Fluid Transfer Valves

Fluid Technology,

Fluid-handling devices are not basically concerned with the modulation of power, but only with the movement of fluid. Choosing a fluid-handling valve used to be easy, because each one had its own area of utility. For on-off, full, or no-flow requirements, ball and gate valves were favored; where tight shutoff was not required, butterfly and slide valves were used. As a result, beliefs were formed which may inhibit the selection of the best valve for a job.

Globe valves are used for throttling purposes and where positive shutoff is required, in sizes up to 6 in. Globe valves have a replaceable plug and seat, and a metal-to-metal seal. Other globe valves are available with elastomeric disc seals. The easy replacement of the plug and seat makes repair simple and inexpensive.

Pressure loss through a globe valve is somewhat high. Globe valves can be used at high pressures, but the higher the pressure, the more difficult the task of sealing around the stem and the greater the torque required to operate it. Other types of globe valves include:

Angle valves in which the fluid makes a 90° turn as it passes through the valve. Pressure loss through an angle valve is less than that through a conventional globe valve.

Y-valves that have a reduced pressure drop because the flow passes straight through the valve.

Needle valves which are functionally similar to globes, but they permit a finer adjustment of flow. The end of the stem is pointed like a needle and fits accurately into the needle seat. The seat is typically metal, although elastomeric seats have been used for very fine adjustments. Needle valves are used for very small, accurately adjustable flows.

Cock, or plug, valves are the oldest type of valve and still enjoy wide use for on-off service. Plug valves are made with both tapered and cylindrical plugs and in lubricated and nonlubricated models. The early forms of the cock valve used metal-to-metal, nonlubricated seals. Plug valves of this type are still used, but problems of sticking and galling limit their usefulness. These difficulties were largely overcome by the development of the lubricated-plug valve. In this valve, the lubricant is forced into the valve under pressure and is extruded between the plug face and the seat in the body. The lubricant prevents leakage between the plug and body, reduces friction and wear between the surfaces when the plug is turned, and also lifts the plug slightly to reduce the torque required to operate the valve.

A nonlubricated-plug valve may use a tapered plug with a mechanical lifting device that unseats the plug before it is turned to reduce the operating torque required. Or it may have an elastomeric sleeve or plug coating with a low coefficient of rubbing friction. Plug valves are available in sizes as large as 34 in. and in pressure ratings as high as 10,000 psi.

Ball valves represent a modification of the plug valves with a spherical instead of a tapered or cylindrical plug. Advances in materials, primarily polymers, plus improvements in design, have reduced the cost and extended applicability of ball valves.

Ball valves are relatively low in cost; they open and close with one-quarter turn of the handle; provide unimpeded flow through the full bore with minimum pressure drop; and their handle position shows immediately whether the valve is open or closed. In addition, they are easy to clean and repair, and the self-wiping action of the seat as the ball-plug rotates prevents any buildup of contamination to impede full closure of the valve.

Ball valves were developed as on-off valves without much attention given to throttling characteristics. However, design improvements have suited ball valves for some types of flow control, such as throttling the flow of air at differential pressures as high as 1,000 psi. The bulk of the control does not occur with a minor movement of the handle. Only 3% of total flow occurs at 10\#161> of handle travel, 10% at 30°, 30% at 56°, 50% at 70°, and 80% at 82° or 91% of full open. Thus, a ball valve has relatively good throttling characteristics at low flow.

The ball valve is somewhat similar in its operation to the butterfly in that a one-quarter turn opens or closes it, and the valve presents little resistance to the flow of fluid through it. It has two advantages over the butterfly, however. It is available in higher pressure ratings, and it provides a clear passage to the fluid. The passage through a butterfly valve is obstructed by the cross section of the disc and, as the pressure rating increases, so must the strength and thickness of the disc. The pressure drop through the same-sized ball valve is, therefore, less, and the benefit increases as the pressure rating of the valve increases.

Ball valves are available in pipe sizes to 42 in. and in pressure ratings to 7,500 psi. Many designs of ball valves are available to satisfy different requirements, including those with all metal seats and seals, and some that are completely lined with plastic.

Butterfly valves were once used for low-pressure service where complete shutoff was not necessary, and they were not used to modulate flow. Butterfly valves had the advantage of small size, light weight, simple design, and low-pressure drop. They also required only a one-quarter turn to change from closed to the fully open position.

Today, butterfly valves retain their traditional virtues. But capabilities have been greatly extended by offset discs and polymeric seals. These and other design innovations have enabled butterfly valves to be used for throttling, tight sealing, and withstanding pressures as high as 1,200 psi while retaining many traditional advantages.

A modern butterfly valve may include a pressure-tight resilient seat and an angularly offset disc. Other butterfly valve designs use a hard seat and an O-ring or piston ring around the disk to seal. Butterfly valves range in size from small to enormous, and are well suited for large flows of gases, liquids, or slurries.

Gate valves include wedge and double-disc valves. Both are typically used in a fully open or fully closed position because close regulation of flow is not possible.

A gate valve can be used for throttling only when the valve is in an almost shut position, where most of the flow reduction occurs. The small, crescent-shaped aperture causes a high flow velocity that can erode seat faces. Repeated movement of the disc near the point of closure against upstream pressure can create drag between the seat on the downstream side and may gall or score the seat faces. In addition, the high-velocity flowing liquid impinging against a partially open disc or wedge produces vibration that can damage seating surfaces and score the downstream side.

Nevertheless, a gate valve is excellent for service that requires either full or no flow. It has essentially no flow restriction when fully open. The flow area at the point of control is equal to the full cross-sectional area of the line. Because flow is straight through the line, pressure drop across a gate valve is only about 1/50 that of a globe valve of comparable size. However, globe valves are preferred if lines must be opened and closed frequently.

Slide valves consist of one or two discs, usually without a spreading mechanism. Fluid pressure on the disc presses its surface against the seat for closure. Some slide valves are made with soft seats to reduce the required manufacturing tolerances for a better seal. Slide valves can be made quite thin for jobs where space is a problem. Ordinarily, slide valves are used to control flows of low-pressure fluids where tight shutoff is not required. They can handle straight-through flow of gases, liquids, slurries, and fluidized solids. Ordinary slide valves are made in sizes from 2 in. and up, and are used at pressures to 400 psi.

The development of a seal that operates in shear has permitted production of special slide valves that operate at pressures to 10,000 psi while retaining the slide-valve advantages of quick opening or closing, unobstructed flow, and low operating torque. This seal principle permits use of erosion-resistant material in the port areas, so these valves show excellent throttling characteristics without undue seal wear.

Lift valves, commonly referred to as control valves, are generally constructed with two ports in parallel. They are made in sizes to 16 in., to give any required relationship between percent of opening (stem travel) and percent of full flow. Complete shutoff is almost impossible.

Diaphragm valves consist of a body, bonnet, and a flexible diaphragm that is pushed down by the stem to effect closure. The principal advantage of this type of valve is that the stem seal is eliminated. Diaphragm valves are used primarily for handling viscous fluids, slurries, or corrosive fluids. They can be used to throttle flow, but because of the large shutoff area, low-flow throttling characteristics are not good. The effective operating temperature range is limited by the properties of the diaphragm and run from -60 to 450°F. Pressure ratings run to 300 psi.