Cycling steam traps

Apart from the oxygen corrosion that results in HW and LP steam heating systems where water losses occur as a result of leaking pump mechanical seals, excess BD, faulty steam traps, and other sources, a subsequent effect is the development of fouling. This effect stems from the production of corrosion debris and (high iron content) sludge that eventually settle out in the boiler. This corrosion debris, sludge, and other foulants must be periodically removed from the boiler by BD, which merely adds to the water loss, and the cycle perpetuates. 

Low Loads and Cycling. The trap should be able to handle loads down to a very small percent of its rating which should be in the realm of 2 to 3% at operating temperature. Cycling, or condensate blowing, should occur at a rate of 3-to4 cyc./min. rate, with a blowing time of 5 to ten seconds. With proper operation, the trap will not allow condensate buildup in the tracer and will blow down completely each time. This, of course, is a bare trap insulation should not be applied to a thermodynamic steam trap. 

With condensate tubes partially flooded, any further variations in steam flow to the reboiler will affect both the AT in the reboiler and the fraction of tube surface covered by condensate. These two often interact, giving rise to a sluggish, sometimes erratic response. The steam trap will offer little to assist with the control of the condensate level. Further, if the reboiler load changes are sudden, the above-mentioned equilibrium will be difficult to establish or sustain. Instead, cycling may develop, with the control valve... 

Cycling (30 s - several minutes duration) instrument fault/condensate in instrument sensing lines/[foaming] /liquid maldistribution/steam trap problems (see Section 5.1)/unsteady vacuum (see Section 2.2). 

It is important to know that steam traps should not be sized based on pipe size. A common problem is poorly sized traps that cause premature failure through excessive cycles and wear on internal parts as well as excessive steam leaks. Therefore, steam traps are selected primarily on duty second, on duty variation for transient cases and third, on equipment requirement (Kenny, 1989). Proper installation and maintenance together with regular vendor service can maintain good steam trap performance and long life. 

This cycling can be eliminated by mounting the control valve in the condensate pipe, but this creates new problems, because when the load decreases, the process is slow steam has to condense before the condensate level is affected, and when the load increases, the process is fast, because blowing out liquid condensate is fast. With such "nonsymmetrical" process dynamics, control is bound to be poor. A better option is to use lifting traps to prevent condensate accumulation. These pumping traps will make temperature control possible even when the heater is under vacuum, but will not improve the problem of low rangeability, and the possible use of two control valves in parallel can still be necessary. 

The sizing of thermodynamic traps is more critical than for other types. Size a trap too small and it will back up condensate too large, eind it will waste steam. Excessive oversizing will cause the trap to destroy itself. This occurs when the trap cannot get enough condensate to fulfill its energy requirements and, therefore, begins to cycle more rapidly partly on live steam. This causes an... 

Both pumping traps and liquid movers use steam as a motive force to pump condensate to the desired location. The steam must be vented between each cycle. For vacuum operation, a vacuum vent is required and usually represents an energy inefficiency unless the vent can be condensed in the process to recover the heat. The steam can be vented back to the calandria steam chest and condensed directly against the process, however, this may represent an operational Instability unless properly designed. The vent rate must be controlled at a rate which does not create problems in the calandria or evaporation system. The receiver must be adequately sized to permit stable operation. Generally, it is better to vent to a separate condensing location. 

Wear and dirt are particular problems with a disk-type trap. Because of the large, flat seating surfaces, any particulate contamination, such as dirt or sand, will lodge between the disk and the valve seat. This prevents the valve from sealing and permits live steam to flow through the discharge port. If pressure is not maintained above the disk, the trap will cycle frequently. This wastes steam and can cause the device to fail prematurely. 

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