Using Pumps With Caution
Different fluids have varying characteristics and are usually pumped under different conditions. It is therefore very important to know all relevant product and performance data before selecting a pump...
- Dr S S Verma
Moving fluids plays a major role in many processes for daily life utilities. Liquids can only move on their own power only from top to bottom or from a high pressure to a lower pressure system. This means that energy to the liquid must be added, to moving the liquid from a low to a higher level. To add the required energy to liquids, pumps are used. There are many different definitions of the name PUMP but the best described one is as: “a machine used for the purpose of transferring quantities of fluids and or gases from one place to another”. A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action. Pumps are used throughout the society for a variety of purposes. Early applications include 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. In biology, many different types of chemical and bio-mechanical pumps have evolved, and bio-mimicry is sometimes used in developing new types of mechanical pumps.
Types of pumps
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. Pumps can be classified into three major groups according to the method they use to move the fluid: direct lift, displacement, and gravity pumps. Pumps operate by some mechanism (typically reciprocating or rotary), and consume energy to perform mechanical work by moving the fluid. Pumps operate via many energy sources, including manual operation, electricity, engines, or solar/wind power, come in many sizes, from microscopic for use in medical applications to large industrial pumps. Mechanical pumps serve in a wide range of applications such as pumping water from wells, aquarium filtering, pond filtering and aeration, in the car industry for water-cooling and fuel injection, in the energy industry for pumping oil and natural gas or for operating cooling towers. In the medical industry, pumps are used for biochemical processes in developing and manufacturing medicine, and as artificial replacements for body parts, in particular the artificial heart and penile prosthesis. There are generally two types of pumps:
• Single stage pump - When in a casing only one impeller is revolving then it is called single stage pump.
• Double/ Multi stage pump - When in a casing two or more than two impellers are revolving then it is called double/ multi stage pump.
Pump types generally fall into two main categories –Rotodynamic and Positive Displacement, of which there are many forms. The Rotodynamic pump transfers rotating mechanical energy into kinetic energy in the form of fluid velocity and pressure. The Centrifugal and Liquid Ring pumps are types of rotodynamic pump, which utilize centrifugal force to transfer the fluid being pumped. The Rotary Lobe pump is a type of positive displacement pump, which directly displaces the pumped fluid from pump inlet to outlet in discrete volumes. In order to select a pump two types of data are required:
• Product/Fluid data which includes viscosity, density/specific gravity, temperature, flow characteristics, vapor pressure and solids content.
• Performance data which includes capacity or flow rate and inlet/discharge pressure/head.
• Different fluids have varying characteristics and are usually pumped under different conditions. It is therefore very important to know all relevant product and performance data before selecting a pump.
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 or Best Efficiency Point (BEP) 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). When a system includes a centrifugal pump, an important design issue is matching the head loss-flow characteristic with the pump so that it operates at or close to the point of its maximum efficiency. Pump efficiency is an important aspect and pumps should be regularly tested for the same.
Many people employ the centrifugal pumps to move liquid or water through a piping system from one place to other. They work on centrifugal force generated by impellers and help to move the fluid. When centrifugal pumps are in operation, they increases the liquid pressure from the inlet points to their outlet points. They increase the pressure by transferring mechanical energy that is generated through the rotating impeller to the fluid, which they have to move out. Generally, these types of pumps are used to pump buildings water supply, hot water circulation and sump pits etc. At domestic level they are applied for maintaining wells water supply and to boost pressure from intake line. They can be applied to move hot water which needs low head in a closed system. Centrifugal water pumps are used worldwide for moving different types of fluid from one location to another. Perhaps they are the most common type of water pumps especially for commercial use but they have drawbacks also.
For most household or light industrial uses, a centrifugal pump is fine. As long as the liquids aren’t too viscous, like mud or waste, and the pump can be totally submerged, it will provide consistent, effective, and reliable operation.
Centrifugal pumps provide a lot of flexibility, are easy to move, and don’t take up a lot of space. Centrifugal pumps are fairly simple in nature. They use the kinetic energy of a motor to move liquids. An engine is attached to the axis, which then rotates the pump impeller, which is reminiscent of an old ship’s 'water wheel'. The rotation moves the water from its entry point through the casing, and finally to the exit. While most pumps are used for water, centrifugal pumps are also used for sewage, petroleum, and chemicals. Incidentally, the reverse of this process is called a water turbine. The impeller is placed in moving water; it can be used as a water turbine which converts the water’s energy into rotational energy. In other words, instead of the motor moving the pump to move the water, the water moves the pump to move the motor. Because of the direct conversion of the motor to rotational energy, the centrifugal pump is a very simple pump. The most common centrifugal pump control methods are
• Stop-Start/Float Level control Operation
• Control Valve Operation
• By-Pass Valve Operation
• Variable Speed Operation
• Hybrid Control (VFD + By-Pass)
• Parallel Operation of Multiple Pumps
• Multiple Speed Motors (2, 3 or 4 Speed)
As with all pumps, there are advantages and disadvantages. The biggest advantage of centrifugal pumps is their aforementioned simplicity. They don’t require any valves, or many moving parts. This makes them easy to produce with many different materials. It also allows them to move at high speeds with minimal maintenance. Their output is very steady and consistent. Most of all, they are very small compared to other types of pumps that create the same output. The main disadvantage is that they use rotation instead of suction to move water, and therefore have almost no suction power. This means that a centrifugal pump must be put under water, or primed, before it will move water. Centrifugal pumps can also develop a phenomenon called “cavitations”. This happens when the speed of the water causes it to vaporize, which causes bubbles in the liquid. A combination of the speed of the vapor bubbles and the implosion of vapor bubbles can be corrosive to the impeller surfaces and pump casing. Advantages and disadvantages of centrifugal pump in general can be summarized as:
Advantages of centrifugal pump
• As there is no drive seal so there is no leakage in pump
• It can pump hazardous liquids
• There are very less frictional losses
• There in almost no noise
• Pump has almost have 100 efficiency
• Centrifugal pump have minimum wear with respect to others
• There is a gap between pump chamber and motor, so there is no heat transfer between them
• Because of the gap between pump chamber and motor, water cannot enter into motor
• Centrifugal pump use magnetic coupling which breakup on high load eliminating the risk of damaging the motor
Disadvantages of centrifugal pump
• Because of the magnetic resistance there is some energy losses
• Unexpected heavy load may cause the coupling to slip ferrous particles in liquid are problematic when you are using magnetic drive. This is because particles collect at impeller and cause the stoppage of pump after some time
Health of pumps for efficiency
It is important to understand the role pumps and valves play in ensuring product safety, and how to clean and maintain them properly. In today’s uncertain economy, keeping pumping systems and stations operating at their optimum capability is vital. However, many organizations responsible for this crucial task do not perform regular health checks on their pump systems. Most people would not run their car or home heating units until they break down or stop working. Similarly, most people maintain their health with visits to the doctor or dentist. The same principles apply to pump systems. They require the same level of preventive care and maintenance so that operators and end users benefit the most from their investments. Preventive maintenance and monitoring provide a clear picture of the pump system’s performance, save end users money and reduce environmental impact by improving energy usage.
The maintenance of a pumping station should not be a one-time event. A robust monitoring system along with regular health checks will deliver an accurate understanding of how a pump system is performing. When end users consider the different options for monitoring performance, they should look for a system that monitors energy use, the whole life-cycle cost of the equipment and how the pump performs against its most efficient duty point.
Also, maintenance records can reveal any fault trends that will help predict or diagnose pump failure, regular breakdowns or loss of performance. This information can assist the operator in planning maintenance and controlling the budget.
Continuously-improving technology has resulted in the increased accuracy of system variable measurements. Monitoring equipment can measure pressure, flow, depth, energy consumption, vibration and temperature, without the need to drastically modify the pump station layout. A modern monitoring system can accurately obtain and record precise data, including trends of all the hydraulic and power inputs, which display in real time as the pump operates. A visual display such as this is more informative than basic numerical data logging and can be invaluable in providing information for system troubleshooting.
Having access to the data obtained from a monitoring system is particularly important because each system or station has a different set of requirements. In general, pumps are selected based on the most efficient duty point designated by the manufacturer.
Selections are most often made from desktop designs and drawings. However, even in the best circumstances, installation of a pump system/station rarely occurs exactly according to plan. This means that the pump will probably operate outside its best efficiency point. Once installed, its performance can be monitored and adjustments can be made, such as an impeller trim or speed change on the variable frequency drive. For pump upgrades and replacements, knowing the precise pumping station system data makes accurate and efficient pump selection easier.
Adjusting the system over time
As time passes, conditions change. Components wear. Parts may be added or removed, and these changes can completely alter the system’s operation. More often than not, particularly if there is no health check in place, these changes are not taken into account. However, they have altered the footprint of the originally installed system. Over time, pipes can become partially obstructed because of silt and debris build up, or local damage may occur in which pressure from the surface has damaged or misshaped a buried pipe.
Unlike our bodies, which provide signals such as pain when they are damaged, the only signals a faulty pipe will give are problems, such as flooding or a reduction in output. In most instances, pump station operators can only see the external picture. They know that the system is not working as it should, but they have no data to support their concerns. Many common pump station issues do not show up immediately. For example, the poor design of wet well benching or in-flow paths can lead to cavitation, pump wear and reduced performance. Analysis of the pump system throughout operation will indicate the telltale signs of performance deterioration. In an ideal world, and particularly when it comes to large pump systems or stations that use a lot of energy, the operator should perform regular health checks through a robust and reliable monitoring system that considers all aspects of the process. This includes a software system that runs seamlessly with the pumping equipment and records data, which can be viewed remotely at any given time for a performance analysis.
Key elements of analysis
Products are available that can show end users how the key elements of their pumping system are operating. These elements include pressure, flow, vibration and temperature. These parameters, combined with audio monitoring, provide operators with the big picture and many small problems before they become big issues. The purpose of a health check and monitoring system is to ensure that pump systems operate at optimum performance as designed. If end users decide to employ a specialist engineering organization to perform this service, it can use the data that it gathers to advise them on their system performance, energy use and efficiencies.
If performance is poor, the service provider should recommend possible causes and remedial actions. Many of us live by the mantra, “If it isn’t broken, don’t fix it.” However, because of this mentality, some pump system operators are pouring money down the sewer. Even if the pump well empties or the station does not flood, does not mean that problems do not exist and improvements cannot be made. If problems are left unresolved, one day the system will fail, which can result in expensive outages and repair or replacement costs. Failure can also cause environmental issues. All of this can be prevented through a regular health and maintenance program.
Small repairs result in large savings
When it comes to poor performance, the issue may not be with the pump but with an associated part—an impeller, for example. A big bill can easily be avoided by having the right system, working in the right way and delivering the desired result. Small repairs and changes can make a big difference to performance.
Analysis before replacement
Replacing a pump with the exact same pump is no longer a practical or viable option. If a pump replacement is essential, then the whole pumping system should be analyzed before investing in a new pump. This is because, over time, the surroundings in which the pump operates is likely to have changed, possibly because of the environment, local construction, changes in weather conditions or a host of other potential causes. Again, a regular health check of the system will have identified these changes on an ongoing basis, supplying the operator with the knowledge required to make an educated pump purchase. Addressing problems in pumping systems is a constant challenge for operators. They should think of their pump system as a finely tuned engine that needs the same level of care and attention as a car’s engine.
With state-of-the-art monitoring technology at their fingertips, improving the performance of pump systems is easier than ever, without the expense of costly replacements or excessive energy use. A significant factor in the design of a pumping system is the flow variation required by the process. Several pumps in parallel, variable speed pumps, pumps with on-off control and pumps with a control valve are some of the methods available for flow variation. A widely used method in the industry is to use control valves, generally located on the pump discharge in the pipe supplying process fluid. The flow could be used for different purposes—such as maintaining the level in a process vessel or in a boiler drum, or maintaining the flow in a pipeline or in the tubes of a fired heater. To understand how flow can be varied by a control valve, the system designer and operator need to understand the basic principles of how control valves behave.
Dr S S Verma
Department of Physics
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