STEAM TRAPS Introduction Steam Trap Selection Guidelines • PROCESS Applications
This guide is intended to provide the user with a "starting point" for the selection of Watson McDaniel steam traps. Steam trap selection can appear to be overwhelming given the range of applications and trap choices available. Selection criteria for a specific application may include pressure & temperature ratings, capacity, physical size & weight, and materials. Process Steam Trap Selection Guidelines
Batch Process - Steady Demand
Batch type processes typically have steady demand as a batch of products is heated to a certain temperature. They tend not to experience rapid changes in steam pressure and steam flow. Common examples of such processes are: • Unit Heaters • Storage Tank Coils • Jacketed Vessels • Pipe Coils The primary steam trap type for process equipment is a Float & Thermostatic. The WFT & FTT Series in Cast Iron and Ductile Iron are the most cost-effective solutions to most applications. The FTE Series is for higher capacity applications. The FT600/601 Series traps are available in Cast Steel or Stainless Steel which may be specified for refineries and higher pressure applications. Since rapid pressure changes do not typically occur with batch processes, Thermostatic Bellows traps can also be selected. The WT2000 , WT3000 & WT4000 Series have Stainless Steel bodies and may be preferred for outdoor applications to Cast Iron F&T traps, particularly when a potential for freezing exists. Continuous Process - Varying Demand • Heating Processes (high to ultra-high capacity) These applications use steam to heat a continuous flowing product. The modulation of the control valve results in rapid change in steam pressure and flow. Common examples of such processes are: • Heat Exchangers • Air Handling Coils • Instantaneous Water Heaters The primary steam trap type for process equipment is a Float & Thermostatic. The WFT & FTT Series in Cast Iron and Ductile Iron are the most cost-effective solutions to most applications. The FTE Series is for higher capacity applications. The FT600/601 Series traps are available in Cast or Stainless Steel which may be specified for refineries and for higher pressure applications. Safety Load Factors (SLF’s) and appropriate rules to size steam traps The largest condensate load occurs when the maximum steam pressure is present in the Heat Exchanger (HX). However, if the steam trap is selected based on the maximum condensate load at maximum pressure, it will not be adequately sized at lower differential pressures. This is because the capacity of a steam trap depends on the differential pressure across the trap (less pressure means less capacity) and trap capacity decreases at a significantly faster rate than condensate load when the steam pressure drops. When temperature control valves are used to control steam flow to a HX, the pressure may reduce to 0 psig or less. The pressure available to discharge condensate would then be based on head pressure of the drip leg. A drip leg length of 14" will produce a head pressure of ½ psig. Use the following rules and safety factors for the three categories of process applications. This should assure the trap has adequate capacity at lower differential pressure and not be drastically oversized when operating at full pressure. For applications NOT containing a Temperature Control Valve and operate at fairly constant steam pressures; select a steam trap that will handle 2X the maximum condensate load at the maximum differential pressure . For Example: if a process will generate 5,000 lbs/hr at 50 psi differential pressure, then choose a trap that can handle 10,000 lbs/hr at 50 psi. For applications WITH a Temperature Control Valve and steam pressures OVER 30 PSI ; select a steam trap that will handle 2.5X the maximum condensate load at the maximum differential pressure . For Example: if a process is expected to generate 5,000 lbs/hr at 50 psi differential pressure, then choose a trap that can handle 12,500 lbs/hr at 50 psi. I II
For applications WITH a Temperature Control Valve and steam pressures UNDER 30 PSI ; calculate the maximum condensate load at the maximum differential pressure; select a steam trap that will handle this maximum amount of condensate at ½ psi differential pressure . For Example: if a process is expected to generate 5,000 lbs/hr at 15 psi differential pressure, then choose a trap that can handle 5,000 lbs/hr at ½ psi differential pressure.The purpose of the ½ psi differential pressure is that this will allow condensate to properly drain when system pressure goes into vacuum. This assumes the installation of a vacuum breaker and a drip leg of at least 14" in length (for ½ psig) below the HX to give proper condensate head pressure to the steam trap, and trap discharge to atmospheric pressure (0 psig).
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Tel: 610-495-5131 • Pottstown PA • USA •• www.watsonmcdaniel.com
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