STEAM TRAP APPLICATIONS DRIP LEG DESIGN
PURPOSE:
Drip Legs are used for removing condensate from steam transmission and distribution lines. This helps ensure high quality steam for use in various plant applications and also will prevent damaging and dangerous waterhammer.
OPERATION:
As steam travels at high velocity through piping, condensate forms as the result of piping heat losses and/or improper boiler control resulting in condensate carryover. Drip legs are therefore located at points where condensate may accumulate to allow for drainage by gravity down to a steam trap for proper discharge from the system. Since condensate drains by gravity, drip legs must be located on the bottom of piping and designed with diameters large enough to promote collection.
INSTALLATION GUIDELINES:
(see Figure 1)
• For drainage of steam transmission and distribution lines, drip legs should be located at bends in piping (direction changes), low points, end of line, and in straight run of piping every 200 feet. • For protection of equipment such as regulators and control valves, drip legs should be installed directly ahead of the regulating or control valve line. • Proper steam trap selection for drip applications is dependent upon application requirements, such as pressure, number of and distance between installed steam traps, ambient conditions, start-up requirements, etc. A commonly accepted practice is to use float & thermostatic (F&T) steam traps for low pressure steam systems up to 30 PSIG, and thermodynamic steam traps for steam pressures over 30 PSIG. • Because condensate drainage from steam systems is dependent upon gravity, drip leg diameter is critical for optimum removal – larger is better. Collection leg diameter (D L ) is recommended to be the same size as the steam main (D), up to 4”. For steam mains above 4”, the collection leg diameter may be half the diameter of the main, but not less than 4”. The length (L) of the drip leg for systems with automatic start-up should be a minimum of 28” to provide approximately 1 PSI head pressure. The length (L) of the drip leg for systems with supervised start-up should be 1.5 x D L , but not less than 8”. • Consider low-cracking pressure (1/4 PSI opening pressure) check valves after steam traps when discharging into condensate return lines. Check valves eliminate the possibility of condensate backing up through the steam trap into the system. • A drain valve is included at the bottom of the collection leg for manual discharge of condensate during supervised start-up. The drain valve should be located at least 6” below the steam trap line. • An isolation valve and strainer should be installed before the steam trap. The isolation valve simplifies maintenance of the trap and the strainer protects the trap from any dirt, debris or scale in the line.
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