Terminology and technical explanations of principles used in the world of precision pumps and fluid handling – compiled in compact form and presented in an easy-to-understand manner.
Comparison of the “average” dispensed value with the actual dispensed value
In the pump world, accuracy is defined as the ability of a given pump to perform relative to the average population. Tighter manufacturing tolerances allow Diener Precision Pumps to control this very well, allowing customers to set tighter control limits on their control software without fear of out-of-box calibration problems.
Precision is a pumps ability to repeat itself at a given performance point. This term applies primarily to metering pumps because they are typically used in high-precision dispense applications. The limits are shown on the product datasheets.
The temperature of the environment outside the pump
Cavitation is a common problem in pumps, caused when local fluid pressures drop below the fluid’s vapor pressure. In general, pump cavitation occurs when the inlet pressures are low. For example, if there is a blocked filter on the inlet, the pump tries to pull fluid, lowering the pressure to overcome the restriction. If the pressure continues to drop (i.e., form a vacuum) the fluid boils and forms vapor bubbles that collapse, damaging the surfaces.
Symptoms of cavitation include high noise, vibration, and unstable motor speed. The noise and vibration come from the collapsing bubbles and the motor speed becomes unstable due to the uneven torque loading. Confirmation of the problem is checked by measuring the absolute pressure at the pump inlet.
Fortunately, cavitation is preventable. Correct pump sizing is critical, as is the design of the inlet/outlet tubing and fittings. Coarse filters are acceptable on the inlet, but fine filters should be avoided because they can plug quickly and form restrictions. Avoid small diameter fittings and long lengths of small I.D. tubing. In general, size components so that the pump can “breathe” easily and remember to increase tube and fitting I.D. for higher viscosity fluids.
The metering pumps inlet stroke of Diener Precision Pumps occurs over only 90 degrees of rotation of the piston, so the fluid momentum is accelerating and decelerating quickly. It’s important to size the inlet tubing sufficiently large to allow the full flow through. It’s good practice to oversize the tube cross-sectional area to account for this.
Coefficient of variation, defined as the standard deviation divided by the mean value (for a group of samples).
The volume of fluid contained within the pump is referred to as the “dead volume”. The dead volume of a metering pump is normally measured with the piston fully retracted. Minimising this volume helps reduce cleaning time and fluids cost. In some pumps from Diener Precision Pumps, we’ve included a “quick rinse” feature that routes the fluid through the magnetic coupling area to further reduce cleaning time.
The condition when the driving and driven magnets disengage
The relative difference between the outlet and inlet of the pump (the pressure “boost” provided by the pump).
The pumps of Diener Precision Pumps are designed to run “dry” for short periods until they self-prime. However, operating a gear or metering pump “dry” for extended periods is not recommended because it generates frictional heat that can damage pumps. The limits are shown on each product datasheet.
A word of caution: starting a gear pump completely dry causes a very loud noise. We always recommend putting a small amount of fluid into the pump prior to start-up (just enough to wet the interior surfaces).
Dry and Wet Lift
The moving components in a positive displacement pumps are typically machined to very small clearances. Once these clearances are filled with fluid, the viscous shear (see Viscosity section) acts to seal the surface and increase the pump’s volumetric efficiency. When the pump is first installed in a system (and presumably dry), the surfaces are not sealed and thus air in the lines can easily get through the spaces. The pumps ability to prime itself is called its “dry lift” capability, usually expressed in mmHg or meters of H2O. The smaller the clearance between moving parts, the higher the dry lift value. Once the pump surfaces are wetted, even with just a thin layer of fluid, the clearances seal better and result in its “wet lift” capability. Positive displacement pumps always have higher wet lift values, so priming the pump prior to start-up is always beneficial: it reduces start-up response time, reduces dry friction, and lengthens pump life.
The amount of time a pump is turned “on”.
Filtering the fluid before it enters the pump is always recommended, although filters must be chosen and maintained carefully to prevent cavitation. Diener Precision Pumps recommends a maximum filter size of 40 micrometer for gear pumps and 2 micrometer for metering pumps. The pumps will pass particles larger than this, but with increasing particle size becomes the potential to damage the pump. Never pump fluids with ferrous particles (iron, steel) through magnetically coupled pumps because the particles can stick to the magnet.
Diener developed a gear pump product line specifically designed to pump fluids with suspended particulates, especially pigmented inks and paints. The gears in these pumps are specifically hardened for this purpose but require engineering review depending on the fluid type and pigment loading. Please consult our factory for application assistance.
The flow through the pump, usually expressed in milliliters/minute.
Internal Pressure Relief Valve
Some gear pumps of Diener Precision Pumps are fitted with adjustable internal relief valves. These serve two purposes: (1) to prevent over-pressurisation and (2) to prevent magnet decoupling. The valve is adjustable via an external screw adjustment. Pumps are provided with the valve completely closed so the customer can fine-tune its position during final installation. Caution: do not over-tighten the adjustment screw as it may damage the internal components.
The life expectancy of a pump will depend on the operating conditions. These include such things as fluid type, temperature, differential pressure, contaminants, and motor speed. Careful selection of the wetted components will help minimize wear without compromising performance.
The metering pump series of Diener Precision Pumps are constructed of ceramic and typically last for the life of the equipment, depending on the accuracy requirement. Metering pump life is generally measured in number of cycles (strokes).
Gear pump life is more likely measured in hours, since they are typically used in continuous operation. Designing the bearings so that they stay within the pressure-velocity limits of the polymers is the key to longevity. The new Silencer series by Diener Precision Pumps operated for 30,000 hours pumping distilled water with no drop in performance. This represents a continuous life expectancy of 3-1/2 years.
Magnetically Coupled Pumps
The gear pumps of Diener Precision Pumps are magnetically coupled to avoid shaft seals. A metallic cup separates the inner and outer magnets, eliminating the need for shaft seals. Magnetic couplings can be split into two categories: inner/outer magnet rings and stator/rotor configurations.
The inner/outer ring configuration is the most common kind of coupling. Both annular magnetic rings are magnetised radially with an equal number of poles. The rings can be made of magnetic ferrites or rare-earth materials. The ferrites are generally less expensive but larger than their rare-earth counterparts. The advantage of rare-earth magnets is the ability to miniaturise the coupling size without a loss of torque.
Ferrite magnet strength weakens with increasing temperature at a rate of approximately 0.2% per degree Celsius. Rare earth magnet strength falls off at about twice this rate. This means a lower decouple point at higher temperatures and a higher decouple points at very low temperatures.
The maximum torque a coupling can sustain is a function of the magnet material, the temperature, and the dynamic loading. When the pump load exceeds the maximum torque, the magnets “decouple”, which means the outer magnet spins at the full speed and the inner magnet stops. The magnets cannot “re-couple” unless the motor is stopped, the pump load reduced, and the unit is restarted. Running a pump in the decoupled condition does not hurt the magnets, but it will slowly generate eddy-current heating in the magnet cup, which reduces the coupling strength until normal operation is resumed.
The second coupling type (stator/rotor configuration) is the most cost effective magnetic coupling. It uses a magnet cup to isolate the fluid, but the outer ring is replaced with a motor stator and the inner magnet ring becomes the motor rotor. The advantage of this design is its simplicity and compactness. Torque limits can be controlled via current-limiting (electronically), eliminating decoupling.
The fluid contains a combination of gas and liquid.
Pump Mounting Suggestions
All products of Diener Precision Pumps can be operated in any position, although there are general guidelines to be aware of that will minimise potential hydraulic and/or hazard problems.
Gear pumps are usually configured with inlet/outlet ports in the same plane.
We define the locations as follows: side, deck, face ports.
Pumps with moulded ports and/or Diener-supplied port fittings are sized for optimum pump performance. We recommend that customer-supplied fittings be carefully sized and applied to avoid the following problems:
- The inside diameter of the fittings should be large enough to minimise the chance of cavitation.
- Tapered pipe thread fittings should be used with a liquid thread sealant or Teflon tape.
- Parallel pipe fittings should include a shoulder O-ring or thread sealant.
- Plastic tube fittings swell with increasing temperature, so select them carefully if operating across a wide temperature range.
Pump’s ability to prime itself.
The pump inlet is “starved” for fluid, usually due to an inlet restriction or no priming flow.
Static Case Pressure
The pressure inside the pump when the pump is turned off.
The term used to describe a piston that will not move in the cylinder.
The fluid and ambient temperature ranges are shown on each product datasheet . These limits can be customised by Diener Precision Pumps engineering team for elevated temperature or cryogenic applications.
The gears in most of our pumps are usually constructed of engineering thermoplastics with a fibre reinforcement to increase strength and control thermal expansion. The clearances in these pumps are kept relatively small to improve volumetric efficiency, which means the specific flow may increase as the gears expand. This is not a problem unless the temperature increases so much that the gears bind in the cavity. Staying within the specified temperature ranges will ensure reliable operation.
The ceramics used in the metering pumps are thermally stable so the displacement will remain constant across the datasheet temperature range.
Viscosity is a fluid’s resistance to shear stresses and is sometimes referred to as the “thickness” of a fluid. Viscosity is typically measured as Dynamic or Kinematic. Dynamic viscosity is a fluids shear resistance to itself (usually expressed as centipoise, where 1 cP = 1 mPa-s = 0.001 Pa-s) and Kinematic is the ratio of the Dynamic viscosity to its density and typically has the units of Stokes (m2/second).
We often compare a fluid’s viscosity relative to water. A fluid thicker than water is more viscous, while thinner fluids are less viscous.
Pumping higher viscosity fluids requires slowing the pump speed and increasing the size of interior pump openings and tube diameters. Please contact us for practical viscosity limits for Diener Precision Pumps products.