Pumping systems account for nearly 20% of the world's electrical energy usage. In some process plants, the VFD for pumps and fans together account for between 50% and 80% of the total electricity used. Very rarely have I come across pumps operating with fully open discharge valves. The same is the case for fans with dampers. Partially closed discharge valves or dampers confirm that pumps and fans use more energy than the process actually needs. A critical review of these operations may lead to energy-saving opportunities.
Centrifugal pumps and fans, driven by electrical motors, are common in most plants. In this first column in a two-part series, we'll look at some steps that can help identify where in your pump system you spend more energy than necessary and find ways to optimize.
Step 1: Walk through the plant and note the discharge valve positions of all pumps that are continuously in service. For some pumps, a recirculation bypass valve may be open at all times instead of a discharge valve. As a rule, pumps are always designed to discharge at more than the required head — even at full capacity operation.
About 90% of VFDs in a plant maintain a constant speed; hence, the only way to control the required head is to throttle the pump's discharge. Constant speed operation adds more energy into the fluid at the pump. However, throttling the fluid to the required level excessively wastes the added energy. If the pump discharge is continuously throttled, opportunities exist to optimize the pump's energy use.
Switching to variable speed operation and leaving the discharge valve open at all times is the best way to optimize energy usage in a centrifugal pump operating at partial loads continuously. Several methods exist to convert to variable speed operation:
- Install a suitable VFD (See how to select the VFD?)) to supply power to the AC induction VFD.
- If the VFD consists of a high-rpm motor that is throttled extensively, consider switching to a lower rpm motor.
- If the pump is always throttled to a constant level, replacing it with a trimmed impeller would optimize the pump's energy use.
- In small- and medium-size VFDs, it may be worthwhile to explore adding a variable speed soft starter or an auto transformer to facilitate motor speed control.
- It may be possible to consider rotor resistance control, if the VFD is a large slip-ring induction motor.
The operation of centrifugal pumps and fans are analyzed using the affinity law (when speed is reduced, the power consumption reduces to the 3rd power). In other words, if the pump's speed reduces by half, the power consumption decreases to 1/8th.
Step 2: Look for cooling water system pumps that continually circulate — even as demand goes down. This is a common problem in plants with many batch processes. Adding a VFD or installing a smaller capacity pump in parallel could reduce excess energy use.
Step 3: Consider optimizing multiple pumps connected to a common header in which all pumps are in active service. For example, a pulp and paper plant had six pumps transferring water from the clarified river water tank to a common plant header. Plant expansion over the years had added three of the six pumps and all of them were in continuous service. Analysis of the system's resistance and switching to high efficiency pumps with sequence controllers brought the number of pumps online down to four.
Step 4: Identify and fix double pumping with variable frequency drive, whenever possible. It isn't uncommon to notice double-pumping of the same fluid in some process plants. For instance, in a recent plant study, we noticed that steam condensate was pumped from one receiver to another receiver and pumped again to a nearby final receiving vessel. The steam could have been routed directly to the final receiver.