Book - School of Science and Technology

Condensate handling equipment 221. Miscellaneous,includingLabyrinthThermodynamicImpulse.Some of these types are illustrated in Figure 8.18 but specialist texts* and manufacturers'literature should be consulted for specific details. It is usual to add auxiliarycomponents to the trap proper in order to make up what is called a trap set and thesewill probably include some or all of: a strainer at entry; a check valve at exit; a sight glassand isolating valves when appropriate. The choice of a trap type suitable for anyparticular application depends upon a number of factors; load characteristics (constantor fluctuating); inlet and outlet pressures; associated thermostatic or other controls to thesteam supply and the relative levels of trap and condensate piping, to name but a few.While it is not possible to generalise for all varieties of application, the trap patterns listedin Table 8.1 are generally suited to equipment within the scope of this book.In an ideal situation, a trap would discharge into a condensate main run below it andthe main would then fall in level back to the boiler plant. Such an arrangement maysometimes be possible in an industrial building but it is very often the case that a trapmust discharge into a main above it; this is quite practicable provided that the steampressure at the inlet to the trap is always adequate to overcome the back pressure imposedby the water in the discharge pipe, Figure 8.19. In simple terms, an available steampressure of at least 10 kPa is required for each metre height of vertical `lift'.In instances where the steam using equipment is fitted with some type of thermostaticor other automatic control which takes the form of a throttling valve, it must beremembered that such a valve acts by causing a reduction in steam pressure to theequipment (and thus to the steam trap). In consequence, although the initial steam supplypressure to the control valve may be adequate to provide the required lift, this will not bethe case at low loads. It is always good practice, therefore, to avoid a situation wherecondensate has to be lifted from a trap serving equipment fitted with automatic control ofsteam input.Table 8.1 Steam traps: general applicationApplicationDrain points on steam mainsFan convectors (large)Natural convectors and radiatorsOil storage tank coils and outflow heatersOil tracing linesPlenum heating or air-conditioning heater batteriesStorage and non-storage calorifiersUnit beaters (small)Unit heaters (large)Type of steam trapMechanical open top, inverted bucket orthermodynamicMechanical ball float or inverted bucketThermostatic balanced pressureMechanical open top or inverted bucketThermostatic bi-metallic or thermodynamicMechanical ball float or inverted bucket: may bemultiple for large unitsMechanical ball float or open bucketThermostatic balanced pressureMechanical ball float or inverted bucket* Northcroft, L. G., Steam Trapping and Air Venting. Hutchinson, 1945. Spirax-Sarco. Various excellentinstructional courses and texts.