Flow coefficient
The Kv - value is also referred to as a flow factor or a flow coefficient. It is a measure of the recoverable flow rate of a liquid or a gas through a valve and is used for selection and dimensioning of valves. The Kv - value is given in units of m³ / h and can be interpreted as the effective cross-section.
Definition
The Kv - value corresponds to the flow of water through a valve ( in m³ / h) at a pressure difference of 1 bar (just 0.98 bar) and a water temperature of 5 ° C - 30 ° C. Depending on the valve size, it is often expressed in l / min.
A Kv value is only valid for the associated stroke ( opening degree) of a valve. The Kv value of a valve at rated travel (100 % opening degree) is referred to as Kv value. Based on the Kv - value of the maximum possible throughput can be determined at a valve.
For example, a fixed adjustable regulating valve with five adjustment levels has a different Kv value, but only a Kv value, namely the flow at maximum level for each individual stage.
This means, for example, for valves that the Kv - value is used to express the maximum possible throughput of any valve. It features and therefore different valves based on their capacity and is in accordance with DIN IEC 534: the value at the maximum open valve K100 with a tolerance of ± 10 %.
The determination of the Kv value is regulated by the technical rule VDI / VDE 2173 Fluidic characteristic quantities of control valves and their determination.
For liquids the minimum required Kv - value is determined for a valve from the required operating data of the application according to the following equation when the pressure drop is 0.35 to 1 bar:
With:
- Kv = Flow Coefficient
- Q = volume flow
- Ap = pressure difference ( inlet pressure - outlet pressure)
- ρ = density of the fluid
For water as the medium ( ρ = 1000 kg / m³) simplifies the formula to:
Strictly speaking, this relationship applies only to cold water, since the density decreases with increasing temperature. Water at 100 ° C was about 4 % lower density.
With a known Kv value can be the throughput for arbitrary densities and pressure differences determine. The following applies in general ( assuming incompressibility )
This means for example:
In non-metric systems, a Cv value is often given (flow coefficient value). Converted is the Cv value about the 1.16 times the Kv value. (Kv = Cv x 0.86 ).
In the natural gas trade is often counted rather than the Kv value, with the KG- value. The KG- value refers to natural gas with a density ratio of d = 0.64 corresponding to a standard density of = 0.82752 (approx. 0.83) kg/m3 at p1 = 2 bar ( absolute), p2 = 1 bar (absolute ) and a Gaseintrittstemperarur of tu = 15 ° C and T = 288.15 K, according to the European standards EN 334 for gas pressure regulators and EN 14382 for safety shut. The KG value ( m³ / h) / indicated in the unit bar.
The conversion to KG gives = 33.62 x Kv.
General fluidic representation and use of the kv- value
Launched by the company Mason - Neilan Regulator Co., Boston 1953 valve coefficient Cv has prevailed in the field of control engineering with metric units as Kv value quickly and was included in the above-mentioned standards. In the general fluid mechanics it is, however, not common. This should mainly be due to the above definition as a tailored size equation. General validity is the representation of the kv- value as the flow coefficient as follows:
Substituting the values in SI units one, the kv- value has the dimension of an area. Physically, this we understand a corrected flow-through, which is by beam waist, vortex formation, etc. is less than the geometric narrowest cross section. However, it has also with the introduction of SI units this universal, scientifically correct representation not enforced.
Between the above-defined Kv value in m³ / h and the general kv- value in square millimeters, the value is:
The kv - value allows a simple calculation of series and parallel circuits of resistors and flow valves to a resulting kv total value and the conversion to the pressure loss coefficient.
See also
- Valve
- Pressure loss
- Pressure loss coefficient