The KVS value: How to define and calculate

Whether in the chemical and petrochemical industries, in food production or in power plants and refineries: Control valves, be they 2-way or 3-way valves, are installed in a wide variety of plants. They are used to control any kind of flow, e.g. of liquids, steam or gas. For optimum control behavior, the design and dimensions of a valve must be precisely determined. The KVS value is one of the most important parameters for this.

What is the K_VS-value?

In answer this question, we have to take a step back and first take a closer look at the K_V-value and the K_V100-value:

The K_V-value = Flow coefficient

The K_V-value of a valve describes the flow rate of liquid or gas at a pressure drop of one bar. This value is also called flow factor or flow coefficient.

An example (with water flow):

If you want to determine the K_V value of a simple valve manually, you regulate the pressure reducer at the installation down to one bar. The valve to be tested is mounted directly behind the pressure reducer. The outflowing water is now collected and the volume determined.

Important: Not only the volume, but also the time in which, for example, ten liters of water flow out of the valve is critical.

In detail, this means that if ten liters of water have flowed out after about 20 seconds, there would be a much higher K_V value than if the volume had only run out after 30 seconds. To be more precise, a pressure drop at the valve of one bar would fill a ten-liter bucket in 20 seconds. Converted to one hour, this results in a volume of 1,800 l/h or 1.8 m³/h.

In summary: The KV value is independent of the nominal size and describes the volume flow rate of a liquid through a component in m³/h at a pressure drop of one bar.

The K_V100-value

Another value in this context is the K_V100. This describes the flow coefficient at a degree of opening of 100%, i.e. when the valve is completely open. This can be used to determine the maximum possible flow rate for a valve.

The K_VS-value

The pressure loss of valves in the open state is calculated via the valve coefficient K_VS. It refers specifically to the series and the valve type and varies depending on the manufacturer. The K_VSvalue corresponds to the water flow through an open valve (in m³/h), at a pressure difference of 1 bar and a water temperature of 5 - 30 °C.

Good to know: When sizing a flow valve, a design reserve of 20 - 30 % should be added to the K_VSvalue.

Calculation of the K_V value for liquids

Essential for the calculation of the flow coefficient for liquids are the volume flow, the density of the medium and the pressure loss. The formula for this is:

Calculation of the K_V value for gases

When calculating the K_Vvalue for gaseous media, a distinction is made between a subcritical and a supercritical flow condition.

Subcritical is given when the flow rate depends on p₁ (= abs. pressure upstream of the valve) and p₂(abs. pressure downstream of the valve). In a supercritical condition, the flow rate depends only on p₁, given by the narrowest point in the valve.

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The KVS value: How to define and calculate

What is the KVS-value?

The KV-value = Flow coefficient

The KV100-value

The KVS-value

Calculation of the KV value for liquids

Calculation of the KV value for gases