What are Pumping Systems?
Pumping systems account for nearly 20% of the world’s electrical energy demand. Furthermore, they range between 25-50% of the energy usage in certain industrial plant operations.
The use of pumping systems is widespread. They provide domestic, commercial and agricultural services. In addition, they provide municipal water and wastewater services, and industrial services for food processing, chemical, petrochemical, pharmaceutical, and mechanical industries.
Pumps have two main purposes:
- Transfer of liquid from one place to another place (e.g. water from an underground aquifer into a water storage tank)
- Circulate liquid around a system (e.g. cooling water or lubricants through machines and equipment)
The main components of a pumping system are:
- Pumps (different types of pumps are explained in section 2)
- Prime movers: electric motors, diesel engines or air system
- Piping, used to carry the fluid
- Valves, used to control the flow in the system
- Other fittings, controls and instrumentation
- End-use equipment, which have different requirements (e.g. pressure, flow) and therefore determine the pumping system components and configuration. Examples include heat exchangers, tanks and hydraulic machines
Pumping System Characteristics:
Pressure is needed to pump the liquid through the system at a certain rate. This pressure has to be high enough to overcome the resistance of the system, which is also called “head”. The total head is the sum of static head and friction head.
Static head: Static head is the difference in height between the source and destination of the pumped liquid (see Fig. 1)
Static head is independent of the flow (see Fig. 1)
Static head consists of:
- Static suction head (hS): resulting from lifting the liquid relative to the pump center line. The hS is positive if the liquid level is above pump centerline, and negative if the liquid level is below pump centerline (also called “suction lift)
- Static discharge head (hd): the vertical distance between the pump centerline and the surface of the liquid in the destination tank
The static head at a certain pressure depends on the weight of the liquid and can be calculated with this equation as shown in Fig. 1:
- This is the loss needed to overcome that is caused by the resistance to flow in the pipe and fittings.
- It is dependent on size, condition and type of pipe, number and type of pipe fittings, flow rate, and nature of the liquid.
- The friction head is proportional to the square of the flow rate as shown in the Fig. 2.
- A closed loop circulating system only exhibits friction head (i.e. not static head).
In most cases the total head of a system is a combination of static head and friction head as shown in the Fig. 2. Left figure is system with a high static head (i.e. the destination reservoir is much higher than the source). Right figure is system with a low static head (i.e. the destination reservoir is not much higher than the source).
Pump performance curve:
The head and flow rate determine the performance of a pump, which is graphically shown in the Fig. 2 as the performance curve or pump characteristic curve.
The Fig. 2 (Top Left) shows a typical curve of a centrifugal pump where the head gradually decreases with increasing flow.
As the resistance of a system increases, the head will also increase. This in turn causes the flow rate to decrease and will eventually reach zero. A zero flow rate is only acceptable for a short period without causing to the pump to burn out.
Pump operating point:
The rate of flow at a certain head is called the duty point. The pump performance curve is made up of many duty points.
The pump operating point is determined by the intersection of the system curve and the pump curve as shown in the Fig. 3
The Best Efficiency Point (BEP) is the pumping capacity at maximum impeller diameter, in other words, at which the efficiency of the pump is highest. All points to the right or left of the BEP have a lower efficiency.
Pump Suction Performance:
Cavitation or vaporization is the formation of bubbles inside the pump. This may occur when at the fluid’s local static pressure becomes lower than the liquid’s vapor pressure (at the actual temperature). A possible cause is when the fluid accelerates in a control valve or around a pump impeller.
Vaporization itself does not cause any damage. However, when the velocity is decreased and pressure increased, the vapor will evaporate and collapse. This has three undesirable effects:
- Erosion of vane surfaces, especially when pumping water-based liquids
- Increase of noise and vibration, resulting in shorter seal and bearing life
- Partially choking of the impeller passages, which reduces the pump performance and can lead to loss of total head in extreme cases.
The Net Positive Suction Head Available (NPSHA) indicates how much the pump suction exceeds the liquid vapor pressure, and is a characteristic of the system design.
The NPSH Required (NPSHR) is the pump suction needed to avoid cavitation, and is a characteristic of the pump design.
Type of pumps:
- Pumps come in a variety of sizes for a wide range of applications. They can be classified according to their basic operating principle as dynamic or positive displacement pumps
- In principle, any liquid can be handled by any of the pump designs.
- Centrifugal pump is generally the most economical but less efficient.
- Positive displacement pumps are generally more efficient than centrifugal pumps, but higher maintenance costs.
Positive displacement pumps are distinguished by the way they operate: liquid is taken from one end and positively discharged at the other end for every revolution.
In all positive displacement type pumps, a fixed quantity of liquid is pumped after each revolution. So if the delivery pipe is blocked, the pressure rises to a very high value, which can damage the pump.
Positive displacement pumps are widely used for pumping fluids other than water, mostly viscous fluids.
Positive displacement pumps are further classified based upon the mode of displacement:
- Reciprocating pump if the displacement is by reciprocation of a piston plunger. Reciprocating pumps are used only for pumping viscous liquids and oil wells.
- Rotary pumps if the displacement is by rotary action of a gear, cam or vanes in a chamber of diaphragm in a fixed casing. Rotary pumps are further classified such as internal gear, external gear, lobe and slide vane etc. These pumps are used for special services with particular conditions existing in industrial sites.
Dynamic pumps are also characterized by their mode of operation: a rotating impeller converts kinetic energy into pressure or velocity that is needed to pump the fluid.
There are two types of dynamic pumps:
- Centrifugal pumps are the most common pumps used for pumping water in industrial applications. Typically, more than 75% of the pumps installed in an industry are centrifugal pumps.
- Special effect pumps are particularly used for specialized conditions at an industrial site. Refer part 2 of this article by clicking here for further details….