Are you installing a new centrifugal pump?
After carefully selecting the proper size and materials, make sure the new pump is configured with the proper installation. Setting up the base correctly and aligning the pump is crucial. It is also important that the pump piping is correct.
Pump piping design is sometimes underestimated when setting up new installations. The focus is more on the equipment than the pipe that is supplied. However, when pumping equipment is installed with improper piping arrangements, the pump can experience premature and repetitive failures throughout its life. Maintenance crews will regularly repair the pump, effectively treating the symptom rather than the actual problem.
Knowledge and sources of information are limited on this subject, except what is found in the operation and maintenance manual (IOM), which is minimal. But by following these six simple rules, users can avoid premature pipeline-related pump failure.
1. Keep suction pipe as short as possible
Include a straight pipe 5 to 10 times the pipe diameter between the pump inlet and any obstruction in the suction line. Obstructions include valves, elbows, tees, etc.
Keeping the suction pipe short ensures that the pressure drop at the pump inlet is as small as possible. Straight pipe gives a uniform velocity along the pipe at the pump inlet. Both are important to achieve optimal suction.
2. Suction pipe diameter must be equal to or larger than the pump suction port size.
Pipe size is a balance between cost and friction losses. Larger pipes cost more, while smaller pipes impose higher friction losses on the system. In terms of diameter, the discharge pipe should normally match the pump discharge flange but can be larger to reduce friction losses and reduce system pressure.
In the suction, the diameter can be the same size, but some engineers select one or two larger diameters, thus requiring an eccentric reducer. Larger suction pipe diameters are usually preferred if the viscosity of the liquid is greater than that of water. This also helps produce a smooth flow to the pump and prevent cavitation.
3. Use of eccentric reducers in suction
Consider using eccentric reducers on the pump suction when a transition in pipe diameter sizes is required. Install the flat side of the reducer at the top when the liquid comes from below the pump. If the fluid comes from the top, the flat part of the reducer must be mounted at the bottom of the pipe. The flat part is designed to prevent an air pocket from forming at the pump suction.
4. Eliminate elbows mounted on or near the pump inlet nozzle
Include 5 to 10 diameters of straight pipe between the pump inlet and the elbow. This helps remove load from the pump impeller and creates a uniform axial load to the bearings.
5. Eliminate potential air inlets in the suction pipe
Maintain adequate levels in the tanks to eliminate the formation of vortices and air inlets. Avoid high pockets in the suction line, which can trap air. Keep all pipe and accessory connections tight under suction and vacuum conditions to prevent air from entering the pump.
6. Ensure piping arrangement does not cause stress on pump volute
Pumps should never support pump suction or discharge piping. Any stress on the pump volute from the system piping greatly reduces pump life and performance.
Keep in mind that increasing the performance of the pump will help compensate for errors in the piping on the discharge side of the pump. Suction side problems, however, can be the source of repetitive failures, causing problems if not treated properly. The suction piping causes most pump problems.
Piping design is an area where basic principles are frequently ignored, resulting in increased vibration and premature seal and bearing failure. Improper plumbing has long been ignored as the reason for these failures due to the many other reasons this equipment can fail. Many experienced engineers can argue that incorrectly piped pumps still work and work as they should. This argument, while valid, does not make questionable piping practices correct.
Why is an expansion joint needed in the pipeline?
The same bomb failed again. A manufacturer experienced repeated failures on the same pump and although the pump had been properly repaired, installed and aligned, it was still experiencing chronic vibration problems.
A closer look determined that the vibrations were not being emitted from the pump, but were affected by vibrations from other equipment as they traveled through the pipes. What can be done to prevent undesirable vibrations from affecting the pump? A pipe expansion joint might help.
What is a pipe expansion joint?
An expansion joint is installed in piping systems for a couple of reasons. They can be used to absorb vibration and shock and to relieve anchor stress, reduce noise and compensate for misalignment. Some expansion joints are also designed for thermal expansion in hot applications. Expansion joints allow pipes to move in three different ways: axial compression or extension, lateral displacement, or angular deflection.
They typically come in three different materials to cover a variety of applications.
- Metal – Metal Expansion Joints are primarily used in applications where thermal expansion is an issue. As the temperature of the pipe increases, the metal expansion joint compresses to compensate for the movement, taking stress off the anchors and the pipe. A metal expansion joint is an excellent example of how to handle hot expanding pipes.
- Rubber: Rubber can be used for thermal expansion and also absorbs vibrations and shock waves well. That is why this type of expansion joint is ideal to minimize the transmission of noise, vibration from other equipment to protect equipment such as pumps. They also function as shock absorbers to minimize trauma caused by water hammer, pressure spikes, and seismic events.
- Braided: Braided stainless steel hoses with flexible or metallic coatings are not technically an expansion joint, but rather a flexible connector. They work well in high pressure and temperature applications that require vibration damping or pipe misalignment. These are often used in pumps and other equipment to help eliminate thrust loading on equipment nozzles due to thermal expansion. Removing that load is critical to pump performance. The braids provide lateral and angular movement. They also absorb vibrations.
The whole process is intertwined and each piece affects the next. Having a reliable piping system is key to a process that maximizes uptime. Careful management of a piping system will keep the pipes in excellent working order and will also extend the life of the equipment connected to them.
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