Subscribe to our

Pumps & Systems, April 2007

Have you ever experienced this? The pump that you purchased for one specific performance is placed in service and operates at another point that is completely different from its original design point or BEP. Here's one way to resolve this problem.

This problem is all too common. In addition to being very inefficient, when you operate the pump away from the original design point or best efficiency point (BEP), it causes a multitude of other issues, including excessive noise and vibration, shaft oscillation, cavitation, premature wear, and failure of mechanical seals, bearings, rings, sleeves and impellers.

In extreme cases, the shaft will break right behind the impeller from the excessive radial forces that occur when you operate a pump away from the original design point. Damage to these pump internals and poor reliability are a real and direct result of such operation.

These problems can be resolved by reengineering the impeller to operate at the new system design point so that the BEP will be the true operating point in the plant or ship system. This will improve both the efficiency and the reliability of the pump.

An impeller that uses traditional metallic technology.

A reengineered impeller that uses a graphite combination-fiber reinforced composite material with phenolic resin.

Operating a pump away from the BEP has a detrimental effect on pump efficiency and wastes a tremendous amount of money, since 85 percent of the total cost of owning a pump is the operational cost (maintenance cost plus the cost of energy). The larger the pump, the more energy is wasted when a pump operates off the original design point.

When a centrifugal pump operates to the left of the originally designed BEP or to the right of the BEP, many bad things happen. First, radial thrust grows exponentially (see Figure 1), resulting in significant shaft deflections and oscillations that lead to premature mechanical seal failures, bearing failures, excessive bushing, ring, and sleeve wear and even shaft failure (breakage).

Figure 1. Radial Thrust

Also, a hydraulic phenomenon called rotating stall sets-in, which is essentially a back-flow that leaves the impeller eye and progresses backwards. This can result in violent piping vibrations, pressure pulsations, and premature wear of the components.

Another very common hydraulic problem that occurs when the pump operates at a performance which is different from the original design point is an occurrence called recirculation cavitation. When two flow paths within a fluid are moving in opposite directions and they are in close proximity to each other, vortices form between the two directions of flow causing high fluid velocities and turbulence, resulting in pockets of low pressure where cavitation occurs.

Suction recirculation cavitation occurs when fluid entering the pump suction is reversed, resulting in high velocity vortexes in or near the impeller eye. These high velocities produce low localized pressures at the center of the vortex, resulting in cavitation damage which occurs in the impeller eye in-between the impeller vanes, on the pressure side of the inlet vanes near the impeller eye. This recirculation cavitation damage increases the  farther away from the BEP that the pump operates.

Discharge recirculation cavitation occurs when fluids leaving the impeller discharge may be reversed, causing high velocity vortexes between the two flow directions, in turn causing low-pressure areas. Cavitation occurs when these low-pressure areas drop below the vapor pressure of the fluid being pumped. Discharge recirculation cavitation occurs on the discharge side of the impeller. This damage increases the farther away from the BEP that the pump operates.

The problem intensifies when a hydraulic parameter called suction specific speed (NSS) is high. Suction specific speed is the geometric relationship between the impeller eye diameter, the impeller outside diameter, and the NPSHR. It is an indirect indication of the impeller eye being too large, but it also depends on several other factors related to design, installation and application.

There are certain engineering rules and principles related to minimum allowable flow: as a function of pump energy, specific speed (NS), suction specific speed (NSS), and other factors. When violated, these rules and principles can cause trouble and problems.

Example 1: A Small Pump

Let's consider a relatively small pump that needs to produce 40-gpm at 140-ft of head. A 1 x 1.5-6 pump size (with an approximately 6-in impeller diameter) may be selected from hydraulic curves. The pump will work, but unfortunately will not be operating at its optimum design point or BEP.

Figure 2. Original hydraulic curve for a 1 x 1.5-6 pump.

As is evident from the hydraulic curve for this pump size (see Figure 2), this pump will have 40 percent efficiency (yellow circle). However, the optimum design point or BEP (red angle) is at 58 percent efficiency. The result is that the pump operates to the left of its BEP for the impeller diameter required to achieve the desired head. This will result in radial thrust, vibrations, and premature wear.

What effect will this have on energy consumption? Note that the horsepower line (see Figure 2) that passes near the operating point is approximately at 4-hp, which is roughly 3-kW. How much does it cost to operate this pump if it is running continuously, 365 days per year at, say, $0.10 per kilowatt-hour? It is 3 x 24 x 360 x $0.10 = $2,592.
 

Now, what would it cost if the efficiency were somehow improved to the 48 percent efficiency that this pump would enjoy if the system operating point were to become the best efficiency or design point by redesigning the impeller? See Figure 3. Obviously, if a pump runs more efficiently, it will take less power. In fact, the power (and thus cost) would be inversely proportional to efficiency: $2,592 x (40/48) = $2,160. The net electrical savings would thus be $2,592 - $2,160 = $432 per year, which is 17 percent less.

Figure 3. The performance curve of the 1 x 1.5-6 pump using a reengineered impeller.

However, when the pump is operating at 40-gpm at 155-ft, instead of 100-gpm at 120-ft, the pump is operating in the danger zone, 60 percent away from BEP. At this point, the pump is subject to high radial loading, which causes tremendous noise and vibration and excessive shaft oscillation that leads to premature bearing, mechanical seal, and impeller failure.

 Using the chart in Figure 4, you can see that when the pump operates 60 percent away from design point (40-gpm at 155-ft, such as the example in the pump curve above), the pump failure rate has increased drastically, by 5.2 times. In other words: 833 days/163 days = 5.2 times or 520 percent higher cost!