A Primer on Metering Pumps


HPD Preshaped Composite Diaphragm

A HPD features a preshaped PTFE/elastomer composite disc diaphragm. On the process side, the chemical resistance of PTFE is utilized. On the hydraulic side, the elastomer imparts favorable elastic and mechanical factors.
A composite diaphragm eliminates the inherent problems of pure PTFE diaphragms. PTFE tends to cold flow when compressed between two metal parts (such as those required to seal the hydraulic side from the process side). A HPD composite diaphragm features an integral O-ring seal around the perimeter of the diaphragm, which provides a better seal between hydraulic and process fluids than conventional diaphragm materials. A HPD is capable of handling pressures up to 3025-psi and temperatures up to 300-deg F (with special modifications).

Metering Pump Drive Mechanisms

Ideally, drive mechanisms feature gears that are submerged in an oil bath to assure long life. Capacity can be adjusted while the pump is running or stopped, ± 1.0 percent accuracy over a 10:1 turndown ratio.

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.

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See also:

Upstream Pumping

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