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.

In this scenario, the detrimental financial impact to the bottom line was substantial—in the range of high seven figures annually.

The primary solution for this application was the implementation of variable speed pressure control. The pump system normally consumed around 200 horsepower, with the end-user valves highly throttled (20 to 40 percent open) and the vibration levels were about 0.6 inches per second. After variable frequency drive (VFD) implementation, the pump normally consumes 75 horsepower. In effect, the excess 125 horsepower, above that required to move the fluid, was “beating up” the pump and contributing to unreliability.

Variable Speed Pressure Control Is Vital

While this level of unreliability may seem uncommon, studies show that these issues exist throughout the process. A 1996 Finnish Research Center study of centrifugal pump performance found that the average pumping efficiency was less than 40 percent for the 1,690 pumps reviewed across 20 different plants, including all market segments

It was also revealed that 10 percent of the pumps were less than 10 percent efficient. Considering this sizable efficiency loss, you can expect that 10 percent of the pumps in any continuous process plant are candidates for optimization. Quite likely, the real number is higher (possibly 20 to 25 percent of pump systems can be cost justified for some type of mechanical and control modification). Ideally, every continuous process plant would identify its problem systems and, using a documented management process, continuously implement solutions. If pump optimization is considered an ongoing improvement process, similar to Six Sigma, this will have an enormous impact on facility pump and process reliability over time.

In addition, pump optimization activities will increase the level of condition monitoring through the broader use of VFDs and wireless vibration monitoring, among other data rich tools, that offer real-time information on pump system performance.

Today, pumps are not considered to be an integral component of the process automation architecture. As a result, plant information systems typically lack continuously monitored data for trends and diagnostics. While the distributed control system (DCS) monitors most of the key process parameters, still, up to 60 percent or more of the pump systems lack a flow measurement on the discharge line. For practical purposes, almost all the work orders and asset information is manually entered. The underlying assets—including compressors, blowers, fans and control valves are rarely connected to the computerized maintenance management system (CMMS). Therefore, this situation becomes a missing link in an e-manufacturing strategy and large potential cost savings are unrealized. According to the ARC Advisory Group, up to 40 percent of manufacturing revenues are devoted to maintenance and up to 60 percent of scheduled maintenance checks and motor systems are unnecessary.

Best Efficiency Performance a Key

The three primary determinates of pump reliability are speed, distance from best efficiency point (BEP) and impeller diameter.

When considering the plant-wide implications of optimizing up to 30 percent of existing pump systems, key indicators—such as overall plant availability—will increase while pump seal and bearing failures will significantly decrease. Coupling the reliability benefits with energy savings, it is unquestionable that quantum leaps in process performance can be realized.

Today, these benefits can be realized by taking a life-cycle-cost approach when assessing systems that are over- or under-sized and that exhibit symptoms of excess mechanical energy.

Once the amount of excess energy is quantified, it will serve the first leg of the stool that represents project justification. Reliability improvements can be predicted and past work orders and CMMS records can be used to estimate annual maintenance costs.

In many cases, process control can be identified in terms of reduced raw material variability and life-cycle-cost savings can be estimated based on current costs versus optimized costs.

Making decisions that are based solely on long-term operating costs versus having a large “safety margin,” to make sure operations can always produce more flow than will ever be needed, will open a window to the plant of the future—one that is available, adaptable and sustainable. I assume that is what is meant when someone says the future must be lean and green. It is no easy task to get there, but the benefits are too compelling not to shake off the inertia and move ahead.

 

Pumps & Systems, September 2011