How to size valves using the KV value

The Kv value, or valve flow coefficient, is one of the most important parameters for identifying the best valve for your system.

As already mentioned in a prevous article dedicated to the pressure loss, the Kv value is obtained from the correlation between the flow rate and the pressure loss that is generated when a liquid passes through a valve. It is defined by the amount of water (expressed in m3/h) that generates a pressure loss of 1 bar between the entry and exit points of a given valve when flowing through it.

The formula used to determine the Kv value, which connects the pressure loss with the flow rate, is Kv = Q / √ΔP, where Q (m³/h) is the flow rate and ΔP (bar) the pressure loss between the valve entry and exit points.

By bringing together flow rate and pressure loss, both of which are essential when sizing valves and, more in general, systems, the Kv value gives important indications that can be used to compare valves of different size, type, and characteristics.

Valve flow coefficient: an ideal instrument for choosing valve sizes

We have chosen to include the Kv value in the technical data sheets of most of our shutoff and check valves because it helps identify the most suitable valve size for the required water flow, essential for system efficiency and stability.

Calculating the Kv value is very simple: find the system flow rate, and divide it by the square root of ΔP, the maximum acceptable pressure loss. The result can then be used to find the most suitable valve in the catalogue, namely the one whose Kv value is closest to the calculated value.

Larger valves naturally have greater flow coefficients, but the Kv value is not tied only to the nominal valve diameter. Not all valves with the same diameter have the same pressure loss: even smaller valves can reach a given flow coefficient if they hold pressure loss. This is the case with our Idrja check valves, studied in detail to be extremely efficient (they are less prone to wear, slim, TIG-welded, and their passage is particularly wide and smooth).

An example of valve sizing using the Kv value

Imagine that a check valve has to be inserted and that the right size needs to be found. Imagine that the system flow rate is 100 l/min = 6 m3/h and that the pressure loss according to the design is 0.1 bar. Apply the formula, and the resulting Kv value is: 6/0.316(square root of 0.1) = 18.98.

At this point we can select the most suitable model from the catalogue. The Idrja check valve series offers a particularly high Kv value, so the flow coefficient can be guaranteed with a 1” model. With the CROMAX AP series, however, whose strong point is not hydraulic efficiency but its resistance to high pressures, a bigger valve is needed. More precisely, the valve size required to guarantee a Kv value of 18.98 is 1½”.