Select a steam trap with a pressure rating equal to or greater than the pressure in the steam supply main but with a capacity based on the estimated pressure at the trap inlet. The pressure at the inlet of the steam trap can be considerably less than the pressure in the steam supply main.
If the steam trap is connected into a common piping return system, it may have to operate against a certain amount of static pressure. This static (back) pressure can cause a reduction in the operating capacity of the steam trap. Table 10-2 illustrates the effect of back pressure on steam trap capacity.
The safety factor for a steam trap is the ratio between its maximum discharge capacity and the condensation load it is expected to handle. The actual safety factor to use for any particular application will depend upon the accuracy of the estimated condensation load, the accuracy of the estimated pressure conditions at trap inlet and outlet, and the operational characteristics of the trap.
The application for which a steam trap is to be used is also an important factor in its selection. For example, a float and thermostatic trap is recommended for use as a steam-line drip trap at pressures ranging from 16 psig to 125 psig. For the same application at pressures of 126 psig or above, an inverted bucket trap is suggested. Unusual operating conditions may also influence the choice of a steam trap for a particular application. A careful reading of the selection guide and related literature provided by the manufacturer will greatly reduce the possibility of error in choosing a suitable steam trap.