The linear characteristic valve plug is shaped so that the flowrate is directly proportional to the valve lift (H), at a constant differential pressure. A linear valve achieves this by having a linear relationship between the valve lift and the orifice pass area (see Figure 6.5.
Why an equal percentage valve is called as equal percentage valve?
Even though the constant differential pressure across the valve calls for a linear control valve, this process calls for an equal percentage valve. At low process flow rates, the outlet temperature is very sensitive to changes in steam flow.
What makes a control valve become linear equal percentage or quick opening control valve?
In most applications, when the valve opens, and the resistance due to fluids flow decreases the pressure drop across the valve. This moves the inherent characteristic: A linear inherent curve will in general resemble a quick opening characteristic . An equal percentage curve will in general resemble a linear curve.
Why are equal percentage valves generally selected over linear and quick opening valves?
Equal percentage – flow capacity increases exponentially with valve trim travel. Equal increments of valve travel produce equal percentage changes in the existing Cv. ... Quick opening provides large changes in flow for very small changes in lift . It usually has too high a valve gain for use in modulating control.
What is equal percentage in the equal percentage valve?
Equal Percentage Valves: Equal percent (= % ) valves are used on heat transfer processes because in case of heat transfer (temperature control), when the heat load is low, the heat transfer surface area available to transfer that heat (load) is large.
What are the 3 most common control valve flow characteristics?
There are three most common control valve characteristics viz Linear, Equal % and Quick-Opening .
What is the working principle of control valve?
What are the Working Principles of a Control Valve? Pneumatic Actuated . Pneumatic Actuators use an air or gas signal from an external source to produce a modulating control action . The actuator receives the force of the pneumatic signal through a top port. Then, it distributes the signal across the actuator’s diaphragm ...
How many types of control valves are there?
The two types of control valves are linear motion and rotary motion.
How do you determine control valve characteristics?
- Type of fluid to be controlled.
- Temperature range of fluid.
- Viscosity range of fluid.
- Specific gravity range of fluid.
- Minimum and maximum flow required.
- Minimum and maximum inlet pressure at the control valve.
- Minimum and maximum outlet pressure at the control valve.
Which of the following valves is better for on off control?
Which of the following valves is better for on/ off control? Explanation: Butterfly valve is for flow regulation. Plug valve involves pressure drop during on/ off control. Ball valve involves on/ off control without pressure drop.
Which is a quick opening valve?
Quick Open : A quick opening valve plug produces a large increase in flow for a small initial change in stem travel . Near maximum flow is reached at a relatively low percentage of maximum stem lift. Quick open plugs are used for on-off applications designed to produce maximum flow quickly.
What are installed valve characteristics?
The installed characteristic is the relationship between valve position and flow in the specific system being considered , taking into account any changes in the pressure differential available to the control valve due to the approximately flow squared relationship between flow and piping pressure losses and/or a ...
What do you understand flow lift characteristics of control valves?
- Linear – flow capacity increases linearly with valve travel.
- Equal percentage – flow capacity increases exponentially with valve trim travel. ...
- A modified parabolic characteristic is approximately midway between linear and equal-percentage characteristics.
What does equal percentage mean?
An equal percentage characteristic is when the valve Cv increases by a constant percentage over a given travel value . For example, the Cv may increase by 50% for every 10% increase in valve travel.
What type of valve is a control valve?
A control valve is a valve used to control fluid flow by varying the size of the flow passage as directed by a signal from a controller. This enables the direct control of flow rate and the consequential control of process quantities such as pressure, temperature, and liquid level.
How do you calculate valve CV?
Cv by definition is the number of gallons per minute (GPM) a valve will flow with a 1 psi pressure drop across the valve. For example a valve with a Cv of 10 will flow 10 GPM with a 1 psi pressure drop. The formula used to select the valve Cv with the specified differential pressure is: Cv=GPM/((SQ RT(∆P)).
