Discs of butterfly valves: designs and materials

The disc inside the butterfly valve controls or shuts off the entire medium flow inside the pipe line. To accomplish this, the disc is turned around its rotational axis into the appropriate position, so that the flow is throttled to suit the require ments. As explained in chapter "Construction types" , the disc is turned in a concentric or eccentric way inside the valve body. When the disc is in the closed position, it bears the full medium pressure, as does the body of the valve, so it is considered a pressure-bearing part.

Some manufacturers of concentric butterfly valves offer a range of different disc diameters to suit lower pressure requirements. In this way, the pressure and wear between disc and body liner can be reduced in order to extend the useful lifetime of these components. Also, the torque requirements are reduced, which leads to the use of smaller and more economic actuators. All in all, this results in reduced costs for the user and longer lifetime. The sealing surface of the disc is always polished to reduce wear and ensure tight sealing. The maximum allowable pressure is engraved on the type plate. The flange connection can be designed independently of the maximum pressure rating, so that, for example, a flange for PN 16 can be combined with a maximum rating of 3 bar (43 PSI) for the disc. Depending on the design and application data, the discs can be supplied in different dimensions and materials.

The torque required for opening and closing the disc is transferred from the shaft to the disc. The shaft can be designed as a single through shaft or can consist of two pieces, which are inserted into the disc from both ends. The torque is transferred from the shaft into the disc via square ends, keys or wedge pins, which connect the shaft and disc. In general, with higher pressure requirements and larger diameters, through-shafts are used to support the disc.

The shaft is mounted in a through-hole in the disc, where it is secured by pins or square ends to transfer the torque. With eccen tric discs, the shaft is located outside the sealing line, in contrast to concentric discs, where the shaft interrupts the sealing line.

To prevent medium from accumulating in small cavities between disc and shaft, it is pos sible to offer a one-piece design, where the shaft is an integral part of the disc. This can be especially important for food and beverage applications as well as for the pharmaceuticals industry. It also eliminates corrosion and makes disassembly of the valve easier. Furthermore, the shaft is blow-out proof and does not need any further devices to prevent it from being ejected from the body. On the other hand, the machining of disc and shaft is more difficult, and a split body is required to make assembly possible. This feature is avail able for concentric valves only.

Basically, it is very easy to manufacture a disc for a concentric butterfly valve. It just re quires a flat piece of material with welded ends to accommodate the shafts. This design is suffi cient for lower pressure requirements, and it can be manufactured quite economically.

To increase the pressure-bearing capacity of the disc, it can be reinforced by ribs or, as a casting, can be designed in a lens-shaped form. These lens-shaped dises, especially with larger diameters, can be hollow inside to save weight. Discs for eccentric valves are some what thicker than concentric discs as they need to provide space for the shaft, which is placed outside the sealing line. Therefore, the flow properties of eccentric valves are reduced com pared to concentric valves.

The appropriate disc material for an appli cation is chosen by the customer and depends on operating parameters such as pressure, tempera ture, corrosion properties, etc. Based on these requirements, materials such as cast iron, stain less steel, or even titanium can be used. In con centric butterfly valves, discs can be coated or covered, for example by PTFE, to enhance cor rosion resistance on the one hand, and keep costs low on the other hand. Otherwise, discs made out of cast iron may only be used for non corrosive media. Stainless steel as a disc/body material offers high chemical resistance against most media, but is more costly to manufacture, as the raw material is significantly more expensive than cast iron, for example. For more de manding applications, a case-by-case evaluation of which material to use is mandatory. For ex ample, to handle chlorine applications, titanium would be the material of choice.

Coating

For concentric valves, coating of the disc is a proven method to achieve improved resistance against corrosion or mechanical wear. In the case of chemically aggressive media, a Halar® coating or another fluoropolymer coating would be suitable. Also, nano-particle coatings would be effective for very sticky media because of the lotus blossom effect, which minimises the particle's adhesion to that surface. For very strong chemical applications, the disc could be coated with different elastomers. In this context, the layer thickness is mostly several millimetres and very important for the protection of the disc. In general, coatings for the disc are identi cal to the liner material (PTFE, PVDF or PFA). Coatings are in many cases very effective. They not only improve resistance or sealing capacity, but can also reduce friction and adhesion properties.

or Covering

If abrasive wear is an issue, coatings of PUR, UHMWPE or Hardox are an excellent option. Discs of eccentric valves are usually not coated, but have a galvanic layer to improve wear properties. For discs of eccentric valves, chrome- or nickel-plating as well as hard facing are options to increase wear resistance, espe cially in combination with metal seats. For applications in the food or beverage as well as in the pharmaceuticals industries, mirror finished discs are an option to reduce the pos sibility of sticking. These discs would also be easier to clean.

In almost all cases, especially with eccentric butterfly valves, the basic material of the disc cannot be used for the seat because of friction properties. This means that the disc is machined or even polished on the seat surface area to pro vide a reliable closing and sealing function. In the case of eccentric valves, a seat system con sisting of graphite and stainless steel is used, mounted in the body outside the disc. For con centric valves, an O-ring is fitted to the disc to provide a tight seal. But in this case, the seal sur face is subject to wear from the medium as soon as the disc is in the open position. With abrasive media, this could create major problems.