Choosing a control valve for a particular application is usually used to be simple. It is usually considered only one type of valve (rod shift) for any application. Each manufacturer provided a product suitable for a task and the selection depended on obvious aspects such as cost, delivery, supplier relationships and preferences.

Today considerations are complicated especially for engineers or technicians with limited experience or those who have not kept updated about the development of control valves.

For many applications, there are a variety of ball, rod displacement and butterfly. Some are referred to as "universal" for almost any size and service, while others are considered optimal solution for special needs. As in all decisions, the selection of a valve involves a number of variables. A list of these should include:

• Range of operating pressure and pressure limits of the body

• Size and flow capacity

• Characteristics of flow and turndown

• Temperature Limits

• Environmental factors such as corrosion and abrasion

• Body Materials

• Pressure drop across the valve

• type of mechanical connections

• Life Cycle Cost

• Type of actuator

• Required Accessories

In all these aspects we refer a1 two in particular for purposes of defining the fluid handling by the valve, its capacity and turndown. Then we will see an example of valve sizing calculation involving Cv of it.

Figure representative of Sizing Valves (Chemical & Process Technology Courtesy)

CAPACITY OF A VALVE

Manufacturers have adopted a term to indicate the capabilities of flow variation on the control valves. For this purpose we define the coefficient Cv:

Cv = Q / [(Ap / G) ^ 1 / 2]

Where:

• Q is the volumetric flow through the valve (flow) in gallons per minute

• Dp is the pressure drop through the valve in psi (including losses at the entrance and exit)

• G is the specific gravity of the fluid.

In other words, is the number of gallons per minute of water at room temperature and pass through a restriction with a pressure drop of 1 psi, for example, a control valve which when fully open circulating 25 gpm water with a pressure drop of 1 psi, has a maximum coefficient of 25. The flow is obtained similarly to various valve opening increases and therefore so is the house Cv to increase. It achieves the "characteristic" of the valve.

VALVE turndown

Is defined as the ratio of maximum and minimum flow that can handle a valve. The turndown of the valves varies depending on the valve body used. For example, the turndown globe valves ranges from 30:1 to 50:1, usually. To this turndown is called inherent. As important as the latter is installed or operational turndown. This is defined as the ratio turndown, and pressure drop:

Ro = (q1 / q2) [(ΔP1/ΔP2) ^ 1 / 2]

Where:

• q1 is the initial flow

• q2es final flow

• ΔP1 is the pressure drop across the valve

• ΔP2 is the final pressure drop

VISCOSITY CORRECTION

This is true for non-viscous fluids. In the infrequent cases of high viscosity fluids, it must adjust the calculation of the pressure drop across the valve (AP). The procedure is as follows:

• Calculate the Reynolds number (R)

R = [(3160) (GPM )]/[( D) (centistokes)]

• If R is greater than 2000, then you should make corrections to AP, considered in the following table:

Table 1 Viscosity correction factor

In the next article, we will develop step by step example of sizing a valve.