Fixing oversized pumps can save energy and reduce maintenance costs.
There are several reasons why engineers typically oversize pumps when designing hydraulic systems. First, they want to build in a margin of error to accommodate uncertainties or changes in design or as facilities evolve. Second, engineers know that fouling, rust, sediment, and increased internal running clearances will reduce performance over time.
Oversizing pumps, however, involves tradeoffs. Oversized pumps waste energy and can cause maintenance problems. If those tradeoffs are too great, correcting the issue often pays for itself.
So, how do you know if you have a real problem?
Five Warning Signs
To see if your pumps have a problem, let’s look at the five most common indicators of oversized pumps:
Excessive flow noise. Excessive noise is hard to hear because most operators regard it as a normal operating characteristic. They only notice noise if it gets worse. Excessive noise is often caused by flow-induced pipe vibration, which can loosen flanged connections and other mechanical joints and create fatigue loads on pipe and piping support welds. Manufacturers typically do not publish acoustical levels on their pumps, as there a several issues that can cause excessive noise when a pump is operating. However, most flow-induced noise occurs when pump operation is outside the preferred operating region (POR).
Highly throttled control valves. Operators use throttle valves to: (1) shift, boost, or reduce flow rates in different system branches, or (2) change backpressure to shift the pump operating point along its performance curve.
When a system has oversized pumps, the throttling or control valve(s) typically remain in a restrictive position. Problems arise when control valve restrictions approach or drop below 50 percent. This forces the pumps to operate at higher backpressures than those associated with their best efficiency point (BEP), make the pumps less energy efficient and accelerating bearing wear.
Overly restrictive throttling, especially without proper valve characterization or sizing, can give rise to nonlinear behaviors, backlash, and stiction (static friction). These issues make processes harder to control. Properly sizing both pumps and control valves will provide more uniform and reliable responses to flow changes and reduces process variability.
Heavy Use of Bypass Lines. Many systems, especially in processing industries (chemicals, petroleum, and food), use bypass lines to manage damaging pressure differentials and flow demand by routing excess flow around system equipment or back to the suction side of the pump, see Figure 1. Heat exchangers often use them to manage temperature. When done right, bypass lines may improve reliability by letting pumps run closer to their BEP.
The downside of bypass lines is that they waste energy pumping fluid that is not needed. A system that typically runs with a large number of open bypass lines is performing inefficiently due to oversized pumps, improper balancing of lines, or both.
Frequent bearing and seal replacement. Oversized pumps generate excess system flow and with throttling cause higher backpressures, this will in turn prematurely wear out bearings and seals. Operating too far to the left of a pump’s BEP will exert greater loads on radial and thrust bearings and lead to reduced service life. This type of operation also causes increased shaft deflection, which reduces the reliability of mechanical seals and packing glands.
Intermittent pump operation. Pumps often use level control systems to automatically maintain a set fluid level in tanks and reservoirs. Because oversized pumps move a greater volume of water than smaller pumps, they do this faster and may turn on and off more frequently. The inrush current needed to accelerate the motor can raise temperatures and prematurely overheat and damage its electric windings. It also shortens the life of the controller. In addition, oversized pumps waste energy due to higher friction losses created by higher system velocities.
Oversized pumps waste energy and create costly maintenance problems. While every system is different, there are three general ways to improve efficiency and reliability.
Trim or replace the impeller. Most pump casings and shafts accommodate high-output and low-output impellers. They differ in terms of diameter, number of vanes, and angle. Changing the impeller is the most cost-effective way to alter flow.
There are two ways to make this change. The first, and least expensive, option is to machine the impeller by trimming its outside diameter. This reduces vane length, tip speed, and the velocity of the fluid running through it. This shifts the performance curve of the pump downward and to the left. The second, and more costly, option is to replace the impeller entirely. Both come with a tradeoff: they increase the clearance between the impeller and pump case, reducing the pump’s efficiency.
Consider a variable frequency drive. Variable frequency drives (VFDs) are a smart choice for pumps that deal with highly fluctuating demand. VFDs use electronic circuits to match motor speed and pump performance with system demands. As demand fluctuates, the VFD adjusts the pump speed without the need for throttling or bypassing excess flow. Unfortunately, retrofitting pumps with VFDs is expensive, though not as costly as a new pump. In systems with variable demand, however, energy and maintenance cost savings often justify their use. However, they make less sense in constant speed/constant design point applications, where trimming is probably a more efficient option.
Add smaller pumps. Designers usually size pumps used to maintain fluid levels in tanks or reservoirs for peak (or worst-case) scenarios. This results in pumps that are far too powerful for everyday operating conditions. These pumps are prone to frequent on-off cycles, high friction losses, and operation far from their BEP. Under these conditions, it makes sense to augment a large pump with one or more smaller pumps designed to handle ordinary, day-to-day demands. A smaller pump will operate more efficiently and require less maintenance.
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