requirements often differ depending on the process area, application and end user specifications. Process conditions and the range of parameters also vary widely, including pressure, temperature, controllability, cavitation risk, environmental impact, noise and more. Plus, priorities may differ.
Sizing the valve
When choosing a valve, it is important not to select one that is too big or too small. An oversized valve will not provide the desired control and can cause the system to hunt or cycle. Furthermore, when operated at ten per cent of its flow coefficient over an extended period of time, the seat and closure member may be damaged.
On the other hand, if an undersized valve is used, the desired flow rate will not be achieved even when it is fully open. When a higher pressure is applied to move a higher flow rate across an undersized valve, pump energy use will be unnecessarily high, and the valve can also cavitate and develop flashing.
As a general rule of thumb, a valve should be sized so that it will operate somewhere between 60 and 80 per cent open at the maximum required flow rate, and no less than 20 per cent open at the minimum required flow rate. This allows as much use as possible of the valve’s control range, while maintaining a reasonable safety factor.
It is important to understand well the process including the control valve. one should sufficiently understand the start-up and shut-down of the process itself, including proper conduct in an emergency situation.
Purpose of use:
The control valve is used for different purposes, Control valves are used to control the level in a tank, there are also valves that control a pressure drop from a high-pressure system to a low-pressure system.
There are control valves that control the cut-off and release of fluids, mix two fluids, separate the flow into two directions, or exchange fluids. Therefore, the most appropriate control valve is chosen after determining the purposes of a particular valve.
The time taken to respond to the control valve after changing the manipulation signal is the response time of the control valve. The control valve experiences a period of dead time before the plug stem can overcome friction from the packing and begin to move. There is also a period of operating time needed to move the required distance. It is necessary to consider the effect of these factors on the controllability and safety of the entire system. For good control valve, the response time should be less.
Specific characteristics of the process:
Determine in advance the presence or absence of self-equilibrium, the range of variation in the required flow rate, the speed of response, etc.
The various conditions of the fluid can be obtained from the process data sheet, and these become the basic conditions for the selection of the control valve. The following are the main conditions that will be used:
Name of fluid
Pressure (at both the inlet and outlet ports of the valve)
Density (specific gravity, molecular weight)
The degree of superheating (water vapour)
Many process control environments offer less than ideal conditions for long-term reliability. Moisture-laden atmospheres, corrosive chemicals and regular wash-downs all have the capacity to shorten the service life of a process control valve. One of the potential weaknesses of the actuator is the spring chamber where atmospheric air is drawn in each time the valve operates.
In addition to the above-discussed tips, you can discuss your requirements with the manufacturer. Whenever you are considering these valves for your project, ensure that they are sourced from a reliable supplier like manufacturer Ningbo Lida Pneumatic Complete Sets Co., Ltd, you can click https://www.nblida.com/product/air-treatment/ to learn more information