The Cost of Lost Steam
Let’s examine how a trap passing steam can affect your whole system. First, determine how much the lost steam costs. Since the steam is being lost at saturation conditions (0 psig from the vented receiver) we can determine the amount of Btu’s that are no longer recoverable. 0 psig steam contains 970 Btu/lb. So, for every pound of steam we don’t recover, we lose 970 Btu’s. But we’re losing more than just that latent energy. We are also losing sensible energy. Having lost that pound of steam, we must now replace it with a pound of water and we have to add energy to the new water just to bring it up to saturation condition (for water it is approximately 1Btu/lb°F). Let’s say the water we are introducing is 60°F. Because we lost our steam through a vented receiver, we have to raise its temperature to 212°F. And because the steam we lost had already been treated, there is the additional cost of treating the new water.

Other Effects of Bad Traps
Knowing this information we are able to calculate the loss associated with losing steam through a bad trap. However, there are other indirect costs related to the failed trap that are more difficult to calculate. One is the damage caused by water hammer. As steam enters a condensate return line, there is the chance steam will mix with the condensate and some of the condensate may flash into steam and recollapse into condensate, causing water hammer and equipment damage. A failed trap can pressurize the return main resulting in insufficient differential pressure across other traps draining into the same main as the failed trap. Consequently, condensate will backup in the processes the traps are associated with. Then someone will wrongly diagnose these traps as being defective, possibly even replacing a good trap and still not getting the desired results. Frustrating! Because the trap has no differential pressure due to the pressurized condensate line, there is also the possibility of water hammer occurring in the heat transfer device that cannot drain. Again, the mixing of steam and condensate can cause water hammer.

Higher Steam Temperatures Problems
This is not the last of the problems that a trap passing steam can cause. With steam passing through the trap, the return condensate is at a higher temperature, which sounds like we are saving energy by not having to add as much sensible heat to the condensate to bring it back up to saturation conditions. But the warmer the condensate is, the more flash steam there will be. Even worse, the pumps will handle hotter condensate, and this can have a negative effect on the pump seals. And, the higher the temperature, the less NPSH (Net Positive Suction Head) we will have available at our pump suctionthe less NPSH available, the greater the chance for cavitation to occur in our pump. So, the indirect cost of a trap passing steam may be great. The best solution is to understand the operation of your traps, survey and test them on a regular basis, and repair or replace traps when they fail. The cost will always be justifiable. So the next time you have problems, look at the whole system. Rember that even if the process is still working, a bad trap may actually cause other, more serious problems.