2000 Hook-up Book

Draining Steam Mains

B) Automatic Start Up One traditional method of achiev ing automatic start up is simply to allow the steam boiler to be fired and brought up to pressure with the steam take off valve (crown valve) wide open. Thus the steam main and branch lines come up to pressure and temperature with out supervision, and the steam traps are relied on to automatical ly discharge the condensate as it is formed. This method is generally con fined to small installations that are regularly and frequently shut down and started up again. For example, the boilers in many laundry and drycleaning plants are often shut down at night and restarted the next morning. In anything but the smallest plants, the flow of steam from the boiler into the cold pipes at start up, while the boiler pressure is still only a few psi, will lead to excessive carryover of boiler water with the steam. Such carry over can be enough to overload separators in the steam takeoff, where these are fitted. Great care, and even good fortune, are needed if waterhammer is to be avoided. For these reasons, modern practice calls for an automatic valve to be fitted in the steam supply line, arranged so that the valve stays closed until a reason able pressure is attained in the boiler. The valve can then be made to open over a timed period so that steam is admitted only slowly into the distribution pipework. The pressure with the boiler may be climbing at a fast rate, of course, but the slow opening valve protects the pipework. Where these valves are used, the time available to warm up the pipework will be known, as it is set on the valve control. In other cases it is necessary to know the details of the boiler start up procedure so that the time can be estimated. Boilers started from

cold are often fired for a short time and then shut off while tem peratures equalize. The boilers are protected from undue stress by these short bursts of firing, which extend the warmup time and reduce the rate at which con densation in the mains is to be discharged at the traps. Determining Condensate Loads As previously discussed there are two methods for bringing a steam main “on line”. The supervised start up bypasses the traps thus avoiding the large warm up loads. The traps are then sized based on the running conditions found in Table 5 (page 10). A safety factor of 2:1 and a differential pressure of inlet minus condensate return pressure. Systems employing automat ic start up procedures requires estimation of the amount of con densate produced in bringing up the main to working temperature and pressure within the time available. The amount of conden sate being formed and the pressure available to discharge it are both varying continually and at any given moment are indeter minate due to many unknown variables. Table 4 (page 10) indi cates the warm up loads per 100

feet of steam main during a one hour start up. If the start up time is different, the new load can be calculated as follows: lbs. of Condensate (Table 4) x 60 Warm up time in minutes = Actual warm-up load. Apply a safety factor of 2:1 and size the trap at a differential pressure of working steam pres sure minus condensate return line presure. Since most drip traps see running loads much more often than start up loads, care must be taken when sizing them for start up conditions. If the start up load forces the selection of a trap exceeding the capability of the “running load trap,” then the warm up time needs to be increased and/or the length of pipe decreased.

SYSTEM DESIGN

Warm Up Load Example Consider a length of 8" main which is to carry steam at 125 psig. Drip points are to be 150 ft. apart and outside ambient conditions can be as low as 0°F. Warm-up time is to be 30 minutes.

From Table 4, Warm Up Load is 107 lb./100 ft.

For a 150 ft run, load is 107 x 1.5 = 160.5 lb/150 ft. Correction Factor for 0°F (see Table 4) 1.25 x 160.5 = 200.6 lb/150 ft. A 30 minute warm up time increases the load by 200.6 x 60 30 = 401 lb/h total load Applying a safety factor of 2:1, the trap sizing load is 802 lb/h. If the back pressure in the condensate return is 0 psig, the trap would be sized for a 125 psi differential pressure. This would result in an oversized trap during running conditions, calculated at 94 lb/h using Tabe 5 (page 10). Either increase the warm up time to one hour or decrease the distance between drip traps.

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