Globe & Needle Valves

A globe valve is a valve used for regulating the flow of fluid in a pipeline. One of the most common types of valves, it has a disc attached to a stem with a handwheel, which can be turned to lower the disc until it makes contact with a seat. Fluid travels in through the inlet, up and through the gap between the disc and the seat and out through the outlet and in so doing, the flow is regulated by the size of the gap between the disc and the seat.

When the handwheel is turned clockwise, the disc moves closer to the seat and the flow is reduced, until it eventually makes contact and the flow is completely blocked. Turned anti-clockwise, the disc moves away from the seat, increasing the flow until it is at maximum capacity.

While this altering of the flow's direction by degrees allows its rate to be regulated, it also creates a big drop in pressure and causes turbulence in the line. So a regular Z-body globe valve is not suitable for applications where such conditions would be undesirable.

However variations on this design, the Y-body and angle design globe valve, have reduced the effects of flow resistance and made the globe valve suitable for even severe applications such as transporting slurry or highly viscous fluids.

Globe valves have several advantages over some other types of valves. For example, they seal better, last longer and are less prone to wear than gate valves and are easier to service and maintain, with convenient access from above.

Today they can be found in almost every industry, from power stations and water treatment plants to refineries, food processing and chemical plants. And applications where they are used for regulating flow include cooling water systems, fuel oil systems, boilers, turbines, vents and drains.

Globe valves are often used in applications where throttling or regulating of flow is required. A small turn of the handwheel on top of the body is all that's needed to adjust the rate at which liquid flows through a globe valve.

This fine degree of adjustment is possible largely because the globe valve's seat is mounted parallel to the line of flow, which makes the valve more efficient when throttling and minimises the amount of disc and seat wear.

The globe valve is comprised of a stationary ring seat mounted in the middle of and parallel to the pipe. The seat has an opening that is closed or opened when a disc attached to a threaded stem is lowered or raised by turning a handwheel.

When the disc is fully lowered, the flow is shut off, when it is fully raised, the fluid flows at its maximum rate and when the disc is partially raised, the flow is partially regulated.

The downside to this is that the globe valve's design creates a lot of resistance by forcing the flow to change direction inside the valve, creating turbulence and a big drop in pressure. It is therefore not recommended for use in applications where such conditions might be detrimental to the system.

Globe valves are typically used for:

• Throttling and flow regulation applications
• On/off duties in applications where high resistance from change of flow direction is not an issue
• Applications requiring frequent valve operation, for which it is well suited due to the short disc travel distance between open and closed positions
• Applications where frequent valve maintenance is required, as the disc and seat can be replaced quickly and easily. This might include use in drains and cooling water systems, boiler and steam vents and fuel oil, feedwater, chemical feed and turbine lube oil systems.

Globe valves are valves used to start, stop and regulate flow in a pipeline. They are made up of several key parts including:

• Bonnet - the outer body of the valve
• Cage - surrounds the stem inside the valve
• Stem - connects the disc to the handwheel (actuator)
• Disc - moveable barrier that blocks or unblocks the flow
• Seat - provides the seal between the disc and bonnet

The globe valve is mounted parallel to the pipe and the disc is perpendicular to the seat. The valve is controlled by turning the handwheel, which lowers or raises the disc closer to or further away from the seat.

When turned clockwise, the disc lowers towards the seat, either reducing or fully obstructing the flow, depending on how low it is allowed to move. And when the handwheel is turned anti-clockwise, the disc slowly rises, which increases the flow until it is at maximum capacity. This ability to gradually reduce or increase the flow is what makes the globe valve so useful for throttling and flow regulation.

Unfortunately, the design of a standard (Z-body) globe valve creates a lot of resistance. Forcing the flow to alter its direction inside the valve creates turbulence and a large drop in pressure.

So a Y-body and angle design were also developed, both of which help to remedy this high pressure drop. In the Y-body design, the seat and stem are angled at approximately 45 degrees to the pipe axis. And in the angle design, the flow direction is altered by 90 degrees, with both designs achieving less flow resistance as a result.

Thanks to its throttling ability, the globe valve enjoys widespread use in a range of industries. For example, oil and gas production is a big user of globe valves and the chemical industry is one of the biggest users of valves in general.

Globe valves and gate valves are two of the most common types of valves. But while they have similar body shapes, each has its own unique design and application.

Globe valves are typically used to stop, start and regulate flow. The disc within the valve moves up and down from the seat by means of a handwheel on a threaded stem and because the change in flow is gradual, this valve is ideal for throttling and regulating flow within a pipeline.

Unfortunately, a regular (Z-body) globe valve causes turbulence and pressure drop, which are less than ideal conditions in many systems. But the development of the Y-body and angle globe valve designs have helped to overcome this and globe valves can now be found in all kinds of severe environments including high pressure and high temperature systems and controlling corrosive and viscous fluids.

Unlike globe valves, gate valves are designed to stop and start flow within a pipeline, but they cannot regulate flow as well.

They have parallel or wedge-shaped gates which are raised and lowered into the flow by means of a handwheel on a threaded stem. And it is this design which prevents them from being useful for throttling, as a partially raised or lowered gate is subjected to pressures from the flow which can quickly lead to damage and wear.

Globe valves and gate valves also differ from each other in other ways including:

• Globe valves seal better than gate valves and last longer, although they are more expensive to buy.
• Globe valves are less prone to disc and seat wear than gate valves and provide easier access for part replacement.
• Gate valves have little flow resistance when fully open and pressure drops are low, making them more suitable for frequent opening and closing.
• Gate valves can be installed in various directions, but globe valves can only be installed in one direction.

A needle valve is a special kind of globe valve in which the plug is needle-shaped rather than disc shaped. A needle valve is able to provide a very fine level of flow control which is useful for precision equipment.

Globe valves are mainly employed in applications where throttling or regulation of flow is required and their design helps them to accomplish this task better than some other valves such as ball valves and gate valves.

That's because the globe valve's seat is mounted parallel to the pipe and flow can be gradually reduced or increased with small turns of a handwheel from above.

The downside of this arrangement is that the change of direction in the flow as it travels through the valve causes a large drop in pressure and turbulence in the line, which can mean that a regular globe valve may not be suitable for some types of systems.

Fortunately, as well as the regular design (known as a Z-body design), globe valves are also available in a Y body design and an angle design. Both of these designs provide a partial remedy for the high pressure drop that occurs in globe valves and are therefore better suited for high pressure services, steam systems and systems involving slurry or highly viscous fluids.

Globe valves are versatile valves and as well as throttling, they can be used as pressure relief valves, check valves or shut-off valves and they are commonly found in applications such as:

• Cooling water systems needing flow regulation
• Fuel oil systems where leak-tightness is important
• High point vents and low point drains when leak-tightness and safety are important
• Feedwater or chemical feed systems
• Condenser air extraction and drain systems
• Heater drains and boiler and main steam vents and drains
• Turbine seals and drains
• Turbine lube oil systems.

Globe valves are used in a wide range of different industries across the globe, including power and water plants, paper mills, refineries, chemical plants, oil and gas production, food processing and the pharmaceutical industry.