What is a globe valve?
Globe valves obtained their name due to the shape of their spherical valve body, but this is slightly deceptive as globe valves are not the only type of valve with this body shape.
Globe Valve (globe shaped body)
Globe valves are some of the most common valves currently employed. Other common types include the plug, butterfly, ball and gate type valves.
Globe Valve Designs
Globe valves are linear actuated valves and have four main designs:
- Straight Flow - Z-shaped body. Most common type.
Straight Flow
- Angle Flow - r-shaped body. Lower pressure drop than Z-type due to less right angle flow turns.
- Cross Flow - T-shaped body.
- Oblique Flow - Y-shaped body. Lower pressure drop than Z-type. Suitable for higher pressures.
Straight, angle, cross and Y-Flow Globe Valves
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Globe Valve Discs
Globe valve discs utilise three basic disc designs:
- Ball Disc – suitable for low temperatures and low pressures. Ideal for start and stop operations. Ill-suited for flow regulation.
Ball Disc
- Composition Disc – composites by definition consist of several components. The disc is almost always manufactured from metal whilst a non-metallic ring is mounted onto the disc to form the seating area. The non-metallic ring material is chosen to have good corrosion and erosion characteristics and may be custom made to ensure it is fit for purpose. If the non-metallic ring is chosen correctly, it should allow for better sealing. Composite discs are often employed for steam system applications.
Composition Disc
- Plug Disc – plug discs have a long and tapered shape. The shape of the plug disc makes it much more suitable to throttling than the composite and ball disc type designs.
Plug Disc
Globe valves can be mounted so that system pressure is exerted on either side of the disc when the valve is closed, however, usually it is more desirable to keep the system pressure boundary restricted to the valve body and disc when the valve is closed and not on the other side of the disc where the boundary includes the bonnet and packing.
Depending upon which side the system pressure is acting, the globe valve will be either hindered or helped by the system pressure. For example, more force is required to open a globe valve with the system pressure pressing the disc onto the seat than if the system pressure was applied on the underside of the disc where the pressure would ‘help’ the valve to lift the disc off the seat.
For steam systems, globe valves should be orientated so that steam does not lift the valve disc as it cools (creates a negative pressure).
What are the main parts of a globe valve?
The main parts of a globe valve are the actuator, stem, body, disc, seat, bonnet and some form of sealing arrangement (packing).
The disc seats at a right angle on the seat surfaces and this generally yields a better sealing arrangement compared to gate valves. As the disc is lowered and raised, it is guided by guide vanes, a piston, or the stem.
Packing is installed between the stem and bonnet to ensure the valve does not leak.
Some globe valves connect the stem directly to the disc, but some designs do not. A screw-down non-return (SDNR) valve is one such design where the stem does not connect to the disc. SDNR valves are used for safety critical systems such as ship bilge water systems and boiler feed water systems.
Flanges are installed on the suction and discharge side of the valve so that piping can be easily connected.
How do globe valves work?
The below video is an extract from our Introduction to Valves Online Video Course.
A circular shaped disc attached to the main valve stem is lowered and raised in order to open and close the valve. When the valve is closed, the disc sits upon the valve seat with both parts being machined in order to obtain a tight seal; the sealing surface can be flat or mitred. When the valve is opened, the flowing medium flows between the disc and seat.
What are the advantages of globe valves?
Due to the circular shape of the disc and seat, uneven wear across machined area is unusual.
Globe valves can be used for throttling (flow control), but the valve does not give a great degree of control due to the large circular shape of the disc. If fine flow adjustments are needed, the globe valve design is probably not the best option.
An angled globe valve has one less right angle turn than a straight globe valve, this leads to a lower pressure drop.
What are the disadvantages of globe valves?
Globe valves suffer from a high pressure drop when the valve is throttled and this can lead to damage of the valve’s components.
Globe valves are often jammed open due to incorrect operation. Globe valves should be opened fully then the handle should be rotated 180 degrees in the reverse direction to again close the valve slightly. If this ‘winding back’ procedure does not occur, the valve may seize in the open position and this makes it difficult for operators to ascertain the valve’s position; this leads to operators trying to open an already fully open valve and this can lead to the stem breaking/fracturing.
Note: A tell-tale sign that the valve stem is broken, is that the valve handle will rotate much more easily. The handle rotates easily because the disc is no longer connected to the stem.
3D Model Components
This 3D model contains all common parts of a globe valve, these include:
- Stem
- Bonnet
- Disc
- Seat
- Packing
- Body
Additional Resources
https://en.wikipedia.org/wiki/Globe_valve
http://www.wermac.org/valves/valves_globe-valves_linear-motion-valve.html