2000 Hook-up Book

Steam Trap Selection

Steam Trap Sizing Steam main drip traps shall be sized with a 2 times safety factor at full differential pressure. In most cases, they will be 3/4” size with low capacity orifice or small er unless otherwise shown on the drawings and they shall be locat ed every 200 feet or less. Traps for steam tracing shall be 1/4" to 1/2" size. They shall be located every 100 feet or less. Radiator traps shall be pipe size. Freeze protection traps shall be 1/2" to 3/4" size unless otherwise noted. Traps for equipment drainage are sized with safety factors that reflect the differences of the HVAC and Process industries, such as variations in actual hydraulic head and material con struction of tube bundles. A summary of these typical recom mendations are as follows: HVAC Industry • Non-modulating control sys tems have traps selected with a 2 times factor at full pres sure differential. • Modulating control systems with less than 30 psig inlet pressure have traps selected

for full-load at 1/2 psi pres sure differential, provide 18 to 24" drip leg for condensate to drain freely to 0 psi gravity return. (With drip legs less than 18", consult a Spirax Sarco representative.) • Modulating control systems with greater than 30 psig inlet pressure have traps selected with a 3 times factor at full pressure differential for all preheat coils, and a 2 times factor for others. Process Industry • Non-modulating control sys tems have traps selected with a 2 times factor at full pres sure differential. • Modulating controls systems with less than 30 psig inlet pressure have traps selected for full load at 1/2 psi pres sure differential, provide 18 to 24" drip leg for condensate to drain freely to gravity return at 0 psi. (With drip legs less than 18", consult a Spirax Sarco representative.) • Modulating control systems have traps selected with a 3 times factor at full pressure differential.

A full discussion of steam trap functions are found in the com panion Fluid System Design volume, “STEAM UTILIZATION.” The material covers operation of all types of traps, along with the need for proper air venting and trap selection. Traps are best selected not just on supply pres sure and load requirements, but after reviewing the requirements of the application compared to trap characteristics including dis charge temperature, air venting capability, response to pressure and load change, and resistance to dirt, corrosion, waterhammer and freezing conditions. Answering these questions leads to the selection of the most appropriate generic type of trap and the general recommenda tions found in Table 11 reflect this. This Selection Guide covers most trap uses and the recommended type can be expected to give sat isfactory performance.

SYSTEM DESIGN

Case in Action: Polyvinyl Butyral Extruders

Solution Float & Thermostatic steam traps were recommended for draining the extruders. This would give them immediate condensate removal; therefore maintaining a consistant temperature throughout the length of the extruder, provid ing better control over product melt. Also, upon recommendation, strainers were installed before the traps to help keep dirt out, and cut down on maintenance cost. Benefits • Maintained consistent temperatures with existing equip ment because there is no condensate in the heat transfer area. • There is less maintenance cost due to the strainers installed before the traps.

Condensate removal was needed from 3 polyvinyl butyral extruders at a pressure of 240 psi. Application required that a consistent temperature be maintained the length of the extruder to provide product quality in the melt. There were nine sections per extruder. The customer had used various brands of traps and trap styles to drain the extruders. Most recently they used a competitors bimetallic trap. They were experiencing inconsistent temperatures throughout the length of the extruder because the bimetallic traps subcooled the con densate, which then backed up into the heat transfer area. They were also experiencing high maintenance costs in relation to these traps.

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