Variable speed drives are a way to lower life-cycle costs
Pumping systems account for nearly 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities. Significant opportunities exist to reduce pumping system energy consumption through smart design, retrofitting and operating practices. In particular, the many pumping applications with variable-duty requirements offer great potential for savings. The savings often go well beyond energy, and may include improved performance, improved reliability, and reduced life cycle costs.
Most existing systems requiring flow control, make use of bypass lines, throttling valves, or pump speed adjustments. The most efficient of these is pump speed control. When a pump’s speed is reduced, less energy is imparted to the fluid and less energy needs to be throttled or bypassed. Speed can be controlled in a number of ways, with the most popular type of variable speed drive (VSD) being the variable frequency drive (VFD).
A proper discussion of pumping considers not just the pump, but the entire pumping “system” and how the system components interact. The recommended systems approach to evaluation and analysis includes both the supply and demand sides of the system.
Most systems have a combination of static and friction head. The ratio of static to friction head over the operating range influences the benefits achievable from VSDs. Static head is a characteristic of the specific installation.
Reducing this head whenever possible generally reduces both the cost of the installation and the cost of pumping the liquid. Friction head losses must be minimized to reduce pumping cost, but after eliminating unnecessary pipe fittings and length, further reduction in friction head will require larger diameter pipes, which adds to installation cost.
Proper selection of pumps, motors and controls to meet the process requirements is essential to ensure that a pumping system operates effectively, reliably and efficiently. All pumps are divided into the two major categories of rotodynamic and positive displacement (PD).
The performance of a pump can be expressed graphically as head against flow rate. The rotodynamic pump has a curve where the head falls gradually with increasing flow. However, for a PD pump, the flow is almost constant whatever the head. It is customary to draw the curve for PD pumps with the axes reversed.
For a PD pump, if the system resistance increases, the pump will increase its discharge pressure and maintain a fairly constant flow rate, dependent on viscosity and pump type. Unsafe pressure levels can occur without relief valves. For a rotodynamic pump, an increasing system resistance will reduce the flow, eventually to zero, but the maximum head is limited.
Many pumping systems require a variation of flow or pressure. Either the system curve or the pump curve must be changed to get a different operating point. Where a single pump has been installed for a range of duties, it will have been sized to meet the greatest output demand. It will therefore usually be oversized, and will be operating inefficiently for other duties.
Consequently, there is an opportunity to achieve an energy cost savings by using control methods, such as variable speed, which reduce the power to drive the pump during the periods of reduced demand.
Effects of speed variation
A rotodynamic pump is a dynamic device with the head generated by a rotating impeller. Thus, there is a relationship between impeller peripheral velocity and generated head. Peripheral velocity is directly related to shaft rotational speed, for a fixed impeller diameter. Varying the rotational speed therefore has a direct effect on the pump’s performance. The equations relating rotodynamic pump performance parameters of flow to speed, and head and power absorbed to speed, are known as the Affinity Laws.
For systems where friction loss predominates, reducing pump speed moves the intersection point on the system curve along a line of constant efficiency. The operating point of the pump, relative to its best efficiency point, remains constant and the pump continues to operate in its ideal region. The Affinity Laws are obeyed, which means that there is a substantial reduction in power absorbed accompanying the reduction in flow and head, making variable speed the ideal control method.
It is relevant to note that flow control by speed regulation is always more efficient than by throttling. In addition to energy savings, there could be other benefits to lower speed. The hydraulic forces on the impeller, created by the pressure profile inside the pump casing, reduce approximately with the square of speed. These forces are carried by the pump bearings, and so reducing speed increases bearing life.
It can be shown that for a rotodynamic pump, bearing life is proportional to the seventh power of speed. In addition, vibration and noise are reduced and seal life is increased, provided that the duty point remains within the allowable operating region.
There are many types of pump prime movers available (such as diesel engines and steam turbines) but the majority of pumps are driven by an electric motor. Although this article is principally about pumps and VSDs, it is important to appreciate that, on a typical industrial site, motor-driven equipment accounts for approximately two-thirds of electricity consumption.
Improvements in motor efficiency, by using high-efficiency motors, can offer major energy savings and short payback. Many of the principles outlined in the article apply to all motors on a site, not just those used as pump prime movers.
Variable speed drives
There are several types of VSDs. In applications that require flow or pressure control, particularly in systems with high friction loss, the most energy-efficient option for control is an electronic VSD, commonly referred to as a VFD. The most common form of VFD is the voltage source, pulse-width modulated (PWM) frequency converter (often incorrectly referred to as an inverter). In its simplest form, the converter develops a voltage directly proportional to the frequency, which produces a constant magnetic flux in the motor. This electronic control can match the motor speed to the load requirement. This eliminates a number of costly and energy inefficient ancillaries, such as throttle valves or bypass systems.
Benefits of VSDs
VSDs offer several benefits, some of which are relatively easy to quantify, and others of, which are less tangible, but there are some potential drawbacks, which must be avoided.
With rotodynamic pump installations, energy savings of between 30% and 50% have been achieved in many installations by installing VSDs. Where PD pumps are used, energy consumption tends to be directly proportional to the volume pumped and savings are readily quantified.
Improved process control
By matching pump output flow or pressure directly to the process requirements, small variations can be corrected more rapidly by a VSD than by other control forms, which improves process performance. There is less likelihood of flow or pressure surges when the control device provides rates of change, which are virtually infinitely variable.
Any reduction in speed achieved by using a VSD has major benefits in reducing pump wear, particularly in bearings and seals. Furthermore, by using reliability indices, the additional time periods between maintenance or breakdowns can be accurately computed.
VSDs have potential drawbacks, which can be avoided with appropriate design and application.
Resonance conditions can cause excessive vibration levels, which are potentially harmful to equipment and the environment. Analyses should be performed to predict and avoid potential resonance situations.
The risk of the rotating element encountering a lateral critical speed increases when a VSD is used.
Additional considerations for VFDs include:
- Reinforced insulation “inverter duty” motors may be needed;
- The high rate of switching in the PWM waveform can lead to problems. Filtering the inverter output can eliminate this problem;
- Older insulation systems may deteriorate more rapidly due to the rapid rate of voltage change. Again, filters will eliminate this problem;
- Long cable runs can cause “transmission line” effects and increase voltages at motor terminals;
- Voltages can be induced in the shafts of larger motors, potentially leading to circulating currents and bearing damage;
- Converter losses produce heat, requiring additional ventilation; and
- The converter may require installation in a less onerous environment than fixed speed gear.
Variable speed pumping full report
Further details and specific guidance are available in the complete Variable
Speed Pumping—A Guide to Successful Applications. This comprehensive handbook provides information on the design, specification and operation of efficient, cost-effective variable speed pumping systems. It covers the basic principles of pump, motor, and drive technology as well as more advanced, specific, and detailed concepts, and provides step-by-step guidance on using a systems approach to incorporating variable speed drives in pumping system applications.
The guide contains over 170 pages and has been compiled, written, edited, and critiqued by pump, motor, and drive experts from academia and industries worldwide.
Information for this article was provided by the Hydraulic Institute, Europump and the U.S. Department of Energy. For more information visit www.pumps.org