What is Globe valve?
- A globe valve is a linear motion valve that is used to stop, start, or regulate the flow of fluid.
- The globe valve disc can be totally withdrawn from the fluid or it can completely close or halt the fluid.
- The disc moves perpendicular to the seat when the Globe valve is opened and closed.
- This motion creates an annular space between the disc and the seat ring, which progressively closes when the valve is closed.
- This feature gives the globe valve the necessary throttling capabilities for regulating the flow.
What are the types of Globe Valves?
Based on the type of disc arrangement
- Ball type
- Needle type
Based on body type
- Z type globe valve
- Y type globe valve
- Angle type globe valve
Based on the type of Body Bonnet Connection
- Screwed bonnet
- Welded bonnet
- Bolted bonnet
- Pressure seal bonnet
What are the parts in the globe valve?
- Hand-wheel
- Stem
- Gland and gland stud
- Bonnet
- Gland Packing
- Gasket
- Disc Stem Nut.
- Disc or plug
- Body
- Seat
Hand-wheel
The hand wheel’s role is to rotate the stem and introduce or remove the plug from the seat, enabling or cutting fluid flow. A valve actuator is an example of a hand wheel (the part that controls the stem and disc movement). It is outside the pressure boundary (the pressure induced by the fluid within the pipe and valve) and hence only suffers stresses put on it while operating the valve in service circumstances, namely the torque generated by turning the hand wheel.
Stem
The stem conveys rotational motion from the hand wheel to the plug, and the power screw on it translates the hand wheel’s rotational motion into a vertical motion, placing the disc in its seat and blocking the flow opening. The valve’s stem is joined to the hand wheel through four flat edges at the top, screwing into the power screw and then locking it into place with a nut. At the bottom, the plug is screwed into the stem and secured with the disc stem nut. As a result, the power screw will exert torsional pressures on the stem.
Gland and gland stud
The gland and gland stud compress the gland packing onto the stem and serve as a seal to prevent valve leakage. They are subjected to pressure stresses as a result of compressing the gland packing and, in this instance, are constructed of bronze, most likely to avoid corrosion due to contact with seawater. These parts are most likely manufactured by casting the pieces and then machining the circular portion. This piece should be tested to ensure it can withstand the pressure being applied to it by pressurising it from the outside and determining the maximum pressure it can withstand before yielding.
Bonnet
The bonnet is the component of the valve that closes the opening. It is a pressure-bearing portion that holds the full pressure of the fluid as it flows through the valve with the body as its service loads. A threaded stud and nuts hold it in place. It also serves as a support for the gland, and in this case, constructed of bronze, the bonnet is normally made of the same material as the valve body.
Gland Packing
The gland packing seals the bonnet and stem, preventing water leaks, and is pressed into place by the gland follower, which is a length of string cinched up and gripping the stem. Because it will be in contact with sea water, it will need to be replaced on a regular basis as the string wears out. The string will most likely be constructed of a durable polymer, and its flexibility and elastic coefficient can be determined by performing tensile tests on it.
Gasket
Gaskets are static seals in a joint that expand to fill flaws in the material when clamped on its surfaces. They can be constructed of materials like Teflon or compressed paper, like the one we looked at, which has non-adsorbent binding agents. Because the gasket will be subjected to pressure and seawater wear, it will most likely need to be replaced over the valve’s lifetime. Tensile stress testing on the gasket should be performed to determine where the material breaks. Gaskets are also evaluated in what is known as a “hot compression test,” in which the gasket is crushed in a hydraulic press and the temperature is raised while the thickness of the gasket is measured.
Disc Stem Nut.
The stem is attached to the plug by the disc stem nut. Its threading secures it to the disc, while the stem’s broader plane holds the screw in place. When screwed, the disc will be forced against the disc stem nut’s wider hexagonal plane. It will have service loads on its threads, will be compressed when the valve is closed, and will be exposed to the full pressure of the fluid flowing through the valve. This piece, like the rest of the sections in touch with sea water, is composed of bronze, but stainless steel might be used instead.
Disc or plug
The plug’s job is to prevent the valve from opening, allowing, or restricting fluid flow. It is screwed into the disc stem nut, which secures it to the stem. When lowered, it compresses against the seat and restricts the flow. The plug will withstand the full pressure of the fluid flowing through the valve and must form a tight seal with the seat, while the threading will withstand the load of the fluid’s pressure exerting an upward push on it. Because it will be in contact with sea water, this item will be made of bronze, with stainless steel as an alternate material. It was most likely sand or die-cast and then turned on a lathe to form its thread. Its edges will be machined to ensure a secure fit with the seat. It must be tested for yield point upon compression and thread strength, as well as the greatest force it can withstand as a load.
Body
The valve body serves as the framework to which all of the other components are joined. It is the valve’s main pressure boundary, which means it will bear the full weight of the pressure of the fluid flowing through the valve, as well as loads of the pipes connecting to it. It serves as a container for fluid flow and accepts fluid input and outflow. The valve body is sand cast, and its side is rotated to seal and attach the gasket. It is machined on both sides to accommodate the bolts snugly. The component is milled for a snug fit of the bonnet, with a pilot finding the hole and a flat seat provided for the nuts.
Seat
The seat is the section into which the plug inserts, and in the valve under consideration for this study, the seat is integrated into the body. It is machined into it and must maintain an impenetrable seal with the disc. The seat will be tested alongside the body.
Advantages and Disadvantages of globe valve
Advantages of Globe Valve
- Better shut off than gate valve.
- Better shut off than gate valve.
- Good for repeated operation because seat and disc do not wear out.
- Easy to repair because the seat and disc can be accessible from the valve top.
- It can be operated faster than a gate valve due to the shorter stroke length.
- Typically operated by an automated actuator
Disadvantages of Globe Valve
- Two or more right angle twists of flowing fluid within the valve body cause high head loss.
- High head loss is caused by obstructions and discontinuities in the flow route.
- Large valves demand a lot of electricity to open and make noise while they’re running.
- Weight is high while comparing other valves with the same pressure rating.
- More expensive than a gate valve