The vast majority of pumping systems run far from their best efficiency point (BEP). For reasons ranging from shortsighted or overly conservative design, specification and procurement to decades of incremental changes in operating conditions, most pumps, pipes and control valves are too large or too small. Pumping systems fail to convert the electric power they consume into fluid motion with anywhere near the economy, reliability and control inherently available in the finely engineered individual components.
Far too many pumps run 24/7 generating excess pressure, velocity, noise, vibration and heat that waste energy, tear up mountings, loosen and crack piping, and erode control valves. Others fail to produce the desired flows with the precision needed to maintain uniform, high product quality and process efficiency.
If you wonder how this has come to pass, you need look no further than the back offices.
The cautious and the careless
“Engineers tend to be careful and conservative,” says Greg Romanyshyn, technical director, Hydraulic Institute (www.pumps.org). “Their designs cover all contingencies, including upsets, to be sure the pump will not be the fault in the system -- the pump will deliver. But margins accumulate through the design, so in operation the pump is oversized and does not operate at BEP.”
The engineers’ caution is more than matched by contractors’ indifference to long-term operating costs. “People will design a piping system and analyze the structural aspects to death due to ASME codes, but they ignore fluid dynamics, which is the purpose of the system,” says Trey Walters, P.E., president and chief technology officer, Applied Flow Technology (www.aft.com). “Why is this so? Because it’s not being demanded by the facility owners, who have lost the expertise. The design firms are close on margins and not willing to pay for something the owner doesn’t demand. So there is a lack of demand for efficiency. As long as it works pretty well, no one pays any attention.”
Why care about over and undersized fluid handling systems if the plant seems to run OK? “The difference between running OK and running efficiently varies from pump to pump, process to process and plant to plant,” says Martin Bima, P.Eng., industrial systems engineer, Business Engineering Services, Manitoba Hydro, Winnipeg, Manitoba. “Unfortunately, after the typical pumping system is installed, it is considered commissioned as soon as it is making ‘enough’ flow and head, regardless of efficiency, operating point or power consumption. It is rarely reviewed until production is negatively affected or a catastrophic failure occurs.”
If the plant's goal is to make as much product as possible, the typical pump installation would be adequate, Bima says. “But if the goal is to lower the energy and maintenance costs per unit of product -- the MBA people's ears should be burning about now -- efficient pumping is one of the most important factors to review. Over and undersized pumping systems waste energy, reduce reliability and contribute to runaway maintenance budgets.”
Sending out an SOS
Generally speaking, in pump systems that contain restrictive or no control devices, an undersized pump operates on the right side of its curve and an oversized pump operates on the left side (Figure 1). “In each case, it’s far from its best efficiency point,” Bima says. “One way of thinking about it is for every kilowatt of power input to the pump, less is being transferred to the fluid. Where does the rest of the energy go? It has to be converted and dissipated somewhere, and it goes to noise, heat and vibration. Noise can be tolerated but the other two will reduce the expected life of the pump, increase maintenance cost, and may cause an unexpected failure.
Figure 1. Pump System Curve Uncertainty
“When a pump is running over or undersized, you are paying more for energy input, accelerating the wear on the pump, and reducing the life of its components.”
Operating a pump at part-flow is a primary cause of pump problems, second only to poor lubrication, asserts Heinz Bloch, P.E., principal of Process Machinery Consulting (www.machineryreliability.com). “Operating at flows lower than the BEP is inefficient,” Bloch says. “Moreover, this can cause pumps to experience excessive internal recirculation. Internal recirculation is not unlike cavitation. It can be of little consequence and can also be life-shortening. In high-energy pumps, internal recirculation can cause extreme damage in just a few weeks of operation.”
In the most common scenario, an oversized pump is paired with an oversized control valve. “The system uses, say, 80 gpm, but the pump wants to deliver 120 gpm,” says Romanyshyn. “The head is raised to a pressure that’s not needed, and excess head times flow is the energy wasted. The control valve consumes the wasted power with excess pressure drop, which shortens valve life.”
In a recent survey by Emerson Process Management of more than 200 sites all over the world, some 500 individuals involved in research, process engineering, procurement, operations and maintenance overwhelmingly reported that the number-one control valve problem is, “Our valves are oversized.”
During continuous process operations, control loops run in a very narrow throttling range - essentially steady-state at or near maximum continuous rating (MCR). The control valve dithers as small input signal changes result in small movements of the stem or shaft. “Today's higher-performing digital valve controllers can compensate for valve sizing errors,” says Michael McCarty, engineering manager, Fisher new product development at Emerson. “Their ability to precisely respond to the control system’s smallest requests can give the appearance of acceptable process loop performance.”