Manual Globe Valves
Globe valves have been around for a long time, being one of the earliest types of valves. They have been used consistently in those services for which globe valves are ideally suited. The only changes over the years have been evolutions in materials and designs that have kept pace with changing conditions of service.For a better understanding of the functions of globe valves it is necessary to highlight one essential difference between globe and gate valves.
Gate valves are designed to completely open the line to flow, or to completely stop the flow. They are therefore, used wide open or fully closed, and are not intended for throttling service.
As opposed to a gate valve, a globe valve provides an additional function - it can be used wide open, fully closed, or with the disk in an intermediate position for regulating the flow. The character of the fluid and the degree of control desired determine the particular design of globe valve that should be used.
The predominant usage of globe valves is found in the three, appreciably different, basic types as shown above (ball-type disk, Fig. 1 (composition disk), and Fig. 2 (plug type disk). There are some modifications and variations of these disk and seat constructions, but they do not affect the fundamental differences or the recommended usages of the three basic types.
Ball-type disk: This is the earliest type of disk and seat construction for globe valves. A ball-shaped disk seats against a tapered, flat-surfaced seat in the body. This type is economical in price, and popular for relatively low-pressure services that are not too demanding in scope. Such a valve preferably should be used wide open or fully shut, but it also can be used for moderate throttling of flow.
Composition disk: The composition disk is an improvement over the ball-type disk for many services. The renewable disks are made from compositions that are varied for specific services, such as cold water, hot water, petroleum products, steam and air. The soft composition disk makes this type of valve ideally suited for compressed air and gas services.
Small particles of foreign matter which would damage a ball-type disk seating upon closure are not likely to damage a composition disk valve because the foreign particles will imbed themselves in the relatively soft disk.
In addition, this valve is easily repairable, which is desirable in valves that are subjected to hard service through frequent operation. The simple replacement of the composition disk usually effects a quick repair, and restores the usefulness of the valve without removing it from the line. The slip-on disk holder and union bonnet supplied in better quality valves of this type expedite such repairs.
Plug-type disk: This is the best of the three types for throttling and hard services. The renewable plug type disks and body seat rings comprise designs and materials which withstand exacting services.
The disk is a cone seating into a cone, producing a wide seating surface that is not easily affected by foreign matter or by wiredrawing in throttling services.
Needle point disk: One modification of the disk-and-seat construction, used in the three basic designs of globe valves, is found in the needle point valve. Fig. 5. The diameter of the seat opening in this valve is reduced. The stem threads are finer than usual, so that considerable turning of the stem is required to increase or decrease the opening through the seat. This type of valve is especially suitable when very close and delicate regulation of flow is desired. Needle valves have many applications in instrumentation.
Materials of Construction
Bronze valves are produced in two bronzes, ASTM Specification B 62 or ASTM Specification B 61. The lower pressure valves are made of the B 62 bronze, whereas the higher pressure valves are made of the B 61 bronze.
Iron body valves are made of two cast irons, ASTM Specification A-126, Class A for valves having light sections; and Class B for valves having medium metal thicknesses.
Steel valves, (some cast, some forged), are available in a variety of steel compositions to ASTM Specifications.
For services of a special nature, we encounter such materials as nickel-bearing irons, nickel-bearing steels, several types of stainless steels, Monel, nickel, Hatelloys, plus titanium, and other alloys.
High Pressure and Temperature
Small valves range in size from 1/2 in. to 2 in., inclusive, and have socket-welding ends. They are regularly made in several alloys for basic pressure ratings of 1500 psi and 2500 psi. The normal temperature limit is in the order of 1050-1060°F, but somewhat higher temperatures can be contemplated for special conditions. Likewise, valves are available on order for pressures beyond the capabilities of the 2500-lb pressure class.
Large valve designs that reflect newer concepts for high pressure and high temperature services, are found in pressure-seal bonnet valves.
Internal fluid pressure acting upon the entire underside of the bonnet reacts upon the smaller contacting area of the wedge-shaped ring, sealing the joint. Because of the design, the sealing pressure is always many times greater than the pressure of the fluid in the line; leakage is precluded. The higher the internal pressure, the greater is the sealing pressure.
Pressure-seal bonnet valves are made in carbon steel, and several alloy steels, for basic pressure ratings of 900, 1500, and 2500 psi. The normal temperature limit is 1050-1060°F, but somewhat higher temperatures may be contemplated for special conditions. Valves are available for pressures beyond the capabilities of the 2500-lb pressure class.
The normal range of sizes for pressure-seal globe valves is 1-in. to 8-in.
Materials for Globe Valve Seats
The simplest form of seating is a disk of the same material as the valve, mating with a machined surface which is integral with the valve body.
Two constructions are used for composition disk valves. The composition disk may seat against an integral seat in the body, or it may seat against a seat ring. These two constructions are found in both bronze and iron valves.
In the construction of all iron valves, the all-iron disk may seat against an integral seat in the iron body, or it may seat against a renewable iron seat ring screwed into the body.
Renewable seats: Moving into the realm of superior types of valve construction, we encounter many designs with the disks and body seat rings made of materials that are different than the valve proper, whether the valve be bronze, iron, or steel. This finds valves with the seating parts made of much better materials than the partent valve metal, not necessarily for the renewability feature but for getting better wear-resisting qualities and longer life in these vital parts.
This type of renewable disk and body seat ring construction is found in bronze, iron, steel, and alloy valves.
The more commonly used renewable seating materials are several nickel alloys; a copper-base, nickel-tin alloy, and a nickel-base copper-tin alloy. Also, a chromium-iron, which is specially processed and heat-treated to develop the desired qualities. For special services, these parts are made of Monel, 18-8 stainless steels, Hastelloys, and Stellite.
Bonnet Construction
Scattered references to bonnet construction have appeared previously but not in any comparative vein. With particular reference to bronze and iron valves, attention should be focused on three types of construction used to connect the body and the bonnet of globe valves.
Screwed bonnet: This is the earliest type and the simplest form of bonnet-body joint. It is sturdy and practical but obviously presents some limitations in dismantling and reassembling. Its use is generally restricted to the lower pressure bronze valves.
Union bonnet: The union bonnet provides a strong, tight joint, yet it can be easily dismantled and reassembled without danger of injury to the valve. The union ring reinforces the bonnet joint, and when the valve is being reground, it centers and guides the valve trimmings, thus simplifying this maintenance operation. Withal, the union bonnet accomplishes its tasks in a minimum of space.
Union bonnet construction is applied to bronze and iron valves in sizes 2 in. and smaller. Sizes 2 1/2 in. and larger have bolted bonnets.
Bolted bonnet: The bolted bonnet more or less takes over where the union bonnet stops. It is used on bronze and iron valves in the 2 1/2-in. and 3-in. sizes and on all sizes of large bronze or iron valves (Fig. 7).
The bolted bonnet assures liberal strength and makes a tight joint. It can be dismantled and reassembled readily and repeatedly without damage to the valve.
Small steel valves: Conventional small steel valves (except bar stock) are equipped with union bonnets or bolted bonnets. However, the application of union bonnets is more limited with respect to the range of sizes so equipped.
The 600-lb steel globe valves are fitted with union bonnets in sizes 3/4 in. and smaller, whereas most bolted bonnet valves on the market are available in 1/2-in. to 3-in. sizes. The 1500-lb steel globe valves have bolted bonnets on all sizes from 1/2-in. to 2-in., inclusive.
Means for Operation
Users can avail themselves of a variety of means for operating globe valves. The conventional means of operation utilizes a hand wheel. Chain wheels can be applied for convenience in operating valves located above a normal reach from the floor or operating levels. Extension stems, with or without floor stands, perform a similar function.
Hammer-blow wheels provide the additional power usually needed for the first effort of unseating and to effect tight closure, in larger steel valves.
Globe valves are available either with or without gearing. The size and pressure class have an important bearing on whether or not gearing is necessary. (See Fig. 7).
Gearing is recommended in the 300-lb class for 8-in. and large globe valves; in 600-lb for 5-in. and larger; in 900-lb for 4-in. and larger; in 1500-lb for 3-in. and larger sizes, when they are used for differential pressures in the upper range of their service ratings.
Motor operation of globe valves finds a wide range of application in piping systems. Its use provides safety in emergencies, saves labor, expedites operation of valves in inaccessible or distant locations, makes possible extreme flexibility in the operation of piping systems. Control of operations can be from one or several stations.
Atomic Energy Valves
Valves for atomic energy services are predominantly produced at present in Type 304 stainless steel. It is necessary to achieve a much better than normal interior finish and maintenance of cleanliness also is very important.
The bulk of production is in valves that are hermetically sealed against outward leakage of radioactive fluids. At all stages of production, these valves and their components are subjected to a series of very exacting nondestructive tests (See Fig. 6).
Throttling Services
Globe valves are installed frequently in services where the sole function is to regulate the volume of flow by throttling. A typical example is the valve in the bypass around a pressure regulator or other type of automatic device.
Care should be exercised in sizing such bypass valves; proper sizing is the key to maximum life and better control of flow. If the bypass valve is too large, the disk will only be cracked off its seat for sufficient flow. Under this condition of close throttling, there obviously will be increased velocity and more wear in the form of wire-drawing of the disk and seat. Conversely, a smaller sized valve will pass the same flow but with the disk farther off its seat in a wider open position, resulting in less wear, better regulation of flow, and a minimum tendency to vibrate.
In larger, higher-pressure globe valves used in the bypass line around boiler feed-water regulators, it is good practice to design the seat opening in the bypass valve for the conditions of flow. Thus it is not unusual to find a 6-in. valve with a reduced parabolic disc in the order of 2 1/2 or 3-in. seat size.
Occasionally, in throttling service, the demand calls for such a wide variation in flow that it is inadvisable to expect one valve to do the job and not suffer from abnormal wear and maintenance shutdowns. The combination of two valves not only reduces wear appreciably but permits closer regulation of flow.
Maintenance Tips
Dirt: The greatest enemy of valves is foreign matter - dirt, scale, chips, weld spatter, and the like. Most troubles with valves are attributable to foreign matter.
Every effort should be made to thoroughly clean out piping before it is placed in service. It will prevent a lot of subsequent maintenance on valves. There is justification also for the installation of strainers in strategic locations. A small investment in a strainer can pay big dividends in protection against the costly ravages of foreign matter.
Closing valves: Never put a wrench on the wheel, or apply any form of leverage; it won't help anything, and you may break the stem or the wheel.
If a globe valve starts to leak, brute force will not make it tight. Instead, bring the disk to its seat gently, back it off a turn, and repeat this operation several times. If a piece of foreign matter has lodged on the seat, this action may flush it away. If this fails, take the valve apart and clean it. If the seating surface is cut, you can refinish or replace these parts.
When a globe valve has been closed while it is hot, it is good practice to follow up later with an extra nudge on the wheel to assure final tight closure. This is particularly advisable when a valve is subjected to temperature and has pressure under the disk. After such a valve is closed, the upper parts (stem, etc) cool and contract slightly, thus reducing the thrust on the disk. A following application of additional compensating thrust is the answer for this situation. A slight extra pull on the handle does the trick.
Leakage: Usually caused by:
- Damage to seating surfaces by foreign matter
- Wire-drawing of seating surfaces due to throttling
- Normal wear in seating due to usage
- Normal wear in stuffing box packing and/or stem
- Bonnet joint leakage
Small leaks become bigger leaks unless repaired promptly. It is equally axiomatic that a valve is no better than its seat.
Conclusion
Globes and angles comprise quite a family of valves, as evidenced by the range of sizes, pressure classes, materials, and connections, types of disks and seats, and other varied details of construction. Users, therefore, can avail themselves of considerable latitude in selections.
The benefits available in great latitude of choice require a degree of forethought in evaluating what a valve is expected to do, how it is to do it, what degree of repairability is wanted, what economies are involved in considering first cost as opposed to final cost. When needed, the background of experience acquired by manufacturers should be used.
Haphazard selections usually produce unwarranted trouble in operation and headaches for maintenance crews. Wise choices produce adequate life and good service.