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PUMPS & PUMPING SYSTEMS
1. INTRODUCTION ..........................................................................................................
2. TYPE OF PUMPS..........................................................................................................
3. ASSESSMENT OF PUMPS .......................................................................................
4. ENERGY EFFICIENCY OPPORTUNITIES ...................................................
5. OPTION CHECKLIST ..............................................................................................
6. WORKSHEETS ...........................................................................................................
7. REFERENCES .............................................................................................................
1. INTRODUCTION
This section briefly describes the main features of pumps and pumping systems. 1
1.1 What are pumps and pumping systems?
Pumping systems account for nearly 20% of the world’s electrical energy demand and range from 25-50% of the energy usage in certain industrial plant operations (US DOE, 2004).
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.
(^1) Information was sourced from three US DOE publications: Improving Pumping System Performance – a Sourcebook for Industry (1999); Pump Life Cycle Costs – A Guide to LCC Analysis for Pumping Systems (2001); and Variable Speed Pumping – A Guide to Successful Applications (2004). These publications are recommended for further reading.
Figure 1. A Pumping System in an Industry (US DOE, 2001)
The pump and the prime mover are typically the most energy inefficient components.
1.2 Pumping system characteristics
1.2.1 Resistance of the system: head 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:
a) Static head Static head is the difference in height between the source and destination of the pumped liquid (see Figure 2a). Static head is independent of flow (see Figure 2b). The static head at a certain pressure depends on the weight of the liquid and can be calculated with this equation:
Head (in feet) = Pressure (psi) X 2. Specific gravity
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.
Figure 2a. Static Head Figure 2b. Static Head Versus Flow
b) Friction head (hf) 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 figure 3. A closed loop circulating system only exhibits friction head (i.e. not static head).
Static head
Flow
destination
source
Static head
Figure 5. Performance Curve of a Pump
1.2.3 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 Figure 6.
Figure 6. Pump Operating Point (US DOE, 2001)
1.2.4 Pump suction performance (NPSH) 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
Flow
Head
Static head
Pump performance curve
System curve
Pump operating point
Head
Flow
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.
2. TYPE OF PUMPS
This section describes the various types of pumps.^2 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 (Figure 7).
Figure 7. Different types of pumps
In principle, any liquid can be handled by any of the pump designs. Where different pump designs could be used, the centrifugal pump is generally the most economical followed by rotary and reciprocating pumps. Although, positive displacement pumps are generally more efficient than centrifugal pumps, the benefit of higher efficiency tends to be offset by increased maintenance costs.
2.1. Positive displacement pumps
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. Positive displacement pumps are widely used for pumping fluids other than water, mostly viscous fluids.
(^2) Section 2 is taken (with edits) from Pumps and Pumping Systems, with permission from the Bureau of Energy Efficiency, India
Dynamic
Positive Displacement
Centrifugal Special effect Rotary Reciprocating
Internal gear
External gear
Lobe
Slide vane
Others (e.g. Impulse, Buoyancy)
Pumps
Dynamic
Positive Displacement
Centrifugal Special effect Rotary Reciprocating
Internal gear
External gear
Lobe
Slide vane
Others (e.g. Impulse, Buoyancy)
Pumps
2.2.2 Components of a centrifugal pump The main components of a centrifugal pump are shown in Figure 9 and described below: Rotating components: an impeller coupled to a shaft Stationary components: casing, casing cover, and bearings.
Figure 9. Main Components of a Centrifugal Pump (Sahdev)
a) Impeller An impeller is a circular metallic disc with a built-in passage for the flow of fluid. Impellers are generally made of bronze, polycarbonate, cast iron or stainless steel, but other materials are also used. As the performance of the pump depends on the type of impeller, it is important to select a suitable design and to maintain the impeller in good condition.
The number of impellers determines the number of stages of the pump. A single stage pump has one impeller and is best suited for low head (= pressure) service. A two-stage pump has two impellers in series for medium head service. A multi-stage pump has three or more impellers in series for high head service.
Impellers can be classified on the basis of: Major direction of flow from the rotation axis: radial flow, axial flow, mixed flow Suction type : single suction and double suction Shape or mechanical construction: − Closed impellers have vanes enclosed by shrouds (= covers) on both sides (Figure 10). They are generally used for water pumps as the vanes totally enclose the water. This prevents the water from moving from the delivery side to the suction side, which would reduce the pump efficiency. In order to separate the discharge chamber from the suction chamber, a running joint is necessary between the impeller and pump casing. This joint is provided by wearing rings, which are mounted either over extended portion of impeller shroud or inside the cylindrical surface of pump casing. A disadvantage of closed impellers is the higher risk of blockage. − Open and semi-open impellers (Figure 10) are less likely to clog. But to avoid clogging through internal re-circulation, the volute or back-plate of the pump must be manually adjusted to get the proper impeller setting. − Vortex pump impellers are suitable for solid and "stringy" materials but they are up to 50% less efficient than conventional designs.
b) Shaft The shaft transfers the torque from the motor to the impeller during the startup and operation of the pump.
c) Casing The main function of casing is to enclose the impeller at suction and delivery ends and thereby form a pressure vessel. The pressure at suction end may be as little as one-tenth of atmospheric pressure and at delivery end may be twenty times the atmospheric pressure in a single-stage pump. For multi-stage pumps the pressure difference is much higher. The casing is designed to withstand at least twice this pressure to ensure a large enough safety margin.
A second function of casing is to provide a supporting and bearing medium for the shaft and impeller. Therefore the pump casing should be designed to Provide easy access to all parts of pump for inspection, maintenance and repair Make the casing leak-proof by providing stuffing boxes Connect the suction and delivery pipes directly to the flanges Be coupled easily to its prime mover (i.e. electric motor) without any power loss.
Figure 11. Cut-away of a pump showing Volute Casing (Sahdev)
Figure 12. Solid Casing (Sahdev)
Figure 10. Closed and Open Impeller Types (Sahdev)
3.2 Difficulties in the assessment of pumps
In practice, it is more difficult to assess pump performance. Some important reasons are:
Absence of pump specification data : Pump specification data (see Worksheet 1 in section
- are required to assess the pump performance. Most companies do not keep original equipment manufacturer (OEM) documents that provide these data. In these cases, the percentage pump loading for a pump flow or head cannot be estimated satisfactorily. Difficulty in flow measurement : It is difficult to measure the actual flow. The methods are used to estimate the flow. In most cases the flow rate is calculated based on type of fluid, head and pipe size etc, but the calculated figure may not be accurate. Another method is to divide the tank volume by the time it takes for the pump to fill the tank. This method can, however, only be applied if one pump is in operation and if the discharge valve of the tank is closed. The most sophisticated, accurate and least time consuming way to measure the pump flow is by measurement with an ultrasonic flow meter. Improper calibration of pressure gauges and measuring instruments : Proper calibration of all pressure gauges at suction and discharge lines and other power measuring instruments is important to obtain accurate measurements. But calibration has not always been carried out. Sometimes correction factors are used when gauges and instruments are not properly calibrated. Both will lead to incorrect performance assessment of pumps.
4. ENERGY EFFICIENCY OPPORTUNITIES
This section includes main areas for improving pumps and pumping systems. The main areas for energy conservation include: Selecting the right pump Controlling the flow rate by speed variation Pumps in parallel to meet varying demand Eliminating flow control valve Eliminating by-pass control Start/stop control of pump Impeller trimming
4.1 Selecting the right pump^4
In selecting the pump, suppliers try to match the system curve supplied by the user with a pump curve that satisfies these needs as closely as possible. The pump operating point is the point where the pump curve and the system resistance curve intersect (as explained in section 1.2.3). However, it is impossible for one operating point to meet all desired operating conditions. For example, when the discharge valve is throttled, the system resistance curve shifts to the left and so does the operating point (see Figure 13).
Figure 13 below shows a typical vendor-supplied pump performance curves for a centrifugal pump where clear water is the pumping liquid.
(^4) Section 4.1 is taken (with edits) from Pumps and Pumping Systems. In: Energy Efficiency in Electrical
Utilities, chapter 6, 2004, with permission from the Bureau of Energy Efficiency, Ministry of Power, India.
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. The BEP is affected when the selected pump is oversized. The reason is that the flow of oversized pumps must be controlled with different methods, such as a throttle valve or a by-pass line. These provide additional resistance by increasing the friction. As a result the system curve shifts to the left and intersects the pump curve at another point. The BEP is now also lower. In other words, the pump efficiency is reduced because the output flow is reduced but power consumption is not. Inefficiencies of oversized pumps can be overcome by, for example, the installation of VSDs, two-speed drives, lower rpm, smaller impeller or trimmed impeller (BEE, 2004).
4.2 Controlling flow rate by speed variation
4.2.1 Explaining the effect of speed^5
A centrifugal pump’s rotating impeller generates head. The impeller’s peripheral velocity is directly related to shaft rotational speed. Therefore varying the rotational speed has a direct effect on the performance of the pump.
(^5) Section 4.2.1 is taken (with edits) from Pumps and Pumping Systems. In: Energy Efficiency in Electrical
Utilities, chapter 6, 2004, with permission from the Bureau of Energy Efficiency, Ministry of Power, India.
Figure 13: Typical centrifugal pump performance curve given by suppliers (Bureau of Energy Efficiency, 2004)
Improved system reliability because wear of pumps, bearings and seals is reduced. Reduction of capital & maintenance cost because control valves, by-pass lines, and conventional starters are no longer needed. Soft starter capability: VSDs allow the motor the motor to have a lower startup current.
Figure 14. Effect of VFD (US DOE, 2004)
4.3 Pumps in parallel to meet varying demand
Operating two pumps in parallel and turning one of when the demand is lower, can result in significant energy savings. Pumps providing different flow rates can be used. Parallel pumps are an option when the static head is more than fifty percent of the total head. Figure 15 shows the pump curve for a single pump, two pumps operating in parallel and three pumps operating in parallel. It also shows that the system curve normally does not change by running pumps in parallel. The flow rate is lower than the sum of the flow rates of the different pumps.
Figure 15. Typical performance curves for pumps in parallel (BPMA)
4.4 Eliminating flow control valve
Another method to control the flow by closing or opening the discharge valve (this is also known as “throttling” the valves). While this method reduces the flow, it does not reduce the power consumed, as the total head (static head) increases. Figure 16 shows how the system curve moves upwards and to the left when a discharge valve is half closed.
This method increases vibration and corrosion and thereby increases maintenance costs of pumps and potentially reduces their lifetimes. VSDs are a better solution from an energy efficiency perspective.
Figure 16. Control of Pump Flow by Valve (BPMA)
4.5 Eliminating by-pass control
The flow can also be reduced by installing a by-pass control system, in which the discharge of the pump is divided into two flows going into two separate pipelines. One of the pipelines delivers the fluid to the delivery point, while the second pipeline returns the fluid to the source. In other words, part of the fluid is pumped around for no reason, and thus is an energy wastage. This option should therefore be avoided.
4.6 Start/stop control of pump
A simple and reasonable energy efficient way to reduce the flow rate is by starting and stopping the pump, provided that this does not happen to frequently. An example where this option can be applied, is when a pump is used to fill a storage tank from which the fluid flows to the process at a steady rate. In this system, controllers are installed at the minimum and maximum level inside the tank to start and stop the pump. Some companies use this method also to avoid lower the maximum demand (i.e. by pumping at non-peak hours).
4.7 Impeller trimming
Changing the impeller diameter gives a proportional change in the impeller’s peripheral velocity. Similar to the affinity laws, the following equations apply to the impeller diameter D:
5. OPTION CHECKLIST
This section includes most important options to improve energy efficiency of pumps and pumping systems.
Operate pumps near their best efficiency point (BEP)
Ensure adequate NPSH at site of installation
Modify pumping system and pumps losses to minimize throttling.
Ensure availability of basic instruments at pumps like pressure gauges, flow meters
Adapt to wide load variation with variable speed drives or sequenced control of multiple units
Avoid operating more than one pump for the same application
Use booster pumps for small loads requiring higher pressures
To improve the performance of heat exchangers, reduce the difference in temperature between the inlet and outlet rather than increasing the flow rate
Repair seals and packing to minimize water loss by dripping
Balance the system to minimize flows and reduce pump power requirements
Avoid pumping head with a free-fall return (gravity), and use the siphon effect
Conduct a water balance to minimize water consumption, thus optimum pump operation
Avoid cooling water re-circulation in DG sets, air compressors, refrigeration systems, cooling towers feed water pumps, condenser pumps and process pumps
In multiple pump operations, carefully combine the operation of pumps to avoid throttling
Replace old pumps with energy efficient pumps
T improve the efficiency of oversized pumps, install variable speed drive, downsize / replace impeller, or replace with a smaller pump
Optimize the number of stages in multi-stage pump if margins in pressure exist
Reduce the system resistance by pressure drop assessment and pipe size optimization
Regularly check for vibration to predict bearing damage, misalignments, unbalance,
foundation looseness etc.
6. WORKSHEETS
This section includes following worksheets: Pump Specification Data Pump Efficiency Calculation
Worksheet 1: PUMP SPECIFICATION DATA
No. Parameter Units Pump number
1 2 3
1 Make
2 Type (reciprocating/centrifugal)
3 Discharge capacity m^3 /hr
4 Head developed mmWC
5 Fluid Handled
6 Density of fluid kg/m^3
7 Temperature of fluid 0 C
8 Pump input power kW
9 Pump speed RPM
10 Pump rated efficiency %
11 Specific power consumption kW/(m^3 /hr)
12 Pump motor
Rated power kW Full load current Amp Rated speed RPM Supply voltage Volts Rated efficiency % Rated power factor Supply frequency Hz
13 Bearing type
Pump (driving end) Pump (non-driving end) Motor (driving end) Motor (non-driving end)
14 Lubricant grade
7. REFERENCES
American Council for Energy Efficiency Economy. www.aceee.org
Bureau of Energy Efficiency, Ministry of Power, India. 2004. Pumps and Pumping Systems. In: Energy Efficiency in Electrical Utilities, chapter 6.
Fluide Design Inc. www.fluidedesign.com
GAMBICA Association, BPMA. Variable Speed Driven Pumps, Best Practice Guide. www.gambica.org.uk/pdfs/VSD_Pumps.pdf
Hydraulic Institute. www.pumpschool.org, www.pumpschool.com/intro/pdtree.htm
Pacific Liquid and Air Systems. www.pacificliquid.com
Sahdev, M. Centrifugal Pumps: Basic concepts of operation, maintenance and trouble shooting, Part I. Presented at The Chemical Engineers’ Resource Page. www.cheresources.com. Downloaded from: www.idcon.com/pdf-doc/centrifugalpumps.pdf
The Engineering Toolbox. www.engineeringtoolbox.com
US Department of Energy (DOE), Office of Industrial Technologies. Pump Life Cycle Costs: A guide to LCC analysis for pumping systems. DOE/GO-102001-1190. 2001. http://www1.eere.energy.gov/industry/bestpractices/techpubs_motors.html
US Department of Energy (US DOE), Office of Industrial Technologies. Variable Speed Pumping – A Guide to Successful Applications. Executive Summary. 2004. http://www1.eere.energy.gov/industry/bestpractices/techpubs_motors.html
US Department of Energy (US DOE), Office of Industrial Technologies. Improving Pump System performance, A Source Book for Industry. As part of: Motor Challenge Program. 1999 http://www1.eere.energy.gov/industry/bestpractices/techpubs_motors.html
Copyright: Copyright © United Nations Environment Programme (year 2006) This publication may be reproduced in whole or in part and in any form for educational or non-profit purposes without special permission from the copyright holder, provided acknowledgement of the source is made. UNEP would appreciate receiving a copy of any publication that uses this publication as a source. No use of this publication may be made for resale or any other commercial purpose whatsoever without prior permission from the United Nations Environment Programme.
Disclaimer: This energy equipment module was prepared as part of the project “Greenhouse Gas Emission Reduction from Industry in Asia and the Pacific” (GERIAP) by the National Productivity Council, India. While reasonable efforts have been made to ensure that the contents of this publication are factually correct and properly referenced, UNEP does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the contents of this publication.
