Globe Valve. The term globe valve applies to a valve whose body is shaped like a globe or has characteristics like those of a globe.
Globe valves are generally suitable for throttling applications, as the valve design determines how closely the flow can be regulated.
The part that controls the flow in the globe valve is the disc, which is attached to the stem.
The disc fits precisely into the valve seat.
When the valve is open, the liquid passes through the space between the edge of the disc and the seat.
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.
Special TRIM of valves may be designed for applications that exceed these differential pressure limits.
The image shows a section of a globe valve
See figure N°6.
Types of globe valves include:
Angle globe valve
An angular globe valve changes direction at 90°.
The ends of this globe valve are at a 90-degree angle, and fluid flow occurs with a simple 90-degree turn.
A particular advantage of the angle body design is that it can function as both a valve and a piping elbow.
They have a slightly lower flow coefficient than Y- globe valves.
The image shows a section of an angle globe valve
See figure N°7.
Needle Valve
A needle valve is a valve that is used for precise flow control.
The needle valve is so-called because of the conical stem that acts as a plug and penetrates an orifice of small diameter in relation to the nominal diameter of this valve.
The displacement of the stem, if it is a fine thread, is slow and by the fact that until a good number of turns is turned the flow section of the fluid is minimal, makes this valve a good flow regulator, for its stability, accuracy and the design of the plug that provides a good metal seal with little wear that avoids cavitation at large differential pressures.
In hydraulic power stations, we use the needle valves as a bypass to the butterfly valve or spherical inlet to the turbines.
First, we operate the needle valve, which can work better than the main valve at large pressure differences without cavitation, and once the main valve is at balanced pressures at the ends, we make its opening, avoiding the water hammer of the installation.
The image shows a section of a Needle Valve
See figure N°8.
Wye Valve
It is an alternative to the high-pressure drop of the traditional globe valve.
The seat and stem have an angle of approximately 45 degrees, which gives a straighter flow path at full opening thus offering less resistance to flow.
They are widely used for regulation during start-up operations.
For its smooth flow pattern, a Wye – valve is preferable for use with erosive fluids.
The image shows a section of a Wye Valve
See figure N°9.
Flow Direction of Globe Valves
The valve seat parallel to the flowline is an important characteristic of the globe valve.
This feature makes the globe valve effective during product throttling as well as giving minimal disk and seat erosion.
In most installations the fluid is routed into the valve body from under the valve seat and disc.
This flow direction uses the pressure of the fluid to aid in the opening operation of this valve. It also reduces the wear of the seats and the valve disk.
An exception however is when high temperature steam is routed through this valve.
The extreme temperature of the vapor coming into contact with the valve seat from below creates a significant temperature difference from the top of the seat and the disc.
This temperature difference between the valve seats and the disc causes leaks.
Therefore, when high temperature steam is routed through a globe valve, it is routed into this valve from the top of the disc.
Globe valve operation
The globe valve must never be set in the fully open position.
After a valve is fully opened, the handwheel or actuating lever must be turned toward the closed position approximately one-half turn.
Unless this is done, the pressure is likely to cause the valve to stick in the open position, making it difficult, if not impossible, to close the valve.
Many gates are damaged in this manner.
Read our next article in which the non-return valves are detailed.
Globe valve – Calculate Man Hours.
