According to the U.S. Department of Energy (DOE), pumping systems use a significant amount of energy in process plants (http://goo.gl/TiBA72). To be more specific, let’s look at petroleum refining — pumping systems consume almost 60% of the total energy used by machine drives (mechanical rotating equipment). In the chemicals sector, this number is still a respectable 28–30%. So, shouldn’t we take an energy efficiency “magnifying glass” to our pumping systems and reap some benefits from possible low- or no-cost measures?
The type of analysis you should perform to understand and evaluate a pumping system is no different than that for other energy-intensive systems. A systems approach will always lead to a truly optimized configuration; as my pumping system expert friends say, “it will get you the best wire-to-water efficiency.” More importantly, it will ensure that we do not fall into possible engineering traps relating to fluid mechanics!
Every pumping application starts with understanding the relationship between flow and head (pressure differential) for the system and process requirement. This also is known as the “system curve.” Once that’s properly understood, the next step is to identify a pump (or pumps) that will meet the system demand. If you are evaluating the current system in place, then your pump already is defined and you will need to get the pump curves. Overlaying the system curve on the pump’s curve provides a significant amount of insight on the pumping system operation. Not only will it give information about the horsepower consumed but will also supply a wealth of knowledge on the current operating efficiency of the system, net positive suction head required (NPSHR), etc. If you take some basic information from the operating pump (pressure differential, amps drawn and speed), you’ll have everything you need to make a good assessment of the operations. (For more on pump curves, see “Select the Right Centrifugal Pump.”)
Process plants have hundreds or even thousands of pumps, so there needs to be a simple way of screening and prioritizing these systems. Most often, you should focus on the largest energy hogs or possibly pumps that will give us the highest amount of savings. A screening tool easily can identify 20% of those pumps that possibly can provide 80% of the energy savings at a plant. I have provided below a few characteristics for short-listing your pumping systems:
1. Large horsepower;
2. Long operating hours;
3. Operations with recirculating flow or bypass;
4. Operations with significant throttling downstream of the discharge;
5. Multiple pumps in operation while design specifies one or a lesser number of pumps;
6. Very noisy pump operation (possibly cavitation); and
7. History of periodic pump failure (possibly due to incorrect application).
The next step is to do an energy assessment, starting with pump and motor efficiency. Most times, the motor efficiency is very high unless the motor is operated at extremely low loads (<25%). The pump efficiency can be obtained from the pump curve. Note that similar to motors, pumps also have energy efficiency standards, which are provided by The Hydraulic Institute. Using the operating data from the pumping system and the pump curves, compare the pumping efficiency to the design (or best efficiency point), to get a good indication of the possible savings achievable with the optimized pump and motor combination. However, you shouldn’t stop your investigation here; now is where “the devil is in the details” really applies.
From a process perspective, you should really look at how much energy is needed to go from Point A to Point B for the specific amount of process flow required. That is where you’ll see a significant amount of system savings. Evaluate different scenarios: a lower number of operating pumps; proper impeller sizing or trimming; adjustable speed drives, piping modifications, etc. The opportunities could be endless.
I am going to leave you here with a link to the U.S. DOE Pumping System Assessment Tool (PSAT) which I have used frequently on pumping systems that I run across while doing energy assessments in process industries (http://goo.gl/Zv43ku). I hope you, too, can use it, and save some energy dollars in your pumping systems.
Riyaz Papar, PE, CEM, is director, Global Energy Services, at Hudson Technologies Company, Pearl River, N.Y. He has more than 20 years of experience in industrial energy systems and with best practices. He also is a U.S. Department of Energy (DOE) Steam Best Practices senior instructor and a DOE steam energy expert. He has provided energy consulting services in 100+ industrial plants in the U.S. and internationally. You can email him at [email protected].