Hydra-Cell Metering Solutions
Challenge the Traditional Definition of Metering Pumps
What gives a particular pump special status as a metering pump, and have advancements in technology affected this definition?
Many types of pumps and systems exist for generating controlled liquid flow, from closed-loop centrifugal pump systems to positive displacement rotary pumps. The technology demanded in precision chemical injection, however, has long been the reciprocating metering pump. Although metering pump systems are distinguished for accuracy, linearity and repeatability, traditional metering pumps suffer from operational drawbacks. Inaccuracies in pumping, lost motion, and potential leakage are common during stroke adjustment, and the intermittent, pul- sating flow of metering pumps places strain on the system. A brand of pumps fea- turing electronic flow control and a multiple-diaphragm design overcome these encumbrances, potentially redefining what a metering pump is.
Metering pumps have benefited from advancements in technology in the many years since their predecessor, the lubricator, first made its appearance on steamships. Packed piston and plunger pumps are capable of a wide range of flows and pressures, but contact of the piston or plunger with the pumped medium necessitates compatibility of the materials of construction and fluid. This can result in costly replacement and maintenance. The environmental and safety issues of leak- age also contributed to the decline in popularity of this design over the seal-less advantages of the diaphragm metering pump.
The first Hydra Cell seal less pumps was manufactured in the early 70's, focusing on agricultural applications and the burgeoning car wash market. Featuring a simple design that helped make it reliable and durable, it quickly became a pump of choice in these industries. The reputation of Hydra cell propelled sales into different markets as new users discovered the advantages of seal less pumps.
New applications discovered for seal-less pumps
Many Hydra-Cell pumps were operating successfully in applications demanding accurate dosing. In many cases, Hydra-Cell was replacing traditional hydraulically balanced, diaphragm metering pumps. Like metering pumps, Hydra-Cell pumps feature hydraulically balanced diaphragms, but also offer electronic flow control, and most models have multiple diaphragms.
Evolution to accuracy - electronic flow control
The effects of technological advances are most evident in the manner in which flow is adjusted with a hydraulically actuated metering pump. A metering pump’s capacity is a function of the diameter of the plunger, the effective length of the stroke and the rate or speed of stroking. Since the diameter of the plunger must remain constant in any given pump, varying the stroke length and pump speed are the only ways to adjust flow. Many years ago, manual stroke adjusters were added as a feature to metering pumps. Initially, these adjusters could not be used while the pump was operating. Design improvements would later allow for altering stroke length during process. There are two main classifications for stroke length adjustment. The first, usually referred to as amplitude modulation, varies the radius of eccentricity of the plunger drive mechanism. In basic terms, 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 classification, referred to as 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.
Recently, the use of variable speed drive motors (VFD) to change stroke speed rather than length has grown dramatically. One advantage is that AC and DC drives can respond more quickly, with approximate speeds of 0 to maximum RPM in 0.5 and 1.3 seconds, respectively. Faster flow correction results in greater long-term accuracy.
In addition, VFD motors are often less expensive than the electronic actuator alternative. Improvements to reliability, repeatability and linearity performance are also benefits of the AC drives. 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.
Since full stroke length is considered optimum for metering pump performance, changing speed as opposed to stroke length to alter flow has gained in acceptance, lessening the importance of a manual stroke adjustment mechanism. Hydra-Cell Metering Solutions pumps, for example, address flow changes using only VFD motors and controllers, always at full stroke length.
Multiple-diaphragm design reduces pulsations
Another commonality among most metering pump designs is the single diaphragm configuration, responsible for the non-linear flow accepted as a “necessary evil” of metering systems. Hydra-Cell Metering Solutions pumps have as many as five diaphragms per liquid end, each with a corresponding set of valves and pistons. The virtually “pulse-free” flow characteristics of these multidiaphragm pumps eliminate many issues long considered inevitable by reducing acceleration losses and pipe strain. This can remove the need for dampeners in the system and expand application opportunities to those requiring linear flow.
To illustrate the effects of pulsation, Hydra Cell conducted a test of its Metering Solutions multi-diaphragm pump and a typical, single hydraulically balanced diaphragm metering pump. Operating under identical flow and pressure conditions to record the pressure traces, the results were dramatic.
Traditional metering pumps can minimize pulsations with dampeners and by multiplexing together several pumps, sequencing the diaphragm strokes. These options, however, add significant costs, size and maintenance to the system.
A pump designed like the Hydra-Cell pump offers major advantages because the wet end can remain constant and the gearing, by employing different gearboxes with different ratios, is the only difference covering a wide range of flows and pressures. Changes in process requirements can be addressed with the change of a gearbox alone. This reduces acquisition, maintenance and downtime costs for the pump. In addition, spare part kits remain the same, reducing inventories.
These size increases, which also include the drive cases for many manufacturers, may result in massive footprints at higher flows and pressures. A smaller Hydra- Cell pump is capable of producing the same capacity ratings as large multiplex systems, while still meeting API 675 performance standards for steady-state accuracy, linearity and repeatability.
These changes may provide solutions to user problems, but they also exclude Hydra- Cell pumps from the strict definition of "metering pumps" or "controlled volume, positive displacement pumps."