Electrical energy cost represents some 40% of the total cost of ownership of pumping systems, yet many organisations fail to introduce the proper steps to leverage cost reduction through efficiency improvements.
The following major barriers must be recognised and addressed to solve this dilemma:
Lack of proper metrics: Energy efficiency has traditionally not been used in assessing performance. In most organisations, the responsibilities of energy procurement and efficient operations are separate and consistent or standardised metrics are not used.
Knowledge gap: A lack of awareness in energy efficiency opportunities is prevalent and, as a result, potential savings and other benefits are missed.
Fear of investment: Operations personnel often struggle to present attractive large or even small investments to their finance organisations.
This article demonstrates how deployment of an energy management plan, with limited investment, can provide reductions in pumping systems total cost of ownership (TCO) while maintaining sustainability objectives. Any sound energy plan should take into account the following steps:
Energy efficiency management.
Asset management.
Energy cost management.
For the purposes of this article, the scope of a pumping system will be defined as encompassing all related elements starting from the point of the utility connection down to the point of end-use.
Fig. 1: Energy saved with variable versus fixed speed drives at 100% and 60% flow, according to the static head and pump sizing. The operating point is represented as the intersection of the pump curve with the system curve.
Step 1: Energy efficiency management
The challenge is that the nature of production in industrial environments is in a constant state of flux. Production cycles, for example, are influenced by variables such as market demand, weather, and local regulations. As a result, factory and building operators must understand how and when energy is used to minimise consumption and related costs.
The pump system energy management approach discussed here will review the nature of efficiency loss, not only for individual components within the system, but also for the system as a whole.
In pumping systems, most inefficiency comes from:
A mismatch between the pump deployed and the actual system requirement.
The improper use of throttling valves and damper technologies to control the flow of liquids.
The way pumping systems are controlled plays a major role in how efficiency can be improved. Control systems themselves are composed of both hardware and software components. On the hardware side, variable speed drives (VSDs) are a primary enabler of high efficiency performance.
Fig. 2: Comparison of two efficiency scenarios at different flow rates: 8 to 9% more efficient with variable speed drives at 60% flow.
The example in Fig. 1 compares two installations, one with a variable speed drive (VSD) and one with a fixed drive throttled system in which static heads are different.
At fixed speed (the throttled system example), it is necessary to add a throttle valve in the hydraulic circuit. This adjusts the flow by increasing or decreasing the flow resistance. This will modify the system curve. However, the speed remains the same, so the pump curve does not change. The flow rate is matched but the head is much higher than required, resulting in poor energy savings.
If a VSD is deployed, the system curve does not change. The pump curve is modified according to flow speed and affinity laws. Adjusting the speed matches the process requirement and results in significant energy savings.
Energy savings depends on the static head: the lower the static head, the better the energy savings (and speed variation range). It is necessary to generate enough power to overcome the static head for pumping action to occur. The friction head is the amount of head required to push the liquid through the pipe and fittings. It depends on flow rate, pipe size, pipe length, and viscosity.
Scenario 1: The static head represents 50% of the system head and the pump is rated for the head and flow of the system. At 100% flow, the power used by the pump is the same at both fixed speed and with a VSD. At 60% flow, the energy savings resulting in the VSD use is 46% (see Fig. 1).
Scenario 2: the static head represents 85% of the system head and the pump is oversized by 20%. In real-world scenarios, 75% of pumps are oversised by 10 to 30% to meet anticipated lifetime peak production; to anticipate future needs, or to rationalise spare parts inventory. Therefore, a variable speed drive saves 20% of energy at 100% flow and saves 36% energy at 60% flow.
Changing the operating point on the pump curve also changes the efficiency of the pump itself. The pump performs at maximum efficiency at its full capacity. This corresponds to what is referred to as the Best Efficiency Point (BEP). In terms of installation design and operation, the objective is to work as closely as possible to the BEP. By varying the speed, the pump efficiency remains roughly the same but is applied to a new flow rate. At fixed speed, reducing the flow rate quickly deteriorates the pump efficiency because it works far from the BEP, while adjusting the speed keeps the efficiency close to the BEP (see Fig. 2).
Determining pump efficiency is only a first step in identifying system performance levels. Monitoring efficiencies via software can detect operating points which are not suitable for the pump.
Access to such data can help improve both system energy efficiency and reliability.
Pump efficiency management best practices
The energy efficiency of a pumping system can be improved by implementing the following simple actions:
Replace fixed drives with VSDs to boost the efficiency. Connected to a pump, a VSD can control speed, pressure and flow in conjunction with dynamic process and production requirements.
Monitor production data and energy consumption data via software dashboards. Continuous tracking of the deviation between production output and energy consumed allows for rapid and cost-effective decision-making. Intelligent electronic devices (IEDs) such as VSDs that are tied into the monitoring system, play a major role in reporting data related to operation, production, and energy in real time. Monitoring points should be close to the load because that is where most of the power is used. The closer the monitoring is to the load, the more insights can be acquired relative to cost savings.
Monitor the operating point of the pump and its efficiency on a continual basis to visualise trends. Observance of the trends can then lead to sensible actions which improve efficiency and verify the impact of improvements to the system.
Use proper metrics to identify an increase or decrease in efficiency on particular systems and to compare efficiency performances of different pumps in multiple sites. A recommended key performance indicator (KPI) metric is the specific energy consumption metric (in kWh/m3).
Step 2: Asset management
Physical assets such as pumps must be maintained on an ongoing basis. Maintenance costs represent 25% of TCO and maintenance practices therefore warrant examination in terms of contribution to energy-influenced savings. Maintenance costs are unavoidable due to the wear of components during system operation and, because the cost of downtime attributed to loss of production, would threaten the solvency of the business. In pumping installations, many moving parts mean that proper maintenance of motors, drives, pumps and associated pipes is crucial.
Numerous steps can be taken to ensure that maintenance costs are kept to a minimum while the integrity of the system is kept stable.
All pumps should be operated within the parameters of a given pump’s specifications (often stated in the pump supplier’s instruction manual or data sheet). As discussed, pump efficiency varies according to operational parameters. The pump is designed for optimal operation at the best efficiency point (BEP) but 75% of the pumping systems are oversized by around 30%. Fig. 3 illustrates how pumps begin to waste efficiency when appropriate maintenance practices are neglected. For example, discharge recirculation can occur if the pump operates at 65% of the BEP flow rate, causing damage to the impeller, and a damaged impeller will be less efficient.
Fig. 3: Maintenance-related issues which impact pump performance.
VSDs can help keep the operating point close to the BEP and also protect the pump against destructive forces generated by inefficiencies. Extreme situations such as dry running, low flow operation, or cavitation due to low net positive suction head, which can cause instantaneous damage, are avoided. Monitoring the operating point of the pump and its efficiency provides diagnostics which can help predict when potential system problems will occur.
Fig. 4 illustrates how operating away from the BEP not only decreases the efficiency but speeds up the wear and tear on the pump, thereby reducing reliability. For example, operations which run at 60% of BEP result in:
50% lifetime reduction of seals.
20% lifetime reduction of bearings.
25% lifetime reduction of casing and impeller.
Approximately 100% increase of maintenance cost.
Fig. 4: Effect of the distance from the BEP on reliability.
Wear is unavoidable due to moving mechanical parts and to the action of the fluid being pumped. Erosion is generated by the speed of the fluid and this could be increased by slurries. Corrosion is due to chemical or electrochemical reaction which attacks the pump materials. Even treated drinking water causes corrosion in cast-iron casings as a result of the catalytic effect of bacteria.
Erosion and corrosion mostly impact the pipes, impeller, and the case.
Efficiency drops by 10 to 15% for an unmaintained pump (see Fig. 5). The major loss in efficiency occurs in the first few years of the pump’s life. Regular maintenance avoids losses if efficiency and capacity which can occur before the pump fails.
Fig. 5: Average wear trends for maintained and unmaintained pumps.
Maintenance practices
A number of approaches are available that can help address the issue of maintenance in a cost effective manner. Preventive maintenance implies the systematic inspection and detection of potential failures before they occur. Condition-based maintenance is a type of preventive maintenance which estimates and projects equipment condition over time, using probability formulas to assess downtime risks. Corrective maintenance is a response to an unanticipated problem or emergency.
Fig. 6 illustrates the cost curves of these three types of maintenance. Condition-based maintenance is the most cost effective of the three approaches.
Condition-based-maintenance monitors system data on an ongoing basis and provides an accurate assessment of the health or status of components, devices, or the complete system.
Variables such as suction and discharge pressure, pump speed, power, flow and temperature are monitored to detect loss of efficiency. Identification of the potential problems is possible by combining the efficiency trends and process variables.
Fig. 6: Cost curves of the different maintenance approaches.
VSDs can measure process variables, temperature and power with high accuracy and to assess pump efficiency. If connected to the automation system, they monitor the health of the system continuously and can indicate when maintenance is needed.
Pipes
As part of the overall pumping system, pipes are also subject to issues such as overpressure, leakage, or pipe burst. An overpressure situation can be caused by poor pump control. Water hammer can also occur. This is caused by a pressure or shock wave traveling through the pipes, generated by a sudden stop in the velocity of the water. This sudden acceleration and deceleration on the motor can be avoided with the help of a VSD. Leakage can also be reduced by automatic adjustments to pressure when appropriate.
Motors
Protection against mains voltage and frequency fluctuations can help maintain the integrity and extend the lifetime of motors. In cases where motors are equipped with VSDs, those electrical disturbances are not transmitted to the motor.
Protection against high temperature conditions can also extend the life of the motor. Devices such as thermal relays, PTC or PT100 thermal sensors can help and are manageable through the VSD.
In cases where long motor cables are used in conjunction with motors and VSDs, it is recommended that filters be installed to avoid the dv/dt and motor voltage surge effects.
Note that, for submersible borehole pumps, it is recommended to verify the peak-to-peak voltage and the dv/dt at the motor terminals with the motor-pump supplier.
Step 3: Energy cost management
Building owners, water or wastewater and oil and gas facility operators are presented with utility bills that have multiple components. These can include power demand charges, energy demand charges, time-of-use charges, ratchet clauses, cost of fuel adjustments, power factor penalties, customer service charges and taxe. A misinterpretation of utility rate structures can lead to poor management of electrical usage and to higher costs.
Most energy bills cover similar basic elements (see Fig. 7). Familiarity with the terms can help to understand where the opportunities for cost reductions exist.
Fig. 7: Worn pump curve versus new pump curve.
Following here are some definitions for common terms used:
Customer charge: This is a fixed charge that depends on the size of the connection which links the industrial installation in question to the electrical utility network. The customer charge is calculated according to an anticipated power usage range, and the price of the actual power that is used.
Actual energy charge: This charge corresponds to the active energy usage, which is the cumulative energy used over a given period of time. The kilowatt-hour (kWh) rate depends on the time period over which the energy was used and whether that consumption occurred during peak and or off-peak hours.
Demand charge: This charge represents the highest average power used within any 15-minute time period over the span of a month, tracked by the utility. This number is then multiplied by the demand charge rate to produce the demand charge which appears on the electricity bill. This means users are charged for a peak demand even if it only happened once during the month.
Power factor penalty: The power factor is the ratio between the active power (which generates work) and the apparent power (which could potentially be used to generate work). This means that a certain portion of the power delivered by the utility to the industrial site is not billed because it did not generate work. If the power factor is less than the given value mentioned in the contract (say around 0,9), the end-user is invoiced for the power factor (reactive power). A lot of equipment or devices have power factors lower than 1: motors, induction furnaces, transformers, VSDs, computers and fluorescent lighting.
Best practices for energy cost reduction
The electrical energy bill for the site can be reduced by implementing the following series of simple actions:
Locate and review the utility contract to better understand the charges associated with the bill and how they can be controlled. Up to 10% savings without any capital investment could be achieved with the support of a company specialist in energy management.
Adjust the timing of energy usage from the peak rate period to the off-peak period as much as is possible (e.g. by controlling differently reservoir and pumping operations).
Reduce the monthly peak demand number to reduce the demand charge. In most cases, 75% of the applications are oversized. VSDs, which can reduce power demand by 20%, are a technology which helps organisations to size according to process requirements.
Power factor penalties which are due to motor and which mitigate harmonics at 48% of THDi for 80% load can be canceled out by deploying VSD to pumps.
Reduce the amount of energy used that is not linked with revenue generation. An active control of the leakage will significantly reduce the operational cost.
Conclusion
By pursuing best practices in energy efficiency management, asset management, and energy cost management, TCO of pumping system networks can be reduced by up to 20%. One simple technology, the VSD with embedded energy management functionality, can of be a major contributor to achieving the TCO target.
The VSD is fully integrated in the numerous steps which can be taken to implement an effective energy management plan. These include adopting energy efficient technologies, implementing condition-based maintenance practices, and optimising cost control of the electricity bill. The linking of pumping processes to energy systems helps improve business performance through better energy management.
Organisations which are ill-equipped to jump-start energy efficiency programmes should seek the assistance of mission-critical subject matter experts. The alternative invites unnecessary delay, risk, and expense.
To achieve operational sustainability, organisations must act quickly to assess their current programmes and begin building operational methodologies which emphasise energy-efficiency improvements.
Contact Ntombi Mhangwani, Schneider Electric, Tel 011 254-6400, ntombi.mhangwani@schneider-electric.com
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