Document pmgp9N57QnnR2pN3vQMyzvknd

362 CHAPTER 26 1959 Guide r ut P 1 ~\ rh T 7 N, [j__L a ITT OTA IS CL HUH. Mfl / \T \ \ CUNO TUC-UIMVTCS Rg. 21.... Relation Between Elapsed Time, Steam Pressure, Condensate and Air Elimination- Rates rate is obtained when the system steam pressure is lower than that existing during a period of constant condensing rate. It will also be noted that the peak rate of air elimina tion does not coincide with the higher condensing rate. Steam How Table 2.... Pressure Drops in Common Use for Siring Steam Pipe* (For Commanding Initial Steam Pressures) Initial Steam Pressure, Psig Pressure Drop Per 100 Ft, Pd Total Pressure Drop in Macsi Supply Piping, Pa Sub&tmos. or 1 vacuum return/ 0 1 2 5 10 15 30 50 100 150 2-4 oz 2 oz 2 os 4 oz 8 oz 1 psi 2 psi 2-5 psi 2-5 psi 2-10 psi 1-2 psi 1 os 1-4 os 8 os IX psi 3 psi 4 psi 5-10 psi 10-15 psi 15-25 psi 25-30 psi * Equipment, control valves, cte., most be aeleeted on tbe beaia of delivered The rate of flow of dry steam, or steam with a small amount of water flowing in the same direction, is in ac cordance with the general laws of gas flow, and is a func tion of the length and diameter of the pipe, the density of the steam, and the pressure drop through the pipe. This rela tionship, developed by Unwin and later by Babcock, has been used for years as a means of determining steam flow through pipes. The new charts for weight-flow rate, pres sure drop, and velocity presented in this chapter take into account the Reynolds number and its effect on friction loss, two items which were not considered when tiring the Un win or Babcock formulas. The data presented in Figs. 22, 23, 24, 25, 26 and Table 5, in later sections of this chapter, are based bn the Moody Friction Factor where the absolute roughness of the internal pipe surface is that of new commercial steel or wrought iron. The Reynolds number, as expressed by Equation 9, Chap ter 4, is Nm. dPp n U) where V m velocity of the steam, feet per second, d - internal diameter of the pipe, feet. p m density of saturated steam at a specified saturation pressure, pounds per cubic foot. ft * absolute viscosity, pounds per foot-second. The velocity is determined by the equation W V 3600 pA (2) where W ** given weight-flow rate, pounds per hour. A -- internal pipe area, square feet. The values used for the absolute viscosity of saturated steam are those of Lieb, published in Combustion, Decem ber, 1940. The relative roughness of tbe internal pipe surface is obtained by dividing the absolute surface roughness by the internal pipe diameter. The values for absolute rough ness are those presented by L. F. Moody in Mechanical Engineering, Vol. 69, 1947, p. 1005, and appear in Table 1, Chapter 4. From the Reynolds number and relative rough ness, the friction factor / is obtained from the Moody Fric tion Factor Chart, fig. 4 of Chapter 4. The head loss in feet is given by the equation fLV 2gd (3) where / -- friction factor. L ~ length of pipe (100 ft). g * acceleration due to gravity, 32.174 feet per (second) (second). Table 3.... Comparative Capacity of Steam Lines at Various Pitches for Steam and Condensate Flowing in Opposite Directions* (Pitch of Pipo in Inches par 10 Pt. Velocity in Ft per Sot) Pildi of Pipe.. X X 1 in. IM in- 2 In. 3 in. 4 fat. S fat. Pipe Site Inches Copat* nr Max. Vei. Capac- Max. Vo1. CapacHr Max. Vei. CapacHr- Max. Vei. CapotHr Max. Vf. Capac* Hr Max. Vol. Copor- ifr Max. VoL CapacHr Max. Vei. Capacity Expressed in Pounds per Hour H 6.3 12 7.6 14 9.3 18 10.1 19 10.6 20 11.5 21 11.9 22 12.3 23 1 11.5 12 13.2 15 15.8 17 17.5 20 18.8 22 20.8 23 22.0 25 22.6 26 ix 26.2 18 29.3 20 33.3 23 36.1 25 38.5 27 41.3 28 43.2 29 44.6 31 in 35.7 18 39.8 21 45.3 23 49.1 25 52.3 27 56.0 28 58.7 30 60.7 31 2 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 Tbs Amobcab Bocirrr or Heattvo ajr> Aie-CoxDrnoirixo Ejrazjmss Research Loboretixy. Steam Hearing Systems 363 The pressure drop in psi per 100 ft of pipe is pAh aP - 144 (4) Pipe Sizes The determination of pipe rises for a given load in steam heating depends' on the following principal factors: 1. The initial pressure and the total presure drop which may be allowed between the source of supply and at the end of the return system. 2. The maximum velocity of mn allowable for quiet and dependable operation of the system, taking into consideration the direction of condensate flow. 3. The equivalent length of the run from the boiler or source of steam supply to the farthest, heating unit. 4. The direction of flow of the condensate, whether against or with tbe steam. Initial Pressure and Pressure Drop Theoretically, there are several factors to be considered such as initial pressure and pressure required at the end of the line, but it is most important that: (1) the total pressure drop does not exceed the initial gage pressure of the system, and in actual practice it should never exceed one-half 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 subatinospheric, which normally operate under controlled partial vacuums, and orifice and vapor systems, which at times operate under such partial vacuums as may be obtained due to the condition of the fire; and (4) the rise in water due to pressure drop,does not ex ceed the difference in level, for gravity return systems, be tween the Lowest point on the steam main, the heating units, or the dry-return, and the boiler water line. Table 2 lists pressure drops in common use with corre sponding initial steam pressures for rising steam piping; It is common practice to limit the total drop in the supply piping to approximately Vs of the initial pressure. The allow able pressure per 100 ft is thereby determined by the equiva lent length. Designers may utilize total pressure drops up to Vt the initial pressure when steam velocities and operating pressure requirements of the selected equipment will per mit. For initial pressures of 30 psig and higher many de signers prefer to size steam piping on the velocity method. The total pressure drop should never exceed one-half of the initial gage 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 velocr ity permissible without interfering with the condensate flow. ASHAE Research Laboratory experiments limit this to the capacities given in Table 3 for horizontal pipes at various grades. Maximum Velocity The capacity of a steam pipe in any part of a steam sys tem depends upon the quantity of condensate present, the direction in which the condensate is flowing, and the pres sure 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 conden sate must 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 counterflowing water may produce objectionable sounds, such as w.ter hammer, or may result in the retention of water in Table 4 .... Length in Feet of Pipe to be Added to Actual Length of Run*--Owing to Fittings--To Obtain Equivalent Length length fat Foet to be Added to Run Size of Pipe Inches Standarc Bbow Side Outlet Teeb Cate Globe Vah* Valve* Valve* X 1.3 . 3 0.3 14 7 X 1.8 4 0.4 18 10 2.2 5 0.5 23 12 l> 3.0 6 0.6 29 15 IX 3.5 7 0.8 34 18 2 4.3 8 1.0 46 22 2X - 5.0 11 1.1 54 27 3 6.5 13 1.4 66 34 3X 4 5 6 8 - 10 12 14 ' 8 15 1.6 80 40 9 18 . 1.9 92 .45 11 22 2.2 112 56 13 27 2.8 136 67 17 35 3.7 180 92 21 45 4.6 230 112 27 53 5.5 270 132 30 63 6.4 310 152 * Valve is full open podtioo. b Values fiven apply only to a tee used to divert the Sow is tbe main to the it riser. Fnmpie of leafth in feet of pipe to be added to actual leoftb of run. -CAST RlSCft OR jRAOIATOR Measured Len*tb - 132.0 ft 4 in. Gate Valve -- 1.9 ft M is. Elbows - 38.0 ft S-4 in. Tea - 38.0 ft Equivalent -- 208.9 ft 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 {ripe 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 cer tain conditions act as a restriction in pipe size. Reaming Important It is extremely important that the ends of all pipe be reamed or filed. This insures full pipe area and minimizes disturbance of the steam or condensate stream. Equivalent Length of Run All tables for the flow of steam in pipes, based on pres sure drop, must allow for the friction offered by the pipe, as well as for the additional 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 the stated number of feet of straight run of. the same size of pipe. Table 4 gives tbe num ber 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