Pumps are widely used in industry in boiler rooms, heating systems, cooling water systems and waste water treatment, but also in processes for pumping products and chemicals. It is important to pay attention to the installation design and the selection of pumps. In practice, most pumps are not functioning optimally an can be improved. One individual pump is often negligible in the overall energy consumption, but structural attention for pumps in the complete plant can lead to significant reduction of energy consumption. In addition to energy savings, this will also improve the stability of the process and the life time of the pumps.
Optimisation of pumps
There are several possibilities to optimise pump systems. It starts with selecting the right pump, an efficient pump drive and good installation. Furthermore savings can be achieved with good design of the piping and integration of the pump in the system. Some common optimisations are described below:
- Pump curve
Each pump has a pump characteristic (pump curve). This curve shows the relationship between the pressure head and the flow of a pump, see the figure below. To select the right pump, it is important to know the system characteristic. This system curve shows the relationship between the flow rate through the system and the pressure drop in the system. The intersection between system and pump curve is the operating point of the pump. It is important that the operating point is within the operating range of the pump and the pump has a high efficiency at this point. This is the case with the system curve in the figure below.
When the operating point is far to the left or to the right of the pump curve (curve B in the figure), the pump is used outside the operating range. This results in poor pump efficiency and a shorter life of the pump. Determine the system characteristic is therefore an important step in the selection of a pump. In practice many pumps operate far from the optimal operating point. A common case is that pumps are chosen too large.
- Electric motor
The pump is driven by an electric motor. These motors are available in different efficiency classes. Especially at part load, the efficiency of an electric motor will drop significantly. By selecting a high-efficiency electric motor a lot of energy can be saved. This is especially profitable when a new pump is purchased.
- Pump efficiency
Like an electric motor also a pump has a certain efficiency. This pump efficiency can be influenced by appropriate design of the impeller and the pump housing. By selecting a high efficiency pump, energy can be saved. This is especially profitable when a new pump is purchased.
There are several options to control a pump system. Improvement of the control method can give a lot of energy savings.
- Variable speed control
Many systems contain control valves to influence the water flow. A control valve creates pressure drop and is basically an energy destroyer. By applying a frequency converter, the capacity of the pump can be influenced so that the same effect is achieved with less energy. By adjusting the pump speed, the pump curve is adjusted and therefore the operating point is shifted on the system curve. As a result, the capacity of the pump is adapted to changes in the system. This is shown in the figure below.
When using a speed control, it is important that the system curve and the pump curve fit together in both full load and part load.
- Set point
Many pumps with speed control are controlled based on a fixed pressure or a fixed pressure drop. An easy way to reduce the energy consumption is reduction of the set point, if possible. Another possibility is the application of a dynamic pressure control, whereby the set point for the pressure is adjusted to the required flow rate.
- Pump manager
Many systems contain multiple pumps which are arranged in parallel and equipped with an on / off control or speed control. When the pump characteristic of each pump seperately and the pumps combined is known, a smart start-up strategy can be determined, which gives the most efficient combination of pumps and speeds in each situation.
Each system is unique. A good system design can save much energy. Some points for attention:
- Pipe diameter
The choice of pipe diameters influences the pressure drop in a system and thus the required pump energy. Often pipe diameters are chosen relatively small to save installation costs. However, this will give a high energy consumption throughout the life time of the installation. By good pipe sizing, more the pump energy savings during the lifetime of the system will be much higher than the extra installation costs due to the larger pipes.
- Over flow
Many systems have flow balancing valves and pressure control valves to maintain a minimum flow in the system. A high amount of overflow water will give unnecessary energy consumption. A correct adjustment of such valves is therefore of importance.
- Pipe fittings before- and after pump
The suction and discharge connection to the pump are usually smaller than the required pipe diameter. Therefore it is necessary to install a diameter reducer in the suction and discharge pipe of the pump. A common mistake is to place the valves between the reducer and the pump. Due to the small diameter, the appendages are cheaper, however, the high fluid velocity through the valves will give a larger pressure drop. This extra pressure drop leads to additional energy consumption of the pump. This is shown in the figure below.
By installation of the valves in the pipe part with a larger diameter, the investment costs for fittings are higher, but this the pay back period is short due to the savings in energy consumption.