A Primer on Metering Pumps

Traditional Designs

Traditional designs use a system that refills the chamber when a vacuum is created by the inability of the diaphragm to move beyond the hydraulic contour plate. It also refills when the suction is momentarily or permanently starved by accidental valve closure, insufficient NPSH, or other similar occurrences. When this happens, the hydraulic fluid chamber is overfilled because a vacuum has been created even though the diaphragm has not been able to travel rearward.
 To avoid diaphragm rupture due to overfilled hydraulic oil, a process side contour plate stops the diaphragm's forward travel, and forces the hydraulic relief valve to open, thus expelling the excess fluid. The contour plate is a concave (actually, concavo-convex) disc that supports the diaphragm and limits its travel. The plate has a series of holes bored through it to permit the fluid to come into contact with the diaphragm. The pattern and size of these holes requires careful engineering to maintain the contour plate strength required to withstand the force of the diaphragm experienced at operating pressure.
The hydraulic contour plate does not cause any problems in pump operation since the hydraulic fluid passes easily through the contour plate holes. However, a process contour plate, required by traditional disc diaphragm liquid ends, places limitations on the types of process fluids the pump can handle (such as slurries) since the process fluid must also pass through contour plate holes. The process contour plate also creates a pressure loss which raises the NPSH requirement of the liquid end.

MARS Design

A MARS eliminates the need for a process contour plate by assuring that the hydraulic fluid can only be refilled when the diaphragm has traveled all the way back to the hydraulic contour plate. The diaphragm presses against the MARS valve, which only then permits a poppet valve to open from the vacuum created by insufficient hydraulic fluid. Hydraulic overfill is therefore impossible.
With the process contour plate gone, the straight through path of the process liquid makes a HPD a perfect choice for slurries and viscous materials. It also lowers the NPSH requirements of the pump, since pressure loss through a process contour plate is eliminated.
A MARS also simplifies HPD start-up. Unlike other hydraulic liquid ends, the refill valve does not need adjustment. Additionally, since a HPD hydraulic fluid cannot be overfilled, there is no need to perform delicate procedures to synchronize hydraulic fluid balances (a difficult task required for tubular and other double diaphragm liquid ends). With a HPD, just fill the reservoirs and turn it on.

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.


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See also:

Upstream Pumping

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