General Definitions
The following terms are commonly used and referred to in describing Discflo pumps.
Absolute viscosity: This is a measure of a fluid's resistance to flow, and applies to Newtonian fluids. See viscosity below.
Boundary layer: The layer formed by the fluid being pumped on the surface of the discs in the Discpac. This layer is stationary relative to the discs under laminar flow conditions and acts as a buffer against abrasive wear.
Capacity: Capacity is a measure of the volume of liquid pumped in a given length of time, usually expressed in US gallons per minute (GPM), liters per second (l/s) or cubic meters per hour (m3/h).
Cavitation: This problem can occur in any type of pump, although it is relatively rare in a Discflo pump. Cavitation usually occurs when the pump is trying to discharge more liquid
than is able to enter the pump. Common causes are excessive suction lift, insufficient NPSH or running the pump too fast. Severe cavitation can cause excessive noise and pump
damage, while mild cavitation will produce a small loss in pump efficiency and moderate wear to pump parts.
Consistency: Consistency, also known as apparent viscosity, is a measure of a slurry's resistance to flow when subjected to shear stress. This term is used with non-Newtonian fluids, while the term absolute viscosity is used in conjunction with Newtonian fluids.
Curve: This is a graph showing the friction losses for a system in column height of water at any given flow rate.
Density: The density of a liquid is the amount of mass of that liquid contained in a unit volume. The units of density are pounds per cubic feet (lb/ft3) or kilograms per cubic meter
(kg/m3).
Equivalent length of straight pipe: This term describes the friction loss in various parts of the system, for example elbows, tees, valves, etc. Each part is given a value equal to
a certain equivalent length of straight pipe.
Flow rate: Similar to capacity, the flow rate is the volume of liquid pumped in a given length of time, usually expressed in gallons per minute (GPM), liters per second (l/s) or cubic
meters per hour (m3/h).
Head: Head is a measure of the pressure exerted by a column of liquid on the liquid at the bottom of the column. It is measured in either feet (ft) or meters (m), and is related to
pressure by the equation: Pressure = specific weight of the fluid × head Note: For definitions of total (dynamic) head, suction head, total discharge head and velocity head, see section 1.1.
Head-capacity curve: This graph contains all the performance points for a given pump speed and a given pump size.
Horsepower: Horsepower is a measure of the work performed in pumping. It depends on the weight of the fluid being pumped and the total head or differential pressure being
developed. One horsepower equals 33,000 ft-lb/min. Terms used in hydraulics include: brake (or actual) horsepower; and hydraulic (or theoretical) horsepower.
Brake horsepower is the actual work required to pump a fluid in a given length of time against a given head.
Hydraulic horsepower (always lower than brake horsepower) is the theoretical work required to pump a fluid in a given length of time against a given head. The metric equivalent of horsepower is power. It is usually measured in kilowatts (kW).
Hydraulics: Hydraulics is the study of the behavior of liquids. Unlike a gas, a liquid has a definite volume: it is considered virtually incompressible at low pressures, but changes density at high pressures and as temperatures vary.
Kinematic viscosity: Kinematic viscosity (measured in centistokes [cS]) is derived by dividing the absolute viscosity (consistency) by the mass density of the fluid.
Laminar vs turbulent flow: The flow of liquid through a pipe can be either laminar or turbulent depending on the liquid's velocity and viscosity and the pipe size. Laminar flow exists when the particles of liquid follow separate non-intersecting paths with little to no eddying. Turbulent flow occurs when the particles cross each other paths. For any liquid and pipe size, the relationship between the liquid's velocity and viscosity and the pipe size can be expressed by a dimensionless number called the Reynolds number, R, according to the formula:
R = VD
===v
where,
V = average velocity in ft/s or m/s
D = average internal pipe diameter in ft or m
v = kinematic viscosity of the fluid in ft2/s or m2/s
For values of R less than 2000, the flow is laminar; when R is above 4000, the flow is considered turbulent. For values of R from 2000 to 4000, flow is generally said to be turbulent for the purpose of friction loss or pressure drop calculations.
Newtonian fluids: A liquid is considered Newtonian or "true" if its viscosity is constant, unaffected by the type and magnitude of motion or agitation to which it may be subjected with no change in temperature; examples of Newtonian liquids are water and mineral oil. Note: It is generally accepted that the average hard rock mineral slurry, especially those with particles under 150 microns and concentrations below 30% by volume, behave as Newtonian fluids, when the fluid velocity in the line is high enough to ensure uniform distribution of the solids. In these cases, the pressure losses can be assumed to be like those for water.
Non-Newtonian fluids: In a non-Newtonian liquid, viscosity varies when the fluid is agitated. There are four categories of non-Newtonian fluids:
Thixotropic: The liquid's viscosity decreases as agitation is increased at constant temperature; examples of thixotropic liquids are cellulose compounds, molasses, paints and soaps.
Dilatant: The liquid's viscosity increases as agitation increases at constant temperature; examples of dilatant liquids are clay slurries and candy compounds.
Bingham-plastic: The liquid does not flow until a threshold shear stress is reached. Viscosity decreases with increasing shear rate; examples of Bingham-plastic liquids are
heavy slurry, sewage sludge and tomato ketchup.
Pseudo-plastic fluids: Similar to Bingham-plastic liquids, except that there is no definite yield stress; examples are paper stock and emulsions.
Net Positive Suction Head Available (NPSHa): This is the difference between the total suction head and the vapor pressure of the liquid, in feet of liquid, at the suction flange. See Technical Definition for further information on calculating NPSHa.
Net Positive Suction Head Required (NPSHr): The amount of NPSHa the system must provide to operate the pump without cavitation.
Pre-rotation: This phenomenon refers to the spiraling of fluid in the suction pipe caused by the disc rotation.
Pressure head: The pressure at any point in a liquid at rest is caused by the atmospheric pressure exerted on the surface plus the weight of liquid above that point. In effect, all liquid pressures can be visualized as being caused by the weight of a column of liquid of a certain height. This column of liquid, real or imaginary, is called the pressure head or static head and is usually expressed in feet of liquid.
Rheology: Rheology is the study of deformation and flow of substances.
Specific gravity: The specific gravity of a liquid is the ratio of its density relative to that of water. It is a dimensionless number calculated by dividing the liquid's density by that of water. The water temperature for this purpose is usually 60°F [15°C], at which point its density is 62.371 lb/ft3 [1.00g/cm3].
Vapor pressure: This pressure comes from the vapor formed by a liquid above its free surface. Vapor pressure is a function of the liquid's temperature, and is generally measured in kiloPascals (kPa), Bars or pounds per square inch (psi). Atmospheric pressure at sea level is 14.7 psi [100 kPa].
Viscosity: Viscosity can be defined as the resistance to shear of a fluid. High viscosity fluids require a greater force to shear at a given rate than low viscosity fluids. From this diagram:
Viscosity = Force x Height
`````````````````Area Velocity
The viscosities of most liquids vary appreciably with changes in temperature, while the influence of pressure is negligible. With certain liquids, the viscosity can change when the liquid is agitated. Based on their viscous properties, fluids can be categorized into five basic types: Newtonian; dilatant; thixotropic; Bingham-plastic; and pseudo-plastic. The latter four are all non-Newtonian fluids. For more details, see separate categories for Newtonian and Non-Newtonian fluids.
There are two basic viscosity parameters:dynamic (or absolute) viscosity: and kinematic viscosity.
Dynamic viscosity is measured in lb-sec/ft2 in the US. In the metric system, it is measured in the dyne-sec/cm2, called the poise, usually expressed in centipoise (cP), such that 100 centipoise equals one poise.
Kinematic viscosity is measured in ft2/sec in the US system, and in cm2/sec in the metric system, where the base unit is called the stoke (S). The centistoke (cS), equal to 100 stoke, is commonly used.
Kinematic viscosity (cS) = absolute viscosity (cP)
``````````````````````````````````````````density (g/cm3)
Discflo's preferred way of expressing viscosity is in centipoise. Water has a value of 1 cP (which is 31.5 SSU, Saybolt Seconds Universal). The relationship between absolute viscosity and SSU can be found by the following equation for values over 350 SSU:
Absolute viscosity (cP) = SSU x spec. gravity x 0.21576
Discflo pumps are capable of handling exceptionally high viscosities up to several 100,000cPs.
Viscous drag: The mechanism of transferring energy from one layer to an adjacent layer in a fluid with laminar flow. Also commonly known as friction. The more viscous the fluid, the more efficient the energy transfer. This explains why the disc pump becomes more efficient as viscosity increases.
