Why must centrifugal pumps have reducers at their inlet and outlet?

Thicker pipes can reduce resistance and lower the fluid flow rate. Reduce energy loss. Also, if all the pipes are the same size as the inlets and outlets, there will be so many different pipe specifications ；

As we all know, when a centrifugal pump is installed in a certain piping system and operates at a specific speed, its flow rate and head are not only related to the characteristics of the pump itself but also to the operating characteristics of the piping system. Here is a simplified formula known as the pipeline characteristic equation: H=H0+kQ2 ； Here, H represents the head generated by the centrifugal pump, Q represents the flow rate of the liquid in the pipeline, and k is the pipeline characteristic coefficient, which is related to the pipeline length, diameter, friction coefficient, and local resistance losses ； By plotting the pipeline characteristic curve using this formula, it can be seen that the operating point of the pump depends entirely on the characteristics of the centrifugal pump and the pipeline characteristic coefficients. Therefore, we generally use thicker pipes to reduce pipeline resistance and achieve the required flow rate and head pressure ； Or the local resistance can be increased or decreased through valve regulation. That is why tees are generally used at the outlet of centrifugal pumps.

The main functions of a pump’s diameter variation are: 1. Reducing flow velocity, 2. Decreasing resistance, 3. Minimizing pressure loss, 4. Lowering the net positive suction head.

What was said on the fourth floor was too technical; in fact, a simple understanding is sufficient. Although the liquid pumped by a centrifugal pump cannot be compressed like gas, it can still have an inlet and an outlet distinguished by their diameter, with the wider end being the inlet and the narrower end being the outlet； Centrifugal impellers are generally double-suction types, while non-centrifugal impellers are single-suction types

The explanation on the fourth floor was professional and correct, but it wasn’t relevant to the topic; what was said on the third floor was more on point – the difference lies in the focus of considerations regarding equipment and process design, which results in the pump inlet and outlet sizes not matching those of the process pipelines.

I think it’s mainly to reduce the net positive suction head

Generally, diameter reduction is required at the inlet and outlet of centrifugal pumps. Since the diameter of the centrifugal pump itself does not meet the process requirements, when calculating the pipe diameter, the flow velocity inside the pipe is first set within a reasonable range; the diameter calculated in this way is generally larger than that of the centrifugal pump itself. In other words, if there is no diameter change and the flow velocity inside the pipe is too high, the pressure loss will be significant, which in turn results in high costs for selecting a pump; therefore, centrifugal pumps have diameter changes at both the inlet and outlet. Personal opinion, for reference only. Generally, it is necessary to use reducing tees, and their function is to reduce the flow velocity at the pump’s inlet and outlet. 1. If no reducing tee is used at the inlet, the flow velocity is high, there are many molecules moving in random directions, which makes bubble formation more likely. Reducing tees can be either eccentric or concentric; generally, eccentric tees are used for diameters larger than DN65, while concentric tees are used for smaller diameters. A short section of pipe with a length of 3D should be placed after the reducing tee to provide a buffer for the fluid facing this sudden change in diameter, thereby enabling a stable flow condition at the pump’s inlet. 2. Even without an outlet, the flow rate remains high; for pipes of the same diameter, a high flow rate implies greater resistance, which in turn requires a higher head pressure ； To give a simple example, the last time I saw a company using a centrifugal pump with specifications of 65-50; its flow rate was 12.5 m3/h and its head was 32 M. The distance from the pump’s outlet to the metering tank in the workshop was 120 meters, and the height difference was 18 meters. No reducer was installed at the pump’s outlet, and the liquid was transported through pipes with a DN50 diameter. As a result, it was not possible to pump the liquid to the desired location. I advised them that changing the outlet diameter to DN65 would solve the problem ； If nothing is changed, at least a 50m head is required; can it reach 32m? 3. The inlet and outlet flanges of the pump are generally of a larger diameter size. 4. For pumps with low head, low flow rate, and small diameter, flanges may not be required if the process conditions permit it. By the way, the medium being transported in the examples I mentioned is toluene