A crucial triumvirate for improving the bottom line
A little more than 10 years ago, the U.S. Department of Energy's Office of Industrial Technology issued a report titled U.S. Industrial Motor Systems Market Opportunity Assessment on the use of motor efficiency technologies.
The report contained an in-depth analysis of energy use and savings potential by market segment. In most industries, the report identified centrifugal pumps, as a group, as the largest consumers of motor energy. Also, among all rotating assets in the plant, process pumps had the highest overall potential for electrical energy savings.
Today, despite the fact that process pumps account for approximately 25 percent of total motor system energy in manufacturing and that pumps are at the heart of most industrial processes (they are after all, the second most common machine in the word), inefficient pump systems continue to represent an enormous cost to industry.
While wasteful systems can certainly have a significant effect on operating costs, improving or optimizing those systems can bring substantial savings opportunities—depending on your perspective.
Energy and Reliability Nexus
Typically, depending on the industry segment, continuous process pumps consume from 10 to 60 percent of total motor system electrical energy. In general, the largest savings opportunity for motor systems is pumps, compared to other operating asset groups—such as compressors, fans and blowers. While it is important to assess and monitor all asset optimization opportunities, optimizing pump system operations provides the largest overall savings opportunity.
In the largest continuously operating process plants, these costs and savings opportunities—when all aspects of the system are taken into consideration—can easily represent millions of dollars. Obviously this varies by industry type and plant size, but the impact on profit margins can be large enough to make or break the bottom line.
When trying to achieve optimization of pumping systems, one quickly discovers what is called the “energy and reliability nexus.” Where there is excess mechanical energy not required for moving process fluid through the pipes, it manifests as vibration, heat and noise. This excess energy becomes a destructive force that contributes to pump and process unreliability.
As a result, pump systems routinely have the highest overall maintenance cost compared to other motor systems, as well as control valves, instrumentation and other types of process control equipment. In addition, pumps and valves are the primary process leak paths for fugitive emission.
In addition, due to mis-dimensioning issues, including over- and under-sized pumps plus control valves and the associated piping, industrial process control performance is degraded over time. It is not uncommon for the majority of control loops to actually increase process variability when in automatic control mode, and, as a result, these control loops are often switched into manual mode to stabilize the process.
Clearly, operating on the pump's head-capacity curve, which relates head to flow rate, has a significant impact on energy consumption, reliability and process control. The horsepower consumed is roughly equivalent to the head and flow that the pump is required to deliver.
Beating Up Your Pump System
In the example below, the brake horsepower (BHP) formula (head x flow) equates the amount of wasted energy from an inefficient pump system that is over-sized and throttled.
BHP = Flow rate (gpm) • Head (ft.) • specific gravity
3960 x Pump Efficiency (%)
A pump delivering 5,000 gallons per minute of water at 100 feet requires:
(5000) (100) (1.0) / (3960) (0.70) = 180 HP
(5000) (100) (1.0) / (3960) (0.40) = 315 HP
The difference between a 70-percent and 40-percent efficient pump system is 135 horsepower (75 percent excess energy), which is, in effect, “beating up your pump system” and contributing to unreliability and poor control performance that continuously degrades over time.
In the underbelly of a process plant, tell-tale symptoms of excess energy moving through the system often can be seen.
These conditions take many forms—including a highly throttled control valve in combination with pronounced pipe movement or even a vibrating cat-walk in connection with the infrastructure used to brace the throttled pump. Cavitation that is noted inside the pump, control valve or piping itself is a clear indication that hydraulic turbulence or instability exists.
Case Study: Pulp Mill Bleach Plant
This type scenario occurred with a vat dilution pump in a pulp mill bleach plant that had a 1,180-rpm, 250-horsepower medium-voltage motor driving a double suction pump. The pump had a 14-inch discharge line that branched into three separate 10-inch lines feeding 200-degree F liquor to the end-user systems. Each of the three branches had its own eight-inch control valves that were usually operating in the range of 20 to 40 percent open. The gaskets between the pump discharge flange and pipe frequently failed.
Looking downstream and up to the top of the bleaching towers, each branch line was “rocking and rolling” and, as a result, experienced an inordinate number of cracks. Pipe cracks lead to chemical losses in the sewers and unplanned downtime.
Taken together, each layer of cost associated with the over-sized pump system had the cumulative effect of 36 hours of downtime each month to repair some component or multiple components of the system.