Flow coefficient and cavitation coefficient of valves
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2014-07-08
1、 Flow coefficient of the valve The flow coefficient of a valve is an indicator of its flow capacity, and the larger the value of the flow coefficient, the smaller the pressure loss when the fluid flows through the valve.
Calculation formula based on KV value
In the formula: KV - Flow coefficient Q - Volume flow rate m3/h Δ P - Pressure loss of valve barP - Fluid density kg/m3
2、 The cavitation coefficient of the valve Using cavitation coefficient δ The value is used to select the valve structure type to be used for controlling flow.
Where: H1- downstream (outlet) pressure of valve mH2- difference of saturated steam pressure corresponding to atmospheric pressure and its temperature m Δ P - Pressure difference before and after the valve, m
Due to different constructions of various valves, the allowable cavitation coefficient δ It's also different. As shown in the figure. If the calculated cavitation coefficient is greater than the allowable cavitation coefficient, it indicates availability and cavitation will not occur. If the allowable cavitation coefficient of the butterfly valve is 2.5, then:
as δ> 2.5, then cavitation will not occur.
When 2.5> δ> At 1.5 hours, slight cavitation will occur.
δ< At 1.5 hours, vibration occurs.
δ< Continuing to use at 0.5 will damage the valve and downstream piping.
The basic characteristic curve and operating characteristic curve of a valve cannot be seen when cavitation occurs, let alone at which point it reaches the operating limit. Through the above calculation, it is clear at a glance. So cavitation occurs because during the accelerated flow of liquid, when the rotor pump passes through a gradually shrinking section, some of the liquid vaporizes, and the resulting bubbles then burst in the open section behind the valve, which has three manifestations:
(1) Noise occurs
(2) Vibration (in severe cases, it can cause damage to the foundation and related structures, resulting in fatigue fracture)
(3) Damage to materials (erosion of valve bodies and pipelines)
From the above calculations, it is not difficult to see that there is a great relationship between cavitation and the pressure behind the valve H1. Increasing H1 will obviously change the situation. Improvement methods:
a. Install the valve at the lower point of the pipeline.
b. Install an orifice plate on the pipeline behind the valve to increase resistance.
c. The valve outlet is open, directly storing water in the reservoir, increasing the space for bubbles to burst and reducing cavitation.
Based on the analysis and discussion of the above four aspects, the main characteristics and parameter lists of gate valves and butterfly valves can be summarized for easy selection. Two important parameters play a crucial role in valve operation.
