ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
esign
n
U
Pressures to 58,080 Pounds per Square inch JOHN T. BARRON
AND
ROBERT T. SHEEN
Milfon Roy Co., Philadelphia 7 8, Pa.
URING the past 15 years, controiled voIume pumps . have developed from mere adaptations of forced feed lubricators to precision flowcontrol instruments for use a t pressures as high as 50,000 pounds per square inch. Basically, these pumps consist of a plunger reciprocating in a specifically designed displacement chamber or liquid end with check valves on suction and discharge side (Figure 1). The model illustrated is used for pressures as low as 7600 pounds per square inch. Properly designed, the displacement chamber permits a controlled volume of liquid to be delivered for each plunger stroke, a quantity determined by the plunger diameter and plunger stroke length. Since the volume delivered is a function of the plunger stroke length and the stroking speed, it is possible t o adjust the volume delivered by manually or automatically varying stroke length or speed. The various types of capacity adjustments employed have been fully described in many published articles (5).
Early Design-10,OQO to 30,000 Pounds per Square Inch With the trend toTard higher operating pressures in various fields, the manufacturers of controlled volume pumps designed special liquid ends in order that this equipment would meet the requirements for service in the range 10,000 to 30,000 pounds per square inch. SIuch of the design of the reciprocating mechanism
forging was used; the passages connecting suction and discharge openings and those between one check valve and another mere machined rather than cast. In addition, the total internal volume of the liquid end is maintained as small as possible because of the compressibility effect on liquids a t these pressures, as described in a previous report on this subject ( 2 ) . The major problems of sealing the pressures within the liquid end and particularly around the reciprocating plunger were met. Standard high pressure tubing sealed the discharge outlet connections to the high pressure systems. Mushroom type plugs, machined to provide a tight metal-to-metal seal with the sockets machined in the liquid end of the body, sealed the openings over the check valves. A hardened plunger, superfinished t o &to5 microinches and a special packing arrangement consisting of soft metallic foil packing combined with other materials proved satisfactory up to a maximum pressure of 30,000 pounds per square inch.
Current Design--50,000
Pounds per Square Inch
With the continued trend toward higher operating pressures. redesign was again required to permit operations a t pressures up t o 50,000 pounds per square inch. The stresses a t these high pressures in the pumping cylinder, plunger, and sealing parts posed a challenge. Fatigue stresses were carefully considered,
F i g u r e 1 . S t a n d a r d m o t o r - d r i v e n , c o n t r o l l e d v o l u m e pump for pressures to 7500 Ib./sq. inch of the lower pressure pumps was used by enlarging parts, increasing bearing areas, strengthening parts by changing materials of construction, and redesigning members such as the plunger, where column action had to be considered. Early major changes occurred in the liquid end or displacement chamber where the higher pressures vere encountered (Figure 2). A solid 882
since these pumps can operate a t 100 strokes per minute and, a t each stroke, the pressure changes from a minimum atmospheric pressure to the maximum discharge pressure. This change in liquid pressure inside the pump chamber results in cyclic stresses up to the maximum which is more severe loading than a constant pressure loading or a fluctuating pressure between narrow limits.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 48, No. 5
EXTREME CONDITION PROCESSING Liquid End Details. I n these 50,000-pound per square inch pumps, the liquid end is symmetrical, lending itself t o straightthrough machining and more determinable calculations of the stress loads that are developed in pumping against high pressures (Figure 3). The pump chamber itself is a forged cylinder. The bore which contains the plunger is machined in a straight line, providing a minimum number of sections t h a t must be analyzed t o determine the stresses t h a t the pump cylinder must withstand. The bore of the cylinder is machined to close tolerances and honed to remove any fine cracks or lines t h a t can result in localized stresses. The finish of the cylinder aids in maintaining the seal between cylinder and packing.
Figure 2.
Early design of high pressure liquid end for pressures to 25,000 Ib./sq. inch
Forged materials of construction are used for this cylinder t o provide materials as dense as possible t o withstand stresses. For pressures up to 25,000 pounds per square inch, 316 stainless steel has proved satisfactory. For pressures above 25,000 pounds per square inch, a carefully selected grade of Type 4340 steel is employed after heat treating to provide the physical requirements necessary for pumps working against these high pressures. Care must be exercised as well in selecting the proper materials of construction for the plunger, since t,his must be rigid enough t o overcome column effects and hard enough t o minimize wear from the packing. Obviously, if these requirements were not met, the plunger would tend t o bend and the packing would erode the plunger, resulting in pump leakage. Carbide is used for very high pressures, although research is continuing on the use of ceramic plungers. Ball checks and seats, too, must be furnished in a hard material of construction, such as carbide, t o preclude rapid erosion of these parts through leakage of liquid at high pressure t o a low pressure area. Figure 3. The drive connector is fitted with a hardened steel thrust spacer which takes the thrust load of the plunger (on the discharge stroke) and distributes this stress over a laige area. By using this spacer it becomes possible t o fabricate the drive connector of soft material, thus maintaining economical machining of the crosshead. I n addition t o these problems of stressing of pump parts, careful consideration has been given to design of the internal volume in the liquid end and t o the amount of ball-check rise, since both conditions affect pumping accuracy. Obviously, volumetric efficiency at these high pressures can be adversely affected by May 1956
compression and expansion of a n i volume of liquid remaining in the liquid end after the discharge stroke has been completed. A small clearance area at the extreme forward end of t h e pump chamber contains a spring that gives the packing its initial set and provides openings from the suction and discharge ports into the pump chamber. The clearance between the plunger and cylinder is eliminated by a loosely fitted spacer, as illustrated. Pumping accuracy can also be adversely affected by insufficient or excessive ball-check rise. Accordingly, guides are used t o limit ball-check rise. Overhung load is eliminated in the patented HyROYmetric design (Figure 4) by the use of a crankshaft supported on each end b y heavy duty roller bearings. One of the unique features of this design is the method of stroke length adjustment. The lead screw, turned by the handcrank, causes the trunnion t o ride up and down on the screw between the trunnion guides on either side of the frame. The split trunnion filler in turn moves the left-hand swing lever pivot up or down in the swing box. Note that the arc length of the swing box travel does not change for different stroke lengths, since crank shaft throw and connecting rod travel are constant. This arrangement for stroke length adjustment produces another singular and very important feature. The crosshead always travels full forward for any setting of stroke length. This means t h a t the plunger travels full forward, scavenging the liquid end on each stroke. Complete scavenging of the liquid end, regardless of stroke length setting, gives maximum volumetric efficiency and highest possible metering accuracy. Stroke length adjustment can be made manually by means of a crank, or automatically by means of a control air signal (3 t o 15 pounds per square inch). Pressure Seals and Safeguards. A major design problem involves sealing of the high pressures within the liquid end and particularly around the reciprocating plunger (Figure 5). Inter-
BALL CHECKS
BALL CHECKS
Current design of high pressure liquid end for pressures to 50,000 Ib./sq. inch
ference-angle fits seal the suction and disuharge connections as well as the forward end of the cylinder between the mating pieces. Screw threads effect no sealing but are used only to effect the physical loading on the sealing pieces. The carbide seats are brazed t o high alloy steel heat-treated taper pieces t h a t maintain the liquid seal between ball checks and pump cylinder. These mating surfaces are made by grinding t o a very fine finish and then lapping as the final operation t o provide
INDUSTRIAL AND ENGINEERING CHEMISTRY
883
ENGINEERING. DESIGN, AND PROCESS DEVELOPMENT
Figure 4.
HyROYrnetric drive
continuous sealing, thereby eliminating mire drawing 811 sealing joints are made Kith a self-centering design which helps in the economy of manufacturing this unit Sormal, commercial manufacturing procedures are sufficient to maintain screw threads and lapped sealing surfaces concentric to eliminate leakage on the sealing port members. I N N E R PACKING
There is alva\-s a degree of hazard connected with handling pressures as high as 50,000 pounds per square inch. Accordingly, a bronze steel cap is placed on the forward end of the pumping cylinder. Should pressures become excessive as a result of stoppage in the valves or other parts of the system, the end sealing plug would blow off, but its energy is dissipated by the cap before it ruptures. For the same reason of safety, it 1s advisable to use blowout disks in both suction and discharge lines of high pressure systems. Since low pressure piping is used on suction lines the use of a blom-out disk can prevent this line from breaking should complete failuie of the system occur. The possibility of this condition occurring is, of course, very remote.
Conclusions MALE
OUTER
Figure 5.
ADAPTER
PACKING
1s
FEMALE ADAPTER
SPACER
Cross section of superpressure packing
The problem of sealing p~cssuie;! ai ound the reciprocating plunger Tas solved, after considerable research and testing, by packing (patent applied for) based on the piinciple of an unsupported area. The pressure applied against the end of the packing is transmitted by tapeied surfaces against the softer metal that is used to conform to the plunger and pump cylinder, thereby effecting a seal. Since we are using metal as our sealing medium, we must use extremely abrasion-resistant plungers to withstand wiping action. The plungers must be concentric and true in diameter Extieme care also must be used when the pump is first placed in operation t o properly break in the packing. Considerable heat can be generated if the contact betveen plunger and packing is improper, resulting in high unit loading on the packing. Unless all rings have full intimate contact with the plunger, enough heat can be generated by surface friction t o melt the relatively soft, low melting point metal packing.
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Operating data on installations of the basic design described are not available for release a t this time Horn-ever, the authors recommend that application of this or any other design of high pressure, controlled volume pump, operating continuously under extreme conditions, be related to an acceptable economic life for the equipment Reciprocating shock loads cycling from zero t o maximum impose severe stresses on the containing vessel and more than 12,000,000 cycles can be realized in less than 4 months of continuous operation a t 20 strokes per minute. The design described in this article can be used only with fluids that have some lubricating properties; otherwise wear would allow only a minimum packing life.
References Bridgman, P. W., “Physics of High Pressure,” George Bell and Sons, London, 1949. (2) Jones, D. R., Trans. Am. Soc. N e c h . Engrs. 75, 361-7 (April 1953). (3) Sheen, Robert T., “Instrumentation by and with Controlled Volume Pumps, ” ISA Paper 48-4-3, 1948. (1)
RECEIVED for review October 13, 1955.
INDUSTRIAL AND ENGINEERING CHEMISTRY
ACCEPTEDMarch 27, 1956.
Vol. 48, No. 5
