A Primer on Metering Pumps


Hydraulic Bypass

 Some designs feature a hydraulic bypass mechanism that uses a piston with a constant stroke length that pumps hydraulic fluid, thus transferring the pumping motion to a diaphragm. Therefore, this type of drive can only mate with a hydraulically actuated diaphragm liquid end.
Capacity is varied by changing the location of a hydraulic bypass port over the piston's path of travel. If the port is positioned at 50 percent of the piston's stroke length, hydraulic fluid will be relieved through the port during the first half of the piston's stroke, and pumped against the diaphragm during the remaining half.
This type of drive is often called hydraulic lost motion, because a portion of the piston's travel does not transmit pumping energy when the capacity adjustment is less than 100 percent. Hydraulic bypass style pumps can develop reciprocating piston motion by way of a worm gear set and eccentric.
In these type of pumps, a piston pumps hydraulic fluid, which either forces the diaphragm to flex or is relieved through the bypass port. A control valve positions the port based on a desired capacity setting. A disc diaphragm liquid end can feature simplex or duplex liquid ends, with maximum capacities ranging between 0.43-gph and 85-gph (170-gph Duplex), and maximum pressures up to 1800-psi.
In some hydraulic bypass style pumps, capacity can be varied by positioning a stroke adjust sleeve over bypass ports bored through the hollow piston. When operating at 100 percent the ports are covered, which traps hydraulic fluid in the hydraulic pumping chamber. Once trapped, the piston's pumping action forces the hydraulic fluid to flex the diaphragm.
A cup valve attached to the diaphragm closes all hydraulic paths to the diaphragm when it reaches the full forward position. This eliminates a process contour plate, as well as excessive hydraulic pressure on the diaphragm, since any excess hydraulic fluid in the hydraulic pumping chamber cannot reach the diaphragm and is forced through the internal relief valve to the fluid reservoir.
A hydraulic bypass style pump can be an excellent choice for a mid-range metering pump capacity at low pressure. Its design is more economical than high pressure pumps in the same capacity range, without sacrificing ruggedness and accuracy. The "straight through" process fluid path allows this metering pump to be applied to many of the same services as the HPD (high performance diaphragm) liquid end.

Polar Crank

 Some metering pumps use a polar crank, an advanced and reliable variable stroke length drive available in high pressure/high flow industrial duty metering pumps.
In the polar crank drive, a high speed worm gear reduces the RPM supplied by the motor, and provides the lower RPM to a rotating crank. A connecting rod with spherical bearings on each end links the crank to the crosshead and piston assembly. The worm gear and crank assembly pivots in an arc about the worm shaft center to change stroke length. The piston stroke length is determined by the angle of the assembly.
For example, when the pump is at zero stroke, the worm/crank assembly is in a vertical position. The crank then rotates in a vertical plane, and one end of the connecting rod revolves with it. The crosshead and the piston remain stationary because no reciprocating action is produced. When the pump is adjusted full stroke (or maximum capacity), the rotating crank is moved to its maximum angle from the vertical axis.
At the top of the rotation cycle, the connecting rod is pushed forward, moving the crosshead and piston to the full forward position at the end of the discharge stroke. As the crank continues to rotate, the angle of the crank causes the connecting rod to pull the crosshead and piston until it reaches the full rearward position, at which point the connecting rod has reached the bottom of the rotation cycle.
 Regardless of the stroke length setting, the top of the rotation cycle always forces the crosshead and the piston to the full forward position at the end of each discharge stroke. This assures complete scavenging of the liquid end during each stroke cycle. The angle of the polar crank can be adjusted in infinite increments between zero and maximum stroke for extremely accurate controlled volume pump settings.
A polar crank drive can feature maximum capacity ranges between 0.033-gph (125-mL/hr) and 2510-gph depending on frame size, stroking speed, and plunger diameter. Discharge pressures are rated up to 7500-psi and up to eight pumps can be multiplexed and driven by one motor. Polar cranks can include HPD, packed plunger, disc diaphragm, or tubular diaphragm liquid ends.
To achieve a high thrust capacity and extend component life, some polar crank drives feature a pressurized lubrication system. This positive oil pressure lubrication ensures long bearing life and permits the pump to operate at very high suction and discharge pressures.
As the crosshead moves forward during the discharge stroke, oil from the reservoir is drawn up through a ball check into a cavity in the crosshead. During the suction (rearward) stroke the lubricant is trapped. It is then forced through the crosshead, into the crosshead connecting rod bearing, through the hollow connecting rod, and finally to the crank connecting rod bearing.
By forcing the oil through this path, every moving part is lubricated during every complete cycle of the pump. To reduce the wear of moving parts and extend oil life, a magnetic strainer cleans the oil before it enters the pressurized system.

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