Why is a maximum suction pressure specified for pumps?
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I have a question for everyone: when designing a pump in accordance with the relevant standards, the user is required to provide the maximum suction pressure. What role does this value play in the pump’s design? Is it used for strength calculations? Thank you.
The maximum suction pressure provided by the user is not used for strength calculations; it is primarily used for cavitation checking. After the fluid flows through the pump inlet and the impeller inlet, the pressure continues to drop before rising again. Cavitation occurs when the pressure drops to the vaporization pressure of the gas. Therefore, the pump suction inlet maintains a certain pressure head to prevent cavitation of the fluid.
If it is cavitation, why is it the “maximum” rather than the “minimum” inlet pressure?
Is there a problem with the literal interpretation? Does it mean that the pump may have several inlet pipelines, and that the inlet pressure is generally related to the net positive suction head?
I’m not in the design field, but from the perspective of our equipment maintenance and management, determining the maximum suction pressure for a pump is related to its efficiency. Moreover, high inlet pressure should have an impact on the balance of the axial force acting on the pump, the stress on the bearings, as well as the sealing at the suction end.
Could you provide the contextual background for discussion? Also, since your materials are in English, it seems that translating (Ma*mum suction pressure) as maximum suction inlet pressure isn’t entirely accurate. Are you referring to API 610?
When a centrifugal pump is in operation, there are specified requirements regarding the suction pressure. The specification for the minimum suction pressure is primarily related to the net positive suction head； The regulations regarding the maximum suction pressure are mainly related to the pump’s sealing and outlet pressure ； The design pressure for the pump’s seal is determined based on the pump’s inlet pressure; if this pressure exceeds the specified maximum value, it will cause the seal to operate under excessive pressure, which can easily lead to damage to the seal. Furthermore, if the inlet pressure exceeds the specified maximum value while the pump is operating, and since the head remains constant, this will cause the outlet pressure of the pump to exceed normal levels. This post was last edited by JW4103 on 2008-1-17 13:38 ]
The 8th floor believes that “the specification for the maximum suction pressure is mainly related to the pump’s head.” However, the definition of the head of a centrifugal pump is \"the net work done in transporting a unit mass of liquid from the inlet flange to the outlet flange\", so what does this have to do with the pressure at the inlet?
List of solutions to prevent cavitation in pumps. Cavitation is not allowed to occur when a centrifugal pump is in operation; therefore, it is necessary to ensure that NPSHa–NPSHr > S. When NPSHa cannot meet this requirement, solutions can be found by either the buyer (user) trying to increase the NPSHa value or the seller (pump manufacturer) trying to reduce the NPSHr value. The details are as follows:
**Methods adopted by the buyer:**
1. Reduce the installation height of the pump (raise the liquid level or lower the pump’s installation position); if necessary, use backflow methods. It is advisable to choose pumps that are efficient and easy to maintain. This method incurs additional installation costs, but it is recommended to use it whenever possible.
2. Reduce the resistance in the suction pipeline, such as by increasing the pipe diameter or reducing the number of pipeline accessories, foot valves, elbows, gate valves, etc. This can improve the suction conditions and save energy, although it requires additional investment (in terms of larger pipe diameters).
3. Adding an auxiliary pump can reduce the cost of the main pump and improve its efficiency. However, this increases the need for maintenance of both the equipment and pipelines.
4. Lower the temperature of the liquid being pumped to reduce vaporization pressure. Efficient and easy-to-maintain pumps can be used, but a cooling system is required.
5. Avoid using throttle valves in the inlet pipeline to prevent localized resistance losses.
6. When flow rate and head are constant, using a double-suction pump may help, as it has a lower NPSHr value. This option can also be considered.
**Methods adopted by the seller:**
1. Improve the surface finish of the flow channels by grinding and cleaning them. This is a simple method, but it increases processing costs. It is often used.
2. Increase the diameter at the inlet of the impeller to reduce the inlet flow velocity. This is a simple method, but it increases the risk of backflow, which is not conducive to stable operation. It is generally not used often.
3. Reduce the pump’s rotational speed. This is a simple approach, but at the same flow rate and head, a pump with a lower speed is more expensive and less efficient. It is generally not used often.
4. Add an inducer wheel at the pump inlet. This is a simple method, but it limits the maximum operating range of the pump. It is often used.
5. For mixed-flow or axial-flow pumps with adjustable blades, the angle of the blades can be adjusted. This method is often used.
6. Use materials resistant to cavitation, such as cemented carbide, phosphor bronze, 18-8 steel, Cr-Ni steel, etc., for the flow components. The structure and performance characteristics of the pump remain unchanged, but material costs increase. This method is used less frequently
**Methods adopted by the buyer:**
1. Reduce the installation height of the pump (raise the liquid level or lower the pump’s installation position); if necessary, use backflow methods. It is advisable to choose pumps that are efficient and easy to maintain. This method incurs additional installation costs, but it is recommended to use it whenever possible.
2. Reduce the resistance in the suction pipeline, such as by increasing the pipe diameter or reducing the number of pipeline accessories, foot valves, elbows, gate valves, etc. This can improve the suction conditions and save energy, although it requires additional investment (in terms of larger pipe diameters).
3. Adding an auxiliary pump can reduce the cost of the main pump and improve its efficiency. However, this increases the need for maintenance of both the equipment and pipelines.
4. Lower the temperature of the liquid being pumped to reduce vaporization pressure. Efficient and easy-to-maintain pumps can be used, but a cooling system is required.
5. Avoid using throttle valves in the inlet pipeline to prevent localized resistance losses.
6. When flow rate and head are constant, using a double-suction pump may help, as it has a lower NPSHr value. This option can also be considered.
**Methods adopted by the seller:**
1. Improve the surface finish of the flow channels by grinding and cleaning them. This is a simple method, but it increases processing costs. It is often used.
2. Increase the diameter at the inlet of the impeller to reduce the inlet flow velocity. This is a simple method, but it increases the risk of backflow, which is not conducive to stable operation. It is generally not used often.
3. Reduce the pump’s rotational speed. This is a simple approach, but at the same flow rate and head, a pump with a lower speed is more expensive and less efficient. It is generally not used often.
4. Add an inducer wheel at the pump inlet. This is a simple method, but it limits the maximum operating range of the pump. It is often used.
5. For mixed-flow or axial-flow pumps with adjustable blades, the angle of the blades can be adjusted. This method is often used.
6. Use materials resistant to cavitation, such as cemented carbide, phosphor bronze, 18-8 steel, Cr-Ni steel, etc., for the flow components. The structure and performance characteristics of the pump remain unchanged, but material costs increase. This method is used less frequently
Thank you for the question from floor 9; I have modified the content on floor 8
I think it has something to do with the balance of axial forces on the seal
The revised answer on the 8th floor is basically accurate; one additional point is that the specification of the maximum suction pressure is also related to the pressure-bearing capacity of the pump itself. If the suction pressure is too high, it may cause the internal pressure of the pump to exceed the designed pressure, leading to damage or even accidents!
Many people may not be aware of it, but there is a theory related to centrifugal pumps known as the \"backflow theory.\" Backflow pressure refers to the maximum suction pressure that a centrifugal pump can handle. If this backflow pressure (inlet pressure) exceeds the specified maximum suction pressure, then the seals used in such pumps – whether they are packing seals or mechanical seals – will no longer function properly, resulting in significant leakage of the fluid being transported and damage to the pump. Therefore, when selecting a pump, it is essential that the maximum backflow pressure does not exceed the pump’s maximum suction pressure.
So the pump also has requirements regarding inlet pressure! Normally, we didn’t pay attention to this issue during production! Generally, it involves things like head, flow rate, medium, sealing bearings, and so on
I would like to ask how to determine the Ma*mum suction pressure? Thank you
This should be used to determine the design pressure of the pump, as the design pressure of a pump is obtained by adding the pump’s head to the inlet pressure! If the maximum pressure at the pump inlet is 1 MPa and the pump head is also 1 MPa, then the design pressure for the pump should be 2 MPa multiplied by 1.5. But if the maximum pressure at the pump inlet is 8 MPa with a pump head of 1 MPa, then the design pressure should be 9 MPa multiplied by the safety factor!
The answer was very satisfactory – simple and clear, easy to understand once learned
The main consideration is the balance of the pump’s axial thrust