A pump is a machine which moves a liquid or a gas from one place to another, often upwards. There are many different kinds of pumps. Pumps need some kind of power to make them work. Sometimes the power comes from a person. Sometimes the power comes from a motor.
Drawing of the inside of a metering pump head to show how it works. The piston moves back and forth inside the pump head.
Centrifugal pump
A
centrifugal pump is a rotodynamic pump that uses a rotating
impeller to increase the pressure and flow rate of a fluid. Centrifugal pumps are the most common type of pump used to move liquids through a piping system. The fluid enters the pump impeller along or near to the rotating axis and is accelerated by the impeller, flowing radially outward or axially into a diffuser or
volute chamber, from where it exits into the downstream piping system. Centrifugal pumps are typically used for large discharge through smaller heads.
The screw centrifugal impeller was invented in 1960 by the late Martin Stahle, the founder of Hidrostal AG. He had received an order from the Amial S.A. fish processing factory in Chimbote (Peru) for the development of a system for transporting fish from the nets into a boat, and from the boat into the fish processing plant. The pump was to work reliably without damaging the fish. The result was the pump with the characteristic screw centrifugal impeller. This invention was a great success. It has since been used in many ways throughout the world in countless other fluid handling systems.
The screw centrifugal pump is a popular choice for handling delicate products such as food and crystals. Its low shear characteristic reduces emulsification when pumping mixtures making it ideal for pumping oily water and Return Activated Sludge [RAS] as it does not damage the
floc. The pump's ability to pass long fibrous materials such as rope without clogging makes it a frequent choice for municipal waste water applications. A screw centrifugal pump typically has an operating efficiency of 70% to 85%. It has a relatively steeply rising head/capacity curve shape giving it good flow control capability over its allowable operating range
The impeller has a single blade, axially extended at the inlet and developed around its axis much like a corkscrew. Linking this to a centrifugal outlet allows pumping with the minimum of agitation and shear, essential factors when product bruising, liquid emulsification or clogging is to be avoided.
The screw centrifugal impeller features:
- Large free passages for pumping liquid with solid objects and fibrous materials
- Able to pump liquids and viscosities above values normally possible with conventional centrifugal pumps
- Steep H/Q curves with closed valve twice best efficiency point
- Low NPSH characteristics
- Flat non-overloading power curves
- High hydraulic efficiencies
Screw centrifugal impeller pumps are widely accepted as state of the art pumps for handling raw sewage and sludges on treatment plants and incorporate many features, which benefits the end user. Screw centrifugal impeller pumps are ideal for handling raw sewage, which contains stringy fibrous material and for handling sewage sludge with up to 10% dry solids content. Typical application areas:
- Sump emptying
- Industrial effluent treatment
- Feeding oily water separators
- Transfer of 'live' fish
- Oil and Chemical spillages
- Mine Drainage
- Parts washer equipment
- Processing of waste oils & sludges
- Transfer of fruit and vegetables
- Municipal waste water treatment plants
Centrifugal pumps are most often associated with the radial flow type. However, the term "centrifugal pump" can be used to describe all impeller type rotodynamic pumps including the radial, axial and mixed flow variations.
Applications
Metering pump for
gasoline and additives. Pumps are used throughout society for a variety of purposes. Early applications includes the use of the
windmill or
watermill to pump water. Today, the pump is used for irrigation,
water supply, gasoline supply,
air conditioning systems,
refrigeration (usually called a compressor), chemical movement,
sewage movement, flood control, marine services, etc. Because of the wide variety of applications, pumps have a plethora of shapes and sizes: from very large to very small, from handling gas to handling liquid, from high pressure to low pressure, and from high volume to low volume.
Priming a pump
Liquid and slurry pumps can lose prime and this will require the pump to be primed by adding liquid to the pump and inlet pipes to get the pump started. Loss of "prime" is usually due to ingestion of air into the pump. The clearances and displacement ratios in pumps used for liquids and other more viscous fluids cannot displace the air due to its lower density.
Pumps as public water supplies
One sort of pump once common worldwide was a hand-powered water pump over a
water well where people could work it to extract water, before most houses had individual water supplies.
From this came the expression "
parish pump" for "the sort of matter chattered about by people when they meet when they go to get water", "matter of only local interest". However water from pitcher pumps are more prone to contamination since it is drawn directly from the soil and does not undergo filtration, this might cause gastrointestinal related diseases.
Today, hand operated village pumps are considered the most sustainable low cost option for safe water supply in resource poor settings, often in rural areas in developing countries. A hand pump opens access to deeper groundwater that is often not polluted and also improves the safety of a well by protecting the water source from contaminated buckets. Pumps like the
Afridev pump are designed to be cheap to build and install, and easy to maintain with simple parts. However, scarcity of spare parts for these type of pumps in some regions of Africa has diminished their utility for these areas.
Specifications
Pumps are commonly rated by
horsepower,
flow rate, outlet
pressure in feet (or metres) of head, inlet suction in suction feet (or metres) of head. The head can be simplified as the number of feet or metres the pump can raise or lower a column of water at
atmospheric pressure.
From an initial design point of view, engineers often use a quantity termed the
specific speed to identify the most suitable pump type for a particular combination of flow rate and head.
Pumping power
Main article:
Bernoulli's equation The power imparted into a fluid will increase the energy of the fluid per unit volume. Thus the power relationship is between the conversion of the mechanical energy of the pump mechanism and the fluid elements within the pump. In general, this is governed by a series of simultaneous differential equations, known as the
Navier-Stokes equations. However a more simple equation relating only the different energies in the fluid, known as
Bernoulli's equation can be used. Hence the work, W, done by or on a pump is given by:
where ?P is the change in
total pressure between the inlet and outlet (in Pa), and the fluid flowrate is given in m^3/s. The total pressure may have gravitational,
static pressure and
kinetic energy components; i.e. energy is distributed between change in the fluid's gravitational potential energy (going up or down hill), change in velocity, or change in static pressure. η is the pump efficiency, and may be given by the manufacturer's information, such as in the form of a
pump curve, and is typically derived from either fluid dynamics simulation (i.e. solutions to the
Navier-stokes for the particular pump geometry), or by testing. The efficiency of the pump will depend upon the pump's configuration and operating conditions (such as rotational speed, fluid density and viscosity etc).
For a typical "pumping" configuration, the work is imparted on the fluid, and is thus positive. For the fluid imparting the work on the pump (i.e. a
turbine), the work is negative.e power required to drive the pump is determined by dividing the output power by the pump efficiency. Furthermore, this definition encompasses pumps with no moving parts, such as a
siphon.
Pump efficiency
Pump efficiency is defined as the ratio of the power imparted on the fluid by the pump in relation to the power supplied to drive the pump. Its value is not fixed for a given pump, efficiency is a function of the discharge and therefore also operating head. For centrifugal pumps, the efficiency tends to increase with flow rate up to a point midway through the operating range (peak efficiency) and then declines as flow rates rise further. Pump performance data such as this is usually supplied by the manufacturer before pump selection. Pump efficiencies tend to decline over time due to wear (e.g. increasing clearances as impellers reduce in size).
One important part of system design involves matching the pipeline headloss-flow characteristic with the appropriate pump or pumps which will operate at or close to the point of maximum efficiency. There are free tools that help calculate head needed and show pump curves including their Best Efficiency Points (BEP).
Pump efficiency is an important aspect and pumps should be regularly tested.
Thermodynamic pump testing is one method.
