Understandings Needs of Control Valves

Once this is known, the control valve can be selected based on the required flowing quantity of the process. However, the control valve must be selected to operate under several different conditions. Usually the normal flow and maximum flow are used in size calculations, but they should be based on actual operating conditions, whenever possible, without any factors having been applied to then the valve flow coefficient, or Cv is calculated.

  Cv is a constant related to the geometry of a valve, for a given travel, that can be used to establish flow capacity it is the number of gallons per minute of water at a temperature of 60 degrees Fahrenheit that will flow through a valve, with a one pound per square inch pressure drop. In general, the valve size should preferably be one to two sizes smaller than the pipe size.

The Cv value is then used as a guide in the valve selection. But before a valve can be selected, the type of valve that will be used needs to be determined.

Flow Characteristics

The flow capacity of a valve is related to the type of valve through its "inherent characteristics". This is the term which is used to explain the relationship between the valve flow capacity and the valve travel when the differential pressure drop across the valve is held constant. It provides an elegant and systematic way of comparing one valve characteristic design to another.

Typical valve characteristics conducted in this manner are named Linear, Equal-Percentage, and Quick Opening. Linear has a flow capacity that increases linearly with the valve travel : flow capacity in Equal Percentage increases exponentially with valve travel trim travel - equal increments of valve travel would produce equal percentage changes in the existing Cv : and Quick Opening provides large changes in flow for very small change in lift.

Therefore, when conditions of a constant pressure drop exist, the fluid that flows through the valve then become only a function of the valve travel and the design of the valve trim. This implies that control valve capacity changes as the valve moves through its complete travel.

Hence, valve trims can be designed to meet a large variety of control application needs. Because many control loops are inherently non - linear, it is possible to compensate for this by selecting the appropriate control valve trim.

However, when valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the valve plug moves through its travel. Therefore, for purposes of process optimization, the installed flow characteristic of the entire process, including the valve and all other equipment in the loop, must be taken into consideration.

This installed flow characteristic is defined as the relationship between the flow through the valve and the valve assembly input, when the valve is installed in a specific system, and the pressure drop across the valve is allowed to change naturally, rather than being held constant.

In most cases, the installed flow characteristics of a valve would impact its inherent flow characteristics. For example, a linear curve will, in general resemble a quick opening characteristic, while an equal percentage curve will resemble a linear curve.

However, many valve designs particularly rotary ball valves, butterfly valves, and eccentric plug valves, have inherent characteristic which cannot be easily changed. But most globe valves have a selection of valve cages or plugs that can be interchanged to modify the inherent flow characteristic.

The decision finally comes to as to which valve to use for an application. However, there will be occasional exceptions but it is only possible, by means of a complete dynamic analysis to correctly determine the correct valve that may be needed.

Valve Types

Equal percentage flow characteristics are the most commonly used valve control. They are generally used on pressure control applications and on other applications where a large percentage of the pressure drop is normally absorbed by the system itself, with only a relatively small percentage available at the control valve. Valve with such characteristic should also be considered where highly varying pressure drop conditions are to be expected.

The valves best suited for this types of control are globe valves and butterfly valves, but for different types of applications. Globe valves provide for efficient throttling and accurate flow control.

Aira 4matic Global Valve Automation Pvt. Ltd.(Manufacturer and Exporter of Process Control Valves)
Plot No. 174, Shiv Shakti Estate, Near Prince Hotel , Narol Road, Ahmedabad - 382 405. Gujarat. India.
Phone No.: +91 79 2573 0631
Email : 4maticvalves@gmail.com , Web : www.4maticcontrolvalves.com