James Archer works in marketing at AuCom. AuCom designs and manufactures soft starters for AC induction motors. For more information, visit aucom.com.
Understanding the cause of the potentially damaging pressure surges is the key to finding the right solution.
- pump/motor overload protection to detect burst pipes
- undercurrent protection to detect blocked pipes
- phase rotation protection to prevent reverse rotation of the pump
- phase loss protection to prevent damage from power disturbances
- instantaneous overcurrent protection to prevent pump damage due to debris
- automatic timers and schedulers for control of operation
- operational logs and recording
The effectiveness of electronic speed control in the reduction of water hammer is determined not only by the type of technology within the soft starter or VSD, but also by the curves for the pump and system.
Examining the Curves
The system curve displays the relationship between flow rate and pressure in the system.
It is made up of two components: static head and dynamic head (see Figure 1).
Figure 1. A system curve shows static head and dynamic head. (Graphics courtesy of AuCom)
Flat system curves are more sensitive to changes in speed. A small change in speed will create a big change in flow. This means speed control must be precisely managed to prevent water hammer.
Pump characteristics vary widely, resulting in a range of different possible pump performance curves. With a steep-curve pump, a large change in pressure produces a small change in flow.
Conversely, with a flat-curve pump, a small change in pressure will result in a large change in flow (see Figure 2).
Figure 2. Pump curves can illustrate steep- and flat-curve pumps.
To help abate water hammer in a system, choose a steep-curve pump wherever possible. The relationship between pressure and flow rate for such pumps makes precise control of the flow rate via control of pump speed much easier.
Once electronic speed control has been picked as a solution, next comes the choice between use of a soft starter or a VSD. Both can control the acceleration and deceleration of the motor/pump—and mitigate water hammer.
The best choice will depend on the system characteristics.
Soft starters run the system at full (fixed) speed during operation, controlling the speed during pump starting and stopping only. Once the system reaches full speed, the soft starter is typically bypassed and operates with very high efficiency (losses of less than 0.1 percent), reducing running costs. Soft starters also come at a lower cost than VSDs. Additionally, harmonic generation onto the electrical supply is not an issue with soft starters, so they do not necessitate the use of costly filters.
On the other hand, VSDs control pump speed during operation as well as during start and stop. This additional ability to control run-time speed comes at a cost. VSDs have a considerably higher capital cost than soft starters, often require harmonic filters (costly devices that prevent harmonic distortion on the electrical supply) and typically produce energy losses of 4 to 6 percent, adding to the system’s lifetime cost.
However, in some pumping systems, the ability to control flow during operation will produce other system efficiencies that outweigh the higher capital and running costs. Unless such system efficiencies are proven to exist, soft starters should be the preferred method of electronic speed control for the mitigation of water hammer.
Selecting Soft Starters
With an understanding of water hammer mechanics, the system curve and the pump performance curve, the next step is to explore the correct application of soft start technology to prevent water hammer.
Not all soft starters are created equal. Over the past 40 years, the starting and stopping modes offered by soft starters have considerably evolved. Voltage, current and, more recently, torque control are common approaches to starting and stopping. Each influences acceleration (and deceleration) but none provide direct control.
Figure 3. Certain soft start technology provides selectable acceleration and deceleration profiles, which can be beneficial in pumping applications. This allows the end user to tailor the soft starter settings for optimal starting and stopping results regardless of the inherent system characteristics.
The most advanced soft start (and stop) mode is direct acceleration (and deceleration) control. This mode can be ideal for pumping applications and the elimination of water hammer because it enables selection between a variety of starting and stopping profiles, depending on the unique characteristics of the pumping system (pump and system curves).
Certain technology provides selectable acceleration and deceleration profiles, which can benefit pumping applications. The ability to select and adjust a variety of control strategies makes it simple to tailor operation for optimal results no matter what the system characteristics.
- Joe Evans, Ph.D, “Waterhammer Part 2 – Causes & Variables,” www.pumped101.com
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