Repost and Share: How to Properly Design the Suction Pipe of a Centrifugal Pump

Thread Content

It is believed that many centrifugal pump manufacturers have encountered this situation: under the same or similar operating conditions, pumps of the same model and specification operate well, with normal performance, vibration levels, and noise levels; yet in a very small number of projects, serious problems arise – the pumps not only have insufficient output but also exhibit excessive vibration and noise. What is the reason? For a centrifugal pump to perform at its best within a (system) process, in addition to its own design and manufacturing quality, it is also closely related to external conditions. The external factor considerations here mainly include: basic design and construction, loading due to takeovers and thermal expansion values, the layout and support of pump inlet/outlet pipes, and the design of the suction pipe along with the length of the straight sections. Among them, the most common main factors that can have an adverse effect on the performance of centrifugal pumps are the design of the suction pipe and the length of the straight section. The proper design of the suction pipeline is key to determining the uniformity of the liquid delivered to the pump. Unstable fluid flow can lead to a decline in pump performance, and may shorten the pump’s service life due to vibration and cavitation. During work, it is common to encounter questions from buyers/users asking whether there are any requirements regarding the length of straight sections in the inlet pipeline of centrifugal pumps If so, what is the basis? Does it include the transition section length? Based on the proven engineering practices outlined in various domestic and international manuals, standards, and specifications, as well as some insights drawn from the author’s own practical experience, this text provides summaries and recommendations on how to properly design the suction pipe of centrifugal pumps. It is hoped that these will help prevent abnormal operation of the pump systems due to improper suction pipe design in engineering applications. 1 Design requirements for the suction pipeline of centrifugal pumps according to different manuals/standards/specifications: In many engineering applications, the design and installation of piping systems are often not given enough attention, but certain basic engineering practices must still be followed. An unreasonable design and layout of the inlet pipeline can lead to a series of problems: 1) Insufficient pump output (reduced flow rate, decreased head, and lower efficiency). 2) It generates considerable noise during operation. 3) The vibration of the pump set is above the limit. 4) Premature failure of bearings and mechanical seals. 5) Premature wear and failure of other pump components, such as wear rings. 6) Cavitation damage occurs to the impeller and pump casing. 7) Leakage occurs at the interface flange. Except that there is no unified standard for the length requirement of straight pipe sections, the requirements for the design of the inlet pipeline of centrifugal pumps are largely similar across different manuals/standards/specifications. The \"Manual for the Installation and Design of Process Piping in Petrochemical Plants\" stipulates that: 1) The piping (layout) should be arranged in a manner that gradually rises or falls, so as to minimize or eliminate the formation of air pockets and liquid pockets, and dead zones should be avoided as much as possible. 2) For pipelines transporting solids, in order to prevent particle settlement from blocking the pipes, the pump’s branch pipes can be connected at a large slope or at a 45° angle. 3) For pumps with lateral suction, when liquid enters the pump nozzle, any deviation in flow direction or the formation of vortices can disrupt the balance of fluid flow within the impeller, affecting the pump’s head and shaft power. Additionally, since the angle at which the liquid enters the impeller differs from the design specifications, air resistance occurs, leading to vibration and noise; as a result, the pump’s performance deteriorates and its lifespan is reduced. To prevent this phenomenon, there must be a straight pipe section between the inlet of the centrifugal pump that draws in air from the side and the first fitting, with a length greater than 3 times the pipe diameter, before a elbow can be connected. 4) For double-suction centrifugal pumps, in order to ensure equal thrust on both sides of the pump shaft and balance the impeller, the suction pipeline should have a straight section. When the suction pipe is parallel to the pump shaft and connected to the pump in the same plane, there should be a straight pipe section of at least 7DN in front of the pump’s suction flange, in order to prevent the medium from deviating due to elbows, which could reduce the pump’s efficiency and damage the impeller ； When the suction pipe is at right angles to the pump shaft and connected to the pump’s suction nozzle, the straight pipe section can include elbows (with a certain radius), or tees (transition sections) and shut-off valves can also be considered as part of the straight pipe. If it is indeed difficult to install a straight pipe section, a flow straightener or guide vanes should be installed near the pump nozzle to prevent flow deviation and vortices. The \"Pump and Pipeline Design Specifications SEPD 0111-2001\" issued by Sinopec Engineering Construction Company, and the \"Pump and Pipeline Design Specifications EM-PDW0111-2003\" issued by Beijing Huafu Engineering Co., Ltd., require that: 1) the pump suction pipeline should be as short and have as few bends as possible, while still meeting the requirements related to thermal stress; under no circumstances shall there be any \"air pockets\" in the inlet pipeline. 2) The inlet of a dual-suction pump should have a straight pipe section with a length of not less than 3 times the pipe diameter; for large pumps, this straight pipe section should be at least 7 times the pipe diameter, so as to allow the liquid to enter the pump smoothly and prevent uneven flow and swirling currents that could cause vibration and noise in the pump. 3) When the piping of a dual-suction pump is of the top suction type, there is no need to consider the required straight pipe section above the suction inlet. Vertical pipes can be directly connected to the suction inlet through elbows and reducers... The national standard GB50275-2010 \"Code for Construction and Acceptance of Compressor, Fan, and Pump Installation Projects\" stipulates that: 1) The diameter of the suction pipeline shall not be smaller than the inlet diameter of the pump. 2) When using a reducer, its length should not be less than 5 to 7 times the difference in pipe diameters. 3) There must be no air pockets in the installation of the intake pipeline. 4) The length of the straight pipe section before the pump inlet should not be less than 3 times the inlet diameter. Standard API RP 686, \"Guidelines for the Installation and Installation Design of Mechanical Equipment,\" specifies that there should be a straight section in the pump inlet pipe between the inlet flange and the first elbow, tee, valve, pressure reducer, permanent filter, or any other device that could affect the stability of the flow pattern of the fluid entering the pump. The typical length of this straight section is 5 times the diameter of the pipe. The pipeline design specifications of the French company TECHNIP require that for double-suction pumps with a \"side-in, side-out\" configuration, when the first elbow on the suction side is installed horizontally, there must be a straight section with a length of at least 5 times the pipe diameter between the pump’s suction end and that first elbow. Conversely, when the first elbow on the suction side is installed vertically, the straight pipe section at the pump’s suction end can be ignored. Kawasaki’s pipeline design specifications require that the length of the straight section before the inlet of a single-suction pump should not be less than 3 times the inlet diameter ； The length of the straight pipe section before the inlet of a double-suction pump should not be less than 5 times the inlet diameter. The straight pipe section refers to the distance from the elbow (transition section), and the temporary filter is included in this straight pipe section. For double-suction centrifugal pumps, the Swiss company SULZER requires at least 7 straight pipe sections with a diameter larger than that of the pipe itself. 2 The inlet must have a sufficiently long straight pipe section. As can be seen from the aforementioned manuals/standards/specifications, except for pumps with an upper suction configuration, the length of the straight pipe section at the pump inlet for other types of suction configurations should be 5 to 7 times the diameter of the inlet. However, in practical engineering applications, these requirements cannot be met due to limitations such as on-site space or investment costs. To this end, the recommended minimum length of the straight pipe section is 3 times the inlet diameter, and this straight pipe section does not include the length of the transition section. 3 Avoid unstable flow conditions at the inlet. Turbulence and vortices are the most common flow patterns at the suction inlet of pumps (especially submersible pumps) that pose a threat to the safe and stable operation of the pump system; vortices include free-surface vortices and those beneath the liquid surface. An unstable inlet fluid flow can have the following adverse effects on the pump: 1) changes in flow rate and head characteristics, which impair the pump’s efficiency. 2) When eddy currents pass through the impeller blades, they induce vibration. 3) The radial force on the impeller will increase. 4) Induce cavitation. Measures to prevent the formation of free-surface vortices with air: 1) Improve the flow conditions to avoid rotation of the fluid or an increase in velocity gradient (an appropriate combination of pipe diameter and depth limits the maximum inlet flow velocity to 0.5 m/s; narrow and long flow channels are sufficient to direct the liquid flow evenly toward the pump). 2) Increase the submersion depth. 3) Achieve an intake water level that prevents the formation of air vortices using pig iron blocks. 4) Install vortex-preventing guide vanes in the water level area to prevent the formation of air vortices. Increasing the submersion depth can prevent vortices from forming in the inlet flow, as defined by ANSI/HI 9.8. The minimum submersion depth S required to prevent the formation of strong vortices is based, in part, on a dimensionless flow parameter, namely the Froude number, defined as: where FD = Froude number, a dimensionless value ； V = Inlet flow velocity based on D = Flow rate/Area ； D = Outer diameter of the inlet flange or pipe inlet ； g = acceleration due to gravity. V, D, and g must use consistent units so that FD is dimensionless. The minimum flooding S should be determined according to (Hecker, G.E., 1987): here, the unit of S is the same as that of D. It is appropriate to determine the size of the sump as a multiple of the pump inlet flange diameter D. The geometric similarity of the hydraulic boundary and the dynamic similarity of the flow pattern were determined based on the size of “D”. There are some variations in the flow velocity at the bell mouth depending on the pump type and manufacturer. Measures to prevent underwater vortices: 1) Improve the flow conditions to avoid rotation of the fluid or an increase in velocity gradient. 2) Use a water guiding cone with guide vanes. 3) Influence the fluid flow near the pipe wall through ribs or similar fittings. If multiple pumps are installed in a single suction structure, installing a partition wall between each pump creates better flow conditions than in an open tank. If partition walls are not used, unfavorable flow patterns will occur. For pumps with a design flow rate greater than 315 l/s, partition walls need to be installed between the pumps. For pumps with a design flow rate of 315 l/s or less, there is no need for a partition wall between pumps, and the minimum distance between pumps should be 2D. The large vertical circulation pumps produced by the German company KSB are renowned for their efficiency and reliability (low vibration levels), and they are widely used around the world. However, in a certain pump station in China, severe vibration issues occurred during actual operation. Based on the comprehensive evaluation by the expert group, there are design issues with the pump station’s inlet tank, which result in the formation of large-scale vortex streets, lead to pressure fluctuations, induce vibrations, and trigger cavitation. Later, the inlet tank was modified using the “vortex control technology (combined variable-height guide piers in the inlet tank + X-shaped vortex removal plates below the pump suction inlet),” and the pump resumed normal operation. 4. When arranging the pipes, avoid the formation of air pockets. The presence of such air pockets can not only directly affect the performance of the pump (resulting in insufficient output or even no flow of liquid), but it can also cause cavitation. To avoid the formation of air pockets during pipeline installation, GB50275-2010 \"Code for Construction and Acceptance of Fan, Compressor and Pump Installation Projects\" provides reference examples for the installation of the pump’s suction pipeline, as shown in Figure 1. Figure 1 – Example of pump suction pipe installation. Explanation of the diagram: 1. Air pocket; 2. Descending toward the pump; 3. Concentric reducer; 4. Ascending toward the pump; 5. Eccentric reducer. Design of the inlet for liquids containing solids: For many common liquids that contain solids, a flow velocity of around 1.0 m/s or higher is required to prevent sedimentation in horizontal pipes ； A flow velocity of less than 0.6 m/s is usually sufficient for organic solids. Within 5 times the suction pipe diameter from the pump (starting from the inlet flange), there should be no components that can interfere with flow, such as partially opened valves, tees, elbows with short radii, etc. Fully open valves that do not cause any flow interference, blade elbows, elbows with long radii, and reducers are not considered to be flow-interfering components (see Figures 2 and 3). Figure 2 – Recommended arrangement of the suction pipe (D = pipe diameter). Figure 3 – Examples of suction pipe connections near the pump that require approval from the pump manufacturer. The diameter of the suction pipe is usually larger than that of the inlet pipe on the pump. In this case, a concentric or eccentric reducer is installed to accommodate the difference in pipe sizes. For horizontal suction pipes, the straight side of the eccentric reducer should be located at the top. For vertical pipes without bends near the pump, concentric reducers are recommended. Recommended length of the straight section in the suction pipe for a double-suction pump when the elbow and the impeller shaft are in the same plane. See Figure 4 for details. Figure 4 – Recommended suction pipeline 6 for double-suction pumps with the elbow and impeller shaft in the same plane; experience with unrestricted inlets. Clear recommendations regarding the use of unrestricted inlets are provided in ANSI/HI 9.8. Unrestricted inlets include pumps mounted on platforms or other structures, which lack guide walls, sump walls, or other flow-guiding structures. A typical installation includes the inlets of rivers, canals, or ditches as well as the inlets of lakes, and pumps located on the seawater system platform. Pumps with unrestricted inlets are usually located where unidirectional flow occurs, or on platforms where tidal changes can lead to highly complex fluid conditions around the pump’s inlet. The recommended minimum distance from any obstacle to the pump suction inlet that could cause a wake effect is 5 times the maximum cross-sectional dimension of the obstacle. If there are no issues with debris or bottom sediment, the inlet flare should be located at a height of 0.3D to 0.5D above the bottom to minimize underwater vortices. For applications where bottom sediment suspension may be a problem, it is recommended to use the minimum gap of 5D. For pumps installed on offshore platforms, the suspension of sand during storms is inevitable due to the effect of waves. In some cases, placing a layer of protective stones around the inlet can effectively reduce the suspension of sediments. Since proper protection design requires specialized skills, the design of such protective layers should be carried out with the assistance of engineers experienced in sand transport and rock dumping design. For unrestricted inlets, debris is a matter of particular concern. Light-load debris can be handled by using a filter screen connected to the pump inlet flange ； For heavily loaded debris, special design considerations are required. 7 Summary and Recommendations 1) The diameter of the suction pipeline is usually larger than the inlet diameter of the pump; any reduction in diameter is strictly prohibited. 2) When the piping is end-sucked or side-sucked, the recommended minimum length of the straight pipe section at the suction inlet is 3 times the inlet diameter, and this straight pipe section does not include the length of the transition section. Large double-suction pumps should have straight pipe sections with a diameter that is several times larger, such as 5 to 7 times. 3) When the piping is of top suction type (including double-suction pumps), there is no need to consider the required straight pipe section above the suction inlet. 4) Appropriate preventive measures should be taken to avoid free-surface vortices and vortices beneath the liquid surface. 5) The inlet structure should avoid cross-flow that creates asymmetric flow patterns. For multiple pumps with a design flow rate greater than 315 l/s installed in a single suction structure, partition walls need to be installed between the pumps. 6) The pump suction pipeline should be as short and have as few bends as possible, while still meeting the requirements regarding thermal stress; under no circumstances shall there be any air pockets in the inlet pipeline. 7) For liquids containing solids, a flow velocity of around 1.0 m/s or higher is required to prevent sedimentation in horizontal pipes ； A flow velocity of less than 0.6 m/s is usually sufficient for organic solids. 8) For unrestricted inlets, the gap between the inlet flare and the bottom varies under different operating conditions, and the design of the bottom also differs. 8 References: Zhang Dejiang et al., Manual for the Installation Design of Process Piping in Petrochemical Plants, Volume 1: Design and Calculation (Fifth Edition), Sinopec Press, January 2014. API RP 686, Recommended Practices for Machinery Installation and Installation Design, Chapter 6 – Piping, SECOND EDITION, December 2009. Yi Zhidan, Analysis of the Straight Sections in Pump Suction Pipelines, Chemical Equipment and Piping, Volume 46, Issue 1, February 2009. Sulzer Pumps Ltd, Centrifugal Pump Handbook, Third edition, 2010. Heinz Hellmann, Compendium of Centrifugal Pumps, Tsinghua University Press, 2013. ANSI/HI 9.8-1998, American National Standard for Pump Intake Design, Hydraulic Institute, 1998