All you need to stay in the KNO
Proper selection of centrifugal pumps is more important than ever. Getting it wrong can have drastic consequences on maintenance, reliability and efficiency. However, the selection process remains difficult for many users. Even contracting the work out to a reputable engineering firm brings no guarantee of success. What it takes is a firm understanding of centrifugal pump design, the common pitfalls involved in the selection process, and the consequences of improper selection.
Centrifugal pump technology is relatively mature. It has been around for quite some time without any revolutionary changes. Certainly, there are new alloys and coatings for casings and impellers, and efficiencies have increased. However, their basic design remains largely unchanged over a considerable period. A centrifugal pump from 100 years ago is nearly identical to modern designs. If anything, older designs are more robust than those on competitive market currently due to pump manufacturers cutting costs by eliminating excess material.
Additionally, the pump’s application plays a critical role. A high-quality pump from a reputable manufacturer may perform poorly in certain systems. Even an expensive model made from titanium and designed to NASA specifications for a 30-year life cycle could perform inadequately for certain industrial applications. To fit the right pump to the right application, it is necessary to dig into the basic operating points of centrifugal pumps.
Figure 1: The components of a centrifugal pump.
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 support the pump shaft and absorb these radial loads (Figure 1).
All pumps have a design point where efficiency is maximized, known as 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, however, will generally have a critical speed around 25% over the BEP where its natural frequency is reached and excessive vibration may occur. What this means, in essence, is that the pump will shake itself apart, first going through the wear ring, then the seals, and finally the bearings. This is easy to spot; the pump will vibrate and may begin leaking fluid well before the next scheduled maintenance period.
This is akin to driving a car with the wheels out of balance. When mud or ice are stuck to one side of the rim, imbalance can result. The car may run fine at 25 mph in the city. However, as the speed increases it will reach a point where the impulses from the wheels work in tandem with the shock absorbers, causing the entire vehicle to shake until the offending speed is passed.
The BEP, therefore, should be fully understood as part of the selection of centrifugal pumps.
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 design and operating conditions. The pump’s performance data is provided to the user in the form of pump curves, whose primary function is to communicate or define the relationship between the flow rate and total head for a pump. They are provided by the manufacturer and show the operating characteristics of a specific pump type, size, and speed based on the results from 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:
Figure 2: A stylized pump curve in black with efficiency in green. The Barringer Curve in purple shows the relationship between the pumps operation and MTBF.
For the sake of accuracy, it is critical to have a pump curve for every pump. As mentioned earlier, the peak or maximum efficiency on the pump curve is indicated by the BEP. To operate on 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) on that spot.
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, though, 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. These forces cause stresses which have a 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.
Using the curve above, the closer the pump is operated to its BEP, the greater the MTBF. As the operating flow rate of the pump moves farther to the left or right of the BEP, failures occur more frequently. MTBF is cut in half when the pump is running 20% below the BEP, or 10% above it. When operated to the left of the BEP, problems like a temperature rise, low flow cavitation, bearing issues, reduced impeller life, and suction and discharge recirculation can lead to seal failure and pump downtime. To the right of the BEP, bearing and seal life are also impacted, and cavitation problems occur.
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All you need to stay in the KNO