When should I choose a butterfly valve instead of a ball valve?

The main difference is temperature resistance versus elasticity. Graphite is relatively soft and deformable, so under pressure it conforms to the sealing faces. A graphite/stainless steel seal offers good resistance to high temperatures and chemicals, but is less elastic than rubber. This seal is therefore suitable for steam and hot liquids. An example is the ORANGEPACK® OP61, a graphite gasket sheet with a perforated 316 stainless steel insert.

A rubber/metal seal combines the elasticity of rubber with the strength of metal. Rubber seals well, but has a more limited temperature range. For higher temperatures and aggressive media, special elastomers are available: FKM (Viton®) is resistant up to approximately 200 °C, and FFKM combines flexibility with very high chemical resistance, but is considerably more expensive.

In short: choose graphite/stainless steel for high temperatures and steam, and rubber/metal when elasticity is the priority within a moderate temperature range.

Both valves are primarily intended to open or shut off a pipeline, not to control the flow precisely. The biggest difference lies in operation. A gate valve is operated with a handwheel that takes several turns, so the gate opens and closes gradually. However, a gate valve is not suitable for precise flow control.

A ball valve is fully opened or closed with a lever in a single quarter turn, and is easy to operate pneumatically (with compressed air). If the flow needs to be controlled precisely, a dedicated control valve is the better choice.