Saturday, September 1, 2012

Globe Valve - Good Throttling Valve

What is a Globe Valve

A globe valve is a linear motion valve used to stop, start, and regulate fluid flow.The globe valve disk can be totally removed from the flow path or it can completely close the flow path. Distinguished property or advantage of this flow control valve is throttling of fluids with good characteristics.You might have often see this valve at the discharge side of pumps for flow control. At the pump discharge side,this valve is often used in combination with check valve and gate valve for better operation and isolation.

Principle of globe valve operation

The essential principle of globe valve operation is the perpendicular movement of the disk away from the seat. This causes the annular space between the disk and seat ring to gradually close as the valve is closed. 
Globe valve
Z-body Globe Valve
This characteristic gives the globe valve good throttling ability, which permits its use in regulating flow.Therefore, the globe valve may be used for both stopping and starting fluid flow and for regulating flow. When compared to a gate valve, a globe valve generally yields much less seat leakage. This is because the disk-to-seat ring contact is more at right angles, which permits the force of closing to tightly seat the disk.

Globe valves can be arranged so that the disk closes against or in the same direction of fluid flow. When the disk closes against the direction of flow, the kinetic energy of the fluid impedes closing but aids opening of the valve. When the disk closes in the same direction of flow, the kinetic energy of the fluid aids closing but impedes opening. This characteristic is preferable to other designs when quick-acting stop valves are necessary.
Disadvantages  of simple globe valve 
  • There is generally High head loss in globe valves.
  • In globe valves generally considerable power is required to operate.
  • Globe valve is used noisy in high pressure applications.
  • In globe valve large openings is necessary for disk assembly.
  • Globe valves have heavier weight than other valves of the same flow rating.
  • In globe valves there is cantilevered mounting of the disk to the stem.
Globe Valve Body Designs

1. Z-body Globe Valve

This design of globe valve body provides a symmetrical form that simplifies manufacture, installation, and repair.

2. Y-body Globe Valve

Y-body Globe Valve

This design is a remedy for the high pressure drop inherent in globe valves. The seat and stem are angled at approximately 45°. The angle yields a straighter flow path (at full opening) and provides the stem, bonnet, and packing a relatively pressure resistant envelope. Y-body globe valves are best suited for high pressure and other severe services.

Angle Valve
3. Angle Valve

A particular advantage of the angle body design of globe valve is that it can function as both a valve and a piping elbow. For moderate conditions of pressure, temperature, and flow, the angle valve  loosely resembles the ordinary globe. The angle valve's discharge conditions are favorable with respect to fluid dynamics and erosion.

Globe Valve Seats

Globe valve seats are either integral with or screwed into the valve body. Many globe valves have backseats. A backseat is a seating arrangement that provides a seal between the stem and bonnet. When the valve is fully open, the disk seats against the backseat. The backseat design prevents system pressure from building against the valve packing.

Globe Valve Direction of Flow

For low temperature applications, globe and angle valves are ordinarily installed so that pressure is under the disk. This promotes easy operation of valve , helps protect the packing, and eliminates a certain amount of erosive action to the seat and disk faces. For high temperature steam service, globe valves are installed so that pressure is above the disk. Otherwise, the stem will contract upon cooling and tend to lift the disk off the seat.
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