SUCTION PIPING IN CENTRIFUGAL PUMPS
Failures occurring in pumps represent a direct cost element for many facilities, but depending on the role the pump plays in the plant, any failure that could cause a production stoppage results in an even larger secondary financial loss. Because these failures occur in the pump, the primary focus always appears to be the pump itself. However, in many cases the root cause of the problem is the pump's piping arrangement. Today we have written an article about how pump piping should be designed, the effect of piping errors on the pump, and how these errors can be prevented.
1. Ensure an Adequate NPSH Value
NPSH is a topic widely discussed today, on which a great deal of material can be found across many different platforms. For this reason, rather than examining NPSH in detail in this article, it will be enough to briefly introduce NPSH and discuss its effects on pump piping.
In centrifugal pumps, for the pump to operate healthily without entering cavitation, the fluid reaching the pump impeller is required to be at a certain pressure. We call this required pressure NPSHp (pump) or NPSHr (required). At the same time, due to our piping arrangement and operating conditions, there is a pressure that we are able to provide to the fluid entering the pump. We call this pressure NPSHm (available) or NPSHa (available).
Therefore, in order for the healthy operating condition mentioned above to be met for the pump, the condition NPSHm > NPSHp must be satisfied. Otherwise, the pump will enter cavitation and damage will occur to the pump parts.
The NPSHp value required by the pump is the responsibility of pump manufacturers, who try to keep this value as low as possible by making various adjustments and improvements to pump designs. However, reducing the NPSHp value of an existing pump already in use is both very difficult and costly, and generally requires measures that yield very low returns. Therefore, rather than trying to reduce the NPSHp value, it will be both easier and far less costly to increase the NPSHm value.
2. Reduce Friction Loss
Friction loss will both reduce the NPSHm value mentioned above and cause our energy costs to increase. For this reason, as in all systems, it is very important to keep friction to a minimum on both the suction and discharge sides of pump systems as well.
To keep friction to a minimum, piping components should first be selected with a material construction and technical specifications that offer minimum resistance to flow. The equipment and materials used should (to the extent the fluid allows) have a low friction coefficient and should be positioned in a way that does not disrupt the piping's flow dynamics. In addition, the longer the path the fluid has to travel to reach the pump inlet, the greater the amount of energy wasted as friction loss. For this reason, the pump should be located as close as possible to the source from which it draws suction. Finally, keeping the velocity of the fluid entering the pump below 1.5 m/s will prevent additional friction loss from occurring on the suction side, because as fluid velocity increases, the friction loss occurring in the pipeline and equipment increases as well.
3. Elbows
Elbows are piping components we use in piping systems to change the direction of the fluid at various angles. However, this change of direction also causes a change in the internal dynamics of the fluid, resulting in irregular flow. Today, from various simulations and fluid analyses, we know that fluid passing through an elbow forms an irregular flow, creating turbulence, which in turn causes damage to the pump impeller and results in vibration. In addition to the damage occurring at the impeller, the resulting vibration in the pump also affects the pump bearings and seal, causing problems at these points as well.
To avoid this situation, it is necessary to ensure that the fluid entering the pump is in a regular flow. In this case, if possible, the priority is to avoid using elbows in the piping at all. However, if we must use an elbow, as in many systems, in order to achieve this regular flow at the pump inlet we need to install a straight length of pipe after the elbow, up to the pump inlet, of a length between 5 and 10 times the suction pipe diameter. In this way, the fluid leaving the elbow will regulate its flow along this straight run and will not create any problem in the pump.
4. What Enters the Pump Must Be Fluid, Not Air!
Due to the piping layout, certain high points in the pipelines create cavities where gases entering the piping can accumulate. When the gases accumulated in these cavities enter the pump all at once or at different times, they create a cavitation effect due to pressure differences inside the pump and can damage all parts in contact with the fluid. At the same time, an increase and accumulation of air in these pockets over time restricts fluid flow, and the pump cannot draw a sufficient amount of suction. This situation likewise shortens the pump's service life and causes failures.
Although it is not possible to reduce the amount of air in the piping to zero, it is possible to bring it down to levels that will not create a problem for us. First of all, when the piping is completed, the line is already full of air; before sending fluid into the piping, this air must be completely vented out. Likewise, in an actively used system, the movement of valves and various equipment, along with fluctuations in the fluid, can create localized low pressures. These low pressures cause the air dissolved in the fluid — even if present only in very small amounts — to come out of solution and again accumulate at the high points of the piping. This air must also be removed from the system at regular intervals.
To guard against air pockets, we must first leave as few high points as possible in the piping. Where necessary, these rises can be made using low-angle transitions, even if this lengthens the piping run somewhat. Another precaution is to install air release valves at points in the piping where the slope rises or falls particularly sharply. In this way, these valves can be used when needed to vent the air inside the piping. Finally, to prevent air pockets at pump suctions, an eccentric reducer should always be used, and the suction pipe should be positioned with a slope rising toward the pump.
5. The Pump Is Not a Support Point for the Piping.
Unfortunately, in many facilities pump piping is not properly supported. In some cases, pumps are even used as a support element for the piping. However, this can have quite negative consequences for the pump.
Pumps are machines built to transfer fluids from one point to another. Since the pump achieves this transfer by creating a pressure differential in the fluid, the pump itself is also affected by the resulting pressure, and radial and axial forces develop on its parts. The pump's design and the materials used are manufactured to withstand these forces and to allow the pump to perform its function trouble-free throughout its service life, with appropriate maintenance. However, if the pump piping is not adequately supported and the full weight of the piping, along with stresses that arise in the piping from time to time, is transferred to the pump, the pump will try to transfer fluid under far greater forces than intended. Likewise, if the piping is not properly aligned and piping components are connected to each other by "forcing" them together, the piping will transmit this strain to the pump flange in the same way, and the pump will again be forced to operate under an external load.
These external forces acting on the pump cause vibration, noisy operation and frequent failures in the pump. As a result, the pump's service life is significantly shortened while maintenance costs increase. Particularly on lines where high-temperature fluids are used, expansion and contraction in the piping causes intermittent loads to develop in the piping, both vertically and horizontally. In systems using such fluids, the piping must absolutely be externally supported, and compensators capable of absorbing the piping load must also be installed at the pump's suction and discharge flanges.
6. Conclusion
- Check your NPSH values and keep the NPSHm value high.
- Keep the pump as close as possible to the source it draws suction from.
- Do not allow the fluid velocity in the suction to exceed 1.5 m/s.
- Determine the suction and discharge pipe diameters correctly.
- Avoid elbows as much as possible.
- If using an elbow is unavoidable, provide a straight suction pipe run of at least 5 times the pipe diameter in length, from the elbow to the pump inlet.
- Create as few high points as possible in the piping.
- Install air release valves at points in the piping where the slope rises or falls sharply.
- Always use an eccentric reducer in the pump suction piping.
- Ensure the pump suction pipe rises with an upward slope all the way to the pump inlet.
- Support the pump's suction and discharge piping externally, independent of the pump.
- Make sure the pump piping is properly aligned and that components are not strained during assembly.
- Use compensators at the pump flanges, particularly in systems using high-temperature fluids.
