|Fig. A: Large wye-pattern globe valve (with gear actuator)|
|Fig. B: Angle globe valve with screwed ends|
|Fig. C: A typical large globe valve with flanged ends|
Types of Globe Valves
Wye Pattern globe valves, among globe valves, offer the least resistance to flow. They can be cracked open for long periods without severe erosion. They are extensively used for throttling during seasonal or startup operations. They can be rod through to remove debris when used in drain lines that are normally closed. Refer to Fig. A.
Angle Pattern globe valves turns the flow direction by 90 degrees without the use of an elbow and one extra weld. They have a slightly lower coefficient of flow than wye-pattern globe valves. They are used in applications that have periods of pulsating flow because of their capability to handle the slugging effect of this type of flow. Refer to Fig. B.
Construction of a Globe Valve
Maintenance of globe valves is relatively easy, as the discs and seats are readily refurbished or replaced. This makes globe valves particularly suitable for services which require frequent valve maintenance. Where valves are operated manually, the shorter disc travel offers advantages in saving operator time, especially if the valves are adjusted frequently.
The conventional disc, in contrast to the plug type, provides a thin contact between the taper of the conventional seat and the face of the disc. This narrow contact area tends to break down hard deposits that may form on the seats and facilitates pressure-tight closure. This arrangement allows for good seating and moderate throttling.
In cast-iron globe valves, disc and seat rings are usually made of bronze. In steel-globe valves for temperature up to 750 F (399 C), the trim is generally made of stainless steel and so provides resistance to seizing and galling. The mating faces are normally heat-treated to obtain differential hardness values. Other trim materials, including cobalt-based alloys, are also used.
The seating surface is ground to ensure full-bearing surface contact when the valve is closed. For lower pressure classes, alignment is maintained by a long disc locknut. For higher pressures, disc guides are cast into the valve body. The disc turns freely on the stem to prevent galling of the disc face and seat ring. The stem bears against a hardened thrust plate, eliminating galling of the stem and disc at the point of contact.
Advantages of a Globe Valve
1. Good shut-off capability
2. Moderate to good throttling capability
3. Shorter stroke (compared to a gate valve)
4. Available in tee, wye, and angle patterns, each offering unique capabilities
5. Easy to machine or resurface the seats
6. With disc not attached to the stem, valve can be used as a stop-check valve.
Disadvantages of a Globe Valve
1. Higher pressure drop (compared to a gate valve)
2. Requires greater force or a larger actuator to seat the valve (with pressure under the seat)
3. Throttling flow under the seat and shutoff flow over the seat
Typical Applications of Globe Valves
1. Cooling water systems where flow needs to be regulated
2. Fuel oil system where flow is regulated and leak tightness is of importance.
4. Feed water, chemical feed, condenser air extraction, and extraction drain systems.
5. Boiler vents and drains, main steam vents and drains, and heater drains.
6. Turbine seals and drains.
7. Turbine lube oil system and others.
Download this article as PDF: Globe Valves Types, Construction, Applications and Advantages