Fittings and valves

Friction due to fittings, valves and other disturbances of flow in pipe lines is accounted for by the concepts of either their equivalent lengths of pipe or multiples of the velocity head. Accordingly, the pressure drop equation assumes either of the forms 

Values of equivalent lengths L, and coefficients K, are given in Tables 6.4 and 6.5. Another well-documented table of K, is in the Chemical Engineering Handbook (McGraw-Hill, New York. 1984 p. 5.38). 

SDecification ANSI B16.5 Product Forms All (also valves) Scooe Standard ASTM materials, dimensions, and pressure-temperature ratings 

MSS 5)-SP-44 Flanges Grades F36 through F65, dimensions, chemistry, and mechanical properties, but no impacts 

MSS-SP-75 All welded fittings except flanges (includes headers) Grades Y42 through Y65, full materials specification 

ASTM A381 Welded pipe(al (starting form for welded headers, tees, and elbows) Grades Y35 through Y65, full materials specification 

ASTM A694 Forged flanges, fittings, and valves Grades F42 through F65, full material specification, but no impacts required 

TABLE 6-5 Additional Frictional Loss for Laminar Flow through Fittings and Valves 

SOURCE From curves by Kittredge and Rowley, Trans. Am. Soc. Mech. Eng., 79,1759-1766(1957). 

The correction CQ (Fig. 6-14d) accounts for the extra losses due to developing flow in the outlet tangent of the pipe, of length Lo. The total loss for the bend plus outlet pipe includes the bend loss K plus the straight pipe frictional loss in the outlet pipe 4fLJD. Note that CQ = 1 for LJD greater than the termination of the curves on Fig. 6-14d, which indicate the distance at which fully developed flow in the outlet pipe is reached. Finally, the roughness correction is 

Example 6 Losses with Fittings and Valves It is desired to calculate the liquid level in the vessel shown in Fig. 6-15 required to produce a discharge velocity of 2 m/s. The fluid is water at 20°C with p = 1,000 kg/m3 and ji = 0.001 Pa s, and the butterfly valve is at 0 = 10°. The pipe is 2-in Schedule 40, with an inner diameter of 0.0525 m. The pipe roughness is 0.046 mm. Assuming the flow is turbulent and taking the velocity profile factor a = 1, the engineering Bernoulli equation Eq. (6-16k written between surfaces 1 and 2, where the pressures are both atmospheric and the fluid velocities are 0 and V = 2 m/s, respectively, and there is no shaft work, simplifies to 

Contributing to lv are losses for the entrance to the pipe, the three sections of straight pipe, the butterfly valve, and the 90° bend. Note that no exit loss is used because the discharged jet is outside the control volume. Instead, the V2/2 term accounts for the kinetic energy of the discharging stream. The Reynolds number in the pipe is 

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The flow resistance of pipe fittings (elbows, tees, etc) and valves is expressed in terms of either an equivalent length of straight pipe or velocity head loss (head loss = Kv /2g ). Most handbooks and manufacturers pubHcations dealing with fluid flow incorporate either tables of equivalent lengths for fittings and valves or K values for velocity head loss. Inasmuch as the velocity in the equipment is generally much lower than in the pipe, a pressure loss equal to at least one velocity head occurs when the fluid is accelerated to the pipe velocity. 

Impervious graphite centrifugal pumps, pipe fittings, and valves were developed because most chemical processes require the movement of Hquids. Graphite pipe and fittings in sizes ranging from 25 to 635 mm ID are used to convey corrosive fluids. 

Plain-end pipe used for socket-weld joints (Fig. 10-126) is available in all sizes, but fittings and valves with socket-weld ends are limited to sizes 3 in and smaller, for which the extra cost of the socket is outweighed by much easier alignment and less skill needed in welding. The joint is not so resistant to bending stress as the butt-welded joint but is otherwise equal, except that for some fluids the crevice between the pipe and the socket may promote corrosion. ANSI B16.il—1973,... 

Threaded Joints Pipe with taper-pipe-thread ends (Fig. 10-128), per ANSI B2.1, is available 12 in and smaller, subject to minimum-wall limitations. Fittings and valves with taper-pipe-thread ends are available in most pipe metals. 

Other Types of Piping Joints Packed-gland joints (Fig. 10-135) require no special end preparation of pipe but do reqmre careful control of the diameter of the pipe. Thus the suppher of the pipe shoiild be notified when packed-gland joints are to be used. Cast- and ductile-iron pipe, fittings, and valves are available with the bell cast on one or... 

Push -on joints (Fig. 10-137) require diametral control of the end of the pipe. They are used for brittle materials. Pipe, fittings, and valves are furnished with the bells cast on one or more ends. Considerable force is required to push the spigot through the O ring this is reduced by the extension on the O ring, which causes the friction of the pipe to elongate the cross sec tion of the main portion of the O ring. 

Silver Brazed Joints These are similar to soldered joints except that a temperature of about 600°C (1100°F) is required. A 15 percent silver, 80 percent copper, 5 percent phosphorus solder is used for copper and copper alloys, while 45 percent silver, 15 percent copper, 16 percent zinc, 24 percent cadmium solders are used for copper, copper alloys, carbon steel, and alloy steel. Silver-brazed joints are used for temperatures up to 200°C (400°F). Cast-bronze fittings and valves with preinserted rings of 15 percent silver, 80 percent copper, 5 percent phosphorus brazing alloy are available. 

Chemical-Porcelain Pipe Made of dense, nonporous material and fired at 1230°C (2250°F), chemical-porcelain pipe, fittings, and valves are inert to all acids except hydrofluoric but are not usually recommended for alkalies. Surfaces, except when ground for gasketing, are usually glazed for easy cleaning. Working pressures of 0.3 to 0.7 MPa (50 to 100 Ibftin") are recommended for valves and piping. Temperatures of 200°C (400°F) or more can be used, but sudden thermal shocks must be avoided. 

Pipe Depending on diameter, pipe is insulated with cylindrical half, third, or quarter sections or with flat segmental insulation. Fittings and valves are insulated with preformed insulation covers or with individual pieces cut from section straight pipe insulation. 

A vacuum system typically consists of one or more pumps which are connected to a chamber. The former produces the vacuum, the latter contains whatever apparatus requires the use of the vacuum. In between the two may be various combinations of tubing, fittings and valves. These are required for the system to operate but each introduces other complications such as leaks, additional surface area for outgassing and added resistance to the flow of gas from the chamber to the pumps. Additionally, one or more vacuum gauges are usually connected to the system to monitor pressure. 

The MAWP of the vessel cannot exceed the MAWP of the no valves, and pipe connected to the vessel. As discussed in Volui Chapter 9, pipe flanges, fittings and valves are manufactured in dan dustry standard pressure rating classes. Table 12-2 is a... 

Add R values for individual fittings and valves from Figures 2-12A through 2-16 and Tables 2-2 and 2-3. 

Scope, 52 Basis, 52 Compressible Flow Vapors and Gases, 54 Factors of Safety for Design Basis, 56 Pipe, Fittings, and Valves, 56 Pipe, 56 Usual Industry Pipe Sizes and Classes Practice, 59 Total Line Pressure Drop, 64 Background Information, 64 Reynolds Number, R,. (Sometimes used Nr ), 67 Friction Factor, f, 68 Pipe—Relative Roughness, 68 Pressure Drop in Fittings, Valves, Connections Incompressible Fluid, 71 Common Denominator for Use of K Factors in a System of Varying Sizes of Internal Dimensions, 72 Validity of K Values,... 

Valves, see pipe, fittings, and valves Vapor cloud explosions, 520 Velocities, fluid flow, 85, 89, 90 Vacuum lines, 133 Velocity head, 71... 

A velocity head is u2/2g, metres of the fluid, equivalent to (u2/2)p, N/m2. The total number of velocity heads lost due to all the fittings and valves is added to the pressure drop due to pipe friction. 

The number of velocity heads lost, or equivalent pipe diameter, is a characteristic of the particular fitting or type of valve used. Values can be found in handbooks and manufacturers literature. The values for a selected number of fittings and valves are given in Table 5.3. 

Table 5.3. Pressure loss in pipe fittings and valves (for turbulent flow)...

Nolte (1978) gives detailed methods for the selection of economic pipe diameters, taking into account all the factors involved. He gives equations for liquids, gases, steam and two-phase systems. He includes in his method an allowance for the pressure drop due to fittings and valves, which was neglected in the development of equation 5.12, and by most other authors. 

Table 4-2 contains a list of K values for use in Equation 4-38 for various types of fittings and valves. 

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