Why Is The Size Of The Pump Suction Pipe Usually One Size Larger Than The Size Of The Pump Interface?

1.Why is the size of the pump suction pipe usually one size larger than the size of the pump nterface?

It is common practice in engineering applications that the size (diameter) of the pump suction pipe is at least one larger than the size of the pump suction flange (or nozzle). This transition is usually done with an eccentric reducer, which is usually, but not always, horizontal at the top. With regard to the suction section of the pump, the most critical point is to ensure that the flow line reaches the suction inlet of the pump without the large-scale turbulence that can be caused by the upstream elbow. This is related to the geometry of the pipe, which means that it is better to use a long straight suction pipe. A thicker pipe can reduce the pressure drop caused by friction and provide more pressure at the pump inlet (impeller suction hole), thus providing more energy to the pump.

In the past, for various reasons, people have designed a variety of pump suction pipes, some of which can even play a positive role. However, as a pipe designer, you don't want to be constantly learning from trial and error, you are looking for a reliable way to give you peace of mind. For more information on this, see the article "How to properly design centrifugal pump suction Pipe".

 

2. Why is the control valve usually one size smaller than the pipe diameter?

The main reason: smaller valves cost less and provide better and more precise control than valves with the same pipe diameter, but at the cost of a higher pressure drop.

 

3. For end-suction centrifugal pumps, does the pump inlet always need positive pressure (higher than atmospheric pressure)?

Not really. Some pumps are designed to lift the fluid up from below the center line 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 two main benefits: First, it will keep the system full of media, which can avoid liquid spills and startup delays when the pump stops running. Second, when the pump stops running, it prevents the reverse rotation of the pump caused by the backpouring of the medium.

 

5. What is the ideal pipeline direction of the pump system?

The erratic performance of the pump is sometimes attributed to poor piping. Poor plumbing is not a common cause, but it can happen. A frequent problem is air blockage.

Ideally, starting at the outlet of the pump, the pipe will slope upward until it reaches the bottom of the tank (water tank). In this way, any air that enters the pump can be expelled from the system.

In the real world, the pipe does not slope all the way up, but extends horizontally for a long distance. If air pockets or low and high points (in both cases, air can be trapped) can be avoided, a longer horizontal section of pipe is acceptable.

In addition, the pipe end is rarely connected to the bottom of the storage tank (water tank). In this case, the pipe will usually protrude from a higher position. This means that there will be a high point where air may be trapped. This may or may not be critical to the process/process, and experienced operators and engineers should make this judgment call. If it is critical to the process/process, an exhaust valve must be installed/used.

If a control valve is used at the end of the pipe to control flow, the end of the pipe should be near the bottom of the tank in order to provide some back pressure to the valve and reduce the possibility of cavitation.

 

6. How to measure pump performance?

You may wonder if your pump is performing well. Your only option is to compare the performance of the pump with the predicted value of the characteristic curve at the correct impeller diameter and pump speed.

You need to install a pressure gauge at the front and back of the pump. The pressure gauge should not be too far from the intended measuring point (i.e. the inlet and outlet flanges). The height between the pressure gauge and the center line of the pump should be measured. You will need to install a valve on the gauge (or use an oil-filled shock-proof gauge) to help mitigate any pressure fluctuations that may occur near the pump. Need to measure flow. Ideally, there should be a flow measuring device in the pipeline that can provide this information. If not, other methods must be considered, such as periodic filling of pumping media in a known volume tank (water tank) or other methods. The pressure reading will give you the total pressure head of the pump, and depending on the flow rate, you can compare the results with the characteristic curve at the pump's speed and impeller diameter.

Only the closing head can be measured and compared with the predicted closing head of the characteristic curve. The cut-off head occurs at zero flow, so flow measurement is not required. By checking the closed head, it is possible to test whether the pump is running at the correct speed and whether the impeller with the correct diameter is installed.

Efficiency measurement is more difficult because of the need to install a torque meter 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 by using water under standard conditions. Liquids with a higher viscosity than water can affect pump performance. Total head, flow and power are adversely affected.

When the viscosity reaches or exceeds 400 cSt, the efficiency will drop by 50%, at which time the use of positive displacement pumps should be considered.

 

8. Can the pump operate within the entire flow range shown in the characteristic curve?

No. The operation of the pump should be as close as possible to the BEP (optimum efficiency point). The typical range is to run the pump between 80% and 120% of the optimal efficiency point flow.

Most pump manufacturers do not encourage pump operation at less than 50% BEP flow. If this must be done, there are two options: either install a recirculation line or install a variable speed drive on the pump.

At the high flow end, the pump will be subject to high vibration and potential cavitation due to the high NPSHR of the pump. There is no alternative but to run with reduced flow.