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Comparison table of the relationship between valve size and medium velocity
Column：Valve knowledge Time：2019-06-22
It is well known that the flow rate and velocity of the valve mainly depends on the size of the valve, but also with the valve structure type of the resistance to the medium...

It is well known that the flow rate and flow rate of the valve mainly depends on the size of the valve, and the valve structure type of the resistance to the medium, at the same time with the pressure of the valve, temperature and the concentration of the medium and other factors have a certain internal relationship.Valve flow area and flow velocity, flow rate has a direct relationship, and flow rate and flow are interdependent two quantities.When the flow rate is constant, the flow rate is large, the flow area can be smaller;Small velocity, the flow area can be larger.On the contrary, the flow area is large and the velocity is small.The runner area is small and the velocity is high.

1, the flow rate of the medium is large, the valve diameter can be small, but the resistance loss is large, the valve is easy to damage.High flow rate, the flammable and explosive media will produce electrostatic effect, resulting in danger;The flow rate is too small, inefficient, and uneconomical.For high viscosity and explosive medium, should be a small flow rate.The flow rate of oil and liquid with high viscosity is selected according to the viscosity, generally 0.1 ~ 2m/s.

2。 Generally speaking, the flow rate is known and the flow rate can be determined by experience。Nominal size of valve can be calculated by flow rate and flow rate。

3, the valve size is the same, its structure is different, fluid resistance is not the same.Under the same conditions, the greater the resistance coefficient of the valve, the more fluid flow through the valve velocity, flow rate drop;The smaller the valve resistance coefficient is, the less the flow rate and flow rate decreases through the valve.

Common flow rates of various media are shown in the following table:

 Fluid name Conditions of use Velocity（m/s） Saturated steam DN＞200DN=200～100DN＜100 30～4025～3515～30 Superheated steam DN＞200DN=200～100DN＜100 40～6030～5020～40 Low pressure steam ρ＜1.0（Absolute pressure） 15～20 Medium pressure steam Ρ=1.0～4.0（Absolute pressure） 20～40 High pressure steam Ρ=4.0～12.0（Absolute pressure） 40～60

 Compressed gas VacuumΡ≤0.3（Gage pressure）Ρ=0.3～0.6（Gage pressure）Ρ=0.6～1.0（Gage pressure）Ρ=1.0～2.0（Gage pressure）Ρ=2.0～3.0（Gage pressure）Ρ=3.0～30.0（Gage pressure） 5～108～1210～2010～158～123～60.5～3 oxygen Ρ=0～0.05（Gage pressure）Ρ=0.05～0.6（Gage pressure）Ρ=0.6～1.0（Gage pressure）Ρ=1.0～2.0（Gage pressure）Ρ=2.0～3.0（Gage pressure） 5～107～84～64～53～4 Gas 2.5～15 Semi water gas Ρ=0.1～0.15（Gage pressure） 10～15 Natural gas 30 Nitrogen Ρ=5～10（Absolute pressure） 15～25

 ammonia vacuumΡ＜0.3（Gage pressure）Ρ＜0.6（Gage pressure）Ρ≤2（Gage pressure） 15～258～1510～203～8 Acetylene water 305～6 Acetylene water ρ＜0.01（Gage pressure）ρ＜0.15（Gage pressure）ρ＜2.5（Gage pressure） 3～44～85 chlorine Gasliquid 10～251.6 氯化氢 气体液体 201.5 Liquid ammonia VacuumΡ≤0.6（Gage pressure）Ρ≤2.0（Gage pressure） 0.05～0.30.3～0.80.8～1.5

 Sodium hydroxide Concentration 0～30%Concentration 30%～505Concentration 50%～73% 21.51.2 Sulfuric acid Concentration 88%～93%Concentration 93%～100% 1.21.2 HCl 1.5 Water has a similar viscosity to liquids Ρ=0.1～0.3（Gage pressure）Ρ≤1.0（Gage pressure）Ρ≤8.0Gage pressure）Ρ≤20～30（Gage pressure）Heat network circulating water, cooling water 0.5～20.5～32～32～3.50.3～1 Water has a similar viscosity to liquids Pressure backwater 0.5～2 Water has a similar viscosity to liquids No pressure backwater 0.5～1.2

 Tap water DirectorΡ=0.3（Gage pressure）BranchΡ=0.3（Gage pressure） 1.5～3.51～1.5 Boiler feed water ＞3 Steam condensate 0.5～1.5 Condensate Gravity 0.2～0.5 Hot water 2 Sea water, slightly alkaline water Ρ＜0.6（Gage pressure） 1.5～2.5

Note: the unit of DN value is mm;Ρ value unit is: MPa.

Examples:

The resistance coefficient of globe valves is much larger than that of gate valves, generally between 4 and 7.Y-type globe valve (direct flow) has a small resistance coefficient between 1.5 and 2.Forged steel globe valve resistance coefficient is large, even up to 8.

Plug valve resistance coefficient is small, usually about 0。4 ~ 1。2。

The resistance coefficient of diaphragm valve is generally around 2。3。

Butterfly valve resistance coefficient is small, generally within 0.5.

Ball valve resistance coefficient is small, generally around 0。1。

Remark: the resistance coefficient of the valve is the value when the valve is fully open。

The selection of valve size should take into account the machining accuracy and dimension deviation of the valve, as well as other factors.Valve size should be adequate, generally 15%.In practice, the size of the valve varies with the size of the process line.