ANSI FCI 87-1:2017 pdf free download – CLASSIFICATION AND OPERATING PRINCIPLES OF STEAM TRAPS
3.2.6 Effect of Steam Trap Operating Principle
A typical steam trap is not just a simple orifice but includes an integral, self-actuated valve operated either by temperature difference, buoyancy, or change of phase. This fact must be taken into consideration when comparing steam trap capacity ratings. The capacity of all steam traps increases as the condensate temperature is lowered. Not all steam traps will, however, discharge condensate at steam temperature. Some traps, while being capable of operating at steam temperature, have their best condensate discharging characteristics at lower temperatures. It would, therefore, be impossible to fix an arbitrary standard temperature at which all steam traps should be tested. Thus, each manufacturer determines the best operating temperature range for their product, and the resultant capacity ratings are only meaningful when based on this stated operating temperature. For this reason, manufacturers who use this standard will state operating temperatures for each published trap capacity.
4.1 Principles of Operation
While there are many ways in which steam traps may be classified, it is probable that the basic principles of operation most simply satisfy our needs. The principles most commonly used in steam traps are as follows: Type 1 – THERMOSTATIC (Temperature). A thermostatic element, sensitive to the temperature difference between steam and cooled condensate, operates the valve. Type 2 – MECHANICAL (Buoyancy). The difference in density of steam and condensate operates a bucket or float controlled valve. Type 3 – THERMODYNAMIC (Change of Phase). The generation of flash steam (change from liquid to vapor phase) either throttles the discharge or operates a valve to regulate condensate flow. In the descriptions which follow, the order of presentation has no significance other than to simplify later explanation. This is particularly true in the case of air removal from the steam traps.
Tube A has at its outlet a valve seat B and an inlet connection C for condensate. The metal rod D has a markedly greater coefficient of expansion than A, so that changes in temperature of fluid in the tube A causes the rod to expand or contract and so move the valve, plug E, to or from the orifice. The rod is threaded at F, allowing the adjustment position of the rod to suit operating conditions.
When cold, the rod D is contracted and the valve wide open. Air, forced out as steam enters the heating surface, flows through the orifice. Condensate follows and heats the rod D which expands, closing the valve. By adjustment of F, the trap can be set to discharge condensate at a predetermined temperature. Collection of air or gases in the tube A reduces the temperature as does condensate, opening the valve. Discharge may be continuous and variable, or intermittent according to condensate temperature and load conditions.