Notebook 3 : How do I select a dosing pump?

It is important to have a complete description of the process fluid and operational conditions for proper sizing and selection of a dosing pump. With this information leading manufacturers will guarantee the performance of their pumps under specific conditions of pressure and temperature. Without this data no valid guarantee is possible.

The specific details of the process fluid and installation that are required include:

Chemical composition.
Physical properties under process conditions including specific gravity, viscosity, vapour pressure and solids content, as well as process temperature and inlet and outlet pressures.
Any unique properties such as toxicity, stability, compatibility with common materials of construction.
Installation layout details.

The importance of providing this information cannot be overemphasised. If necessary a secrecy agreement may even be required to protect the purchaser’s interests while enabling the supplier to correctly access the materials of construction and installation criteria. Many leading manufacturers and suppliers will willingly assist at this stage of the selection process giving the purchaser the benefit of their experience. However the extent that this information is not supplied, the risk of unsatisfactory results is directly increased. Chemical composition if not fully available should not be substituted with statement’s such as “corrosion inhibitor”, “solvent” etc. as these do not provide sufficient information for accurate assessment of requirements. To the knowledgeable supplier, however, “10% water solution of alkaline phosphates” is adequate. Otherwise the pump supplier goes for the maximum resistant and possibly uneconomic materials for diaphragm, 0-rings and wetted parts. Acids should be clearly detailed in terms of concentration and details of different combinations will also assist with design criteria. Dissolved chlorine even at the 1% level should be noted.

Specific Gravity, Viscosity and Vapour Pressure are important. In some instances it is necessary to know the SG in order to convert the given viscosity in centipoises / milliPascal [mPas] to centistokes or visa versa.

Viscosity is important and when this is a function of temperature it is called a Newtonian fluid. Flow rates or shear rates are directly proportional to the stress imposed. Non-Newtonian fluids have a viscosity that is directly affected by flow rate or shear it self. Some classes of non-Newtonian fluids have reduced viscosity as flow rate or shear rate increases, such as polyelectrolyte. While with others the viscosity will reduce as a function of time at constant shear rate and still others the viscosity will increase as the flow rate or shear rate increases. Without previous experience with a given fluid it may be necessary to carry out tests to determine the correct requirements for a good result.

Viscous Fluid Behavior

Effective viscosity can be better understood by looking at the behavior of viscous fluids at different shear rates. There are a number of types of viscous fluid behavior.

1. Newtonian Fluids: Viscosity is constant with change in shear rate or agitation. Forces to cause motion increase proportionately as speed increases. Fluids showing Newtonian behavior include water, mineral oils, syrup, hydrocarbons, and resins.
2. Thixotropic Fluids: Viscosity decreases as shear rate or agitation increases. The forces to cause motion are relatively lower as speed increases. Examples are soaps, tars, vegetable oils, shortening, glue, inks, peanut butter, and some slurries.
3. Dilatant Fluids: Viscosity increases as shear rate increases. Forces to cause motion may greatly increase as speed increases. Some liquids showing dilatant behavior are slurries, clay, and candy compounds.
4. Plastic and Pseudo-Plastic Fluids: Viscosity decreases as shear rate increases, but initial viscosity may not be great enough to prevent start of flow in a typical pumping system. Typical plastic fluids are gels, latex paints, lotions, and shortening.

Vapour Pressure is the pressure exerted by a liquid at any given temperature when equilibrium exists between the liquid and vapour phases and is a direct function of temperature. A rule of thumb is that NPSHa should exceed the vapour pressure by a minimum of 35 kPa. Water and must aqueous solutions have a vapour pressure of 7 kPa or less at 38oC and generally are not a concern at ambient temperature. High suction lift, high temperatures and dissolved gases such as ammonia and hydrogen chloride warrant careful consideration.

Solids content requires special consideration as generally diaphragm and check-valve designs are based on handling clear liquids. However, solids content are pumped using expanded valve chambers and valves with soft seats. Dosing pumps are available specifically for dosing slurries and depending on the natural fluidity of the solids involved concentrations can be as high as 50%, where as with lime, carbon and diatomaceous earth 15% by weight is possible.

Pumping temperature generally falls into the band 5oC to 40oC. Higher or lower temperatures should be specified and dosing pumps may be fitted with double diaphragms containing a buffer fluid between them, which enables handling of higher temperatures and there are other methods of adapting to temperature extremes.

Notebook 1 : “What is a dosing or metering pump.?”
Notebook 2 : “How do I select a dosing pump?”
Notebook 3 : “How do I install a dosing or metering pump?”
Notebook 4 : “What does a complete dosing pump installation look like?”
Notebook 5 : Continues the discussion on liquid dosing technology.

If you have any questions please go to liquiddosing@dosetech.co.za