

A preferred construction is shown in the drawings in which:

FIGURE 1 is a diagrammatic cross-section of the in- 'vention.

FIGURE 2 is an explanatory graph.

Referring first to FIGURE 1 of the accompanying drawings, the nature of the problem to be solved will be first discussed. FIGURE 1 is a cross-sectional view of a diaphragm-operated compressor wherein 1 designates the diaphragm the outer periphery of which is stretched between the valve-carrying head 2 and the cylinderfi. The valve-carrying head 2 is provided with bores 4 and 5 inside which are fitted respectively suction and delivery valves which are not illustrated.

The chamber 6 extending over the diaphragm forms a compression chamber, whereas the chamber 7 underneath the diaphragm is filled with a suitable liquid which is compressed through the reciprocation of the plunger 8 so as to bend the diaphragm. The plunger is sealed against the outer atmosphere by the packings 9 and It). The operative liquid contained in the chamber 7 is completely separated by the diaphragm 1 from the liquid to be delivered which is contained in the compression chamber 6.

The plunger 8 is generally controlled in its reciprocation by a conventional crank system. For the execution of the invention, it is of advantage to control said plunger through hydraulic means as may be provided by a driving piston 11, while the chambers above and underneath said driving piston are filled in alternation by the compressed operative fluid.

The arrangement required for this purpose is irrelevant as far as the invention is concerned and, since it forms a technically well-known matter, it is not illustrated in the drawings. It may be constituted, for instance, by a slide valve the movement of which is reversed through usual and conventional means when it reaches the upper and lower ends of its stroke.

In order to disclose the problem implied by the operation of such a machine, it is essential to first consider thevariations in pressure produced by the operation of said machine.

The corresponding graphs of pressures vs. stroke are shown in FIGURE 2, where 12 designates the lower end and 13 the upper end of the plunger stroke. The area between the ordinate lines passing through 12 and 13 shows consequently the volume compressed by the plunger.

3,236,640 Patented Nov. 22, 1966 At the beginning of the compression stroke, the suction pressure 14 prevails in the delivery chamber 6 and, consequently, also in the actuating chamber 7. During the compression stroke, the gas filling the chamber 6 is first compressed to a pressure 15, after which it is urged outwardly through the delivery valve. It is necessary for various reasons for the volume displaced by the plunger 8 to be larger than the volume displaced by the diaphragm 6, which latter can only move between the two incurved bounding surfaces formed respectively on the cylinder 3 and on the valve-carrying head 2. The fluid urged into the actuating chamber 7 is compressible and the walls of the cylinder 3 expand under the action of its high pressure and finally perfect fluid-tightness of the packing 9 can not be considered as reliable. Since, in order to cut out the drawbacks arising therethrough, the volume shifted by the plunger 8 should be larger than that displaced by the'diaphragrn 1, the diaphragm will actually engage for a pressure corresponding to the point 16, the upper surface limiting the chamber on the valvecarrying head 2 before the end of the plunger stroke is reached at 13. Since the plunger 8, on the other hand, continues travelling up to the end of its stroke, this will lead to an extremely speedy increase in pressure inside the chamber 7.

During the compression stroke, '14-15 and the delivery stroke 1516 the diaphragm 1 is not subjected to very considerable stresses, since the pressures in the chambers 6 and 7 are practically equal. However, when point 16 has been passed and when the diaphragmhas engaged the upper surface bounding the chamber 7 and the latter has been subjected to a very high increase in pressure, the diaphragm may risk being damaged, since said increase in pressure urges the diaphragm into the openings of the suction and delivery channels 4 and 5, which may perforate the diaphragm to a certain extent. It is therefore necessary to limit the increase in pressure to a value which may be resisted by the diaphragm. This is obtained normally by means of a safety valve which is ad- .justed in a manner such that the driving medium can 'boundary surface somewhat beforehand, say at point 19,

:since for reducedfinal pressures, the compressibility of oil, the elastic expansion of the cylinder and the lackof iiuidtightness of the'packing 9 do not play such a large part. In the case considered, upon continuedmovement of the plunger 8, a very speedy increase in pressure would be obtained and would end only when the safety valve, adjusted for the pressure 17, allows the excess driving medium to flow out. The difference in pressure between the pressures 17 and 19 can however reach several thousand atmosphere and it is therefore no longer allowable for the diaphragm.

Similar conditions appear during the return'stroke of the plunger 8. The gas enclosed above the diaphragm in the dead space expands first from the pressure 16 down to the suction pressure 20 which is equal to the starting pressure 14. When operating with a reduced final pressure 19, the expansion would reach the point 21. In both cases, the suction valve would finally open and a suction stroke is executed in the chamber 6 with the interposition of the diaphragm. Since, however, the prior compression stroke has allowed some of the driving medium to be lost as a consequence of the lack of fluidtightness of the packing 9, the diaphragm 1 will reach already for the pressure at the point 22 the lower concave boundary surface formed on the cylinder 3 on which the diaphragm rests, while a further downward movement of the plunger 8 produces a speedy drop in pressure. When operating with large suction pressures as occurs in particular in the so-called circulation pumps, the diaphragm may again be damaged by the difference in pressure appearing in such a case. Such a difference in pressure urges the diaphragm 1 into the cylinder bore which may lead to a perforation of said diaphragm.

Now, the present invention has for its object to remove these drawbacks in so far that means are provided for limiting the possible excess pressure at the end of the compression stroke and at the end of the suction stroke down to a non-dangerous value, said limitation matching automatically the values of the operative suction and compression pressures.

The invention consists chiefly in that means are provided which act in accordance with the rise in the operative pressure in the compressor cylinder above a predetermined non-dangerous value, so as to stop the compres sion stroke.

To this end, an arrangement is provided by the invention which records the suction and compression pressures prevailing at any moment and storing or so to speak memorizing their values, so as to compare them with the excess pressure or reduction in pressure which may appear at the end of the stroke, after which they call for the required steps adapted to limit said excess pressure or reduction in pressure to the desired value considered as non-detrimental.

Since the pressures in the compression chamber 6 are extremely high and consequently can be measured only by means of intricate and sensitive instruments, the invention replaces such instruments by means measuring the pressures of the driving medium underneath the piston 11. Said pressures are proportionally smaller by the ratio between the cross-sectional area of the plunger 8 and that of the piston 11, but the curve defining them is substantially similar to that shown in the graph of FIGURE 2.

To limit the excess pressure at the end of the compression stroke, it is necessary, in accordance with the above disclosure, to obtain an information as to the value of the pressure of the driving medium obtained when the pressure applied to the diaphragm is that shown at -16. This is obtained through a controlled valve tapping off the actuating liquid under the piston 11 and measuring its pressure during the short period corresponding to the stroke section extending between

points

23 and 24.

Said tapping ofl. valve shown at 25 in FIGURE 1 is actuated by a cam 26 controlled by the piston rod. Said cam shows a notch so as to make the tapping off valve 25 open under the action of the spring 27 during the short time 23-24.

Obviously, the tapping off must be done during the delivery period. The short time 23-24 must therefore be blockaded somewhere between the

points

15 and 16. This can easily be achieved by placing the cam 26 in a suitable position on the piston rod.

Furthermore, it is necessary, as mentioned in the above disclosure, to store said information concerning the value of the normal pressure up to the end of the stroke. This is obtained by means of a hydraulic accumulator 28 which is filled through the arrangement described up to a point corresponding to said pressure prevailing inside the cylinder during the period corresponding to the track 23-24.

During the downward movement of the piston, the tapping off valve 25 also opens during its passage over the track 24-23. However, during said period, only a re duced pressure is obtained in the driving cylinder. In order to prevent the accumulator 28 from emptying, there is provided a cutting off slide valve 29 which releases the connection with the accumulator during only the upward movement of the piston, while it closes same during the downward stroke. The control of this cutting off slide valve 29 is not illustrated in FIGURE 1, since it is irrelevant as far as the inventionis concerned. It can be ensured in a very simple manner in accordance with the position of the control slide valve which is not illustrated.

According to the preceding disclosure of the principle of the invention, it is necessary to make use of the stored information concerning the pressure for the stoppage of the compression stroke, with the diaphragm 1 engaging the upper surface limiting its movement. This is obtained by means of a control piston 30 subjected on its terminal surface 31 to the pressure prevailing underneath the driving piston 11, while the other terminal surface 32 of said piston 30, the size of which is equal to that of the surface 31, is subjected to the pressure prevailing in the accumulator 28 and to the thrust of a spring 33.

As long as operation extends between the

points

16 and 15 of the graph, the pressure is the same on both

surfaces

31 and 32, so that the piston 30 is urged by the spring 33 into its left-hand position as illustrated. As soon as the diaphragm has passed beyond the position corresponding to the point 16 of the graph and engages its upper limiting surface, the pressure increases speedily underneath the piston 11 and this increase acts immediately on the lefthand surface of the control piston 30. The surface 32 on the right-hand side of the latter remains however, as precedingly, subjected to the action of the stored pressure 28. As soon as the increase in pressure on the track between 16 and 17 rises above the value corresponding to the thrust exerted by the spring 33, the control piston enters its right-hand terminal position.

This movement of the control piston can serve for executing a reversal of the movement of the piston 11 and thereby terminate the compression stroke. The means provided for this purpose are well-known and are irrelevant as far as the invention is concerned and, consequently, they are not illustrated in FIG. 1. The main control slide valve which is also not illustrated and which serves for feeding the oil alternatingly on both sides of the piston 11 can be reversed for instance by mechanical means or else by a well-known hydraulic follower system, or again a combination of electric switches and electromagnetically controlled valves may be used.

It is obvious that such an arrangement operates exactly in the same manner when the terminal pressure of the machine is modified.

When, for instance, the machine operates with a terminal pressure such as that illustrated at 18, 19, said pressure is stored through the tapping off valve in the accumulator 28 during the short period corresponding to the track 34-35. After the piston has passed beyond the position corresponding to point 19, the control piston 30 is shifted towards its right-hand position by the excess pressures appearing at this moment.

It is also obvious that the excess pressure appearing at 36 with reference to 19 must be equal to the previously referred to excess pressure appearing at 17 with reference to 16, which means that it should correspond as precedingly to the thrust exerted by the spring 33. By suitably selecting this thrust exerted by said spring 33, it is an easy matter to reduce said extra pressure to a value which is not dangerous for the diaphragm. This leads to the desired result consisting in the protection of the diaphragm against an overload at the end of the compression stroke, independently of the varying values of the final pressures.

In case, by reason of a considerable modification in the graphs, it is necessary to modify the position in time of tapping off

period

23, 24, or 34, 35, as illustrated by the graph on a purposely exaggerated scale, this modification in time can be also obtained readily by shifting the cam 26 with reference to the piston rod.

The protection of the diaphragm against overloading at the end of the suction stroke is obtained in a similar manner.

To this end, there is provided a further tapping off slide valve 36, which is controlled in a manner similar to that disclosed hereinabove by a cam 37 so as to open for a very short time said valve'under the action of the spring 38 which allows a sample of compressed fluid to be tapped off at this moment. The cam 37 is designed and adjusted in a manner such that the removal of said pressure sample is obtained during the passage over the track between the points 39 and of the graph.

The pressure tapped off is stored in a manner similar to that disclosed hereinabove in an accumulator 41. A slide valve 42 is also provided so as to shut off the accumulator 41 and to connect it only during the suction stroke, while it closes the connection in contradistinction, during the compression stroke, said slide valve 42 being controlled correspondingly by means which are not illustrated. The pressure prevailing in the accumulator 41 acts on a pressure amplifier in which the areas of the larger piston 43 and of the smaller piston 44 are in the same ratio as the large piston 11 with reference to the plunger 8. Since, however, the pressure exerted by the spring acts in antagonism with the pressure exerted on the larger piston 42, the pressure eXerted through the smaller piston 44 is somewhat lower than the suction pressure exerted by the machine at point 22.

As soon as the diaphragm engages its lower surface limiting its movement as defined by said point 22, the pressure of the actuating medium begins to sink rapidly. As soon as said 'pressurehas sunk down to the value 46, the pressure above the springurged packing 9 drops under the pressure prevailing underneath it. The packing 9 is thus lifted above its support and allows the driving medium'fed out of the pressure amplifier 44 to enter the cylinder. The pressure in the cylinder can therefore not drop underneath the value 46.

It is readily apparent from the preceding disclosure that the allowed drops in pressure between the

points

22 and 46 depend on the power of the spring 45. A suitable selection of the spring 45 makes it an easy matter to reduce the drop in pressure between 22 and 46 to a value small enough for the diaphragm not to be damaged thereby.

The pressure amplifier 45 is not required during the compression stroke and consequently the connection between the piston 43 of the pressure multiplier and the accumulator 41 during said pressure stroke, is interrupted by a slide valve 47 which substitutes for said connection a connection with the atmosphere. The spring 45 urges then the piston 43 rearwardly so that thepressure amplifier can be filled again with fresh fluid. The slide valve 47 can be readily controlled in a manner such that it connects, the amplifier during the suction stroke with the accumulator 41 and with theatmosphere during the compression stroke. The means for executing such a control are well-known and are irrelevant as far as the invention is concerned, and for this reason they have not been illustrated in FIGURE 1. This control of the slide valve 47 'can be associated for instance ina simple manner with the 'movement of the main slide valve which controls the alternate feed of oil towards the opposite sides of the piston 11 When the piston 11 has reached the lower dead end of the stroke, the cam pushesthe valve 49 to the right. Oil delivered by the pump 48 will therefore flow under the pilot piston of the valve 51, and push it to the upper position as shown. Therefore, oil delivered from the pump 48 will enter on the lower side of the piston 11, and oil from the upper side of the piston 11 will escape back to the tank so that the piston will perform an upward movement.

As soon as this upwardmovement has started, the cam 50 will move away from the roller and will no longer press the valve 49 to the right, but the valve 49 will stay in its position as there is no force to change this, and the upward movement of the piston 11 will therefore continue. At a certain portion of the stroke, let us say the distance between 34-35 or 23-24 in FIG. 2, the valve 25 will open under the influence of the spring 27 due to the action of the negative cam 26. The following valve 29 will be open under the influence of its pilot piston which is in connection with the pressure under the piston 11, and therefore the pressure under the piston 11 will enter the accumulator 28.

In the accumulator 28 there will therefore be stored the pressure under the piston 11 during the period 34-35 or 23-24 according to the working pressure of the compressor. Evidently, the valve 25 will open a second time when the piston 11 makes its downward movement, but at thismoment, the pressure under the piston 11 will be small, and therefore the valve 29 will'be closed so that the pressure in the accumulator 28 cannot be altered.

As previously pointed out, the membrane will come to the'end of its stroke before the piston 8 comes to the end of its stroke. Therefore, a sharp pressure rise will happen which will show in the diagram as the pressure rise from 16-17. Now, there will be acting on the right side of the piston 30 the pressure 16 which has previously been stored in the accumulator 28. The-re will be acting on the left side in the chamber 31 the pressure 17 which is rising sharply, and this pressure will move the piston 30 to the right against the influence of spring 33. This movement of the piston30 will swing over the lever 52 and put the slide valve 49 in the opposite position. Therefore, the pressure on'the pilot piston of slide valve 51 will be released and slide valve 51 will come under the influence o f its spring to the lower position. The oil delivered by the pump 48 will go to the opposite side of piston 11 and the oil under the piston 11 will escape so that the piston reverses its movement and goes downward.

The moment of reversal of the motion therefrom does not depend on the position of the piston but on the moment where the sharp pressure rise occurs.

If during the suction stroke the pressure prevailing under the piston 11 would be completely relieved to the atmosphere, thepressure to be stored in the accumulator 41 during the period 39-40 would be substantially equal to the atmospheric pressure and would in no way be proportional to the suction pressure prevailing in the chamber 6. Therefore the oil displaced by the piston 11 must not directly escape to the atmosphere, but through a pressure regulating valve 53 provided with an adjusting screw 54. In this way the pressure prevailing under the piston 11 is on the one hand maintained substantially proportional to the suction pressure prevailing in the chamber 6 and, on the other hand, low enough to ensure the correct movements of the valves 42 and 47 in their openingposition and of the valve 29 in its closing position.

During the downward movement of the'piston, the valve 36 opens for a short moment under the influence of the spring 38 when its roller falls into the negative cam 37. This short opening will correspond with, let us say the period 39-40. At this moment, there will be a communication between the room under the piston 11 and the accumulator 41, because the valve 42 will be open.

The valve 36 will ofcourse open a second time during the upward movement of the piston 11, but at this moment, no communication between the room under the piston 11 and the accumulator 41 is established, the valve 42 being closed under the influence of the pressure.

During the upward stroke of the piston 11, the pressure under the piston 11 will press the valve 47 to the left side, thus allowing the piston 43 to do a suction stroke under the influence of the spring 45.

During the suction stroke, the slide valve 47 under the influence of the spring will go to the other end position, and the pressure in the accumulator 41 will act on the piston'43 and drive it to the right. The piston 44, therefore, will create an oilpressure which can easily be made equal to the oil pressure made by piston 8, if the relation between the piston surfaces 8 and 11 and 44 and 43 are made identical.

If now the membrane 1 comes to the end of its stroke before the piston 8 comes to the end of its stroke, the continued movement of the piston 8 cannot create any depression under the membrane, because the piston 44 will go forward and replace just the necessary amount of oil to avoid the creation of such a depression. Therefore, the membrane will be protected against one-sided pressure.

It is thought that the invent-ion and its advantages will be understood from the foregoing description and it is apparent that various changes may be made in the form, construction and arrangement of the parts without departing from the spirit and scope of the invention or sacrificing its material advantages, the form hereinbefore described and illustrated in the drawings being merely a preferred embodiment thereof.

I claim:

1. In combination with a fluid-delivery machine including a chamber, a diaphragm subdividing said chamber into an operative chamber containing the fluid to be delivered .and an actuating chamber and a hydraulically control-led piston adapted to execute periodically a forward stroke feeding a driving fluid into the control chamber to urge the diaphragm into the operative chamber and a return stroke sucking the driving fluid back to urge the diaphragm rearwardly, the provision of means tapping off a sample of the low pressure driving fluid at a selected moment during the delivery period before the diaphragm reaches its outermost position, means of storing said sample, and means comparing the pressure of the stored sample with the actual pressure of the driving fluid at other moments and adapted to stop the forward stroke of the hydraulic piston when said actual pressure has risen above the stored pressure by a predetermined amount.

2. In combination with a fluid-delivering machine including a chamber, a diaphragm subdividing said chamber into an operative chamber containing the fluid to be delivered and a control chamber and a hydraulically controlled piston adapted to execute periodically a forward stroke feeding a driving fluid into the control chamber to urge the diaphragm into the operative chamber and a return stroke sucking the driving fluid back to urge the diaphragm rearwardly, means tapping off a sample of the driving fluid at the moment of the forward stroke at which the control chamber reaches its maximum expansion under a predetermined pressure of the driving fluid, means storing said sample, means comparing the pressure of the stored sample with the actual pressure of the driving fluid at other moments and adapted to stop the forward stroke of the hydraulic piston when the actual pressure V has risen above the stored pressure by a predetermined amount, and means controlled by the sinking of the pressure of the driving fluid in the control chamber by a predetermined amount underneath the pressure corresponding to the maximum expansion of the operative chamber, and urging a volume of driving fluid into the control chamber to restore the pressure therein to a value corresponding to said amount underneath lastamentio-ned pressure.

3. In combination with a fluid-delivering machine including a chamber, a diaphragm subdividing said chamber into an operative chamber containing the fluid to be delivered and a control chamber and a hydraulically controlled piston adapted to execute periodically a forward stroke feeding a driving fluid into the control cham ber :and a return stroke sucking the driving fluid back to urge the diaphragm rearwardly, the provision of means tapping off a sample of the driving fluid under the pressure prevailing at the moment of the return stroke at which the diaphragm reaches its position corresponding to a maximum expansion of the control chamber, means storing said sample, and means comparing the pressure of the stored sample with the actual pressure of the driving fluid at other moments and adapted to stop the return stroke of the hydraulic piston when said actual pressure has sunk underneath the stored pressure by a predetermined amount.

4. In combination with a fluid-delivering machine including a chamber, a diaphragm subdividing said chamber into an operative chamber containing the fluid to be delivered and an actuating chamber and a hydraulically controlled piston' adapted to execute periodically a forward stroke feeding :a driving fluid into said actuating chamber and a return stroke sucking the driving fluid back to urge the diaphragm rearwardly, the provision of means tapping off a sample of the driving fluid under the pressure prevailing at the moment of the return stroke at which the diaphragm reaches its position corresponding to a maximum expansion of the control chamber, means storing said sample, a pressure amplifier fed by said storing means and the amplifying ratio of which is equal to the ratio between the two stages of the hydraulic piston, and means comparing the amplified stored pressure at the output of the pressure amplifier with the actual pressure of the driving fluid beyond the first stage of the hydraulic piston and adapted to stop the return stroke of the hydraulic piston when said actual pressure has sunk underneath said amplified stored pressure by a predetermined amount.

5. In combination with a fluid-delivering machine including a chamber, a diaphragm subdividing said chamber into an operative chamber containing the fluid to be delivered and a control chamber and a hydraulically controlled piston adapted to execute periodically a forward stroke feeding a driving fluid into the control chamber to urge the diaphragm into the operative chamber and a return stroke sucking the driving fluid back to urge the diaphragm rearwardly, the provision of means tapping off a sample of the driving fluid at the moment of the forward stroke :at which the control cham'ber reaches its maximum expansion under a predetermined pressure of the driving fluid, means storing said sample and adjustable eans comparing the pressure of the stored sample with the actual pressure of the driving fluid at other moments and adapted to stop the forward stroke of the hydraulic piston when the actual pressure has risen above the stored pressure by a predetermined amount.

6. In combination with a fluid-delivering machine including a chamber, a diaphragm subdividing said chamber into :an operative chamber containing the fluid to be delivered and an actuating chamber and a hydraulically controlled piston adapted to execute periodically a forward stroke feeding a driving fluid into the actuating chamber and :a return stroke sucking the driving fluid back to urge the diaphragm rearwardly, the provision of means tapping off a sample of the driving fluid under the pressure prevailing at a selected moment during the suc tion period before the diaphragm reaches its innermost position, means of storing said sample, a pressure ampli fier fed by said storing means and the amplifying ratio of which is equal to the amplifying ratio of the main hydraulic piston, said pressure amplifier feeding into the actuating chamber, when the pressure of said chamber has sunk underneath said amplified stored pressure by a predetermined amount.

References Cited by the Examiner UNITED STATES PATENTS 1,301,485 4/1919 Mueller 103-.44- 2,807,215 9/1957 Hawxhurst 103- 44 2,975,599 3/1961 Bennett 10344 ROBERT M. WALKER, Primary Examiner,

Claims

1. IN COMBINATION WITH A FLUID-DELIVERY MACHINE INCLUDING A CHAMBER, A DIAPHRAGM SUBDIVIDING SAID CHAMBER INTO AN OPERATIVE CHAMBER CONTAINING THE FLUID TO BE DELIVERED AND AN ACTUATING CHAMBER AND A HYDRAULICALLY CONTROLLED PISTON ADAPTED TO EXECUTE PERIODICALLY A FORWARD STROKE FEEDING A DRIVING FLUID INTO THE CONTROL CHAMBER TO URGE THE DIAPHRAGM INTO THE OPERATIVE CHAMBER AND A RETURN STROKE SUCKING THE DRIVING FLUID BACK TO URGE THE DIAPHRAGM REARWARDLY, THE PROVISION OF MEANS TAPPING OFF A SAMPLE OF THE LOW PRESSURE DRIVING FLUID AT A SELECTED MOMENT DURING THE DELIVERY PERIOD BEFORE THE DIAPHRAGM REACHES ITS OUTERMOST POSITION, MEANS OF STORING SAID SAMPLE, AND MEANS COMPARING THE PRESSURE OF THE STORED SAMPLE WITH THE ACTUAL PRESSURE OF THE DRIVING FLUID AT OTHER MOMENTS AND ADAPTED TO STOP THE FORWARD STROKE OF THE HYDRAULIC PISTON WHEN SAID ACTUAL PRESSURE HAS RISEN ABOVE THE STORED PRESSURE BY A PREDETERMINED AMOUNT.