Fixing Oversized Pumps to Save Energy and Reduce Maintenance Costs.

Engineers often incorporate oversized pumps when hydraulic systems are designed to allow for design changes, uncertainties, and facility growth. However, oversizing can come with drawbacks. Oversized pumps can lead to increased energy waste, higher maintenance expenses, and other issues. This article will examine the indicators of oversized pumps and suggest three solutions to enhance efficiency and dependability.

Warning Signs of Oversized Pumps

Excessive flow noise: 

It's often difficult to perceive excessive noise as a problem since many operators consider it a natural part of the equipment's operation. As a result, they only tend to notice when the noise becomes particularly bothersome. One of the leading causes of excessive noise is flow-induced pipe vibration, which can cause flanged connections and other mechanical joints to loosen and create fatigue loads on the pipe and piping support welds. Manufacturers typically do not provide acoustical levels for their pumps, as several factors can lead to excessive noise during pump operation. However, flow-induced noise occurs when the pump operates outside the preferred operating region (POR).

Highly throttled control valves: 

Throttle valves are a crucial tool for operators, who can use them to shift, boost, or reduce flow rates in different system branches. Additionally, these valves can modify backpressure, changing the pump operating point along its performance curve. However, when a system has too large pumps, the control valve can become restrictive, which can cause problems when the restriction approaches or drops below 50 percent. This can cause the pumps to operate at higher backpressures than their best efficiency point (BEP), leading to decreased energy efficiency and faster bearing wear.

Heavy use of bypass lines: 

Numerous processing systems in industries such as chemicals, petroleum, and food rely on bypass lines to regulate pressure differentials and flow demands. By redirecting excess flow around equipment or back to the pump's suction side, bypass lines can enhance reliability by allowing pumps to operate closer to their Best Efficiency Point (BEP). However, one major drawback of bypass lines is their tendency to waste energy by pumping fluid that isn't required. In addition, inefficient system performance due to oversized pumps, improperly balanced lines, or a combination of both can result in many open bypass lines.

Frequent bearing and seal replacement: 

Large pumps that exceed the required flow rate can cause issues within the system. For example, back pressure can increase beyond normal levels when throttling regulates the flow. This can lead to premature wear and tear on the bearings and seals, ultimately reducing their service life. Additionally, operating a pump too far to the left of its BEP (best efficiency point) can place excessive loads on the bearings, causing increased shaft deflection and decreased reliability of mechanical seals and packing glands.

Intermittent pump operation: 

Level control systems are commonly employed in pumps to maintain a desired fluid level in tanks and reservoirs automatically. However, when pumps are oversized, they tend to operate more quickly and cycle on and off more frequently. This can result in a higher inrush current needed to accelerate the motor, leading to elevated temperatures and premature damage to the electric windings. The controller's lifespan may also be shortened; oversized pumps waste energy due to the higher friction losses caused by the elevated system velocities.

What can you do to make the pump more efficient?

Trim or replace the impeller.

A pump's efficiency relies heavily on its impeller, which generates fluid flow. However, an oversized impeller can cause the pump to overwork, leading to increased energy consumption and equipment damage. To address this, one solution is to trim or replace the impeller.

Trimming the impeller involves reducing the outer diameter of the vanes, resulting in lower tip speed, vane length, and fluid velocity. This reduces the pump's performance curve, moving it towards the left and downwards, resulting in optimal flow rate and head for the system's requirements.

Replacing the impeller with one of a different diameter, angle, or number of vanes is another solution. However, it's more expensive than trimming, and it's only necessary when the current impeller is too worn or damaged to be salvaged. One thing to remember when considering impeller changes is the potential increase in clearance between the impeller and the pump case, which can reduce the pump's efficiency. This is especially true for smaller-diameter impellers, so it's crucial to consult a professional and weigh the pros and cons before making any modifications.

Consider a variable frequency drive.

When dealing with highly fluctuating demand, Variable Frequency Drives (VFDs) can be a wise pump control option. These electronic circuits match the motor speed and pump performance with system requirements, allowing the VFD to adjust the pump speed in real-time without throttling or bypassing excess flow. As a result, VFD use can significantly reduce energy consumption and equipment wear and tear.

While retrofitting pumps with VFDs can be costly, they are often a worthwhile investment in systems with variable demand, where the cost savings in energy and maintenance can be substantial. However, trimming the impeller in constant speed/design point applications is likely a more effective solution than installing a VFD.

It is important to note that careful selection and installation of VFDs are crucial. A qualified professional should evaluate the system and suggest the best VFD for the application. Furthermore, it is essential to ensure that the VFD is compatible with the pump and the motor and is programmed correctly to maximize energy efficiency.

Add smaller pumps

When pumps are sized solely for peak demand, they may be too large for day-to-day operations, leading to frequent on-off cycles, high friction losses, and poor pump efficiency. One or more smaller pumps can be added to the system to handle everyday demands to avoid this issue. 

These smaller pumps are designed to operate efficiently at lower flow rates and heads, thereby reducing energy consumption and maintenance costs. They can also run parallel to larger pumps, providing extra capacity when required. This approach benefits systems with varying demands, such as irrigation systems or water treatment plants.

It is essential to exercise caution when adding smaller pumps to a system. They should be carefully sized and selected to ensure compatibility with the existing system and to meet the necessary flow and head requirements. Additionally, it is recommended to consult a professional to ensure proper system design and installation.