STEADILY rising energy costs have focused attention on steam trap performance. Several methods used for evaluating trap operation-including pyrometers, thermometers, pellets, and crayons that melt at predetermined temperatures — have little or no usefulness.

Measuring temperature ahead of a trap has little value. Upstream temperature is invariably equal or close to that of saturated steam. And if the trap has failed closed, the piping ahead of the trap will be cold. Temperature downstream of the trap always corresponds to the saturated vapor pressure in the return line. Because pressures in return lines are often high (sometimes 100 psig or more), downstream temperature measurements are of little value. These conditions rule out temperature measurement as a general trap method.

Only when the steam pressure is very low (in the order of 2 to 5 psi as, for example, in a low-pressure steam heating system) are temperature measurements useful in trap testing. In a low-pressure heating system, thermostatic traps that discharge at temperatures below 212 F are generally used. Although there may be pressure in the return line, the temperature of the condensate will be below 212 F because there is no flashing. In a vacuum system, however, flashing may occur and temperature measurements are of no value.

A trap operating at high pressure discharges large volumes of flash steam. Flash steam results from the evaporation of hot-condensate as it passes into a lower pressure zone. Percentages of condensate evaporated at various initial steam pressures upstream of the trap during discharge to pressurized return lines (flash tank pressure) are shown in Table 1.

Suppose a trap is operating at 125 psig and is discharging condensate to atmosphere at a rate of 500 lb per hr. Table I indicates the percentage of flash is 14.8; therefore, steam discharge is 500 X 0.148 = 74 lb per hr. The specific volume of steam at atmospheric pressure is 26.8 cu ft per lb, so volumetric discharge of steam is 74 X 26.8 = 1983.2 cu ft per hr. The weight of water discharged is 500 – 74 = 426 lb per hr. Specific volume of water at 212 F is. 0.01672 cu ft per lb. Thus, the volume of water discharged is 426 X 0.01672 = 7.1 cu ft per hr. The percentage of steam by volume in this mixture is: steam_equasion201 Valving the trap discharge to vent to atmosphere is a fairly accurate way to determine trap performance, Fig. 1. This trap is operating properly at 125 psi, discharging to atmosphere. The discharge will be 99.6 percent steam and only 0.4 percent water.

Fig. 1. An atmospheric
discharge line joined to the condensate return line may
be used for a visual test of steam trap operation. Valve
A (normally open) is closed, and valve B (normally closed)
is opened to divert discharge to atmosphere.