The casing of fluid machines such as pumps, compressors, blowers, and the like is often produced by casting cast iron, or other metals. However, casings made of cast iron and like metals tend to be damaged by water hammer or freezing and also tend to be broken by the action of external forces at the site of use. In addition to these problems, there is the problem of corrosion resistance: red rust appears and rusting occurs. To avoid this, it is necessary to use high-grade metals such as high-nickel alloys, so the casing is expensive and there are economic problems. Moreover, there are the drawbacks that it is heavy and its installation work is complicated.
Hence, casings made of plastic have recently been developed with a focus on the fact that they have superior corrosion resistance and are lighter than casings made of cast iron. However, conventional casings made of plastic are made of polyester, furan resin, polyurethane, nylon, and similar plastics that are reinforced with glass fibers, i.e., FRP (fiber reinforced plastics), so they have drawbacks.
That is, FRP casings are assured of rigidity because they are strengthened or reinforced with fibers, which is a reinforcing material. But on the other hand, there are the drawbacks in that the mechanical strength is directional because of the orientation of the fibers so pressure resistance to fluid pressure acting in all directions is insufficient. Also, flexibility is lacking so the casing is susceptible to the impact of water hammer. In the case of glass fibers, there is also the drawback that adhesion between the fibers and plastic is not always sufficient so the glass fibers inside the plastic that comprises the casing are corroded by the fluid and the durability of the casing declines. Moreover, because glass fibers are mixed in, there are limits to weight reduction of the casing.
Furthermore, when preparing such FRP casings, the casings are produced by the hand lay-up method which requires manual labor. This introduces the concern that the glass fibers will scratch the hands of the workers at that time. Thus, the working environment is not ideal and along with that, productivity is inferior.
Also, the plastics used in conventional methods have polarity so there are concerns about their water resistance. There is also the problem that dimensional changes in the casing may be too great for the plastics presently used for such applications.
When the present inventors conducted an intensive investigation of plastic casings for fluid machines where pressure resistance is demanded, they discovered that if a specific norbornene type polymer is used as the material of the casing, it has sufficiently high rigidity without mixing in glass fibers, or other reinforcing materials, and the drawbacks that accompany the mixing in of glass fibers can be solved in one stroke. At the same time, the norbornene polymer also excels in impact resistance, water resistance, and chemical resistance. Also, by working out the casing shape, a so-called flexible structure casing that is easily deformed by fluid pressure but has very superior pressure resistance can be obtained.