The following are considerations associated with steam systems that have special significance for the steam trap user and designer alike. While these are common problems, their adverse effects can be minimized by good planning and equipment selection.
Condensate will always collect in the low points of a steam system unless special effort is made to drain it away or to eliminate the low point. Below figure shows a sagging steam main that has allowed condensate to accumulate.
Steam flowing in the main, often at surprisingly high speeds (90 miles per hour is not unusual), will pick up slugs of condensate and slam them into valves, elbows, steam traps or other such equipment with devastating affect. Steam trap designers seek to create robust products that will withstand water hammer. Steam trap users are best advised to correct water hammer at its source by following good piping practice and promote good drainage and prevent the accumulation of water that makes water hammer possible.
Boilers and steam systems are full of air prior to start-up. An especially important part of getting any steam system operating efficiently is the removal of air from it. Air is a poor conductor of heat, and mixtures of air and steam have less heat content than steam alone at the same pressure. Both of these factors have an especially adverse affect on heat transfer rates. Air is eliminated from the steam system by thermostatic air vents and by steam traps. Some traps are much more effective air eliminators than others, a subject which can be discussed in greater detail in another post.
Some mechanical and thermodynamic traps have difficulty differentiating between steam and air. When such traps restrict the proper venting of air and delay the heating up of the system, they are considered to be "air binding." Thermostatic air vents and thermostatic traps are commonly used to improve air venting.
Carbon Dioxide and Oxygen are both present in steam systems. Free oxygen is a normal constituent of water but it is principally the boiling process that volatilizes the carbonates in water to produce carbon dioxide. Both gases foster corrosion. An important function of a steam trap is to assist in the purging of these non-condensable gases from the steam system.
Certain applications, piping configurations and steam trap types tend to create conditions in which steam at the trap keeps it closed, thereby preventing condensate which has formed upstream of the trap from being drained.
Small levels of backpressure typical of a properly designed condensate return system are not generally a problem. It is the elevated levels of backpressure found in the inadequate return system that creates drainage problems.
All steam systems and their associated components suffer from the effects of corrosion. Corrosion attacks boiler tubes, steam mains, heat exchangers, valve components and fittings such as steam traps. Over time all these items succumb. The primary defense is a carefully monitored and maintained boiler feedwater treatment system that controls the gases (oxygen and carbon dioxide) which promote corrosion. Carbon dioxide by itself is not corrosive, but it can combine with free hydrogen to form carbonic acid which is corrosive. A principal reason stainless steel is used extensively in steam traps is to resist the effect of corrosion and prolong the life of the trap.
Corrosion is best controlled by proper boiler water treatment but any piping arrangements that interfere with good drainage increases the potential for corrosion problems.
The trash and accumulated debris in a newly piped steam system must be seen to be believed. In older systems dirt, corrosion products, and sealants from the maintenance repair of a leaky joint, continue to plague such components as small valves, instruments and steam traps. These devices with their small clearances and vulnerable seating surfaces are especially susceptible to dirt related failures. Dirt which prevents the free movement of internal parts or which gets caught between the valve and seat sealing surfaces leading to erosion damage is a major source of problems. With good reason, the knowledgeable user places a pipeline strainer upstream of each steam trap.
By their nature steam traps generally have small passages that are subject to obstruction. Corrosion products and pipeline trash are the usual culprits. A clogged steam trap means trouble because it is no longer able to protect or drain the equipment it was meant to serve. Dirt pockets and strainers help to protect the trap.
A shutdown in'freezing conditions of a system that drains poorly, for whatever reason, is an invitation to trouble. In extremely cold conditions poorly insulated condensate return systems can freeze. Even ifequipment is not damaged by ice, seldom the case, start -ups become extremely tedious because ice blockages prevent the circulation of steam necessary to bring the system up to temperature. Valuable production time is lost.
A good knowledge of the properties of steam and the problems of steam systems is an essential foundation to a good understanding of steam trapping.
All steam systems must deal with problems of corrosion, air and gas venting, dirt (usually corrosion products) and water hammer. Steam traps are both a victim of these problems as well as potential solution contributors. It is knowledge of good practice that will decide whether they are part of the problem or part of the solution.