Safety Drains Prevent Flooded Heat Exchangers
11/17/2011
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When steam supply pressure is insufficient to lift condensate to an overhead return, the condensate will accumulate in the piping and the heat exchanger until there is sufficient pressure to push it out and up to the return.
This condensate drainage issue can affect your steam system in a number of ways such as by freezing your preheat coils. In addition, poor condensate drainage can cause erratic heat exchanger temperatures which can trigger destructive water hammer. 
For most condensate drainage issues, a simple safety drain will address this problem. The job of a safety drain is to dump condensate when pressure differential is too low to lift it to an overhead return. The safety drain functions as an overflow or as a liquid level control to prevent condensate from accumulating above a specified point in the drip leg. However it should be noted that to avoid any unnecessary waste of condensate it should operate only when the accumulation of condensate reaches a danger point. .
There are many options when choosing the type of safety drain to use. For instance you can choose a ball float drainer, a float and thermostatic trap or an inverted bucket trap to solve condensate accumulation issues. It boils down to personal preference but testing shows that when the steam pressure modulates below 5 PSI, a ball float or float and thermostatic trap is more suitable.
Whatever safety drain you choose, it must be connected to the drip leg so that the floating line of the trap bucket or float is above the inlet to the primary trap (about 4”). For best results, the drip leg should be at least 12” long between the heat exchanger and the primary trap inlet. Drip leg diameter should be at least the full size of the heat exchanger drain.
Sizing the safety drain is very important as well. It must have the capacity to handle all the condensate that will accumulate above the drain take off level at the pressure then in existence. To determine the required capacity, the condensing rate of the heat exchanger when steam supply pressure is throttled down below the point at which it can lift the condensate to the return should be determined. The safety drain should be able to handle this volume at the pressure of the static head of condensate which would be about ¼ PSI with length of the drip leg as previously noted.
For more information about safety drains please contact Affiliated Steam and Hot Water at 800-424-5392 today.
Comments
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On
11/18/2011
Steve Ashby
said:
It should also be noted that, while a safety drain serves an important purpose in preventing flooded heat exchangers, condensate is directed to drain which can be costly. Careful selection of a condensate return pump can effective drain heat exchangers while return the valuable condensate back to the boiler house.
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