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The whole system savings approach
36
Feature
WORLD PUMPS
March 2013
Energy efficiency
The whole system savings approach Power consumption of pumping systems should not be considered in isolation when seeking to reduce operating costs. Hydraulic efficiency has a relatively small impact on savings, yet establishing the exact process demand to correctly size and operate the system inevitably leads to whole system savings.
P
umping 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. Centrifugal pumps are major power consumers within the processing, power generation, water utilities and building industries. Buildings and factories are the main focuses for energy efficiency because they represent practically 80% of the world’s energy usage. Despite this, pumping facilities are very rarely considered as potential energy savers. Energy cost for running a pump is perceived as incompressible and generally accounted as a fixed cost. However, 20%
to 80% of this energy could be saved using a system approach, turning it into a variable cost. Auditing the installed base in Great Britain could potentially save more than 10 TWh of electricity per year, avoiding the emission of approx. 5.5 million tons of CO2. Installing an efficient pump and motor will only provide small energy savings. In order to achieve overall efficiency, the complete pumping system must be taken into account (as illustrated by The Sankey Diagram, Figure 1). Applying the system approach in order to save energy can somewhat be likened to
Figure 1: Energy usage in a pumping system
www.worldpumps.com
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
'lean manufacturing.' Basically, fluid is only pumped when and where it is needed, following the principle of “just in time." The methodology aims to eliminate what is not needed in order to make significant process improvements. Energy efficiency is the primary focus when assessing an installed base; it is the route to Whole System Savings. Energy fees are by far the largest part of a pump’s life cycle cost (Figure 2). Depending on the type and operating condition it ranges from 40% to 90%. Furthermore, inefficiencies in a system cause useful energy to be converted into parasitic heat and vibration. This badly affects the reliability of components. The further away a pump operates from its best efficiency point, the lower the Mean Time Between Failure (Figure 3). Therefore, it can be concluded that saving energy has the potential to reduce maintenance and down time costs. Pumping systems are critical assets for processes. When it comes to failures, the consequences are often far beyond the simple cost of repair. International Paper operates paper mills throughout the U.S. During a one-year survey they experienced 101 pump faults, which generated a total cost of $5 million lost production. With an average cost of $50,000 per failure, a significant part of this amount was attributed to the
WORLD PUMPS
Feature March 2013
Figure 2: Life cycle cost structure of a medium size process centrifugal pump.
consequences of the plants down time. Probably one of the most tragic examples of a pumping facility breakdown was that of the Fukushima Dai-Ichi Nuclear Power Station, which was hit by a tsunami in 2011. After the cooling plant was flooded, pumping systems failed, and as a consequence three damaged reactors emitted vast amounts of radiation.
Whole system savings Systems Theory defines a system according to its boundaries and interactions with the external world (assuming it is open). In most cases a pumping system is open and often rather complex. Therefore, it is necessary to gain a clear understanding of the boundary and the dynamics of inputs and outputs. Using a simple graphical summary clearly illustrates the main characteristics (Figure 4).
Figure 3: Pump curve sensitivity for centrifugal pump reliability. In grey: Mean Time Between Failure as a function of operating flow around Best Efficiency Point. In blue: Failure modes related to operation out of BEP.
A. Define the pumping system’s present duty.
Pump behaviour
E. Fit the supply (pump and system) to the demand (process).
Hydraulic behaviour of a pump is commonly represented by a performance curve, while a network’s characteristics are represented by a system curve. Both curves are generated by plotting head against flow rate. The duty point indicates the hydraulic condition of the pump, and this point sits at the intersection of the two curves. Pump and system work together in a symbiotic manner, with the network leading the way within the limitations imposed by the pump.
F. Control the effectiveness of improvements carried out.
Pump performance curves change from one pump type to another and are
B. Establish the real need. C. Investigate where the main points of energy loss exist. D. Reduce energy losses accordingly.
Assessing the energy consumption of a pumping facility requires adequate methodology and tools. Appropriate guidance can be found within the Standard ASME EA-2-2009 - Energy Assessment for Pumping Systems. Furthermore, the Pumping System Assessment Tool (PSAT) training from the U.S. Department of Energy can provide the required skills and knowledge. Assessing a pumping facility is simple when following these steps:
Figure 4: Open pumping system.
www.worldpumps.com
37
38
Feature
WORLD PUMPS
March 2013
technological choices, the use of high safety margins, and so on. Often interrelated, the symptoms become hard to separate in order to establish the root and quantify the effect. Some examples of energy wasting sources: • Friction losses in pipes and accessories – This is the most famous root of inefficiency and is directly linked to the speed of fluid in pipes and accessories. Selecting the right size can save huge amounts of energy (Figure 6). Further, clean pipes and a streamlined layout also help. • Inadequacy of the pump with the system – The pump is not operating at its best efficiency point, causing reliability issues and wasting energy (as shown in Figure 3). This constantly happens at the pump selection stage for a new build. Safety margins are over-exercised because a pipe-work simulation cannot give "exact’"information, which leads to the selection of an oversized pump. Purchasing a pump close to its maximum operating flow in order to reduce costs leads to the same problem. Another example is when system characteristics are modified due to changes in processes, yet the pump is left untouched (i.e. no impeller trimming or pump replacement).
Figure 5: System curve, pump curve and operating point
Figure 6: Annual water pumping cost for 1000m of pipe of different size (water at 20oC, 0.07 £/kWh, 8,760 h/year, 94% motor eff., 82% pump eff., clean cast iron pipe).
dependant of rotational speed. The system curve is a mix between intrinsic characteristics (i.e. elevations, pipe losses etc.) and operation variables (i.e. control valves, tank level etc.).
pump features enables sources of loss and related saving potential to be established.
Pump and system curves are calculated by on-site measurements during the assessment, which gives an accurate picture of typical operating conditions. Mainly a non-intrusive process, the assessment barely disturbs the process/ processes at hand. Operational variables must also be established, because a pumping system is seldom a static environment. Identifying network and
Inefficiency sources receive the majority of attention in pumping system assessments. Some are rather obvious, but many are hidden at first sight. Usually the facility looks fine and operates well, but huge amounts of energy can still be wasted.
www.worldpumps.com
Spotting losses
Many of these losses are related to the plant’s history, i.e. modifications,
• Valve throttling and control valves – Inefficiencies are inevitable while using control valves as they rely on friction losses to control the flow rate. The amount of throttling indicates the quantity of energy lost, and is also a good indication of an oversized pump (see 3.1 for real-life example). • Unnecessary static head – The level of energy required to lift the fluid is not always useful and the static head is sometimes excessive with no functional justification. A common example is a discharge tank level kept high while the suction tank level is kept low. Another is a non-return valve replaced by an openair discharge line. • Bypass/re-circulation – There are various reasons for the use of a bypass but they are major wasters of energy. Here, a volume of fluid is pumped and returned to the suction side without being utilised. Bypasses are frequently
WORLD PUMPS
Feature March 2013
fitted to avoid turning off the pump when the process is not in use, bringing us to the next point. • Flow in pipes when the process is not in use – Energy is useless when a flow is present in a pipe for no reason. For example, cooling water is regularly found flowing into loops and heat exchangers when they are not in operation (see 3.2 for example). • Supply higher than demand – Often unnoticed, this is one of the biggest sources of energy-waste. It is when flow rate exceeds process requirements or when a pump is kept operational with no demand at all. Closed loop systems are often operated in this way, as are certain transfer applications. Additional sources of energy loss, such as those mentioned previously, take place when supply is not driven by demand. Energy wasting sources are often interrelated, meaning that their effect is duplicated. Therefore, reducing each and every one amplifies final savings. Adjusting the flow rate and selecting the right pump for the duty is the most straightforward way to achieve this.
Figure 7: Pumping system load profile - Flow rate histogram.
Q. Why does crystallisation rate need to be controlled? A. To get the right crystal size. Conclusion: The duty of the pump is governed by the size of the crystal. Optimum efficiency is reached when a systems final function is still ensured, despite operating at the lowest possible flow rate. Speed of fluid in the pipe is directly proportional to flow rate. Friction loss varies as the square of speed so it
requirements is instrumental when establishing an operating pattern.
Fixed demand: Suitable operation depends on pump selection and system layout. The goal is to select a pump able to provide the required flow rate at its best efficiency point. Head value is determined by the facility’s requirements at the flow rate specified (See 2.2 for example). PSAT software gives an indication of the pump
Process demand Savings can only be achieved if the pumping facility supplies what is needed, when it is needed (Just in Time). This means identifying the genuine purpose of the pump, which in most cases is not to move fluid. For example, a pump on a filtration line can be used to assist with fluid cleaning; however its final function depends on why the fluid is being cleaned in the first place. The “Five Whys” method is useful for understanding the goal of the system. For example, when considering a cooling water pump on a crystallisation process:
"Understanding process dynamics and requirements is instrumental when establishing an operating pattern." varies as the square of flow rate. Therefore, reducing flow rate promptly improves the specific energy - the amount of energy used by unit of volume displaced in the system. Ultimately, the pump set matches the duty in order to meet performance expectations.
and motor efficiency that can be expected for a specified duty point. Effectiveness of this step is improved when the duty and system layout have already been addressed (see 2.4 and 2.5).
Q. Why does heat dissipation need to be increased? A. To cool down a process in a given time.
When demand is fluctuating, the systems operating condition could be changing accordingly. Identifying the load profile over a period of time gives a good idea of the facility’s duty (Figure 7). Main duty points can be identified in order to set up an operating pattern, which means potential savings can be calculated against the changing duty (as opposed to using one fixed duty point).
Facilities experiencing fluctuating demand must target multiple duty points and correctly handle intermediate states. An adequate solution depends on the system’s characteristics (i.e. static head, friction loss) and duty points. Where possible, the fluctuation should be smoothed and simplified, for example by the use of a buffer tank. This potentially reduces peak flow rates and therefore energy consumption.
Q. Why does the process need to cool down in a given time? A. To control crystallisation rate.
Process demands are dependent on the operation of a suitable pumping system. Understanding process dynamics and
Q. Why is there a pump on this system? A. To move cooling water. Q. Why is the cooling water moving? A. To increase heat dissipation.
Fluctuating demand:
When the fluctuation pattern is complex, operation can be difficult, often requiring www.worldpumps.com
39
40
Feature
one or several pumps and a specific control method. Common regulating methods rely on control valves and bypasses. However, modern control technologies avoid the use of inefficiencies as a control method as they are based on control loops backed with sensors and calculated information. They usually stay
WORLD PUMPS
* Annual electricity usage: 590 MW.h, £41,300 annual electricity cost at 0.07 £/kW.h. * Main points of losses: Flow rate is governed by a control valve that is only operating from 10% to 40% aperture. The pump is obviously
"Adopting a system approach lowers the life-cycle cost of a pumping facility by increasing energy efficiency and subsequently addressing maintenance and down time costs." closer to the required duty and provide more flexibility. Using a variable speed drive is one way to cope with fluctuations in demand. However, it is not a universal solution. It is really important to understand where and how to use a variable speed drive, as energy usage can be increased rather than saved if used incorrectly. Optimisation process involves considering actual priorities and constraints. It seldom leads to one unique solution so that several returns on investment schemes can be envisaged. The biggest saving is not obtained by the solution with the highest potential, but by the best implementable solution. The results of the energy audit must be put into action to generate any savings, as only then will reductions in energy use, maintenance and down time costs assist to achieve the calculated ROI.
Real life examples Many Whole System Saving assessments have been conducted in Great Britain, with two examples from the process industry below. Both cases focus on one pumping system out of several hundred operated at each industrial site.
Processing plant, food industry, London * Duty: Process fluid transfer from a mechanical strainer to a supply tank. * Pumps: Two centrifugal slurry pumps with open impeller, alternatively operating. Belt driven by 110 kW electric motors. www.worldpumps.com
massively oversized and the control valve is dissipating the excess energy. * Solution: Downsize the pump and replace the control valve by a variable frequency driver (this facility is suitable for a retrofit). * Results: 93% of energy saved, cutting electricity costs in excess of £44,000 per year (at 8p /kW.h). 298 tons of CO2 emission is avoided.
Processing plant, pharmaceutical industry, Scotland * Duty: Cooling water loop between cooling towers and several processes. * Pumps: Two standard centrifugal pumps with closed impeller, alternatively operating. Close coupled to 75 kW electric motors. * Annual electricity usage: 370 MW.h, £25,900 annual electricity cost at 0.07 £/kW.h. * Main points of losses: The cooling plant was sized when the site was larger and would have allowed for further growth. The demand is now much inferior and the differential of inlet and outlet temperature is extremely low. This demonstrates that the flow rate is too high and that the pump is oversized. Furthermore, the cooling water is flowing in heat exchangers and lines for processes that are not in use. There is no control method to change the system’s duty when isolating unused lines.
March 2013
* Solution: The pump should be downsized and a variable frequency driver set up so that the differential of inlet and outlet temperature drives the flow rate. Then a routine procedure to isolate the lines where processes are not working should be instated. * Results: 86% of energy saved, cutting electricity costs in excess of £20,000 per year (at 8p /kW.h). 172 tons of CO2 emission is avoided.
Conclusion Adopting a system approach lowers the life-cycle cost of a pumping facility by increasing energy efficiency and subsequently addressing maintenance and down time costs. The assessment provides quantified savings potential and practical guidance, highlighting effective solutions and the possible ROI. Processes that have experienced modifications throughout their history can now be streamlined with a short ROI time, ultimately leading to a more flexible system with a higher level of resilience. Pumping systems are critical assets in many processes. Enhancing them reduces plant down time and operating costs, while improving flexibility. The industry is facing rising costs and growing uncertainties, and so the ability to improve bottom line and productivity simultaneously is certainly good news for most. Achieving desired results depends on the quality of the assessment. Therefore, the skills of the assessing team and the methodology used are instrumental. Acknowledged qualifications and accreditations encourage pump users to gain confidence in such a service. However, there are not many recognised bodies that certify auditors for pumping systems – this is still a growing area. Nevertheless, the work carried out by the U.S. Department of Energy and the UK British Pump Manufacturer Association is pointing in the right direction. For a full list of references, please contact the author.
Contact Florent Violain Sterling SIHI Tel: +33 (0)1 34823900 E-mail: florent.violain@sterlingfluid.com
Feature
WORLD PUMPS
March 2013
Energy efficiency
The whole system savings approach Power consumption of pumping systems should not be considered in isolation when seeking to reduce operating costs. Hydraulic efficiency has a relatively small impact on savings, yet establishing the exact process demand to correctly size and operate the system inevitably leads to whole system savings.
P
umping 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. Centrifugal pumps are major power consumers within the processing, power generation, water utilities and building industries. Buildings and factories are the main focuses for energy efficiency because they represent practically 80% of the world’s energy usage. Despite this, pumping facilities are very rarely considered as potential energy savers. Energy cost for running a pump is perceived as incompressible and generally accounted as a fixed cost. However, 20%
to 80% of this energy could be saved using a system approach, turning it into a variable cost. Auditing the installed base in Great Britain could potentially save more than 10 TWh of electricity per year, avoiding the emission of approx. 5.5 million tons of CO2. Installing an efficient pump and motor will only provide small energy savings. In order to achieve overall efficiency, the complete pumping system must be taken into account (as illustrated by The Sankey Diagram, Figure 1). Applying the system approach in order to save energy can somewhat be likened to
Figure 1: Energy usage in a pumping system
www.worldpumps.com
0262 1762/13 © 2013 Elsevier Ltd. All rights reserved
'lean manufacturing.' Basically, fluid is only pumped when and where it is needed, following the principle of “just in time." The methodology aims to eliminate what is not needed in order to make significant process improvements. Energy efficiency is the primary focus when assessing an installed base; it is the route to Whole System Savings. Energy fees are by far the largest part of a pump’s life cycle cost (Figure 2). Depending on the type and operating condition it ranges from 40% to 90%. Furthermore, inefficiencies in a system cause useful energy to be converted into parasitic heat and vibration. This badly affects the reliability of components. The further away a pump operates from its best efficiency point, the lower the Mean Time Between Failure (Figure 3). Therefore, it can be concluded that saving energy has the potential to reduce maintenance and down time costs. Pumping systems are critical assets for processes. When it comes to failures, the consequences are often far beyond the simple cost of repair. International Paper operates paper mills throughout the U.S. During a one-year survey they experienced 101 pump faults, which generated a total cost of $5 million lost production. With an average cost of $50,000 per failure, a significant part of this amount was attributed to the
WORLD PUMPS
Feature March 2013
Figure 2: Life cycle cost structure of a medium size process centrifugal pump.
consequences of the plants down time. Probably one of the most tragic examples of a pumping facility breakdown was that of the Fukushima Dai-Ichi Nuclear Power Station, which was hit by a tsunami in 2011. After the cooling plant was flooded, pumping systems failed, and as a consequence three damaged reactors emitted vast amounts of radiation.
Whole system savings Systems Theory defines a system according to its boundaries and interactions with the external world (assuming it is open). In most cases a pumping system is open and often rather complex. Therefore, it is necessary to gain a clear understanding of the boundary and the dynamics of inputs and outputs. Using a simple graphical summary clearly illustrates the main characteristics (Figure 4).
Figure 3: Pump curve sensitivity for centrifugal pump reliability. In grey: Mean Time Between Failure as a function of operating flow around Best Efficiency Point. In blue: Failure modes related to operation out of BEP.
A. Define the pumping system’s present duty.
Pump behaviour
E. Fit the supply (pump and system) to the demand (process).
Hydraulic behaviour of a pump is commonly represented by a performance curve, while a network’s characteristics are represented by a system curve. Both curves are generated by plotting head against flow rate. The duty point indicates the hydraulic condition of the pump, and this point sits at the intersection of the two curves. Pump and system work together in a symbiotic manner, with the network leading the way within the limitations imposed by the pump.
F. Control the effectiveness of improvements carried out.
Pump performance curves change from one pump type to another and are
B. Establish the real need. C. Investigate where the main points of energy loss exist. D. Reduce energy losses accordingly.
Assessing the energy consumption of a pumping facility requires adequate methodology and tools. Appropriate guidance can be found within the Standard ASME EA-2-2009 - Energy Assessment for Pumping Systems. Furthermore, the Pumping System Assessment Tool (PSAT) training from the U.S. Department of Energy can provide the required skills and knowledge. Assessing a pumping facility is simple when following these steps:
Figure 4: Open pumping system.
www.worldpumps.com
37
38
Feature
WORLD PUMPS
March 2013
technological choices, the use of high safety margins, and so on. Often interrelated, the symptoms become hard to separate in order to establish the root and quantify the effect. Some examples of energy wasting sources: • Friction losses in pipes and accessories – This is the most famous root of inefficiency and is directly linked to the speed of fluid in pipes and accessories. Selecting the right size can save huge amounts of energy (Figure 6). Further, clean pipes and a streamlined layout also help. • Inadequacy of the pump with the system – The pump is not operating at its best efficiency point, causing reliability issues and wasting energy (as shown in Figure 3). This constantly happens at the pump selection stage for a new build. Safety margins are over-exercised because a pipe-work simulation cannot give "exact’"information, which leads to the selection of an oversized pump. Purchasing a pump close to its maximum operating flow in order to reduce costs leads to the same problem. Another example is when system characteristics are modified due to changes in processes, yet the pump is left untouched (i.e. no impeller trimming or pump replacement).
Figure 5: System curve, pump curve and operating point
Figure 6: Annual water pumping cost for 1000m of pipe of different size (water at 20oC, 0.07 £/kWh, 8,760 h/year, 94% motor eff., 82% pump eff., clean cast iron pipe).
dependant of rotational speed. The system curve is a mix between intrinsic characteristics (i.e. elevations, pipe losses etc.) and operation variables (i.e. control valves, tank level etc.).
pump features enables sources of loss and related saving potential to be established.
Pump and system curves are calculated by on-site measurements during the assessment, which gives an accurate picture of typical operating conditions. Mainly a non-intrusive process, the assessment barely disturbs the process/ processes at hand. Operational variables must also be established, because a pumping system is seldom a static environment. Identifying network and
Inefficiency sources receive the majority of attention in pumping system assessments. Some are rather obvious, but many are hidden at first sight. Usually the facility looks fine and operates well, but huge amounts of energy can still be wasted.
www.worldpumps.com
Spotting losses
Many of these losses are related to the plant’s history, i.e. modifications,
• Valve throttling and control valves – Inefficiencies are inevitable while using control valves as they rely on friction losses to control the flow rate. The amount of throttling indicates the quantity of energy lost, and is also a good indication of an oversized pump (see 3.1 for real-life example). • Unnecessary static head – The level of energy required to lift the fluid is not always useful and the static head is sometimes excessive with no functional justification. A common example is a discharge tank level kept high while the suction tank level is kept low. Another is a non-return valve replaced by an openair discharge line. • Bypass/re-circulation – There are various reasons for the use of a bypass but they are major wasters of energy. Here, a volume of fluid is pumped and returned to the suction side without being utilised. Bypasses are frequently
WORLD PUMPS
Feature March 2013
fitted to avoid turning off the pump when the process is not in use, bringing us to the next point. • Flow in pipes when the process is not in use – Energy is useless when a flow is present in a pipe for no reason. For example, cooling water is regularly found flowing into loops and heat exchangers when they are not in operation (see 3.2 for example). • Supply higher than demand – Often unnoticed, this is one of the biggest sources of energy-waste. It is when flow rate exceeds process requirements or when a pump is kept operational with no demand at all. Closed loop systems are often operated in this way, as are certain transfer applications. Additional sources of energy loss, such as those mentioned previously, take place when supply is not driven by demand. Energy wasting sources are often interrelated, meaning that their effect is duplicated. Therefore, reducing each and every one amplifies final savings. Adjusting the flow rate and selecting the right pump for the duty is the most straightforward way to achieve this.
Figure 7: Pumping system load profile - Flow rate histogram.
Q. Why does crystallisation rate need to be controlled? A. To get the right crystal size. Conclusion: The duty of the pump is governed by the size of the crystal. Optimum efficiency is reached when a systems final function is still ensured, despite operating at the lowest possible flow rate. Speed of fluid in the pipe is directly proportional to flow rate. Friction loss varies as the square of speed so it
requirements is instrumental when establishing an operating pattern.
Fixed demand: Suitable operation depends on pump selection and system layout. The goal is to select a pump able to provide the required flow rate at its best efficiency point. Head value is determined by the facility’s requirements at the flow rate specified (See 2.2 for example). PSAT software gives an indication of the pump
Process demand Savings can only be achieved if the pumping facility supplies what is needed, when it is needed (Just in Time). This means identifying the genuine purpose of the pump, which in most cases is not to move fluid. For example, a pump on a filtration line can be used to assist with fluid cleaning; however its final function depends on why the fluid is being cleaned in the first place. The “Five Whys” method is useful for understanding the goal of the system. For example, when considering a cooling water pump on a crystallisation process:
"Understanding process dynamics and requirements is instrumental when establishing an operating pattern." varies as the square of flow rate. Therefore, reducing flow rate promptly improves the specific energy - the amount of energy used by unit of volume displaced in the system. Ultimately, the pump set matches the duty in order to meet performance expectations.
and motor efficiency that can be expected for a specified duty point. Effectiveness of this step is improved when the duty and system layout have already been addressed (see 2.4 and 2.5).
Q. Why does heat dissipation need to be increased? A. To cool down a process in a given time.
When demand is fluctuating, the systems operating condition could be changing accordingly. Identifying the load profile over a period of time gives a good idea of the facility’s duty (Figure 7). Main duty points can be identified in order to set up an operating pattern, which means potential savings can be calculated against the changing duty (as opposed to using one fixed duty point).
Facilities experiencing fluctuating demand must target multiple duty points and correctly handle intermediate states. An adequate solution depends on the system’s characteristics (i.e. static head, friction loss) and duty points. Where possible, the fluctuation should be smoothed and simplified, for example by the use of a buffer tank. This potentially reduces peak flow rates and therefore energy consumption.
Q. Why does the process need to cool down in a given time? A. To control crystallisation rate.
Process demands are dependent on the operation of a suitable pumping system. Understanding process dynamics and
Q. Why is there a pump on this system? A. To move cooling water. Q. Why is the cooling water moving? A. To increase heat dissipation.
Fluctuating demand:
When the fluctuation pattern is complex, operation can be difficult, often requiring www.worldpumps.com
39
40
Feature
one or several pumps and a specific control method. Common regulating methods rely on control valves and bypasses. However, modern control technologies avoid the use of inefficiencies as a control method as they are based on control loops backed with sensors and calculated information. They usually stay
WORLD PUMPS
* Annual electricity usage: 590 MW.h, £41,300 annual electricity cost at 0.07 £/kW.h. * Main points of losses: Flow rate is governed by a control valve that is only operating from 10% to 40% aperture. The pump is obviously
"Adopting a system approach lowers the life-cycle cost of a pumping facility by increasing energy efficiency and subsequently addressing maintenance and down time costs." closer to the required duty and provide more flexibility. Using a variable speed drive is one way to cope with fluctuations in demand. However, it is not a universal solution. It is really important to understand where and how to use a variable speed drive, as energy usage can be increased rather than saved if used incorrectly. Optimisation process involves considering actual priorities and constraints. It seldom leads to one unique solution so that several returns on investment schemes can be envisaged. The biggest saving is not obtained by the solution with the highest potential, but by the best implementable solution. The results of the energy audit must be put into action to generate any savings, as only then will reductions in energy use, maintenance and down time costs assist to achieve the calculated ROI.
Real life examples Many Whole System Saving assessments have been conducted in Great Britain, with two examples from the process industry below. Both cases focus on one pumping system out of several hundred operated at each industrial site.
Processing plant, food industry, London * Duty: Process fluid transfer from a mechanical strainer to a supply tank. * Pumps: Two centrifugal slurry pumps with open impeller, alternatively operating. Belt driven by 110 kW electric motors. www.worldpumps.com
massively oversized and the control valve is dissipating the excess energy. * Solution: Downsize the pump and replace the control valve by a variable frequency driver (this facility is suitable for a retrofit). * Results: 93% of energy saved, cutting electricity costs in excess of £44,000 per year (at 8p /kW.h). 298 tons of CO2 emission is avoided.
Processing plant, pharmaceutical industry, Scotland * Duty: Cooling water loop between cooling towers and several processes. * Pumps: Two standard centrifugal pumps with closed impeller, alternatively operating. Close coupled to 75 kW electric motors. * Annual electricity usage: 370 MW.h, £25,900 annual electricity cost at 0.07 £/kW.h. * Main points of losses: The cooling plant was sized when the site was larger and would have allowed for further growth. The demand is now much inferior and the differential of inlet and outlet temperature is extremely low. This demonstrates that the flow rate is too high and that the pump is oversized. Furthermore, the cooling water is flowing in heat exchangers and lines for processes that are not in use. There is no control method to change the system’s duty when isolating unused lines.
March 2013
* Solution: The pump should be downsized and a variable frequency driver set up so that the differential of inlet and outlet temperature drives the flow rate. Then a routine procedure to isolate the lines where processes are not working should be instated. * Results: 86% of energy saved, cutting electricity costs in excess of £20,000 per year (at 8p /kW.h). 172 tons of CO2 emission is avoided.
Conclusion Adopting a system approach lowers the life-cycle cost of a pumping facility by increasing energy efficiency and subsequently addressing maintenance and down time costs. The assessment provides quantified savings potential and practical guidance, highlighting effective solutions and the possible ROI. Processes that have experienced modifications throughout their history can now be streamlined with a short ROI time, ultimately leading to a more flexible system with a higher level of resilience. Pumping systems are critical assets in many processes. Enhancing them reduces plant down time and operating costs, while improving flexibility. The industry is facing rising costs and growing uncertainties, and so the ability to improve bottom line and productivity simultaneously is certainly good news for most. Achieving desired results depends on the quality of the assessment. Therefore, the skills of the assessing team and the methodology used are instrumental. Acknowledged qualifications and accreditations encourage pump users to gain confidence in such a service. However, there are not many recognised bodies that certify auditors for pumping systems – this is still a growing area. Nevertheless, the work carried out by the U.S. Department of Energy and the UK British Pump Manufacturer Association is pointing in the right direction. For a full list of references, please contact the author.
Contact Florent Violain Sterling SIHI Tel: +33 (0)1 34823900 E-mail: florent.violain@sterlingfluid.com