Diaphragm Construction

7 Jun

The operating mechanism of a diaphragm valve is not exposed to the media within the pipeline. Sticky or viscous fluids cannot get into the bonnet to interfere with the operating mechanism. Many fluids that would clog, corrode, or gum up the working parts of most other types of valves will pass through a diaphragm valve without causing problems.  Conversely, lubricants used for the operating mechanism cannot be allowed to contaminate the fluid being handled.  There are no packing glands to maintain and no possibility of stem leakage.   There is a wide choice of available diaphragm materials.  Diaphragm life depends upon the nature of the material handled, temperature, pressure, and frequency of operation.

Some elastomeric diaphragm materials may be unique in their excellent resistance to certain chemicals at high temperatures.  However, the mechanical properties of any elastomeric material will be lowered at the higher temperature with possible destruction of the diaphragm at high pressure.  Consequently, the manufacturer should be consulted when they are used in elevated temperature applications.

All elastomeric materials operate best below 150 F.  Some will function at higher temperatures. Viton, for example, is noted for its excellent chemical resistance and stability at high temperatures.  However, when fabricated into a diaphragm, Viton is subject to lowered tensile strength  just  as  any other  elastomeric material would  be at elevated  temperatures. Fabric bonding strength is also lowered at elevated temperatures, and in the case of Viton, temperatures may be reached where the bond strength could become critical.

Fluid concentrations is also a consideration for diaphragm selection. Many of the diaphragm materials exhibit satisfactory corrosion resistance to certain corrodents up to a specific concentration and/or  temperature. The elastomer may also have a maximum  temperature limitation based on mechanical properties which could be in excess of the allowable operating temperature depending upon its corrosion resistance.  This should be checked from a corrosion table.

Diaphragm Valve Stem Assem blies

Diaphragm valves have stems that do not rotate.  The valves are available with indicating and nonindicating stems.The indicating stem valve is identical to the nonindicating stem valve except that a longer stem is provided to extend up through the handwheel. For the nonindicating stem design, the handwheel rotates a stem bushing that engages the stem threads and moves the stem up and down.  As the stem moves, so does the compressor that is pinned to the stem.  The diaphragm, in turn, is secured to the compressor.

Diaphragm Valve Bonnet Assem blies

Some diaphragm valves use a quick-opening bonnet and lever operator. This bonnet is interchangeable with the standard bonnet on conventional weir-type bodies.  A 90   turn of the lever moves  the diaphragm from full open to full closed. Diaphragm valves  may also be equipped with chain wheel operators, extended stems, bevel gear operators, air operators, and hydraulic operators.

Many diaphragm valves  are used in vacuum service. Standard bonnet construction can be employed in vacuum service through 4 inches in size.  On valves 4 inches and larger, a sealed, evacuated,  bonnet should  be employed. This is recommended to guard against  premature diaphragm failure.

Sealed bonnets are supplied with a seal bushing on the nonindicating types and a seal bushing plus O-ring on the indicating types. Construction of the bonnet assembly of a diaphragm valve is illustrated in Figure 15.  This design is recommended for valves that are handling dangerous liquids and gases. In the event of a diaphragm failure, the hazardous materials will not be released  to the atmosphere. If  the  materials  being handled  are extremely hazardous,  it is recommended that a means be provided to permit a safe disposal of the corrodents from the bonnet.

Reducing Valves

Reducing valves automatically reduce supply pressure to a preselected pressure as long as the supply pressure is at least as high as the selected pressure. As illustrated in Figure 16, the principal parts of the reducing valve are the main valve; an upward-seating valve that has a piston on top of its valve stem, an upward-seating auxiliary (or controlling) valve, a controlling diaphragm, and an adjusting spring and screw.

Reducing valve operation is controlled by high pressure at the valve inlet and the adjusting screw on top of the valve assembly. The pressure entering the main valve assists the main valve spring  in  keeping  the  reducing  valve  closed  by  pushing  upward  on  the  main  valve  disk. However, some of the high pressure is bled to an auxiliary valve on top of the main valve.  The auxiliary valve controls the admission of high pressure to the piston on top of the main valve. The piston has a larger surface area than the main valve disk, resulting in a net downward force to open the main valve. The auxiliary valve is controlled by a controlling diaphragm located directly over the auxiliary valve.

The controlling diaphragm transmits a downward force that tends to open the auxiliary valve. The downward force is exerted by the adjusting spring, which is controlled by the adjusting screw. Reduced pressure from the main valve outlet is bled back to a chamber beneath the diaphragm to counteract the downward force of  the adjusting spring. The position of  the auxiliary valve, and ultimately the position of the main valve, is determined by the position of the diaphragm. The position of the diaphragm is determined by the strength of the opposing forces of the downward force of the adjusting spring versus the upward force of the outlet reduced pressure.   Other reducing valves work on the same basic principle, but may use gas, pneumatic, or hydraulic controls in place of the adjusting spring and screw.

Non-variable reducing valves, illustrated in Figure 17, replace the adjusting spring and screw with a pre-pressurized dome over the diaphragm. The valve stem is connected either directly or indirectly to the diaphragm. The valve spring below the diaphragm keeps the valve closed. As in the variable valve, reduced pressure is bled through an orifice to beneath the diaphragm to open the valve. Valve position is determined by the strength of the opposing forces of the downward force of the pre-pressurized dome versus the upward force of the outlet-reduced pressure.

Non-variable reducing valves eliminate the need for the intermediate auxiliary valve found in variable reducing valves by having the opposing forces react directly on the diaphragm. Therefore, non-variable reducing valves are more responsive to large pressure variations and are less susceptible to failure than are variable reducing valves.

Pinch Valves

The relatively inexpensive pinch valve, illustrated in Figure 18, is the simplest in any valve design. It is  simply an industrial  version  of  the pinch  cock used in the laboratory to control the flow of fluids through rubber tubing.

Pinch  valves  are  suitable  for  on-off and throttling services. However, the effective throttling range is usually between 10% and 95% of  the rated flow capacity.

Pinch valves are ideally suited for the handling of slurries, liquids with large amounts   of   suspended   solids,   and s y s t ems      that     convey      s o lids pneumatically.  Because the operating mechanism is completely isolated from the   fluid,   these    valves    also    find application where corrosion or metal contamination of the fluid might be a problem.

The pinch control valve consists of a sleeve molded of rubber or other synthetic material and a pinching mechanism.  All of the operating portions are completely external to the valve.  The molded sleeve is referred to as the valve body.

Pinch valve bodies are manufactured of natural and synthetic rubbers and plastics which have good abrasion resistance properties.  These properties permit little damage to the valve sleeve, thereby providing virtually unimpeded flow. Sleeves are available with either extended hubs and clamps designed to slip over a pipe end, or with a flanged end having standard dimensions.

Pinch Valve Bodies

Pinch  valves  have  molded  bodies  reinforced  with  fabric.    Pinch  valves  generally  have  a maximum  operating  temperature  of  250oF. At  250oF,  maximum  operating  pressure  varies generally from 100 psig for a 1-inch diameter valve and decreases to 15 psig for a 12-inch diameter valve.   Special pinch valves are available for temperature ranges of -100oF to 550oF and operating pressures of 300 psig.

Most pinch valves are supplied with the sleeve (valve body) exposed.     Another style fully encloses the sleeve within a metallic body.  This type controls flow either with the conventional wheel and screw pinching device, hydraulically, or pneumatically with the pressure of the liquid or gas within the metal case forcing the sleeve walls together to shut off flow.

Most exposed sleeve valves have limited vacuum application because of the tendency of the sleeves to collapse when vacuum is applied.  Some of the encased valves can be used on vacuum service by applying a vacuum within the metal casing and thus preventing the collapse of the sleeve.

Butterfly Valves

A butterfly valve, illustrated in Figure 19, is a rotary motion valve that is used to stop, regulate, and start fluid flow. Butterfly valves are easily and quickly operated because a 90o rotation of the handle moves the disk from a fully closed to fully opened position. Larger butterfly valves are actuated by handwheels connected to the stem through gears that provide mechanical advantage at the expense of speed. Butterfly valves possess many advantages over gate, globe, plug, and ball valves, especially for large valve applications. Savings in weight, space, and cost are the most obvious advantages. The maintenance costs are usually low because there are a minimal number of moving parts and there are no pockets to trap fluids.

Butterfly valves are especially well-suited for the handling of large flows of liquids or gases at relatively low  pressures  and for  the  handling  of  slurries  or  liquids  with  large  amounts  of suspended solids.

Butterfly valves are built on the principle of a pipe damper.  The flow control element is a disk of approximately the same diameter as the inside diameter of the adjoining pipe, which rotates on either a vertical or horizontal axis.  When the disk lies parallel to the piping run, the valve is  fully  opened.   When  the  disk  approaches  the  perpendicular  position,  the  valve  is  shut. Intermediate  positions,  for  throttling  purposes,  can  be  secured  in  place  by  handle-locking devices.

Butterfly Valve Seat Construction

Stoppage of flow is accomplished by the valve disk sealing against a seat that is on the inside diameter periphery of the valve body.  Many butterfly valves have an elastomeric seat against which the disk seals.  Other butterfly valves have a seal ring arrangement that uses a clamp-ring and backing-ring on a serrated edged rubber ring.  This design prevents extrusion of the O-rings. In early designs, a metal disk was used to seal against a metal seat.  This arrangement did not provide a leak-tight closure, but did provide sufficient closure in some applications (i.e., water distribution lines).

Butterfly Valve Body Construction

Butterfly valve body construction varies.    The most economical is  the wafer type that fits between two pipeline flanges.  Another type, the lug wafer design, is held in place between two pipe flanges by bolts that join the two flanges and pass through holes in the valve’s outer casing. Butterfly valves are available with conventional flanged ends for bolting to pipe flanges, and in a threaded end construction.

Butterfly Valve Disk and Stem Assem blies

The stem and disk for a butterfly valve are separate pieces.   The disk is bored to receive the stem.  Two methods are used to secure the disk to the stem so that the disk rotates as the stem is turned.  In the first method, the disk is bored through and secured to the stem with bolts or pins.  The alternate method involves boring the disk as before, then shaping the upper stem bore to fit a squared or hex-shaped stem.  This method allows the disk to “float” and seek its center in the seat.  Uniform sealing is accomplished and external stem fasteners are eliminated.  This method of assembly is advantageous in the case of covered disks and in corrosive applications.

In order for the disk to be held in the proper position, the stem must extend beyond the bottom of the disk and fit into a bushing in the bottom of the valve body.  One or two similar bushings are along the upper portion of the stem as well.  These bushings must be either resistant to the media being handled or sealed so that the corrosive media cannot come into contact with them.

Stem seals are accomplished either with packing in a conventional stuffing box or by means of O-ring seals. Some valve manufacturers, particularly those specializing in the handling of corrosive materials, place a stem seal on the inside of the valve so that no material being handled by the valve can come into contact with the valve stem.  If a stuffing box or external O-ring is employed, the fluid passing through the valve will come into contact with the valve stem.

Needle Valves

A needle valve, as shown in Figure 20, is used to make relatively fine adjustments in the amount of fluid flow.

The distinguishing characteristic of a needle valve is the long, tapered, needle- like point on the end of the valve stem. This “needle” acts as a disk. The longer part of the needle is smaller than the orifice in the valve seat and passes through the orifice before the needle seats. This arrangement permits a very gradual increase or decrease in the size of the opening. Needle valves are often used as component parts of other, more complicated valves. For example, they are used in some types of reducing valves.

Needle Valve Applications

Most constant pressure pump governors have needle valves to minimize the effects of fluctuations in pump discharge pressure. Needle valves are also used in s ome components of automatic combustion control systems where very precise flow regulation is necessary.

Needle Valve Body Designs

One type of body design for a needle valve is  the bar stock body.   Bar stock bodies  are common, and, in globe types, a ball swiveling in the stem provides the necessary rotation for seating without damage.  The bar stock body is illustrated in Figure 21.

Needle valves are frequently used as metering valves.  Metering valves are used for extremely fine flow control.  The thin disk or orifice allows for linear flow characteristics.  Therefore, the number of handwheel turns can be directly correlated to the amount of flow.  A typical metering valve has a stem with 40 threads per inch.

Needle valves generally use one of two styles of stem packing: an O-ring with TFE backing rings or a TFE packing cylinder.  Needle valves are often equipped with replaceable seats for ease of maintenance.

Check Valves

Check valves are designed to prevent the reversal of flow in a piping system.    These valves are activated by the flowing material in the pipeline.                               The pressure of the fluid passing through the system opens the valve, while any reversal of flow will close the valve.  Closure is accomplished by the weight of the check mechanism, by back pressure, by a spring, or by a combination of these means.   The general types of check valves are swing, tilting-disk, piston, butterfly, and stop.

Swing Check Valves

A swing check valve is illustrated in Figure 22.  The valve allows full, unobstructed flow and automatically closes as pressure decreases.  These valves are fully closed when the flow reaches zero and prevent back flow.  Turbulence and pressure drop within the valve are very low.

A swing check valve is normally recommended for use in systems employing gate valves because of the low pressure drop across the valve.  Swing check valves are available in either Y-pattern or straight body design.  A straight check valve is illustrated in Figure 22.  In either style, the disk and hinge are suspended from the body by means of a hinge pin.  Seating is either metal-to- metal or metal seat to composition disk. Composition disks  are usually recommended for services where dirt or other particles may be present in the fluid, where noise is objectionable, or where positive shutoff is required.

Straight body swing check valves contain a disk that is hinged at the top.  The disk seals against the seat, which is integral with the body.  This type of check valve usually has replaceable seat rings.  The seating surface is placed at a slight angle to permit easier opening at lower pressures, more positive sealing, and less shock when closing under higher pressures.

Swing check valves are usually installed in conjunction with gate valves because they provide relatively free flow.  They are recommended for lines having low velocity flow and should not be used on lines with pulsating flow when the continual flapping or pounding would be destructive to the seating elements.            This  condition can be partially corrected by using an external lever and weight.

Tilting Disk Check Valves

The tilting disk check valve, illustrated in Figure 23, is similar to the swing check valve.  Like the swing check, the tilting disk type keeps fluid resistance and turbulence low because of its straight-through design.

Tilting disk check valves can be installed in horizontal lines and vertical lines having upward flow.   Some designs simply fit between two flange faces and provide a compact, lightweight installation, particularly in larger diameter valves.

The disk lifts off of the seat to open the valve.  The airfoil design of the disk allows it to “float” on the flow.  Disk stops built into the body position the disk for optimum flow characteristics. A large body cavity helps minimize flow restriction.  As flow decreases, the disk starts closing and seals before reverse flow occurs.  Backpressure against the disk moves it across the soft seal into the metal seat for tight shutoff without slamming. If the reverse flow pressure is insufficient to cause a tight seal, the valve may be fitted with an external lever and weight.

These valves are available with a soft seal ring, metal seat seal, or a metal-to-metal seal.  The latter is recommended for high temperature operation.  The soft seal rings are replaceable, but the valve must be removed from the line to make the replacement.

Lift Check Valves

A lift check valve, illustrated in Figure 24, is commonly used in piping systems in which globe valves are being used as a flow control valve.  They have similar seating arrangements as globe valves.

Lift check valves are suitable for installation in horizontal or vertical lines with upward flow. They are recommended for use with steam, air, gas, water, and on vapor lines with high flow velocities.  These valves are available in three body patterns:  horizontal, angle, and vertical.

Flow to lift check valves must always enter below the seat.  As the flow enters, the disk or ball is raised within guides from the seat by the pressure of the upward flow.  When the flow stops or reverses, the disk or ball is forced onto the seat of the valve by both the backflow and gravity.

Some types  of  lift check valves  may be installed horizontally.     In  this  design, the ball is suspended by a system of guide ribs.  This type of check valve design is generally employed in plastic check valves.

The  seats  of  metallic  body  lift  check  valves  are  either  integral  with  the  body  or  contain renewable seat rings.  Disk construction is similar to the disk construction of globe valves with either metal or composition disks.  Metal disk and seat valves can be reground using the same techniques as is used for globe valves.

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