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Steam Traps 

     As a general rule of thumb, steam traps are devices that allow us to recycle condensate while allowing steam to continue its purpose, but what are the working mechanics behind them?

     Following the boom of steam and its use during the industrial revolution, steam traps entered the scene in the early 1800’s. Only speculating, I would guess that plant personnel grew tired of manually having to release condensate from piping and realized that condensate is essentially “pure” water that can be recycled. Although they are critical to the overall efficiency of a boiler system, steam traps are nothing more than automated valves that separate condensate and non-condensable gasses from the steam line. Steam traps can be classified into three major types: Mechanical, Thermostatic, and Thermodynamic.


Mechanical Steam Traps

     Mechanical steam traps were the first on the scene to separate condensate and are still utilized today. They operate by utilizing the difference in density between steam and condensate water, the most popular types being the inverted bucket and float style traps. In other words, they work in direct relation to the amount of condensate present in the trap. As the name implies, the inverted bucket type includes an upside-down bucket used as a float device that controls when the discharge valve is opened or closed (Figure 1). The float type designs have an internal ball float which regulates a modulating discharge valve when condensate levels rise (Figure 2). This ensures that no steam is allowed through with condensate. These traps are simple in design yet are consistent and are some of the most reliable devices. One advantage that mechanical steam traps have over the others is their ability to operate proportionately to the amount of condensate return. Another benefit to these traps is their capability to withstand variance in steam/condensate loads and pressures to the designed degree. Since there is a distinct difference in temperature between condensate and steam, one easy verification method that mechanical traps are working properly is to take temperature readings from the vapor phase and compare between the liquid phase. Typically, the temperature difference should be 20-30°F and can be done using a non-contact digital thermometer.

Inverted Bucket Steam Trap

Figure 1: Inverted Bucket Steam Trap

Float Style Steam Trap

Figure 2: Float Style Steam Trap

Thermostatic Steam Traps

     Thermostatic steam traps operate on steam’s temperature difference from cooled condensate and air. Steam increases the pressure inside the thermostatic element, causing the trap to close (Armstrong). There are several designs, but the three most popular are the liquid expansion traps, bimetallic and balanced pressure thermostatic traps. Liquid expansion steam traps utilize internal oil that expands when heated, closing the condensate release valve. These valves are useful during startup when an excessive amount of cold condensate may initially be returning. As these traps are designed based on working temperatures, they are typically not useful during normal operating conditions and often require a mechanical steam trap in conjunction.

Liquid Expansion Steam Trap

Figure 3: Liquid Expansion Steam Trap Diagram

Balance pressure traps have an internal diaphragm and a contained solution that also regulates condensate separation while still using the thermostatic effects of steam. The thinly designed diaphragm wall allows for a rapid response to changes in temperatures and pressures but is more prone to corrosion failures. These have an advantage over the liquid expansion types in that they are more compact and contain fewer components that could fail.

Thermostatic Steam Trap Diagram
balanced pressure steam trap

Figure 4: Thermostatic Steam Trap Diagram

Thermodynamic Steam Traps

     The thermodynamic trap is an extremely robust steam trap with a simple mode of operation. The trap operates by means of the dynamic effect of flash steam as it passes through the trap (Spirax). Although they discriminate between liquids and gasses, they cannot distinguish steam from air or other non-condensable gases and have a reduced ability to bleed-off unwanted gasses. The only moving part is the disc above the flat face inside the contorl chamber, depicted in (Figure 4). They are compact, simple, and can be used in high pressure settings

Thermodynamic Steam Trap

Figure 5: Thermodynamic Steam Trap diagram.

     When it comes to deciding which steam trap(s) will work best for a given boiler plant, maintaining the desired performance of the system is paramount. Traps will need to be chosen based on the amount of condensate return, operational pressures, and amount of non-combustible gasses. And as for the performance of individual traps, they should be tested on a regular basis to ensure they are in good working order. Today, traps can be tested using an ultrasonic trap tester that can verify whether a trap is leaking. This should be left to a trained professional that understands the noise feedback associated with working steam traps. As mentioned earlier, temperature testing can be a simple test on mechanical traps. For instance, if a trap is blocked and is colder than the rest of the system, that might justify taking the trap offline for inspection. Steam trap selection, in conjunction with the proper maintenance, will allow for a boiler system to continue working properly and efficiently 


     Jed Kosch





List of Resources:


[1] Cold Weather Operation of Cooling Towers. (2016). MARLEY - Thermal Science, 1–8.


[2] Operating Cooling Towers In Freezing Weather. (2007). Technical Report - SPX, 1–8.


[3] BAC. (2015). Minimizing Energy Costs with Free Cooling. BAC Technical Resources, V, J50–J61.


[4] TRANE. (1999). Centrifugal Water Chillers. A Trane Air Conditioning Clinic, 1–70.


[5] Lane, R. W. (1993). Control of Scale and Corrosion in Building Water Systems (1st Ed). McGraw-Hill.

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