Control Valves Interview Questions
why are equal percent flow characteristics better than linear?
For equal percent flow characteristics, the Cv increase is about 43% for each 10% of travel. That means the flow increases exponentially, providing better control and high rangeability. Please note that below 5% of travel it is difficult to achieve good control. Useful terms:
- Inherent flow characteristic
- Installed flow characteristic
what process data do i need to size a valve?
FOR LIQUIDS -
Flow rate, inlet pressure (P1), pressure drop (∆P), design pressure, design temperatures, inlet and outlet temperatures, vapour pressure (Pv), critical pressure, specific gravity or density and viscosity.
FOR GASES -
In addition to the above – with the exceptions of vapour pressure (Pv), critical pressure and viscosity – you will need molecular weight, ratio of specific heats and compressibility.
Ideally, a better sizing and selection can be made if you have the flow cases: i.e. maximum flow, normal flow, minimum flow.
what is the rule of thumb for valve size compared with line sized?
It is good practice to have the valve not less than half line size. If the valve size is very small compared to the line size, too much stress may be created on the valve neck, which can be caused by twisting of the pipes, misalignment of the drilled holes and thermal expansion of the pipe. In addition, there will be some pressure loss within the system due to pipe reducers.
What is KV?
Kv is a flow coefficient expressed in MKS units. It is obtained by dividing Cv by 1.16 (Kv=Cv/1.16).
What is the typical pressure loss across the valve?
Typically 0.3 bar for a correctly sized valve
What is the advantage of a parabolic plug design?
A parabolic plug design gives you a more accurate regulation and stability at low flows
What is the advantage of independent adjustment of opening and closing speed?
You can eliminate fluctuations within sensitive networks, and avoid water hammer and surge problems. You also get full control on level control valves. The valve can close fast and open slowly, and vice versa.