Tuesday, October 15, 2013

Globe Valves Types, Construction, Applications and Advantages

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Conventional globe valves may be used for isolation and throttling services. Although these valves exhibit slightly higher pressure drops than straight through valves (e.g., gate, plug, ball, etc.), they may be used where the pressure drop through the valve is not a controlling factor. 

Also, wye-pattern (Fig. A) 
Fig. A: Large wye-pattern globe valve (with gear actuator)
and angle-pattern (Fig. B)
Fig. B: Angle globe valve with screwed ends
globe valves exhibit improved flow characteristics over the tee-pattern (Fig. C) globe valve. 

Fig. C: A typical large globe valve with flanged ends
Because the entire system pressure exerted on the disc is transferred to the valve stem, the practical size limit for these valves is NPS 12 (DN 300). Globe valves larger than NPS 12 (DN 300) are an exception rather than the rule. Larger valves would require that enormous forces be exerted on the stem to open or close the valve under pressure. Globe valves in sizes up to NPS 48 (DN 1200) have been manufactured and used.

Globe valves are extensively employed to control flow. The range of flow control, pressure drop, and duty must be considered in the design of the valve to avert premature failure and to assure satisfactory service. Valves subjected to high-differential pressure-throttling service require specially designed valve trim. Generally the maximum differential pressure across the valve disc should not exceed 20 percent of the maximum upstream pressure or 200 psi (1380 kPa), whichever is less. Valves with special trim may be designed for applications exceeding these differential pressure limits.

Types of Globe Valves

Tee Pattern globe valves have the lowest coefficient of flow and higher pressure drop. They are used in severe throttling services, such as in bypass lines around a control valve. Tee-pattern globe valves may also be used in applications where pressure drop is not a concern and throttling is required. Refer to Fig. C.

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

A typical large globe valve with flanged ends is illustrated in Fig. C, and a large wye-pattern globe is illustrated in Fig. A. Globe valves usually have rising stems, and the larger sizes are of the outside screw-and-yoke construction. Components of the globe valve are similar to those of the gate valve. This type of valve has seats in a plane parallel or inclined to the line of flow.

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 principal variation in globe-valve design is in the types of discs employed. Plug-type discs have a long, tapered configuration with a wide bearing surface. This type of seat provides maximum resistance to the erosive action of the fluid stream. In the composition disc, the disc has a flat face that is pressed against the seat opening like a cap. This type of seat arrangement is not as suitable for high differential pressure throttling.

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

The following summarizes the advantages of globe valves:

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

The following are some shortcomings inherent in globe valves:

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

The following are some of the 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.

3. High-point vents and low-point drains when leak tightness and safety are major considerations.

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.
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