In this screw-down stop valve or globe valve as it is commonly called, the axis of the stem is at right angles to the body seat face. The relatively short movement required of the stem to open or close the valve and the very positive seating action combine to make this type of valve well suited for duties as a stop valve or for the close regulation or throttling of flow. All contact between body seat and disk ends as soon as the disk is moved from the closed position so that mechanical wear of the sealing faces is minimized. Whatever wear may occur during service does not create a major problem as the body seat and disk in most globe valves can be repaired or replaced with a minimum of difficulty and without having to remove the valve from the pipeline, a distinct advantage where welded-in valves are concerned. The minimum resistance to the flow of a globe valve is higher than in most other valves because of the changes in direction of flow as the fluid passes through the valve. This may vary considerably, however, depending on the body design and the disposition of the stem relative to the inlet and outlet ports.
In common with gate valves, both inside and outside screw arrangements are used for globe valves. The larger valves are usually of outside screw design, while the inside screw arrangement predominates for the smaller ones.
Various seating designs are available to suit different service requirements and provide different flow characteristics. The conventional narrow seat type has a tapered or spherical shaped surface on the disk and a flat angled surface on the body seat and there is narrow line contact between disk and seat. Provided suitable materials are used for disk and seat, this line bearing breaks down hard deposits that may form on the seat on some services and assures pressure-tight closure. There is little protrusion of the disk into the valve orifice so that full bore flow is obtained with only a short lift of the disk, which makes this form of seating unsuitable for close regulation of flow. Preferably, such a valve should be used wide open or fully closed. To achieve better flow characteristics the disk is made to project into the body seating orifice. The disk may be of the plug type, which is conical in shape with matching conical seat in the body, giving reasonable flow control and exceptional resistance to galling, erosion, and wire drawing under throttling conditions, or it may be contoured to provide specific flow characteristics, e.g. percentage flow equal to percentage lift. Other specialized designs may use hollow disks with vee-shaped or contoured body seatings to achieve a variety of flow conditions. Soft forms of seating are also employed, including PTFE (or other plastic) seat or disk inserts and the very popular composition disk, which is used frequently for steam and gas services, particularly in low-pressure bronze valves. Soft seatings provide tight shutoff with the minimum of effort but they are not suitable for throttling duties as they can be quickly damaged by wire drawing. Disk replacement is a simple matter and, providing the body seat face is undamaged, seating performance can be quickly restored to 'as new' condition. Also, by using disks of different materials a valve may be made suitable for other classes of service.
Generally, the disk and stem are separate components and connected together in such a manner that the disk is free to revolve independently of the stem and is able to swivel. This allows the disk to sit squarely on its seat and avoids frictional contact that might damage the seating surfaces. In some small valves, e.g. needle valves, the disk and stem may be integral, and while these will give close and stable regulation of flow under high-pressure drop conditions, they are not best suited for use as shut-off valves. Although much larger sizes are made, standard lines of globe valves do not usually extend beyond 200 mm or 250 mm. Beyond this, depending on the service pressure, the axial load imposed on the stem by the pressure of the fluid acting over the whole of the exposed area of the disk can make direct manual operation very difficult or even impossible. To overcome this problem some form of additional mechanical advantage can be provided, such as hammer-blow wheels, gearing, or power operators. Special designs are also available which incorporate means for balancing or partially balancing the axial load on the disk.
