What is a valve
A valve is a device that can be used to control the flow of the fluid, a valve can regulate, direct or control the flow of fluid. The controlling of the flow is done by opening or closing of the passageways. The valve is the major component of the piping system. Valves are able to do certain functions like stop and start the fluid flow, they can vary the amount of fluid flow, the valve can also control the direction of the fluid flow. A valve can relieve overpressure in any component or piping.
What is a control valve and how does it work
Control valves are devices which can control certain conditions like pressure, temperature, flow, and liquid level by opening and closing according to the signal received by the controller, the control valve is also known as the final control element. The controller sends the signal by comparing a set point to a process variable. The values for process variables are provided by sensors and it also monitors the conditions. In-process control industries control valves are the final control element. The control valve can control the flow of the fluid-like gas, steam, water so that it can compensate for the load disturbance.
The closing and opening of a control vale are done automatically with the help of electric, hydraulic or pneumatic actuators. The actuators closing and opening are controlled by positioners and the positioners do this according to the electrical or pneumatic signals. A control valve consists of internal trim parts, valve body, actuators that can provide the required amount of power to operate the valve and certain other accessories like positioners, transducers, pressure regulators, and limit switches. Control valves are used in pipelines to create externally variable and adjustable restrictions.
Explain the operating principle of the control valve
The operation of the control valve is based on the balanced forces between pneumatic forces from the diaphragm against a mechanical force produced by the actuator spring. The control valve operates with a pneumatic signal 3 to 15psi. In case if the pressure is higher than 3 psi which is applied to the diaphragm then it will start overcoming the spring force and moves the diaphragm plate that will move the connected stem and the plug is operated. Control valves also act as a safety device in the process.
What are the required features for a control valve
- Fluid must be contained without external leakage
- It must have adequate capacity for the intended service
- It must be capable to withstand the corrosive and temperature influence of the process
- It must incorporate appropriate end connections so that it can mate with adjacent pipelines
- Actuator attachment is needed, it is the actuator thrust transmission to the rotary shaft or to the valve plug stem
What is the purpose of control valves
The major purpose of a control valve is to provide the means of implementing or actuating a control strategy for given process operation. Control valves are valves that are considered to provide a continuous variable flow area for the purpose of regulating or adjusting the steady-state running conditions of a process. In certain conditions, it must also do mixing or routing of the fluids.
How do you choose a control valve
In order to select a control valve for an application, certain principles must be known they are
- How fluid flow and pressure determine what happens inside a control valve
- Control valves are selected according to how they can vary the pressure and flow conditions in a process
- Types of valves commonly available
- Size and capacity of the valves must be checked for the following
- It must be selected according to how actuators and positioners drive the control valve
- We must ensure the process requirements are properly defined
- The required flow capacity must be calculated over the operating range
- Limitation of the valves must be determined or certain unfavorable conditions like cavitation and noise and how to handle it.
- While using a control valve in a pipeline we must first know, how this control valve creates a pressure drop by looking at the fundamentals of flow in a pipeline and through a restricted area
What are the characteristics of the control valve and their features
The capacity of the control valve is normally measured in CV, flow coefficient which is defined as the number of gallons per minute of water at 60 degrees F that will flow with a 1 psi pressure drop through a valve.
The rangeability of the control valve is the ratio of maximum to minimum controllable flow.
Control valve characteristics
The flow characteristic of the control valve is the relationship between control valve capacity and valve stem travel.
Linear characteristics of the control valve
The flow capacity increases linearly with valve travel in which the valve differential pressure drop is constant over the travel range. Linear valve plug for liquid level control and control application needs constant gain.
Equal percentage characteristic of the control valve
There will be a rapid increase in flow capacity with the valve trim travel. Equal increment in valve travel creates an equal percentage change in the CV. In order to reduce the system, a drop-in control valve equal percentage valve can be used.
Quick opening characteristics
It provides an enormous change in flow for very small changes in the lift or in which a maximum CV is achieved with minimal closure member travel. Valve gain is too high. A small change in valve stroke will create a large increase in inflow.
What are the parts of the control valve
- Stem guide
- Spiral guide plate
- Multiple springs
- Spring adjustment screw
- Travel indicator
- Glands packing
- ,12 Plug
13. Seat ring
What is cavitation in control valve and how to avoid it
If the speed is high in the valve, then the liquid pressure will drop and this will cause the fluid to start bubble or flash. During the recovery of the pressure, the bubble will collapse. Cavitation could be noisy and it would only have low intensity and frequency. Cavitation is harmful to control valves it could wear out the trim and body parts of the control valve. Cavitation can be avoided by using more than one control valve.
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