2000 Hook-up Book

Testing Steam Traps

from a pressure of 125 psi. The steam tables show that each pound of water carries 324.7 BTU which is a144.5 BTU more than it can carry as liquid at atmospheric pressure. As the latent heat at 0 psig is 970.6 BTU/hr., then 144.5/970.6 lbs. of flash steam are released per pound of conden sate, or 14.29%, which is some 74.45 pounds per hour. The vol ume of steam at 0 psig is 26.8 cu. ft. per pound, so some 1,995 cu. ft. per hour of flash steam is released. The remaining water, 500 - 74.45 = 425.55 lbs. has a volume of about 7.11 cu. ft. per hour. Thus, the discharge from the trap becomes 1995/1995 + 7.11 = 99.65% steam and 0.35% water, by volume. It is sometimes claimed that an observer can distinguish between this “flash” steam and leakage steam by the color of the steam at the discharge point. While this may be possible when a trap is leaking steam but has no condensate load at all, so that only steam is seen at the discharge, it is obvious that the presence of any condensate will make such differentiation virtually impossible. It would be like trying to distinguish between 99.65% steam with 0.35% water, and perhaps 99.8% In a closed piping system, trap discharge sounds may be a good indicator of its operation. A simple stethoscope will be of little value, but the sound produced at ultra high frequencies measured by an ultrasonic instrument eliminates background noise interference. Live steam flow produces a greater and steady level of ultra sound, while flashing condensate tends to have a crackling sound and the level changes with the trap load. The problem is that the instrument requires the operator to make a judgement as to trap condition which will only be as reliable as his training and expe rience provide for. steam with 0.20% water! Trap Discharge Sounds

Increasing attention is being paid in modern plants to means of assessing steam trap perfor mance. While it is important to know if a trap is working normally or is leaking steam into the con densate return system, most of the available methods of assess ing trap operation are of much more restricted usefulness than is appreciated. To explain this, it is necessary to consider the mode of operation of each type of trap when operating and when failed, and then to see if the proposed test method can distinguish between the two conditions. Temperature Test Methods One well established “method” of checking traps is to measure tem perature, either upstream or downstream. People use pyrome ters, remote scanners and temperature sensitive crayons or tapes, while generations of mainte nance men have thought they could assess trap performance by spitting onto the trap and watching how the spittle reacted! Certainly, if a trap has failed closed, the tem perature at the trap will be lower than normal, but equally the equip ment being drained will also cool down. The trap is not leaking steam since it is closed, and this failure is only a cause of problems in applications like steam main drips where the condensate not discharged at the faulty trap is car ried along the steam line. More usually, the temperature on the inlet side of the trap will be at or close to the saturation temperature of steam at whatever pressure is reaching the trap. Even if the trap were blowing steam, the tempera ture remains much the same. The one exception is in the case of a temperature sensitive trap, especially one of the bimet al pattern. If this fails open, then the temperature at the inlet side will rise from the normal sub cooled level to saturation values, and this rise may be detectable if the steam pressure is a known, constant value.

Measuring temperatures on the downstream side of a trap, by whatever method, is even less likely to be useful. Let’s look first at a trap discharging through an open-ended pipe to atmosphere. The pressure at the trap outlet must be only just above atmos pheric, and the temperature just above 212°F. With any condensate present with the steam at temperatures above 212°F on the inlet side, the condensate, after passing through the trap will flash down to 212°F and this temperature is the one that will be found. Any leaking steam will help evaporate a little more of the condensate without increasing the temperature. Again, the only exception which may be encountered is the low pressure steam heating system where thermostatic traps normal ly discharge at temperatures below 212°F into atmospheric return. A temperature of 212°F here may indicate a leaking trap. Discharge of condensate into a common return line is more usual than discharge to an open end, of course. The temperature in the return line should be the saturation temperature corre sponding to the return pressure. Any increase in this temperature which may be detected will show that the return line pressure has increased. However, if trap “A” discharging into a line blows steam and the pressure in the line increases, then the pressure and temperature at traps “B” and “C” and all others on the line will also increase. Location of the faulty trap is still not achieved. Visual Determinations The release of flashing steam from condensate nullifies the effectiveness of test cocks, or three-way valves diverting a trap discharge to an open end for test purposes. It also restricts the infor mation which can be gained from sight glasses. Consider a trap dis charging to an open end some 500 lbs. per hour of condensate

SYSTEM DESIGN

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