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Gate Valve

With starting and stopping flow its prime function, the gate valve is intended to operate either fully open

Table 3-1 Representative Equivalent Length, in Pipe Diameters, of Various Valves and Fittings

Equivalent Length

Description of Product in Pipe Diameters,

L/D

Conventional With no obstruction in flat, bevel, or plug type seat Fully open 340

Globe With wing or pin-guided disc Fully open 450

Valves _{(No obstruction in flat, bevel, or plug type seat)}

Y-pattern With stem 60° from run of pipeline Fully open 175 With stem 45° from run of pipeline Fully open 145

Angle With no obstruction in flat, bevel, or plug type seat Fully open 145

Valves Conventional With wing or pin-guided disc Fully open 200

Conventional wedge Fully open 13

disc, double disc, Three-quarters open 35

or plug disc One-half open 160

Gate One-quarter open 900

Valves Fully open 17 Pulp stock Three-quarters open 50 One-half open 260 One-quarter open 1200

Conduit pipeline Fully open 3a

Conventional swing 0.5 (3.5)b_{. . .Fully open 135}

Check Clearway swing 0.5 (3.5)b _{. . .Fully open 50}

Valves Globe lift or stop 2.0 (13.8)b _{. . . Fully open Same as globe}

Angle lift or stop 2.0 (13.8)b_{. . . Fully open Same as angle}

In-line ball 2.5 (17.3) vertical and 0.25 (1.7) horizontalb _{. . . Fully open 150}

Foot Valves with Strainer With poppet lift type disc 0.3 (2.1)b _{. . . Fully open 420}

With leather-hinged disc 0.4 (2.8)b _{. . . Fully open 75}

Butterfly Valves (6 in. and larger) (152.4 mm and larger) Fully open 20

Straight-through Rectangular plug port area equal to Fully open 18 100% of pipe area

Cocks Rectangular plug port area equal to Flow straight through 44

Three-way

80% of pipe area (fully open) Flow through branch 140

Source: Extracted from a table published by Crane Co.

a _{Exact equivalent length is equal to the length between flange faces or welding ends.}

or fully closed. When fully open, it has the least resistance to flow of all the valve types, as illus- trated in Table 3-1 and Figure 3-1.

From an examination of Figure 3-2, it be- comes readily apparent how the gate valve got its name. A gate-like disc, actuated by a stem screw and hand wheel, moves up and down at right angles to the path of flow and seats against two faces to shut off flow. As the disc of the gate valve presents a flat surface to the oncoming flow, this valve is not suited for regulating or throttling flow. Flow through a partially open gate valve creates vibration and chattering and subjects the disc and seat to inordinate wear.

There is a wide variety of seats and discs to suit the conditions under which the valve is to operate. For relatively low pressures and tem- peratures and for ordinary fluids, seating materials are not a particularly difficult prob- lem. Bronze and iron valves usually have bronze or bronze-faced seating surfaces; iron valves may be all iron. Nonmetallic “composition” discs are available for tight seating or hard-to-hold flu- ids, such as air and gasoline.

Gate discs can be classified as solid-wedge discs, double discs or split-wedge discs. In the solid-wedge design, a single tapered disc, thin at the bottom and thicker at the top, is forced into a similarly shaped seat.

In the double and split-wedge disc designs, two discs are employed back to back, with a spreading device between them. As the valve wheel is turned, the gate drops into its seat (as with any other gate valve), but on the final turns of the wheel, the spreader forces the discs out- ward against the seats, effecting tighter closure. Bypass valves should be provided where the differential pressure exceeds 200 psi (1378 kPa) on valves sized 4 to 6 in. (101.6 to 152.4 mm),

and 100 psi (689 kPa) on valves 8 in. (203.2 mm) or larger. Bypass valves should be ½ in. (12.7 mm) for 4-in. (101.6-mm) valves, and ¾ in. (19.1 mm) for 5-in. (127-mm) valves or larger.

Globe Valve

The globe valve (which is named for the shape of its body) is much more resistant to flow than the gate valve, as can be seen by examining the path of flow through it (Figure 3-3) and the data in Table 3-1 and Figure 3-1. Its main advantages over the gate valve are its use as a throttling valve to regulate flow and its ease of repair.

Because all contact between seat and disc ends when flow begins, the effects of wire draw- ing (seat erosion) are minimized. The valve can operate just barely open or fully open with little change in wear. Also, because the disc of the globe valve travels a relatively short distance between fully open and fully closed, with fewer turns of the wheel required, an operator can gauge the rate of flow by the number of turns of the wheel.

As with the gate valve, there are a number of disc and seat arrangements. These are clas- sified as conventional disc, plug type, and composition disc.

The conventional disc is relatively flat, with beveled edges. On closure it is pushed down into a beveled, circular seat.

Plug type discs differ only in that they are far more tapered, thereby increasing the contact surface between disc and seat. This character- istic has the effect of increasing their resistance to the cutting effects of dirt, scale, and other foreign matter.

Problem

Based on the use of Schedule 40 pipe, find the equivalent length in pipe diameters (L/D), resis- tance factor (K) and equivalent length in feet (m) of pipe (L) for 1, 5 and 12-inch, fully opened, con- ventional type gate valves.

Solution

To Find Procedure (note dashed lines in charts) 1-in. 5-in. 12-in.

L/D See Table 3-1. Read L/D value for conventional

type gate valve, fully opened. 13.00 13.00 13.00

K See Figure 3-1. From Schedule 40 pipe sizes (1, 5, 12

in.) draw lines upward from L/ D value (13). Draw

horizontal line. Intersecting points are K values. 0.30 0.20 0.17

L Connect L/D value of 13 to Schedule 40 pipe

sizes (1, 5, 12 in.) on d scale. At intersecting

points on center scale read L values. 1.10 5.50 13.00

Figure 3-1 Valve Data for Use with Table 3-1 Source: Extracted from a table published by Crane Co.

The composition disc differs from the others in that it does not fit into the seat opening but over it, much as a bottle cap fits over the bottle open- ing. This seat adapts the valve to many services, including use with hard-to-hold substances such as compressed air, and makes it easy to repair.

Figure 3-4 Angle Valve

Angle Valve

Very much akin to the globe valve, the angle valve (Figure 3-4) can cut down on piping installation time, labor, and materials by serving as both valve and 90° elbow. It is less resistant to flow than the globe valve, as flow must change direction twice instead of three times. It is also available with con- ventional, plug type, or composition discs.

Ball Valve

The ball valve derives its name from the drilled ball that swivels on its vertical axis and is oper- ated by a handle, as shown in Figure 3-5. Its advantages are its straight-through flow, mini- mum turbulence, low torque, tight closure, and compactness. Also, a quarter turn of the handle

Figure 3-3 Globe Valve Figure 3-5 Ball Valve

Note: A word of caution regarding the swing check: There have been instances when a swing check stayed open a few seconds after the re- versal of flow began, allowing the velocity of backflow to rise to such a point that, when clo- sure finally did occur, it was instantaneous and the resulting shock to the valve and system caused serious damage. Good insurance against such a possibility is a lever and weight or a spring to ensure immediate closure upon rever- sal of flow.

The lift check, Figure 3-8, is primarily for use with gases or compressed air or in fluid sys- tems where pressure drop is not critical. makes it a quick-closing or -opening valve. Reli-

ability, ease of maintenance, and durability have made the ball valve popular in industrial, chemi- cal, and gas transmission applications.

Butterfly Valve

Figure 3-6 illustrates a butterfly valve, the valve most commonly used in place of a gate valve in cases where absolute, bubble-free shut-off is re- quired. It is manufactured in nominal diameters from 1 to 72 in. (25.4 to 1828.8 mm).

In addition to its tight closing, one of the valve’s advantages is that it can be placed into a very small

space between pipe flanges. It is available with several types of operator, motorized and manual, and a variety of component material combinations. Screwed-lug type valves should be provided so that equipment may be removed without draining down the system.

Check Valve

Swing checks and lift checks are the most com- mon forms of check valve. Both are designed to prevent reversal of flow in a pipe. The swing check, Figure 3-7, permits straight-through flow when open and is, therefore, less resistant to flow than the lift check.

Figure 3-7 Swing Check

Figure 3-6 Butterfly Valve