The double-beat globe valve is an old design which is very useful for special applications. Figure 3.22 shows the basic construction.
Figure 3.22. A double-beat globe valve
Globe valves are very good for low leakage shut-off and regulation. As the valve size and the operating pressure increase, the hydraulic load on the stem becomes greater making the valve difficult to operate manually. This problem was first experienced with steam engines as sizes and steam pressures increased to produce more power. The double-beat globe valve reduces the hydraulic load by balancing the forces on the discs.
The upper disc has inlet pressure on the top; the lower disc has inlet pressure on the bottom. The areas are not quite equal, because of the difference between the inner and outer seat diameters. This difference is very small and produces only a small thrust for large high pressure valves. Operation by ungeared handwheel with rising stem valves can be considered for applications impossible with other valves. They are also very good for use on sites without power or for emergency back-up systems.
As with other globe valves there is some flow resistance, but this may not be as high because two seats are used. Double-beat globe valves cannot be cleaned with rods or “pigged”. Operation with clean fluids is preferred to reduce the possibility of differential seat/disc wear.
Essentially the valve has two pressure casings. The outer casing being at inlet pressure and the inner at outlet pressure. The incoming flow circulates around the casing and flows through both seats into the inner casing. Flow in either direction is possible; sealing in both directions is equal. The discs require a degree of flexibility to ensure both seat properly. The body is relatively complicated and casting is the preferred method of construction. However fabrication and forging is possible if necessary. Connections do not necessarily have to be in-line. They can be at 90° without significantly affecting the flow losses.
Double-beat globe valves are reserved for special applications and probably made to order. Typical applications include main isolating valves for steam and water turbines.