Subscribe to our

There are similarities, because some of those seal-less pumps and traditional metering pumps both feature hydraulically balanced diaphragms. Key differences, however, were that the newest seal-less pumps offered electronic flow control and, with most models, multiple diaphragms.

Like traditional metering pumps, the newest metering pump technology offers the advantages of a seal-less, hydraulically balanced diaphragm design. However, unlike traditional metering pumps, it also has electronic flow control to improve accuracy and a multiple-diaphragm design to reduce pulsations.

In traditional hydraulically-balanced diaphragm pumps, the diaphragm is balanced between two fluids, the process liquid and the actuating medium. This balanced design allows for higher discharge pressure and higher flow rates than the mechanically actuated diaphragm design.

These hydraulically balanced diaphragm metering pumps share several inherent features:

• Primary flow adjustment through change in stroke length via manual control;
• Single diaphragm per liquid end;
• Pulsating, intermittent flow;
• Limited flow and pressure range per plunger, diaphragm and liquid end combination;
• Dramatic footprint increases in proportion to flow and pressure capabilities.

Evolution to Accuracy: Electronic Flow Control

The flow rate of a metering pump can only be adjusted by varying the stroke length or speed. The first manual stroke length adjusters could not be used while the pump was operating, but later improvements would allow for altering the stroke length during the process.

One method of adjusting stroke length is amplitude modulation, varying the radius of eccentricity of the plunger drive mechanism. A slider crank allows the stroke length to be altered by changing the length of a pivot arm, similar to the movement of a pendulum. This is attached to the piston, the stroke length of which corresponds to the size of the arc of the pendulum.

The other method, lost motion, can be further subdivided into mechanical and hydraulic lost motion. In mechanical lost motion design, the motor turns a worm shaft, which rotates an eccentric gear. A cam rotates with the gear and actuates the plunger through a cam follower. As the plunger moves forward on the discharge stroke, it displaces the fluid behind the diaphragm, which, in turn displaces the medium being pumped. A spring then retracts the plunger to its original position. Limiting the rearward travel of the plunger changes the stroke length and the resulting flow rate.

Hydraulic lost motion involves a change in the effective, as opposed to the actual, stroke length. In this design, the plunger reciprocates the entire length of the stroke, but a portion of the actuation fluid is deflected through a bypass valve.

With the growth of automation, pneumatic and electronic actuators were attached to the stroke adjustment mechanism for both amplitude modulated and lost motion metering pumps. Although convenient, there are resultant pumping inaccuracies during the adjustments due to the slow rate of change (typically one second per one percent of stroke length).

A recent improvement incorporated by the newest metering pump technology is the use of variable speed drive motors to change stroke speed instead of stroke length. AC and DC drives can respond more quickly, with approximate speeds of zero to maximum rpm in 0.5-sec and 1.3-sec, respectively. Faster flow correction results in greater long-term accuracy.

VFD motors are often less expensive than the electronic actuator alternative, and AC drives improve reliability, repeatability and linearity. Many of these drives are available with turn down ratios of 1000:1, as good or better than those that can be achieved using the electronic actuator in conjunction with the manual stroke adjuster.

Variable frequency drive (VFD) motors and controllers are usually less expensive than electronic actuators and provide full stroke length, thus improving accuracy, eliminating lost motion, and eliminating a potential leak path in metering and dosing applications.