Advanced Drive Technology
A new innovation in metering pumps combines advanced gear reduction design with electronic variable speed drives to achieve twice the accuracy over a turndown ratio (100:1) ten times greater than traditional designs, with ± 0.5 percent steady state accuracy.
The most significant difference from traditional systems is the close relationship between the driver and drive mechanism and how they enhance overall pump performance. The operating characteristics of this technology are the result of a unique constant stroke length drive mechanism that depends on a special electronic variable speed drive to vary pump flow rate.
Traditional designs utilize worm gear sets to convert motor rotation to reciprocating motion through an eccentric or similar mechanism. Worm gears are the best choice when the drive mechanism is required to incorporate a variable stroke length adjustment. They operate well at high rotary speeds, due to an oil shield that develops between the gear surfaces. Unfortunately, they lose that shield at lower speeds, which causes wear and raises motor torque requirements. This limits worm gears to a 10:1 turndown ratio.
This new technology utilizes a special helical gear set. Since there is no need for stroke length adjustment, the gear set is very simple. Helical gears are known for operating with low noise and low friction. Combined with a basic scotch yoke mechanism to develop reciprocating motion, this gear set runs quietly and efficiently with few moving parts. All moving parts are submerged in oil to ensure long life.
Figure 1 indicates the greatest advantage of the special gear design. The flat torque curve (vs. the worm gear set) allows the technology to operate at or below 1 percent of speed without placing extra demand on the motor, thus permitting a 100:1 turndown ratio. The simple drive mechanism also allows the product to be easily duplexed within the same housing by adding a second piston and liquid end opposite the primary liquid end. The capacity can therefore be doubled economically and efficiently.
Conventional AC and DC variable speed drives are limited to turndown ratios between 5:1 and 30:1. This is insufficient to take advantage of the turndown capabilities of this gear design. New technology in brushless DC motors and controllers has created advanced drives capable of strong operation at low speeds. The variable speed drive can deliver rated torque at less than 1 percent (100:1 turndown) of maximum rated RPM while maintaining steady state speed control at better than ± 0.1 percent. It works well with the drive mechanism.
The flexibility of a 100:1 turndown ratio permits these metering pumps to be applied where a wide range of dosage rates are required. It can also provide built-in growth potential by pumping efficiently in systems requiring a fraction of the pump's capacity for the short term during startup or early phases of an expanding project. These pumps can provide all this without compromising accuracy or drive power.
This new technology can respond instantly to changes in dosage rate. Its constant stroke length does not upset the balance within the liquid end hydraulic system; that is, hydraulic fluid volume remains constant. When the hydraulic balance is disturbed, as in variable stroke length designs, the full result of dosage changes can take minutes or hours. The instant response can provide smoother operation in closed loop or automated systems, and also assure proper dosage at all time in systems requiring very close dosage tolerance.
The ± 0.5 percent steady state accuracy over the full turndown ratio is a result of the constant stroke length and the precise speed control of the drive. This level of accuracy provides maximum chemical economy while assuring stable automatic operation and optimum process quality.
Metering Pump System Components
Proper metering pump system operation depends upon the selection of appropriate system components suited for the application requirements.
Safety Relief Valves
Most piping systems require the use of an external safety valve to protect the piping from over-pressure. Diaphragm pumps feature internal safety valves to protect the pump, but external safety valves are still recommended. Safety relief valves should match the operating pressures of the pumps being used. Typical valve materials include specialty steel, 316SS, alloy 20, and PVC.
Back Pressure Valves
To prevent unmetered liquid from free-flowing through the pump, metering pump systems require a greater pressure in the discharge line than the suction or inlet line. When the process does not supply a minimum of 25-psi above the suction pressure, a back pressure valve is required. Typical valve materials include specialty steel, 316SS, alloy 20 and PVC.
The metering pump's reciprocating motion provides a pulsating discharge flow. Applications requiring a steady flow can eliminate over 90 percent of the pulsations with a pulsation dampener. Typical dampeners are available for pressures to 1000-psi. Sizing is based on cubic inch/stroke displacement of the specific pump.
Metering pumps should be factory tested. Once installed, pump calibration should be periodically determined to verify proper operation, especially after the performance of any maintenance. Calibration columns can provide an inexpensive means of assuring pumping accuracy.
Accurate dosing requires proper mixing of the solution being pumped. Typical mixers are direct drive, high speed units designed for mixing medium and low viscosity fluids and dispersion of light solids.