A lot of times when looking at the efficiency of a boiler, facility operators will focus on optimizing the performance of their fuel and heat exchange systems, overlooking the entire systems; leaving steam traps out of the picture. In fact, the US Department of Energy explains that among steam systems that have not been maintained for between 3 and 5 years, about 15% – 30% of the installed steam traps may have failed. That leads to leakage of live steam into the condensate stream.
What is a steam trap?
A steam trap is an industrial device used to remove condensate and non-condensable gases from networks with live steam. They do this by various mechanisms and with minute losses of energy from steam lines.
Before going further, let’s understand steam and the action of steam traps.
Why is steam important?
Steam is an affordable mover of energy, being easily generated and transported in steam lines. It carries a lot of energy because it contains large amounts of sensible and latent heat. After doing useful work (in heaters, turbines etc), it is condensed back to water, and either recycled back to the boiler or channeled to a drain.
And what of condensate?
Condensate should always be separated from the steam loop whenever possible because not only can it cause corrosion of the lines, but it can also cause water hammer, compromising on effectiveness.
Types of steam traps
There are 4 types of steam traps in the market today:
Mechanical steam traps
Rely on density difference between the live steam and condensate to separate them. It contains a float that moves up or down depending on the quantity of condensate deposited inside it. There are 2 types here:
- The ball float type, which opens once a specific level of flooding in the device is reached. It also has an air vent on the topside to allow when the steam loop is being started. Visually, it will have a large bulge that houses the float; making it susceptible to heat loss by radiation. It is therefore advisable to lag it to prevent this loss, especially if the steam traps are exposed to outdoor weather conditions.
- The inverted bucket type, which lifts the bucket when steam flows in, closing the condensate outlet. At reduced steam levels, the condensate outlet is open when the bucket is lowered. Under low load conditions, it will experience steam losses of around 0.5kg/h if the trap is of 0.5inches. Lagging is also recommended here
Thermostatic steam traps
This type uses difference in temperatures between the steam and condensate to effectively separate them. Examples include Thermostatic traps, Temperature control traps (TB) and Bi-Thermostatic steam traps.
It simply holds the condensate until it reaches a set temperature before being released into the condensate line. Mixture of the condensate with live steam is avoided to prevent more heat loss from the steam. To achieve this, a cooling leg is usually incorporated into the trap system. A video illustration of its operation is shown here.
Thermodynamic (TD) steam traps
These will rely on the pressure and volume variations of both steam and condensate to make the separation successful. Their operation is controlled by the formation of flash steam that builds pressure in the top cap/bonnet and causes the plate to close condensate flow. Upon reduced bonnet temperature because of heat loss, condensate pressure once again pushes the plate up allowing flow.
Orifice/venturi steam traps
Their operation relies on the two-phase nature of the fluids so as to separate them. Passing the steam and condensate through various stages means the denser liquid throttles the nozzle and blocks any flow of steam through it. This results in a continuously ‘flooded’ nozzle that constantly discharges condensate as it forms. It also has no mechanical parts meaning that it’s rugged with a long service life.
Installation of steam traps
Along with the presence or dirt and pressure surges, oversizing and poor installation of steam traps are some of the main causes for their failure.
Some things to consider when installing steam traps are:
- Steam traps should be supplied by condensate using gravity.
- Install them with separation distances of 15m (50 feet) to 45m (150 feet) depending on your requirement and local regulations
- For steam traps with diameters of 2.5cm (1 inch) or less, use tube connectors as opposed to threaded connections to avoid steam leaks from corrosion, contraction & expansion action that the threads face over their lifetime
- The pipe diameter before the trap should never be less than the trap’s inlet diameter. Similarly, after the trap, the pipe diameter should at least match that of the trap, if not larger.
- Ensure your steam traps have strainers to reduce the effects of corrosion. These can either be separately installed before the trap or come integrated into it.
Assessing steam trap performance
Having a preset schedule for steam trap maintenance will help you track the performance of each trap and ready you for any repairs or replacements. Methods of inspecting steam traps include:
- Visual method – requires an expert who will look at individual traps, and through a sightglass, note the presence of flash steam and live steam. From such observations, trap performance can be quantified
- Sonic method – using a trained ear or sonic meter, the various sound signatures produced as live steam flows across the trap can be used to assess how well the device is working
- Temperature method – for this method, temperature changes across the trap are monitored and performance can be computed based on noted variations. However because of closely similar temperatures of the saturated steam and condensate, it is difficult to realize significant temperature changes.
Standards to observe
There are two standards you should check to help you quantify the performance of your steam traps:
This standard by the Association of Mechanical Engineers (ASME) outlines the procedures used for conducting analyses that quantify the performance of steam traps operating in steam systems.
This standard by ISO highlights two test methods for steam traps
Steam traps’ main objectives are to remove condensate as it forms and remove air and non-condensable gases, all with a negligible loss of energy. Therefore, their design, installation, and proper maintenance is very important and can save your facility from unnecessary energy losses.