Butterfly Valve Selection Principle

Selecting the right type of butterfly valve and materials for various applications is crucial for ensuring optimal performance and longevity. This comprehensive guide explores the selection criteria for different types of butterfly valves and materials based on specific operating conditions and requirements.

Selection of Butterfly Valve Types

1.The flow characteristic of butterfly valves is approximately linear, whether it is a concentric or an eccentric type, the difference is not significant. The concentric type is suitable for regulating and small-caliber process valves.

2.How to choose between double eccentric and triple eccentric butterfly valves? Both of these structures have the characteristics of quick detachment and tighter closure. So, how should these two structures be chosen?

Double eccentric is more convenient in terms of processing and manufacturing for soft sealing. On the other hand, the manufacturing process of triple eccentric is more complex, but its characteristic of tighter closure is more prominent, and it is less prone to interference. Therefore, it is more suitable for use in metal hard sealing.

Currently, there is no comprehensive analysis in the market. It is not correct to recommend the use of triple eccentric metal hard sealing in all situations, especially in large-diameter normal temperature water systems. There is no need for it. Because replacing metal hard sealing rings is much more difficult than replacing soft sealing rings, soft sealing should be used for large-diameter water valves in water engineering. Only in situations where maintenance and replacement are inconvenient, and there are higher temperatures and harsh operating conditions, should metal hard sealing be used. In order to facilitate the replacement of metal sealing rings in harsh conditions, an extended section is generally added to the valve body, and a middle port is opened to facilitate the disassembly of the sealing ring (see Figure 1).

Figure 1 Butterfly valve for easy seal removal

3.Selection of Vacuum Butterfly Valves

Vacuum butterfly valves are available in two types: rubber soft sealing and metal hard sealing. Structurally, they can be concentric, eccentric, or lever type. Generally, for applications requiring high vacuum, soft sealing is preferred. Metal sealing is necessary for applications with certain temperatures. To achieve good results, the machined surface should undergo processing with a good surface roughness and cleanliness. Painting or applying general rust-proof oil is not allowed; only vacuum grease is permitted.

II. Selection of Butterfly Valve Diameter

The selection of butterfly valve diameter is mainly determined by the medium flow, flow rate, or velocity passing through the valve.

For liquids, the flow velocity should generally not exceed 5m/s, with a maximum of 7-8m/s. Typically, the economic flow velocity for water is 2-3m/s, while for low-pressure gases it is recommended between 2-10m/s, and for medium-pressure gases between 10-20m/s. For steam, the recommended velocities are low-pressure steam at 20-40m/s, medium-pressure steam at 40-60m/s, and high-pressure steam at 60-80m/s.

Sometimes, to reduce pressure loss, a larger valve may be selected to lower the flow velocity. However, this may not be suitable for precise control at low flow rates.

Cavitation, a phenomenon of rapid evaporation and condensation of liquid in a low-pressure area, may occur when a fluid flows through an area with extremely low absolute pressure. This phenomenon does not occur in airflow because the state of the gas remains unchanged under low pressure. Measures should be taken to prevent cavitation.

Limit the flow velocity to prevent extremely low pressure.

Introduce atmospheric pressure into the low-pressure area.

Reduce the pressure difference (ΔpT≤FL2(p1-pVC)), where pVC represents the pressure on the throttling section caused by the obstructed flow.

When the pressure difference on the valve is less than 1.5MPa, even if cavitation occurs, it does not cause severe damage to the material, so no special measures are needed.

From a material perspective, harder materials generally have better resistance to cavitation.

Butterfly valves generally have a cavitation coefficient σ≥2.5 when cavitation does not occur.

III. Selection of Butterfly Valve Materials

The main material of the butterfly valve body should be determined based on the properties of the medium, operating temperature, and flow conditions (presence of particles, two-phase flow, etc.). Under normal circumstances, users are required to specify the material of the valve body, especially for special or harsh operating conditions. In most cases, the material of the valve body is the same as or slightly higher than that of the pipeline.

The applicable temperatures for steel valves can be found in ANSI B16.34. Carbon steel is undoubtedly a commonly used economical material, with a temperature resistance of up to 425°C. Cast iron and ductile iron are widely used in low-pressure valves. The temperature limits for general valve body materials are listed in Table 1.

Table 1 Temperature limits for valve body materials

For valves resistant to seawater, different requirements dictate various approaches based on the operating conditions and importance:

1.Manufacture using titanium alloy.

2.Utilize duplex stainless steel.

3.Incorporate rubber lining or polytetrafluoroethylene (PTFE).

4.Apply epoxy or ceramic coatings.

5.Employ low-nickel cast iron or low-alloy cast iron.

Sometimes, a combination of the above materials is also used for cross-composite applications. Typically, lightweight quality is required on ships, making titanium alloy preferable, while ordinary power plants and chemical plants may opt for lining or low-alloy ductile iron with coatings.