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The flow rate and flow velocity of a valve mainly depend on its nominal diameter, and are also related to the resistance of the valve structure to the medium. In addition, they are inherently associated with many factors such as pressure, temperature and medium concentration.
The flow area of a valve has a direct relationship with flow velocity and flow rate, and velocity and flow rate are interdependent quantities.When the flow rate is constant, a higher velocity allows a smaller flow area, while a lower velocity requires a larger flow area.Conversely, a larger flow area results in a lower velocity, and a smaller flow area results in a higher velocity.
A high medium flow velocity allows a smaller valve nominal diameter, but leads to greater pressure loss and makes the valve prone to damage.High velocity can generate static electricity for flammable and explosive media, creating hazards.Conversely, an excessively low velocity results in low efficiency and poor economy.For viscous and explosive media, a lower flow velocity should be selected.For oil and other high-viscosity liquids, the flow velocity is chosen according to the viscosity, generally in the range of 0.1–2 m/s.
Generally, the flow rate is known, and the flow velocity can be determined by experience.The nominal diameter of the valve can be calculated from the flow velocity and flow rate.For valves with the same nominal diameter but different structural types, the flow resistance will be different.Under the same conditions, the larger the valve flow coefficient, the more the flow velocity and flow rate decrease when fluid passes through the valve;the smaller the valve flow coefficient, the less the flow velocity and flow rate decrease.
List of Typical Flow Velocities for Various Media
| Fluid Name | Operating Conditions | Flow Velocity |
| Saturated Steam |
DN>200 DN=200~100 DN<100 |
30~40 25~35 15~30 |
| Superheated Steam | DN > 200 DN = 200~100 DN < 100 |
40~60 30~50 20~40 |
| Low-Pressure Steam | p < 1.0 (absolute pressure) | 15~20 |
| Medium-Pressure Steam | P = 1.0~4.0 (absolute pressure) | 20~40 |
| High-Pressure Steam | P = 4.0~12.0 (absolute pressure) | 40~60 |
| Compressed Gas | Vacuum P ≤ 0.3 (gauge pressure) P = 0.3~0.6 (gauge pressure) P = 0.6~1.0 (gauge pressure) P = 1.0~2.0 (gauge pressure) P = 2.0~3.0 (gauge pressure) P = 3.0~30.0 (gauge pressure) |
5~10 8~12 10~20 10~15 8~12 3~6 0.5~3 |
| Oxygen | P = 0~0.05 (gauge pressure) P = 0.05~0.6 (gauge pressure) P = 0.6~1.0 (gauge pressure) P = 1.0~2.0 (gauge pressure) P = 2.0~3.0 (gauge pressure) |
5~10 7~8 4~6 4~5 3~4 |
| Coal Gas | 2.5~15 | |
| Semi-Water Gas | P = 0.1~0.15 (gauge pressure) | 10~15 |
| Natural Gas | 30 | |
| Nitrogen | P = 5~10 (absolute pressure) | 15~25 |
| Ammonia Gas | Vacuum P < 0.3 (gauge pressure) P < 0.6 (gauge pressure) P ≤ 2 (gauge pressure) |
15~25 8~15 10~20 3~8 |
| Acetylene in Water | 30 5~6 |
|
| Acetylene Gas | ρ < 0.01 (gauge pressure) ρ < 0.15 (gauge pressure) ρ < 2.5 (gauge pressure) |
3~4 4~8 5 |
| Chlorine | Gas Liquid |
10~25 1.6 |
| Hydrogen Chloride | Gas Liquid |
20 1.5 |
| Liquid Ammonia | Vacuum P ≤ 0.6 (gauge pressure) P ≤ 2.0 (gauge pressure) |
0.05~0.3 0.3~0.8 0.8~1.5 |
| Sodium Hydroxide | Concentration 0~30% Concentration 30%~50% Concentration 50%~73% |
2 1.5 1.2 |
| Sulfuric Acid | Concentration 88%~93% Concentration 93%~100% |
1.2 1.2 |
| Water & Similar Viscous Liquids | P=0.1~0.3 (gauge pressure) P≤1.0 (gauge pressure) P≤8.0 (gauge pressure) P≤20~30 (gauge pressure) District heating circulating water, cooling water Pressure return water Non-pressure return water |
0.5~2 0.5~3 2~3 2~3.5 0.3~1 0.5~2 0.5~1.2 |
| Tap Water | Main line P=0.3 (gauge pressure) Branch line P=0.3 (gauge pressure) |
1.5~3.5 1~1.5 |
| Boiler Feed Water | > 3 | |
| Steam Condensate | 0.5 ~1.5 | |
| Condensate Water | Gravity flow | 0.2 ~ 0.5 |
| Superheated Water | 2 | |
| Seawater / Slightly Alkaline Water | P < 0.6 (gauge pressure) | 1.5 ~ 2.5 |
Notes:
Unit of DN: mm
Unit of P: MPa
Gate valves have a low flow resistance coefficient, only in the range of 0.1–1.5.
Large-size gate valves have a resistance coefficient of 0.2–0.5.
Reduced-bore gate valves have a relatively higher resistance coefficient.
Globe valves have a much higher resistance coefficient than gate valves, generally between 4–7.
Y-type (straight-through) globe valves have the lowest resistance coefficient, at 1.5–2.
Forged steel globe valves have the highest resistance coefficient, even up to 8.
The resistance coefficient of check valves depends on the structure:
Swing check valves are normally about 0.8–2, among which multi-disc swing check valves have a higher resistance coefficient.
Lift check valves have the highest resistance coefficient, up to 12.
Plug valves have a low resistance coefficient, usually about 0.4–1.2.
Diaphragm valves generally have a resistance coefficient of around 2.3.
Butterfly valves have a low resistance coefficient, generally within 0.5.
Ball valves have the lowest resistance coefficient, generally around 0.1.
The above resistance coefficients are for valves in the fully open position.
