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528 CHAPTER 20 1958 Guide tant that: (1) the total pressure drop does not exceed the initial gage pressure of the system, and in actual practice it should never exceed onehalf of the initial gage pressure; (2) the pressure drop is not so great as to cause excessive velocities; (3) there is a constant initial pressure, except on systems specially designed for varying initial pressures, such as the sub-atmospheric, which normally operate under controlled partial vacua and orifice and vapor systems, which at times operate under such partial vacua as may be obtained due to the condition of the fire; and (4) the rise in water due to pressure drop does not exceed the difference in level, for gravity return systems, between the lowest point on the steam main, the heating units, or the dry-return, and the boiler water fine. The present tendency in steam heating unmistakably points toward a constant lowering of initial pressures, even to those below atmospheric, and to the use of reasonably small pressure drops because' a system de- Table 3. Compabative Capacity op Steam Lines at Vabious Pitches foe Steam and Condensate Flowing in Opposite Dibections* Pitch of Pipe in Inches per 10 Ft. Velocity in Ft per Sec Pitch or Pipe M IN. H IN. 1 IN. iy$ in. 2 IN. 3 IN. 4 IN. 5 IN. Inches > >. >> > >> >* -- a a0a3} > 3 & !> a a> `3 > *3 > `3 > *3 > "3 > ai i a Ksi a i & 1 a OO UOO O O Capacity Expressed in Square Feet E D R % 25.0 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 15 63.0 17 70.0 20 75.2 22 83.0 23 87.9 25 90.2 26 1H 104.9 18 117.2 20 133.0 23 144.5 25 154.0 27 165.0 28 172.6 29 178.2 31 142.6 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 Capacity Expressed in Pounds per Hour 6.3 12 7.6 14 9.3 18 10.1 19 10.6 20 11.5 21 11.9 22 12.3- 23 l 11.5 12 13.2 15 15.8 17 17.5 20 18.8 22 20.8 23 22.0 25 22.6 26 1H iy* 2 26.2 18 29.3 20 33.3 23 36.1 25 38.5 27 41.3 28 43.2 29 44.6 31 35.7 18 39.8 21 45.3 23 49.1 25 52.3 27 56.0 28 58.7 30 60.7 31 59.0 19 65.9 20 74.9 23 81.4 25 86.6 27 92.4 28 97.1 29 100.3 30 From The Amebican Society op Heating and Aib-Conditioning Engineebs Research Laboratory. signed in this manner will operate under higher pressures without dif ficulty. When a system designed for a relatively high initial pressure and a relatively high pressure drop is operated at a lower pressure, it is likely to be noisy and have poor circulation. The total pressure drop should never exceed one-half of the initial gage pressure when condensate is flowing in the same direction as the steamWhere the condensate must flow counter to the steam, the governing factor is the velocity permissible without interfering with the condensate flow. ASHAE Research Laboratory experiments limit this to the ca pacities given in Table 3 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 condensate present, the direction in which the con densate 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 condensate must Steam Heating Systems 529 flow against the steam, even in limited quantity, the velocity of the steam must not exceed limits above which the disturbance between the steam and the counter-flowing water may produce objectionable sounds, such as water hammer, or may result in the retention of water in certain parts of the system until the steain 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 horizontally; (3) the quantity of condensate flowing against the steam; and (4) freedom of the piping from water pockets which under certain conditions act as a restriction in pipe size. Reaming Important It is extremely important that the ends of all pipe be reamed or filed. Table 4. Length in Feet of Pipe To be Added to Actual Length of Run--Owing to Fittings--to Obtain Equivalent Length Size op Pipe Inches Length in Feet to be Added to Run Standard Elbow Side Outlet Teeb Gate Valve* Globe Valve* Angle Valve* H 1.3 3 0.3 14 7 % 1.8 4 0.4 18 10 i 2.2 5 . 0.5 23 12 m 3.0 6 0.6 29 15 m 3.5 7 0.8 34 18 2 4.3 8 1.0 46 22 2J4 5.0 11 1.1 54 27 m3 6.5 13 1.4 66 34 8 15 1.6 80 40 4 9 18 1.9 92 45 5 11 . 22 2.2 112 56 6 13 27 2.8 136 67 8 17 35 3.7 180 92 10 21 12 27 14 30 45 4.6 230 112 53 5.5 270 132 63 6.4 310 152 b '"> open position. Mensured Length tK i U*G8-glven appv only to a tee used to divert the flow in the main 4 in. Gate Valve 4*4 in. Elbows 2-4 in. Tees = 132.0 ft = 1.9 ft = 36.0 ft = 36.0 ft Equivalent ** 205.0 ft LAST RISER OR RADIATOR This insures full pipe area and minimizes disturbance of the steam or condensate stream. Equivalent Length of Run *?T tables for the flow of steam in pipes, based on pressure drop, must *or the friction offered by the pipe, as well as for the additional sta'fo^11'06 fittings and valves. These resistances generally are j ln terms of straight pipe; in other words, a certain fitting will thp UCe a .^roP ffi. pressure equivalent to so many feet of straight run of uriisiI?1116!]8^6 P'Pe- Table 4 gives the number of feet of straight pipe allowed for the more common types of fittings and valves. In all /_i'".,slz'.nS tables in this chapter the length of run refers to the equivalent J run as distinguished from the actual length of pipe in feet. The