Implementing flow control in centrifugal pumps and fans by means of variable speed AC drive instead of the traditional mechanical flow control devices is capable of providing energy savings from 10% to 60% depending on the application. This typically provides a return on investment (ROI) of 6 to 24 months, depending on the process load and if this is a new design or a retrofitting of existing installation.
VFD Application for Energy Saving in Pumps and Fans
Above mentioned ROI, is not considering the other economic benefits of using variable speed AC drives, such as:
- Reduced maintenance costs,
- reduced process variability,
- improved plant overall power factor,
- and elimination of valves as sources of fugitive emissions.
The Energy Crises
Egypt is experiencing one of its most series energy crises for decades, the rising population and the intense heat waves lead to electricity demand around 20% more than the available generation capacity. This is calling for innovative -industry proven- solutions to enhance energy efficiency that would lead to reduction of the overall demand as well as reduction of CO2 emissions and carbon foot-print.
This paper discusses the use of variable speed AC drives with pumps and fans for flow control applications instead of a control valve or louver, and how this can help in achieving high levels of energy savings, increase equipment life-time and allow for safer and more environment friendly operation.
Traditional Flow Control Approach
Controlling the flow of centrifugal pumps and fans is traditionally achieved by means of mechanical flow control devices (i.e. control valves and louvers). Although such technique has been the standard of all process control text books over the years, they fail short in providing any significant reduction in energy consumption in response to a reduced process demand, figure 1 illustrates the relation between power consumption against a varying flow rate by means of a control valve.
As illustrated in figure 1, reducing the flow by 20% by means of control valves or louvers will have minimal or no impact on the rotating equipment energy consumption. This lack of direct relation between product flow rate and its energy consumption contributes directly to the increase of per-unit production cost, and the increase of plant’s carbon foot-print.
Another drawback for the use of such traditional method of flow control is an increase of operating pressure of the rotating equipment, which is a major contributor to frequent leaks, increased vibration and motors overheating. Figure 2 explains the relation between valve control and operating pressure.
Figure 2 – Pump/System curve operating point change with throttling valve
The above mentioned factors have a direct impact in increasing the plant operation and maintenance costs as well as increasing the unplanned maintenance activities.
Variable Frequency Drives, the Smart Solution
Variable frequency drives (VFD) are becoming more common place and more widely used in applications. They are capable of varying the output speed of a motor without the need for mechanical pulleys, thus reducing the number of mechanical components and overall maintenance. But the biggest advantage that a VFD has is the ability to save the user money through its inheriting nature to save energy by consuming only the power that’s needed.
A VFD is similar to the motor to which it’s attached, they both convert power to a usable form. In the case of an induction motor, the electrical power supplied to it is converted to mechanical power through the rotation of the motor’s rotor and the torque that it produces through motor slip. A VFD, on the other hand, will convert its incoming power, a fixed voltage and frequency, to a variable voltage and frequency. This same concept is also the basis to vary the speed of the motor without the need of adjustable pulleys or gearing changes.
Figure 3 – Variable Frequency Drive vs Throttling Control Valve
Energy savings by using a VFD is derived from the laws of Affinity that states:
- Flow is proportional to shaft speed
- Head (pressure) is proportional to the square of shaft speed
- Power is proportional to the cube of shaft speed
Applying the affinity low to the power consumption curve of centrifugal pump or fan shows the amount of energy that can be saved at the same operating point of the throttling valve approach; figure 5 shows energy saving of almost 50% in response to a 20% reduction in flow rate.
When looking at the VFD control from the pump/system curves point of view, it shows the additional benefits of reduced operating pressure at the same flow rates, and thus reduced vibrations, leakage potential and mechanical failures.
VFD Flow Control Limitation and Solution
The main limitation in flow control using VFD/pump combination is presented in processes where very wide range of flow rates is required to be achievable, for example a range from 20% to 100% of nominal flow rate. Operating the pump at very low speeds will result in very low produced pump head, this head may not be enough to overcome the static system head, and so flow may never build-up.
The solution for the above limitation, a combination of VFD and a control valve can be utilized, with a split range control algorithm, so the VFD regulates the high flow rates down to a certain minimum, and when the process demands lower flow-rates, the control valve starts adjusting the system curve to achieve the requested lower flow rate.
VFD Flow Control Economic Benefits
Dramatic energy cost reduction obtained by elimination of the pressure drop across the pump
If the energy savings were small, then this would be an irrelevant point. However, there are so many liquid flow pumps in processes that pump up to a high head, only to take that energy loss as pressure drop across a control valve, that this source of energy loss may be as much as 60% of the pump energy and up to 25% of the total energy used to operate a refinery, paper mill or chemical plant. Then too, if we designed our processes from the beginning with the VFD/pump, we could use much smaller electric drive motors.
Better control leading to reduced process variability and increased in-spec product
It is well known that the most often cited problem in tuning of PID control loops is the variable hysteresis introduced into the flow loop by control valve stickiness. The control valve also introduces a lag due to mechanical motion of the valve actuator itself. Control loops that have been optimally tuned at plant startup must gradually be detuned as the stickiness of the control valve builds, or the loop may become unstable. Today, we seem to accept this kind of sub-optimal process performance, but we can do better by replacement of the major source of error—the control valve—with a VFD/pump.
Reduced overall plant maintenance cost
From one hand, removing “or neutralizing” the control valve, which is a component with a high demand on maintenance resources is a great source of cost reduction. On the other hand, operating the pumps at lower pressure levels increases the life time of the pump itself, reducing stresses and vibrations on all mechanical components of the system and reduces possibilities of leakage as well.
Eliminating of a source of fugitive emissions
The benefit obtained from the elimination of fugitive emissions known to originate at the packing of the control valve stem is obvious when the control valve is eliminated.
Improvement in plant power factor
A less obvious benefit of using a VFD rather than discrete motor starters is operation at a near-unity power factor. Many plants use expensive capacitor banks to enhance their overall power factor, these capacitor banks also require regular maintenance and replacement. Thus, the use of VFDs eliminates the need of most of plant’s capacitor banks.
Conclusion, VFD is Better than FCV
It seems clear that using a VFD/pump combination to replace the use of an AC line-powered electric motor and a control valve is a superior way to do flow control and reap large benefits in lower energy consumption with better energy efficiency. There is also the potential for improved process control from elimination of the problems of control valves.