Straight globe valve
These globe valves, with flat discs and seats, are sometimes called "screw down stop valves". Straight globe valves are called "straight" because the connections are in line. However the flow path is far from straight and is fairly tortuous. The flow must change direction through 90 ~ in order to pass through the seat and then another 90 ~ to return to the original direction. The shape and area of the passages in cast valves varies considerably with valve size and pressure rating. A typical construction is shown in Figure 3.14.
Valves with closed die-forged bodies or machined from barstock can have severe limitations placed on the port sizes. Port machining is only possible through the connections and the bonnet opening. Inlet and outlet passages must be drilled at an angle through the connection openings. The small diame- ters possible increase the fluid flow losses. Additionally, the sharp corners formed by manufacture may induce cavitation in liquid applications.
Some valves are described as "full-bore" and "reduced port" but this is irrelevant. Check fluid losses on a case by case basis.
Globe valves can be used for flow regulation as well as isolation but this extra facility is at the cost of increased pressure drop. Flow losses are higher than gate valves but sealing can be better. Because of the internal passage construction, globe valves cannot be cleaned with rods. They are also not recommended for solids handling applications or dirty fluids. Globe valves should only be operated with flow passing up through the seat. Flow in the other direction will create much higher flow losses.
Straight globe valves can have cast or closed die-forged bodies or be machined from barstock. Some forged valves have flanges attached by friction or butt welding, so check weld quality assurance. The seat is at right angles to the centre-line of the connections so that all assembly can be performed through the bonnet opening. Seats are usually screwed in. A special key is required to engage with flats in the seat bore.
The disc can be one of two types -- flat or crow-foot guided with shaped ports. The flat disc type has lower pressure losses but is not as good for flow regulation. The flat disc creates high velocities across the full seat periphery when throttling. The crow-foot guided disc divides the flow stream into multiple small streams with consequently higher losses at lower velocities.
The crow-foot guided disc is bottom guided, as well as top guided from the stem, and is a more rigid assembly. The flat disc tends to deflect the stem away from the seat centre due to fluid circulating from the inlet side to the outlet. This circulation is worst when the valve is throttling, creating high velocities. Guided discs can be fitted with an isolation seating which is separate from the throttling portion. The disc should be attached to the stem with a self-aligning connection so that closing forces are applied to the seat evenly. The disc does not rotate against the seat and there are no galling tendencies between similar materials. Small brass and bronze globe valves may have an integral hemispherical tip on the stem and no disc.
Some steel valves may have an option for a needle-shaped disc for better regulation characteristics. Steel valves will generally have a bolted bonnet with low pressure bonnets located by a spigot and high pressure ones by a ring-type joint. Special valves may have a clamped connection.
A substantial backseat bush is necessary to provide good guidance for the stem. Some valves can achieve a good seal between the backseat bush and the stem when wide open to allow repacking the box on-line. Bolted glands are common with reinforced graphite packing. Live-loaded glands, using disc spring stacks, may be optional. Outside screw types with rising stems are standard. Geared hand wheels may be standard over DN100 depending upon the pressure rating.
Brass and bronze valves, with screwed bonnets/glands and connections, suitable for 10 barg are used extensively for clean water applications. Popular cast and forged steel straight globe valve ranges are listed in Table 3.3.
Larger valves are available, and alternative materials can be used. The standard material trim is 11/13 Cr but some valves specifically intended for continuous regulation, such as those for balancing circuits, can have 17 Cr precipitation-hardened seats and disc. The best versions of these valves will have skirt-guided or crow-foot guided discs.
Small bar stock globe valves can be over-centre toggle operated, see Figure 3.15.
The stem is spring-loaded from the bonnet to close the valve. Unidirectional operation is specified with the incoming fluid entering under the fiat plug which is fitted with a trapped PTFE soft seal. Shut-off capabilities are limited by the spring strength. Maximum pressure is only 20 barg. There is no packing fitted. The plug is sealed in the burnished bonnet by a Viton TM "O" ring. Toggle operated valves can be bellows sealed. Slightly larger globe valves with hardened stainless steel ball tips for pressures up to 414 barg are made in sizes from 1/8” to ½”NPT, ¼”to ¾” od compression fitting ¼”to ½” nb socket weld.
Small valves can utilise metal diaphragms to seal the bonnet.
Figure 3.16 shows a straight globe valve with bellows sealed bonnet.