Document ga36YnOe6yMNr5y5705krdVLG

484 CHAPTER 23 ; , . : 1949 Guide Table 12. Pipe Sizes fob One-Pipe Up-Feed System; Shown in Fio. 19 Past' or Stbtxm Branches to radiators.. Branches to radiators.. Riser .... Riser______ _______ _ Riser Riser Riser ....... Runout to riser Supply main_________ Branch to suddIv main Dry return main_____ Wet return main_____ Wet return main_____ Wet return main Section or Pro Radiation ScTFLtED EDR Sq Ft Theoretical . Practical Pipe size Pro size (Inches) (Inches) 100 a to 6 b to c c to d.. d toe. t to/.. 50 200 300 400 500 600 / tog , 600 g to A. 600 h to j .600. /to k ,600 k to m- 600 m to n . 600 n to p 600 - 2 IX 2 2X 2X 3. 3 3X ;3 2X IX I 1 1 2 IX 2 2}^ 2X 3 3 3^v :3 :3 2 2 2 2 f"I f1 * PjlM.R Fio. ,19. Riser, Supply . Main, and Return Main of One-Pipe System SIZING PIPING FOR TWO-PIPE HIGH PRESSURE SYSTEMS Steam piping for two-pipe high pressure can be sized for greater pressure drops than that of the return piping. For a system using steam at 30 psig the total pressure drop can be 5 to 10 psi and; for 150 psig systems, 25 to 30 psi. It has been observed that the maximum total pressure in the retums of a 30-psig system is about 5 psi, and that of a 150-psig system is about 20 psi. The pressure in the return mains is, of course, caused by the discharge of traps, oh.by leaky traps and by`flashing of contfensafe into steam due to .the lower pressure in the return line.,, The usual-practice.in^the sizing of ' high pressure returns has been to size, on the. basis of J . psi per 100 ft of pipe for 30-psig systems, and 1 psi per. 100 ft for.,150-psig systems.: . .This.is an average figure which corresponds generally to' several of the previously published tables for the design of. high pressure return, piping. Notes on Two-Pipe High Pressure Systems y; Pitch of mains should not be less than 1 inch in 10 feet. Pitch of horizontal runouts to risers and heating units should be not less than J inch per foot. SIZING PIPING FOR TWO-PIPE LOW PRESSURE SYSTEMS Piping for two-pipe low pressure systems is sized in the same manner as for two-pipe vapor systems, except that the pressure dr,op throughout the system can be based,on j psi to 1 psi drop. SIZING PIPING FOR TWO-PIPE VAPOR SYSTEMS While many manufacturers of patented.vapdr;heatihg accessories have their own schedules for pipe sizing, an inspection.-of these sizing tables ..indicates that in general as small a drop as possible is recommended. The reasons for this are: (1) to have the condensate return to-the.boiler by -Steam Heating Systems and Piping 485 gravity, (2) to obtain a more uniform distribution, of'steam: throughout the system, especially when it.is desirable to carry a moderate; or low fire, and (3) that with large variations in'.pressure the value of graduated valves on radiators is destroyed., For small vapor systems where the equivalent length of run does not ' exceed 200 ft, it is recommended that; the main and any runouts to risers that may be dripped should be sized from Column D, Table 5, while riser runouts not dripped and radiator runouts should employ Column I. The Up-feed steam risers should be taken from. Column,,AT. On the returns, the risers should be. sized from Tables 6 and 7, Column, U, (lower portion) and the mains from Column U (upper portion).- -It should again be noted that the pressure drop in-the steam side of the--system is kept the same as on the return side except where the flow in the'riser'is concerned; On a down-feed system the main vertical riseri.shouid .be sized from Column H, but the down-feed risers' can be taken from Column D al- ' though it so happens that the values.in Columns D and H for small systems correspond. This will not hold true in larger systems. For vapor systems over 200 ft of equivalent-length, the drop should-not exceed | psi to J psi, if possible. Thus, for a 400 ft equivalent' run- the drop per 100 ft should be not over J psi'divided by 4, or ^ psi. In this case the steam mains would be sized' from Column B, the radiator and undripped riser runouts from'Column I\ the' risers from Column B; because Column H gives a drop in excess of' ^ psi.- On a down-feed system, Column B'would have to be used-for; both the main riser and the smaller risers feeding the radiators in, order not to increase the drop over psi-. The return risers would, be sized from the lower portion of Column 0 and the dry return main from the upper portion of the same column; while any wet returns would be sized from Column N. The same pressure drop is applied on both the steam and the return sides of.the system. Notes on Vapor Systems .. ........ 1. Pitch of mains should not be.less than } in. in 10 ft. 2. Pitcb.of horizontal runouts to' risers and radiators should.'not be' less than I in. per foot. Where this pitch cannot be obtained runouts over8 ft in length should be' one size larger than called for in the table. , 3. In general it is not desirable to have a supply main smaller than 2 in. 4. -When necessary, supply main, supply risers, or runouts to supply risers should: be dripped separately into a wet, return,, or, may.be .connected into the dry .return, through a thermostatic drip trap. SIZING PIPING FOR TWO-PIPE VACUUM SYSTEMS 1 Vacuum, atmospheric, sub-atmospheric and orifice'systems are usually employed in large installations and have total drops varying from i to } psi. Systems in which the maximum equivalent length does not exceed 200 ft preferably employ the smaller pressure, drop, while systems over 200 ft equivalent length of run, more frequently are designed for the higher drop, owing to the relatively greater saving in pipe sizes.. For example, a system with 1200 ft longest equivalent length of run would employ a drop per 100 ft of psi divided by 12, or A psi. In this case, the steam main would be sized from Column C, Table 5, and the risere alsp from Column C (Column H could be used as far as critical velocity is concerned but the. drop would exceed the limit of psi). Riser runouts, if dripped, would use Column C but if undripped would use Column /; radiator runouts, Column I; return risers, lower part of Column S, Tables 6 and-7-; return runouts to radiators, one.pipe size largerthan the radiator trap connections;