Why is the size of the pump suction pipe usually one size larger than that of the pump connection?

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1. Why is the size of the pump suction pipe usually one size larger than that of the pump connection? A common practice in engineering applications is to make the size (diameter) of the pump suction pipe at least one size larger than that of the pump suction flange (or fitting). This transition is usually achieved through an eccentric reducer, whose top is typically horizontal, but not always. Regarding the suction section of the pump, the most important thing is to ensure that no significant turbulence caused by upstream elbows occurs when the flow reaches the pump’s suction inlet. This is related to the geometry of the pipe, which means it is best to use a longer, straight suction pipe. Thicker pipes can reduce the pressure drop caused by friction and provide higher pressure at the pump’s suction inlet (the impeller’s suction opening), thereby supplying more energy to the pump. In the past, for various reasons, people designed all kinds of inadequate pump suction pipes, some of which even had certain positive effects. However, as a pipeline designer, you don’t want to keep learning through trial and error; you are looking for reliable methods that will allow you to work with peace of mind. For more information on this topic, please refer to the article “How to Properly Design the Suction Pipe of a Centrifugal Pump”. 2. Why are control valves usually one size smaller than the pipe diameter? The main reason is that smaller valves are cheaper and offer better, more precise control compared to valves with the same pipe diameter, but at the cost of a higher pressure drop. 3. For end-suction centrifugal pumps, is it always necessary for the pump inlet to have positive pressure (above atmospheric pressure)? No. The design of certain pumps allows them to lift liquid above the centerline of the pump. There are many different types of pumps that can do this, including small household pumps and large industrial pumps. 4. Is it necessary to install a check valve on the outlet side of the pump? It is necessary. There are mainly two benefits: first, it keeps the system filled with medium, which prevents liquid from overflowing and delays in startup when the pump stops operating. Secondly, it prevents the medium from flowing back and causing the pump to rotate in the reverse direction when the pump stops operating. 5. What is the ideal piping layout for a pump system? Unstable pump performance is sometimes attributed to blocked pipes. Clogged pipes are not a common cause, but it is still possible. A common problem is air blockage. Ideally, starting from the pump outlet, the pipeline will slope upward continuously until it reaches the bottom of the storage tank (water tank). In this way, any air that enters the pump can be expelled from the system. In the real world, pipes do not always slope upward; instead, they extend horizontally for a long distance. Longer horizontal sections of pipe are acceptable if it is possible to avoid the formation of air pockets or low and high points (in both cases, air can get trapped). Furthermore, the end of the pipeline is rarely connected to the bottom of the storage tank (water tank). In this case, the pipe usually protrudes from a higher position. This means there will be a peak that may contain air. This may be crucial for the process/flow, or it may not matter at all; experienced operators and engineers should make the judgment in this regard. If it is crucial for the process/flow, an exhaust valve must be installed/used. If a control valve is used at the end of the pipeline to regulate flow, the end of the pipeline should be located near the bottom of the tank, so as to provide a certain back pressure for the valve and reduce the likelihood of cavitation. 6. How to measure the performance of a pump? You might wonder if your pump is performing well. Your only option is to compare the pump’s performance with the predicted values on the characteristic curve at the correct impeller diameter and pump speed. You need to install a pressure gauge in front of and behind the pump. The pressure gauge should not be too far from the intended measurement points (i.e., the inlet and outlet flanges). The height between the pressure gauge and the centerline of the pump should be measured. You need to install a valve on the pressure gauge (or use an oil-filled shock-absorbing pressure gauge) to help reduce pressure fluctuations that may occur near the pump. It is necessary to measure the flow rate. Ideally, there should be a flow measurement device in the pipeline that can provide this information. If not, other methods must be considered, such as periodically pumping in the medium into a tank (water tank) of known volume, or other alternatives. The pressure reading will provide you with the total head of the pump; based on the flow rate, you can compare the result with the characteristic curves for that specific pump speed and impeller diameter. It is possible to measure only the shut-off head and compare it with the predicted shut-off head from the characteristic curve. The shut-off head occurs at zero flow rate, so there is no need for flow measurement. By checking the shut-off head, it is possible to test whether the pump is running at the correct speed and whether an impeller of the correct diameter is installed. Measuring efficiency is difficult because a torque meter needs to be installed on the pump shaft. 7. What is the effect of liquid viscosity on pump performance? The performance or characteristic curve of the pump is determined using water under standard conditions. Liquids with a viscosity higher than that of water can affect the performance of the pump. The total head, flow rate, and power will all be adversely affected. When the viscosity reaches or exceeds 400 cSt, efficiency will drop by 50%; in such cases, considering the use of a positive displacement pump is advisable. 8. Can the pump operate across the entire flow range indicated by the characteristic curve? No. The pump should operate as closely as possible to the BEP (Best Efficiency Point). The typical range is to operate the pump at 80% to 120% of the flow rate at the optimal efficiency point. Most pump manufacturers do not encourage operating pumps at flow rates below 50% of the BEP. If this must be done, there are two options: either install a recirculation pipeline or fit a variable-speed drive on the pump. At high flow rates, the pump will experience high vibration and potential cavitation due to its high NPSHR at that time. There is no other way but to run with reduced traffic.