1. The Mysterious NPSHA

The most important thing is the NPSHA of thedouble suction pump. If the user does not correctly understand NPSHA, the pump will cavitate, causing more costly damage and downtime.

2. Best Efficiency Point

Running the pump away from the Best Efficiency Point (BEP) is the second most common problem affecting the double suction pumps. In many applications, nothing can be done about the situation due to circumstances beyond the owner's control. But there is always someone, or the time is right, to consider changing something in the system to allow the centrifugal pump to operate in the area it was designed to operate. Useful options include variable speed operation, adjusting the impeller, installing a different size pump or a different pump model, and more.

3. Pipeline Strain: Silent Pump Killer

It seems that ductwork often is not designed, installed or anchored correctly, and thermal expansion and contraction is not considered. Pipe strain is the most suspected root cause of bearing and seal problems. For example: after we instructed the on-site engineer to remove the pump foundation bolts, the 1.5-ton pump was lifted by the pipeline by tens of millimeters, which is an example of severe pipeline strain.

Another way to check is to place a dial indicator on the coupling in the horizontal and vertical planes and then loosen the suction or discharge pipe. If the dial indicator shows movement of more than 0.05 mm, the pipe is too strained. Repeat the above steps for the other flange.

4. Start Preparation

Double suction pumps of any size, except for low-horsepower rigid-coupled, skid-mounted pump units, rarely arrive ready to start at the final site. The pump is not "plug and play" and the end user must add oil to the bearing housing, set the rotor and impeller clearance, set the mechanical seal, and perform a rotation check on the drive before installing the coupling.

5. Alignment

Alignment of the drive to the pump is critical. No matter how the pump is aligned at the manufacturer's factory, alignment can be lost the moment the pump is shipped. If the pump is centered in the installed position, it may be lost when connecting the pipes.

6. Oil Level and Cleanliness

More oil is usually not better. In ball bearings with splash lubrication systems, the optimal oil level is when the oil contacts the very bottom of the bottom ball. Adding more oil will only increase friction and heat. Remember this: The biggest cause of bearing failure is lubricant contamination.

7. Dry Pump Operation

Submersion (simple immersion) is defined as the distance measured vertically from the surface of the liquid to the centerline of the suction port. More important is necessary submergence, also known as minimum or critical submergence (SC).

SC is the vertical distance from the fluid surface to the double suction pump inlet required to prevent fluid turbulence and fluid rotation. Turbulence can introduce unwanted air and other gases, which can cause pump damage and reduce pump performance. Centrifugal pumps are not compressors and performance can be significantly affected when pumping biphasic and/or multiphase fluids (gas and air entrainment in the fluid).

8. Understand the Pressure of a Vacuum

The vacuum is a subject that causes confusion. When calculating NPSHA, a thorough understanding of the topic is especially important. Remember, even in a vacuum, there is some amount of (absolute) pressure - no matter how small. It's just not the full atmospheric pressure you normally know working at sea level.

For example, during an NPSHA calculation involving a vapor condenser, you might encounter a vacuum of 28.42 inches of mercury. Even with such a high vacuum, there is still an absolute pressure of 1.5 inches of mercury in the container. A pressure of 1.5 inches of mercury translates to an absolute head of 1.71 feet.

Background: A perfect vacuum is approximately 29.92 inches of mercury.

9. Wear Ring and Impeller Clearance

Pump wear. When the gaps wear and open, they can have negative effects on the double suction pump (vibration and unbalanced forces). usually:

Pump efficiency will decrease one point per thousandth of an inch (0.001) for clearance wear of 0.005 to 0.010 inches (from original setting).

Efficiency begins to decrease exponentially after the clearance wears down to 0.020 to 0.030 inches from the original clearance.

In places of severe inefficiency, the pump simply agitates the fluid, damaging bearings and seals in the process.

10. Suction Side Design

The suction side is the most important part of the pump. Fluids do not have tensile properties/strength. Therefore, the pump impeller cannot extend and draw fluid into the pump. The suction system must provide the energy to deliver the fluid to the pump. The energy may come from gravity and a static column of fluid above the pump, a pressurized vessel/container (or even another pump) or simply from atmospheric pressure.

Most pump problems occur on the suction side of the pump. Think of the entire system as three separate systems: the suction system, the pump itself, and the discharge side of the system. If the suction side of the system supplies enough fluid energy to the pump, the pump will handle most problems that occur on the discharge side of the system if chosen correctly.

11. Experience and Training

People at the top of any profession are also constantly striving to improve their knowledge. If you know how to achieve your goals, your pump will run more efficiently and reliably.