Optimize pump systems to save electrical energy

S. Schofield, Europump Executive Council Member, Brussels, Belgium

Given the global rise in energy costs, there has never been a better time to review and assess the efficiency of pumping systems—whatever their size, complexity or sphere of operation.

Pump systems account for 20% of the world’s electrical energy demand and 25%–50% of the electrical energy use in certain applications. Pumps are the single largest user of electricity within industry across the European Union (EU), consuming more than 300 TWhr/yr of electricity, which in turn accounts for more than 65 MMtpy of CO₂ emissions.

It is also well-documented that rotodynamic pumps, which account for 80% of the installed base, are oversized 20%–30% of the time, indicating a major energy savings potential if these pumps are properly sized and operated.

Where to start? Assess the potential savings by understanding the pumping system. To identify whether a pumping system is sized correctly, check the following:

1. Is liquid flow being wasted?
2. Is additional pressure being created?
3. What is the monitoring and controls philosophy?
4. Are oversized pumps installed for the required duty?
5. Are significantly old pumps employed in the system?
6. Is there excessive noise in the system?
7. Are throttling valves installed in the system?
8. Is the correct size of pipework installed?
9. Inadequate maintenance lowers pump system efficiency, so what maintenance procedures are in place?
10. Is a proactive equipment replacement program adopted to current EU legislation?

Available technology. Choosing the right equipment requires an understanding of drive technology. Use the latest EU regulations to achieve the best possible drive efficiency with:

1. Variable-speed drive (VSD)
2. Integrated control and monitoring
3. Permanent magnet technology.

System design—making the right choices. Most existing pump systems operating today were not originally designed with energy conservation as a major consideration. If pump systems are initially designed on an energy efficient basis and pumps are correctly applied and sized, the energy savings can often exceed 50%.

To design an energy efficient pump system, the following criteria should be considered:

1. Basic plant layout
2. Pipe size, configuration and restrictions to limit pipe work losses
3. Information on the liquid (viscosity, density, aggressiveness, temperature, etc.)
4. System characteristics and pump selection
5. Pump/system control
6. Avoid oversizing the pump units
7. Select the most energy-efficient pumps at the best efficiency point (BEP)
8. Fit VSDs to match the system design and demand.

Low energy costs are a direct result of knowing the minimum flow and pressure required by the system to operate successfully (system demand), selecting the correct sized pump and matching it to the system (FIG. 1).

When selecting a pump, it is important to determine the required flow and pressure generated by the pump. The flow may be determined by a process requirement, by the heating or cooling required in the system, or by the peak water demand for utilities. The pressure required may be to elevate the liquid within the system or to overcome the pressure losses in the system created as liquid passes through it.

It is important to know as much about the pump system as possible and to create a pressure/flow profile for the system. The energy required to drive the pump is directly related to the flow and pressure required. Generating high pressures usually leads to designs that may be inefficient; therefore, it is important that neither the flow nor the pressure are over specified.

VSDs can save energy. Significant energy savings are being seen by using VSDs and high-efficiency motors. Generally, VSDs are used to continually adjust the speed of the pump to match demand. Savings can be determined by the affinity laws (Eqs. 1–3):

Q₂ / Q₁ = n₂ / n₁               (1)

H₂ / H₁ = (n₂ / n₁)²          (2)

P₂ / P₁ = (n₂ / n₁)³        (3)

where, Q = flow, H = head, P = power and n = rotational speed.

When building a new pumping system, most pumps are selected with a "safety factor" in play to satisfy potential future uprates, or to allow for wear in the pump or fouling of the system. Often, many different parties are involved in specifying and building a system and the safety factor can grow exponentially. This results in the pump delivering much higher flows than required. It may also be necessary to vary the flow due to process conditions or varying heating and cooling needs within buildings.

Traditionally, throttling is used to regulate flow in a pumping system. While throttling reduces the flow, the motor is still running at full speed and works even harder as it must work against a restriction. By reducing the speed of the motor, the VSD ensures no more energy than necessary is used to achieve the required flow. A centrifugal pump running at half speed consumes only one-eighth of the energy compared to one running at full speed. Using an electrical VSD is the simplest and most economical way of controlling the pump and matching it to the pump system.

Key considerations when optimizing the energy use in pumping systems include:

1. Has an alternative configuration been considered? In some cases, moving from an existing pump layout to an alternative configuration can offer potential energy savings.
2. Is pump performance monitored? By looking for early signs of pump wear (i.e., increased noise, vibration, power consumption), significant energy and maintenance savings can be achieved.
3. Has ease of maintenance been considered? When designing or replacing pumps, ease of future maintenance requirements should be considered.
4. VSD considerations. When fitting a VSD to a pumping system, output and input filters should be considered, along with insulated motor bearings.
5. Maintenance. When maintaining pumping systems, it is recommended to use original equipment manufacturer (OEM) recommendations and parts.
6. Reliability/security. Efficient, well-maintained pumps are more likely to be reliable and unlikely to fail prematurely, causing loss of production or services.
7. Valves. Valves can be a source of wasted energy within a pumping system; if installed or replaced, they should be checked for correct operation.
8. Pumps not in use. Standby pump units or pumps with no demand should be switched off to save energy.
9. Is the pump working most of the time close to its BEP? Rotodynamic pumps operating away from their BEP not only waste energy, but also reduce the life expectancy of the pump.
10. System alterations. When upgrading, changing or expanding a pumping system, the demand may have changed and existing pumps may not be the most efficient solution.
11. Purchasing the correct pump set. When purchasing a pump set, price should not be the deciding factor. If a pump is sized correctly, the return on investment is shorter; likewise, if a pump is oversized and wasting energy, it will incur additional cost for the anticipated life of the pump set.
12. Has an energy check been undertaken on existing systems? The installed base of pumps is 10 times the number sold each year. Energy audits in accordance with ISO 14414 can identify substantial energy savings in existing pumping systems.

When specifying a new pump, ask for a high-efficiency motor to be fitted. If replacing or rewinding a motor, evaluate the cost of fitting a high-efficiency motor, remembering to factor in the running savings that will pay back any increase in cost. Invest in an energy audit, review utility bills and understand the energy being used. Conduct an audit if:

1. Energy bills are high
2. Pumps are in continuous operation
3. The system comprises many pumps
4. Processes have varying flows
5. Throttled pumps are used
6. Pumps are on bypass
7. There are noisy valves or pipework
8. Critical systems have been subject to breakdowns.

The purpose of an energy audit is to reduce operating costs by reducing energy consumption. The UK government has estimated that most companies can reduce their energy consumption by 10%–20%. Energy audits carried out in accordance with ISO 14414 have shown that savings of 30%–50% are not unusual. When deciding whether to carry out an energy audit, a good starting point is to assume a savings of > 10% of current energy consumption. Reviewing utility bills provides an indication of the potential savings and the investment necessary into the auditing process.

Across most industrial sites, some two-thirds of the total energy consumption is used to power electric motors. Furthermore, the overall cost associated with operating these essential pieces of equipment throughout their entire lifespan can be broken down as: 5% for the initial purchase/installation costs, 10% for ongoing maintenance, and a massive 85% for the energy used to run them. Clearly, any reduction in the energy consumed by electric motors is important; with modern designs, that reduction can be as much as 30%. It is also evident that many pumps and motors are constantly operated at full power, irrespective of process needs. Consequently, there is potential for significant energy savings across Europe’s installed base—savings that are reflected in the bottom line and increased profitability.

Europump. Europump is the European Association of Pump Manufacturers and was established in 1960. It represents 16 national associations in 12 EU member states, Russia, Switzerland, Turkey and the UK. Europump members represent more than 450 companies with a collective production value of more than €10 B and an employee base of some 100,000 people across Europe. HP

STEVE SCHOFIELD is Director and Chief Executive Officer of the British Pump Manufacturer’s Association Ltd. (BPMA), the UK trade association representing the interests of UK manufacturers of liquid pumps. Schofield is responsible for all BPMA activities and manages a small team that offers information to members on technical, training, standards, legislative, marketing and energy-related activities. Since joining the BPMA, he has also been actively involved in many European Commission energy programs, such as Pump SAVE, Motor Challenge, ProMot and DEXA. From 2000–2002, he was involved in the working group that produced the “International guide on pump lifecycle costing.” In 2003, he worked with Future Energy Solutions to produce the “Best practice guide on variable speed pumps” and subsequently assumed the role of Secretary to produce the “International guide for variable speed pumping.” In 2005, he again assumed the role of Secretary to produce the “International guide for system efficiency in rotodynamic pumping systems.” In the UK, he has worked closely for many years with government departments, such as the Department for Environment Food and Rural Affairs (DEFRA) and the Department of Energy and Climate Change (DECC). In Europe, he serves as Secretary for Europump on Lot 29, a study on various types of pumps, as well as Secretary to its Marketing Commission. His main responsibility at Europump over the past few years has been working with the European Commission to ensure a systems approach on pumping systems becomes a reality across Europe. Schofield is now also a member of Europump’s Executive Council. He is the Convenor for ISO TC 115 WG 7 working on the ISO 14414 pump system assessment standard, as well as a member of the working groups writing the ISO 50001 and EN 16247 series of energy management and audit standards. Schofield has close links with the U.S. Department of Energy on pumps systems training and recently carried out a review of pump system training for the United Nations Industrial Development Organization (UNIDO). He has presented papers on energy-related issues in the UK, Europe, U.S., South Africa and Singapore. Prior to joining the BPMA in 1998, he worked in the pump industry for 25 yr in positions with Mather & Platt, Weir Pumps, Durco, Flowserve and Hayward Tyler.