Proper pump selection is more important than ever, with drastic consequences for maintenance, reliability and efficiency. The selection process remains difficult for average users, and even contracting the work to a reputable engineering firm does not guarantee success. In this column, we will cover centrifugal pump design, common pitfalls in the selection process and consequences of improper selection.
Centrifugal pump technology has been around for centuries without any revolutionary changes. There are new alloys and coatings to build the casings and impellers from, and efficiencies have increased. Bur the basic design remains largely unchanged. Unlike other 21st century technologies, a centrifugal pump from 100 years ago is nearly identical to modern designs. If anything, the older designs are more robust, with the current competitive market forcing manufacturers to cut costs by eliminating excess material.
A pump from a reputable manufacturer may perform poorly in a system regardless of the pump’s specific quality. A pump made from titanium and designed for a 30-year life cycle, while expensive, could perform inadequately for another industrial application. Therefore, it is crucial to fit the right pump to the right application. To understand why failure can occur, let’s dig into the basic operating points of centrifugal pumps.
As the pump shaft spins, it turns the impeller inside the casing, which adds energy into the process fluid. This allows the impeller to act as a cantilever with a wear ring, seals and bearings that keep everything in place and fluid from leaking out. The spinning impeller changes the incoming fluid’s direction, which can cause intense radial loads on the pump. The bearings not only reduce rolling friction, but also support the pump shaft and absorb these radial loads. This may be seen in a detailed view of a centrifugal pump in Image 1.
All pumps have a design point where the efficiency is maximized, called the best efficiency point (BEP). This is where the pump runs the smoothest and radial forces are minimized. The further away from the BEP, the higher the radial loads on the pump. The pump will generally have a critical speed around 25 percent over the BEP where its natural frequency is reached and excessive vibration may occur. The pump will essentially shake itself apart, first going through the wear ring, then the seals and finally the bearings. This is usually easy to spot since the pump will vibrate and may begin leaking fluid far before any scheduled maintenance.
Pump curves demonstrate the strong relationship between pump life, pump reliability and where the pump operates on its curve.
The performance of individual pumps is a combination of the pump design and the operating conditions. The pump’s performance data is provided to the user in the form of pump curves, with the primary function to communicate or define the relationship between the flow rate and total head for a specific pump. They are provided by the manufacturer and show the operating characteristics of a specific pump type, size and speed based on results of standardized tests and test conditions. A healthy pump maintains the defined relationship between the head and flow at all times.
The pump curve is required for:
- Proper pump selection. Using a pump curve will ensure the pump selected is matched to the system requirements.
- Monitoring the health of the pump. If the pump is not operating on the published curve, something is wrong.
- Troubleshooting the operation of the entire piping system. The pump provides energy into the system, and knowing the energy going in is a critical clue for identifying issues. Without a pump curve, it is extremely difficult to determine what is causing a problem in the system and what should be done to correct the problem.
For accuracy, it is critical to have a pump curve for every pump.
Image 2 shows a stylized pump curve in black with efficiency in green. To operate at the BEP, the system must either control the pressure at the outlet of the pump or the flow through the system to keep the pump operating point (indicated by the red arrow).
For example, if the system causes the pressure at the discharge to surpass the pressure at the BEP, the operating point will move to the left up the curve and flow will reduce. If the system causes the pressure at the pump’s discharge to drop, the operating point will move down and to the right. Moving to the left or right of the BEP causes forces on the impeller to increase, and these forces cause stresses that have a significant negative effect on the life and reliability of the pump.
If we overlay the expected life of the pump as a function of where the pump is operating, we get a “Barringer Curve,” which shows the mean time between failure (MTBF) as a function of BEP flow rate. This curve was created by Barringer & Associates in a study of seal failures in centrifugal pumps.