Why look at pump efficiency? The greatest cost of owning a pump is the energy to run it. In today’s world of reducing ‘carbon footprints,’ facility designers and operators should be proactive in reducing energy consumption, not just for the economic impact to the operator, but to reduce energy consumption overall and the accompanying release of greenhouse gases in electrical generation.
By Steve Walden, Senior Rotating Equipment Engineer for AXG Contracting, Wever, Iowa USA
If the pump operator can reduce the energy consumption of moving liquid through the production system, over the life of the process, significant savings can be achieved. For example, in a facility with 40 pumps with an average kW rating of 56 kW (75 HP), the average cost of electricity ($/Eur 0.15/kWhr) to run those pumps with a 90% on-stream time is about USD $2.65 Million per year. A relatively achievable efficiency increases of 2% would yield savings of about USD $53,000 per year. The cost of an ANSI pump, motor, and base of 56 kW installed is approximately USD $100,000. The capital pay-off of a pump every two years would make a significant profit. The reduction in CO2 emissions (U.S. average of 0.857 lbs. or 0.389 kgs CO2e per kWh) would be approximately 302,000 lbs (137,000 kgs) per year.
Consequently, pump efficiency can be increased in a few ways. First, design the system for efficiency. Second, reduce the energy consumption of the systems. What is the definition of pump efficiency? The direct pump efficiency is the brake horsepower delivered to the pump shaft by the driver divided into the brake horsepower of the hydraulic output of the pump, measured at the discharge flange.
This is too simplistic of a definition. An operator can have the most hydraulically efficient pump, but, not have an efficient system. It all begins with Front-End Engineering and Design (FEED). After a preliminary process design has been developed, the project manager begins to obtain the equipment needs by creating block flow diagrams showing all the steps in the process and the basic equipment needed with the beginnings of the specifications for that equipment. For the pumps, the project manager (PM) works with the customer to define what the process requirements are; flow and delivery pressure/head, as well as the temperature to the next component of the system. Given these requirements and the available real estate, the PM then lays out the basic facility and concurrently develops the Process and Instrumentation Diagrams (P&IDs). With the plant layout in hand and the required process flow rate and delivery pressure and temperature, the basic hydraulic calculations tell the PM what size pump to specify.
Also, with these specifications come the fluid properties that become part of the design calculations, viscosity, pH, solids inclusion, suction temperatures & pressures, fluid vapor pressures at suction, discharge conditions, and so on. These parameters define the materials of construction, type of pump needed, and size.
In the past, to accommodate future expansion and to introduce a bit of overdesign to ensure meeting the performance requirements, many engineering design and construction (EDC) firms specified pumps that would not operate at the Best Efficiency Point (BEP).
For a centrifugal pump, this might mean going to the next size up and having the impeller trimmed to accommodate the head/flow required. Historically, this has been one of the major introductions to the inefficiency of pumping systems.
Having an impeller near the bottom diameter range gives the pump room for process changes, but with the larger vane tip to volute cutwater clearance, allowing for more hydraulic losses.
With a more nuanced design, using computer-aided design, computational fluid dynamics and other tools, EDCs move the BEP much closer to the hydraulic design for the pumps specified. To counter that, many EDCs are specifying variable frequency drives (VFD) as one way to decrease the head and flow to operate in the process comfortably at the design parameters and still allow for future expansion, with impellers sized for better efficiency. AVFD uses only the power necessary to operate the pump at its setpoint.
Today’s EDC firms should also look closely at the life cycle cost of systems:
Life Cycle Cost
- Operating Energy
In the first part of this series, the focus will be the FEED efficiency influence and then the operating energy costs will be discussed. The second part of this series will deal with practical methods for increasing the hydraulic efficiency of a pump.
FEED Influences on Pump
Efficiency Front End Engineering and Design has the first job of increasing efficiency in the system. Limiting the distances fluid is pumped, the number of direction changes, and minimizing the rise in elevation, all work together to increase the energy efficiency of the system.
The issue becomes one of real estate and structure. What if a pump is required to move 1,000 gallons per minute of water to the top floor of the 828-meter-tall Burj Khalifa? A onepump system would have an incredibly high-pressure pump with the associated heavy wall piping. Alternatively, pumps could be installed on every tenth floor and an atmospheric receiving tank on each of those floors, sized appropriately to contain the use for the floors below, as well as room for the next pump’s suction volume. Obviously, in this case, a complex system of pumps and receivers can move the required water to the top floor. However, the multi-level pump and receiver system installed cost is much higher than just one massive pump and heavy wall piping. In terms of energy efficiency, it is a compromise.
Now that the process is known, the type of pump can be specified. Positive displacement (PD) or centrifugal? Although this article focuses more on centrifugal, the efficiency of PD pumps can use much of the same design initiatives to increase the system efficiency.
Now the design engineer needs to identify the pipe specification and what schedule and types of fittings are available to use. Here is another opportunity to increase efficiency. The use of poly pipe in low-pressure systems can reduce internal pipe friction and increase efficiency. Steel pipe is always a good choice, but it will have surface roughness from the mill and subsequent corrosion over time. Stainless steel may not have corrosion issues, but its cost is significantly higher. This is to be accounted for in the design.
Every joint, every change of direction, every change of diameter, and every valve contributes to the friction losses in the system. The design engineer must choose wisely to maximize efficiency. Should the engineer use a full port ball valve instead of a gate valve? The selection made should ideally pay off in the future.
The designer must consider many potential issues, such as friction loss being a function of fluid velocity squared, should the design engineer use a larger-sized pipe. Will the increased cost of the larger pipe pay off in the long term? What other issues may arise from that choice? Reduced erosion/ corrosion? Sediment drop-out? But what if the task is to improve an existing system?
Once the FEED, subsequent construction and commissioning is complete, the plant engineer is saddled with what the project and design engineers have given in the existing physical plant. The upcoming installment of this series will have some real-world examples of projects that were proven to increase pump efficiency, focusing on improving the hydraulic efficiency of the pump itself.
About the Author
Steve has over 30 years’ experience in the petro-chemical, refining and aerospace markets solving rotating equipment issues contributing to plant on-stream time in the pursuant fields. He holds a BS in Mechanical Engineering from Oklahoma State University. He has held Certified Maintenance and Reliability Professional and Machinery Lubrication Analyst II.