frequently asked questions
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.
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.
Kv is a flow coefficient expressed in MKS units. It is obtained by dividing Cv by 1.16 (Kv=Cv/1.16).
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
The flow characteristics only apply when:
- the pressure drop remains constant with the flow
- there is no onset of flashing, cavitation or sonic flow
- the valve is not installed between reducers
- the flow curves normally have a tolerance band of +/-5%
Kent Introl looks at each customer’s application requirements and sizes the valves accordingly. However, there are standard rules for certain types of trim, as follows:
Cage-guided trims
The preferred flow direction for liquids is over the plug and for gases is under the plug.
Top-guided trims
Such as contoured and microspline, are always flowed over.
On high pressure-drop gas applications, KKI would carefully study the Joule-Thomson effect and advise on the materials and trim design to avoid icing or material failure caused by low temperatures.
KKI uses state-of-the-art engineering tools to analyse flow erosion mathematically before specifying any solution. KKI developed its patented HF-LCV trim 20 years ago and successfully used it in applications with high sand-particle contamination within the fluid. Angle style valve bodies are normally recommended, but if this is impractical then a globe style valve body with a sacrificial seat exit diffuser is used to protect the valve body from flow erosion.
Kent Introl manufactures the ‘Vector’ velocity control trim. This velocity control trim limits harmful flow velocities by separating the flow into smaller individual channels, staging the full pressure drop across multiple turns in the fluid path. Increasing the drag within the trim results in much lower velocities, which are well within the erosion threshold for most trim materials. In addition to the Vector trim, Kent Introl manufactures HFQ class trims which enable the optimum flow area to increase as the flow passes through each stage of the trim, resulting in a very low trim exit velocity and very high levels of noise attenuation.
