SUCTION SYSTEM IN CENTRIFUGAL PUMPS
Although malfunctions in pumps are a direct cost element for many facilities, any malfunction that may cause a stoppage in production, in line with the duty of the pumps in the facility, causes a secondary and much larger financial loss. Since these malfunctions occur in the pump, the primary focus always appears to be the pump. However, the root cause of the problems is, in many cases, the plumbing structure of the pump. Today, we wrote an article about how the pump installation should be, the effect of errors made in the installation on the pump, and how these errors can be prevented.
1. Ensure Adequate NPSH Value
NPSH is a subject that is widely discussed today and where we can find many resources on many different platforms. For this reason, rather than examining NPSH in detail in our article, it will be sufficient to talk about the effects of NPSH on pump installations and briefly introduce NPSH.
In centrifugal pumps, the fluid coming to the pump fan is required to be at a certain pressure in order for the pump to operate properly without cavitation. We call this desired amount of pressure NPSHp (Pump) or NPSHr (required). However, due to our installation structure and operating conditions, there is a pressure that we can provide to the fluid that will enter the pump. We call this pressure NPSHm (Available) or NPSHa (available).
Therefore, in order to ensure healthy working conditions for the pump mentioned above, the NPSHp condition must be met. Otherwise, the pump will enter cavitation and damage to the pump parts will occur.
Therefore, in order to ensure healthy working conditions for the pump mentioned above, the NPSHp condition must be met. Otherwise, the pump will enter cavitation and damage to the pump parts will occur.
2. Reduce Friction Loss
Friction loss will both reduce the NPSHm value mentioned above and cause our energy costs to increase. For this reason, it is very important to keep friction to a minimum on both the suction and discharge sides in pump systems, as in all systems.
In order to keep friction to a minimum; First of all, installation elements with material structure and technical features that will provide minimum resistance to flow should be selected. The friction coefficient of the equipment and materials to be used should be low (as much as the fluid allows) and should be positioned so as not to disrupt the dynamics of the installation. However, the longer the fluid travels to the pump inlet, the more energy will be wasted as friction loss. For this reason, the pump should be as close as possible to the source from which it is suctioning. Finally, ensuring that the fluid velocity entering the pump does not exceed 1.5m/s will prevent additional friction loss in the suction. Because as the speed of the fluid increases, the friction loss in the pipeline and equipment will also increase.
3. Elbows
Elbows are plumbing elements we use in plumbing systems to change the direction of the fluid at different angles. However, this change in direction also causes a change in the internal dynamics of the fluid, causing the fluid to flow in an irregular form. Today, we know from various simulations and fluid analysis that a fluid passing through the elbow creates an irregular flow, creating turbulence and causing vibration, causing damage to the pump fan. In addition to the damage to the fan, the vibration in the pump will also affect the pump bearings and shaft seal, causing problems at these points.
To avoid this situation, it is necessary to ensure that the fluid entering the pump is in a regular flow. In this case, it is a priority not to use elbows in the installation if possible. However, if we have to use an elbow, as in many systems, in order to provide this regular flow to the pump inlet, we need to put a straight pipeline after the elbow to the pump inlet with a length of minimum 5 and maximum 10 times the suction pipe diameter. Thus, the fluid coming out of the elbow will regulate its flow along this line and will not cause any problems in the pump.
4. It Must Be Fluid Entering the Pump, Not Air!
4. It Must Be Fluid Entering the Pump, Not Air! Due to the structure of the installation, some peaks in the pipelines create gaps where gases entering the installation can accumulate. When the gases accumulated in these spaces enter the pump suddenly or at different periods of time, it creates a cavitation effect due to pressure differences within the pump and may damage all parts in contact with the fluid. At the same time, the increase and accumulation of air in these pockets over time restricts fluid passage and the pump cannot provide sufficient suction. This situation also shortens the working life of the pump and causes malfunctions.
Although it is not possible to reduce the amount of air in the installation to zero, it is at least possible to reduce it to levels that will not cause problems for us. First of all, when the installation is completed, the line is already full of air. This air must be completely evacuated before sending the fluid to the installation. Again, in an actively used system, partial low pressures can be created by the movements of valves and various equipment and fluctuations in the fluid. These low pressures cause the air in the fluids, which carry even a very small amount of dissolved air, to evaporate and accumulate at the peak points of the installation. This air must be removed from the system at regular intervals.
In order to be protected from air pockets, it is necessary to leave as few peaks as possible in the installation. When necessary, these elevations can be made by slightly extending the length of the installation with low-angle transitions. Another precaution we take is to place air release valves in the installation, especially at points where the slope increases or decreases rapidly. Thus, when necessary, the air in the installation can be evacuated by using these valves. Finally, to prevent air pockets in pump suction, eccentric reduction should always be used and the suction pipe should be positioned on an upward slope towards the pump.
5. The Pump is Not a Support Point for the Plumbing.
Unfortunately, in many facilities pump installations are not properly supported. In some cases, pumps are even used as a support element in the installation. However, this may have very negative consequences on the pump.
Pumps are machines produced to transfer fluids from one point to another. Therefore, since the pump ensures this transfer by creating a pressure difference on the fluid, the pump is also affected by the resulting pressure and radial and axial forces occur on the parts. The pump design and the materials used are manufactured to withstand these forces and ensure that the pump performs its duty without any problems with certain maintenance throughout its life. However, if the pump installation is not adequately supported and the entire weight of the installation and the tensions in the installation that occur from time to time are transferred to the pump, the pump will try to transfer fluid under much more force than anticipated. Likewise, if the installation is not properly aligned and the installation equipment is connected to each other by "force", the installation will transmit this strain to the pump flange in the same way and the pump will try to operate under an external force.
These external forces acting on the pump cause vibration, noisy operation and frequent malfunctions in the pump. Therefore, pump life is significantly shortened and maintenance costs increase. Especially in lines where fluids at high temperatures are used, expansion and contraction in the installation causes loads to occur at intermittent intervals both vertically and horizontally in the installation. In systems where such fluids are used, the installation must be supported externally, and compensators that can absorb the installation load must be placed on the pump suction and discharge flanges.
6. Conclusion
- Check NPSH values and keep NPSHm high.
- Keep the pump as close to the source of suction as possible.
- Do not allow the fluid velocity in the suction to exceed 1.5m/s.
- Determine the suction and discharge pipe diameters correctly.
- Avoid elbows as much as possible.
- If it is necessary to use an elbow, ensure a straight line with a minimum length of 5 times the suction pipeline from the elbow to the pump inlet.
- Create as few peaks as possible in the installation.
- Place air release valves at points in the installation where the slope increases and decreases rapidly.
- Be sure to use eccentric reduction in the pump suction system.
- Make sure that the pump suction pipe has an increasing slope up to the pump inlet.
- Support the pump's suction and discharge lines externally from the pump.
- Make sure that the pump installation is aligned and that the parts are not strained during assembly.
- Use compensators on pump flanges, especially in systems where high temperature fluids are used.
