Why is the dredge pump suction pipe size usually one size larger than the dredge pump Suction size?

1. Why is the size of the pump suction pipe usually one size larger than the pump connection size?
A common practice in engineering applications is that the size (diameter) of the pump suction pipe is at least one size larger than the size of the pump suction flange (or nozzle). This transition is usually accomplished by an eccentric reducer, the top of which is usually level, but not always. Regarding 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 large-scale turbulence that may be caused by the upstream elbow. This is related to the geometry of the pipe, which means it is better to use a longer, straight section of the suction pipe. Thicker pipe reduces pressure drop due to friction and provides greater pressure at the pump suction (impeller suction hole), thus providing more power to the pump.

In the past, due to various reasons, people designed various useless pump suction pipes, some of which even played a certain positive role. However, as a piping designer, you don¡¯t want to always be learning by trial and error, you¡¯re looking for a reliable method that will give you peace of mind.

2. Why are control valves usually one size smaller than the pipe diameter?

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

3. For end-suction centrifugal pumps, does the pump suction always require positive pressure (above atmospheric pressure)?

no. Some pumps are designed to lift liquid up from below the centerline of the pump. There are many different types of pumps that can do this, including small household pumps and large industrial pumps and also dredge pump ,some dredge pump or sand mining pump can suction depth for max 15m ,that is negative pressure .and for dredge booster pump can get the positive pressure for delivery slurry  .
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 like water or slurry , which can avoid liquid overflow and start-up delays when the pump stops running. Second, when the pump stops running, it prevents the medium from flowing back and causing the pump to rotate in reverse.
5. What is the ideal piping route for a pump system?

Unstable pump performance is sometimes blamed on poor piping. Bad pipes are not a common cause, but they can happen. One problem that often arises is air obstruction.
Ideally, the pipe would slope upward from the outlet of the pump to the bottom of the storage tank (tank). This way, any air that enters the pump can be removed from the system.
In the real world, pipes do not slope all the way up, but extend horizontally for long distances. Longer horizontal sections of ducting are acceptable if air pockets or low and high spots can be avoided (in both cases, air can be trapped).

Also, the end of the pipe is rarely connected to the bottom of the storage tank (tank). In this case, the pipe will usually protrude from a higher location. This means there will be a high point that may entrain air. This may or may not be critical to the process/process and should be judged by experienced operators and engineers. If critical to the process/process, a vent valve must be installed/used.
If a control valve is used at the end of the pipeline to control flow, the end of the pipeline should be close to the bottom of the tank to provide a certain back pressure for the valve and reduce the possibility of cavitation.

6.How to measure pump performance? 

You may have doubts about whether your pump is performing well. Your only recourse is to compare the pump performance with the characteristic curve predictions for the correct impeller diameter and pump speed.
You need to install a pressure gauge before and after the pump. The pressure gauge should not be located too far from the intended measurement points (i.e. 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 pressure gauge (or use an oil-filled shock-proof pressure gauge) to help dampen any pressure surges that may occur near the pump. Flow needs to be measured. 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 regular filling of pumped media in a storage tank (water tank) of known volume or other methods. The pressure reading will give you the total pressure head of the pump and based on the flow rate you can compare the results to the characteristic curve at the pump's speed and impeller diameter.
It is possible to measure only the closing head and compare it with the predicted closing head of the characteristic curve. Shut-off head occurs at zero flow, so flow measurement is not required. By checking the shut-off head, you can test whether the pump is running at the correct speed and whether the impeller of the correct diameter is installed.
Measuring efficiency is difficult because a torque meter needs to be installed on the pump shaft.
7. How does liquid viscosity affect pump performance?

The performance or characteristic curve of a pump is determined 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 viscosity reaches or exceeds 400 cst, efficiency drops by 50% and a positive displacement pump should be considered.

8. Can the pump be operated over the entire flow range shown in the characteristic curve? 

cannot. The pump should be operated as close as possible to the BEP (Best Efficiency Point). A typical range is to operate the pump between 80% and 120% of the best efficiency point flow.
Most pump manufacturers discourage operating pumps below 50% BEP flow. If this must be done, there are two options: either install a re-circulation line or install a variable speed drive on the pump.
At the high flow end, due to the high NPSHR of the pump at this time, the pump will be affected by high vibration and potential cavitation. There is nothing to do but run at reduced flow.