1. Field of the Invention
This invention relates generally to pumps and in particular to metering pumps.
2. Description of the Prior Art
Fluid metering is used in virtually every process where liquids are either mixed or used for specific purposes in manufacturing or various processes. Accuracy and consistency is important, because the efficacy of the process may be affected if the exact amount of additive liquid chemical is not applied to a process stream or process reservoir of liquid. Further, the cost of various chemicals that are blended or metered may be quite high, so for a system to be cost effective, the process of metering must be accurate. Additionally, metering or blending of various chemicals into process streams or process reservoirs must be consistent. Slugging or injecting with intermittent pulses is not desirable, because the consistency of the final mixture varies with uneven delivery of the injected fluid.
Additives for all types of motor and aviation fuels, additives for sanitizing water, additives for oilfield production and pipeline delivery are only a few of the many applications where consistent, accurate metered injection of various liquids is required.
Prior art metering pumps have included high speed reciprocating devices that utilize diaphragms, pistons or plungers as a means for moving the fluid and rely on check valves located on the suction and discharge side of the pumping chamber to isolate the suction from the discharge cycle. Such pumps usually have small displacement, and rely on the movement of fluid to aid in closing and opening the check valves on the suction and discharge side of the pump.
Any leakage of the valves whatsoever compromises the accuracy and consistency of the pump. Slow movement of such pumps is disadvantageous, because slow moving fluid reduces the amount of fluid energy available to seal or seat the valves. As a result, most such pumps run at relatively high speed. Some diaphragm pumps may only cycle periodically, but the pumping speed during that cycle is very fast in order to effect good valve sealing.
Such low-volume, high-speed pumps share common problems. They tend to cavitate if operated with less than ideal suction conditions. Changes in fluid viscosity are also a problem, because such pumps usually have small passageways and valves that are easily clogged or become less efficient when heavier fluids are introduced. Some fluids become more viscous when temperatures are lowered, making it difficult to hold injection ratios over a wide range of operating speed and temperature.
Another common problem with prior art metering pumps is loss of efficiency due to cavitation or leakage through poorly seated valves.
Another problem is the lack of precise control over delivered flow rates.
Another problem of prior metering pumps is in the complexity of piping between reservoir and pump along with potential problems arising from friction loss in pipe and fittings.
Another problem of prior metering pumps is that when used with certain chemicals and when subjected to a negative pressure, such as in suction lift applications, the liquid chemical goes through a phase change to gas, causing the pump to lose its prime, or pump with less efficiency.
Another problem of prior metering pumps is the fact that the pump is usually mounted outside of the fluid tank requiring separate mountings for the pump and tank, requiring connecting pipe and fittings between the pump and tank with the chance of inducing air into the suction of the pump and ineffectiveness due to friction loss of flow through the fittings or connecting pipes or tubing.