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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 KVS-value?

In answer this question, we have to take a step back and first take a closer look at the KV-value and the KV100-value:

The KV-value = Flow coefficient

The KV-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 KV 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 KV 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 KV100-value

Another value in this context is the KV100. 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 KVS-value

The pressure loss of valves in the open state is calculated via the valve coefficient KVS. It refers specifically to the series and the valve type and varies depending on the manufacturer. The KVS value 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 KVS value.

Calculation of the KV 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 KV value for gases

When calculating the KV value 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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Andreas Schalwig

Andreas Schalwig

Technical Sales

I’m happy to help you with projects and enquiries in the field of valve technology as well as measurement and control technology. I have more than 15 years of experience in the dimensioning of control valves and actuators. What’s more, I help customers from all over the world to modernise their industrial plants.

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