Document 6bqgQpXmVGp51xmvy6bLJeoY6

252 Chapter 14 _________________ 1945 Guide -- The total-pressure drop-shonid - never exceed one-half-of the-initial-- pressure when condensate is flowing in the same direction as the steam. Where the condensate must flow counter to the steam, the governing factor is the velocity permissible without interfering with the condensate flow. A.S.H.V.E. Research Laboratory experiments limit this to the capacities given in Table 2 for horizontal pipes at varying grades. Maximum Velocity _ The capacity of a steam pipe in any part of a steam system depends upon the quantity of condensation present, the direction in which the "condensate is flowing, and the pressure drop in the pipe. Where the quantity of condensate is limited and is flowing in the same direction as the steam, only the pressure drop need be considered. When the con densate must flow against the steam, even in limited quantity, the ve locity of the steam must not exceed limits above which the disturbance between the steam and the counter-flowing water may produce object ionable sounds, such as water hammer, or may result in the retention of Table 2. Comparative Capacity of Steam Lines at Various Pitches for Steam and Condensate Flowing in Opposite Directions3 ' Pitch of Pipe in Inches per 10 Ft Pitch or Pipe X IK. X VI. 1 IN. IMm. 2 IN. 3 nt. 4 IN. 5 IN. Pipe Size Inches Sq Ft Rad. Based on 240 Btu a 8q Ft Rad. Based on 240 Btu sa Sq Ft Rad. Based on 240 Btu Ma Sq Ft Rad. Based on 240 Btu j a s Sq Ft Rad. Based on 240 Btu sa Sq Ft Sq Ft Rad. Rad. 5 Based on 240 Btu S Based on 240 Btu <3 SqFt Rad. Based on 240 Btu 3 a X 25.6 12 30.3 14 37.3 18 40.4 19 42.5 20 46.1 21 47.5 22 49.3 23 1 45.8 12 52.6 13 63.0 17 70.0 20 75.2 22 83.0 23 87.9 25 90.2 26 tx 104.9 18 117.2 20 133.0 23 144.5 25 154.0 27 165.0 28 172.6 29 178.2 31 1M 142.6 18 159.0 21 181.0 23 196.5 25- 209.3 27 224.0 28 234.8 30 242.6 31 2 236.0 19 263.5 20 299.5 23 325.5 25 346.5 27 371.5 28 388.4 29 401.1 30 Data from American Society of Heating and Ventilating Engineers Research Laboratory. water in certain parts of the system until the steam flow is reduced sufficiently to permit the water to pass. The velocity at which such disturbances take place is a function of (1) the pipe size, whether the pipe runs horizontally or vertically, (2) the pitch of the pipe if it runs hori zontally, (3) the quantity of condensate flowing against, the steam, and (4) freedom of the piping from water pockets which under certain con ditions act as a restriction in pipe size. Reaming Important Three factors of uncertainty always exist in determining the capacity. of any steam pipe. The first is variation in manufacture, which appar ently cannot be avoided. The second is the care used in reaming the ends of the pipe after cutting. The effect of both of these factors increases as the pipe size decreases. According to A.S.H.V.E. Research Laboratory tests, either of these factors may affect the capacity of a 1-in. pipe as much as 20 per cent. The third factor is the uniformity in grading the pipe line. All of the capacity tables given in this chapter include a factor of safety. However, the factor of safety referred to does not cover abnormal defects or constrictions nor does it cover pipe not properly reamed. Equivalent Length ol Run All tables for the flow of steam in pipes', based on pressure drop,.must allow for the friction offered by the pipe as well as for the additional Steam Heating Systems and Piping 253 resistance-of the fittings and- valves.--These resistances-generally. are stated in terms of straight pipe; in other words, a certain fitting will produce a drop in pressure equivalent to so many feet of straight run of the same size of pipe. Table 3 gives the number of feet of straight pipe usually allowed for the more common types of fittings and valves. In all pipe sizing tables in this chapter the length of run refers to the equivalent length of run as distinguished from the actual length of pipe in feet. The length of run is not usually known at the outset; hence it is necessary to assume some pipe size at the start. Such an assumption frequently is considerably in error and a more common and practical method is to Table 3. Length in Feet of Pipe to be Added to Actual Length of Run-- Owing to Fittings--to Obtain Equivalent Length Inches Standard Elbow Side Outlet Tee Gate Valve Globe Valve Angle Valve A H l 1mM 2m 3 * 3'A 4 5 6 8 10 12 14 1.3 1.8 2.2 3.0 3.5 4.3 5.0 6.5 8 9 11 13 17 21 27 30 "Valve in full open position. Example of length in feet of pipe to be added to actual length of run. z' 4` 5 .6 7 8 11 13 15 18 22 . 27 35 45 53 . 63 1 0.3 0.4 0.5 0.6 0.8 1.0 1.1 1.4 1.6 1.9 2.2 2.8 3.7 4.6 5.5 6.4 14 18 23 29 34 46 54 66 80 92 : 112 136 180 230 270 310 7 10 12 15 18 22 27 34 40 ' 45 56 67 92 112 ` 132 152 Measured Length = 132.0 ft 4 in. Gate Valve = . 1.9 ft . ,, " 4--4 in. Elbows = 36.0 ft ---------Y [Equivalent Length *= 169.9 ft '................... H assume the length of run and to check this assumption after the pipes are sized. For this purpose the length of run usually is taken as double the actual length of pipe. TABLES FOR PIPE SIZING2 One factor determining the size of a steam pipe and its allowable limit of capacity is the direction of the flow of condensate; whether against or with the steam. Tables 4 and 5 are based on the actual inside diameters of the pipe and the condensation of x/i lb (4 oz) of steam per square foot of equivalent *Pipe size tables in this chapter have been compiled in simplified and condensed form for the convenience of the user; at the time all of the information contained in previous editions of The Guide has been retained. Values of pressure drops, formerly expressed in ounces, are now expressed in fractions of a pound.