Steam Traps

Condensate creation in steam lines is an inevitable phenomenon. Due to the pipe’s heat loss, there is always a portion of steam converted into condensate. This condensate causes many problems to the pipeline such as corrosion and erosion. It is also the reason of high pressure drop of the line. Moreover, if there is a pocket (low point) in the pipe’s route, condensate would trap there. As a result, it harms the fluid hydraulic plus pipe’s internal surface.

Steam traps are vital components to extract the condensate from steam lines (for line sizing criteria click here). They can be found in all the steam generation and handling units. As their name suggests, they are utilized to remove the liquid condensate by trapping the steam. Without them, steam is also wasted alongside with the condensate removal. They are almost at the size of the corresponding valves. Based on their applications, there are several types of steam traps such as thermostatic, float & thermostatic, thermodynamic and inverted bucket.

Steam traps have three primary usages:

• Removing the condensate from steam lines.
• Liquid drainage of process equipment
• Trapping the steam at the outlet of heat exchangers while allowing condensate to pass. Applicable for those exchangers that consume steam as hot source (reboilers). Therefore, no steam would pass the exchanger without complete condensation.

In this article, we will study the working principle of various types of steam traps available in oil and gas industry. In addition, we will provide guidance on selecting the proper type of steam traps based on their characteristics. At the end, their design techniques will briefly be discussed.

Thermodynamic (disk) steam traps are mostly used at main steam headers. They are small-sized steam tarps with a maximum size of one inch. They are used for intermittent condensate removal application in a cyclic on/off operation. They only discharge small amount of air (non-condensable) and therefore are not used for process applications. Due to their robust design, they are resistant to water hammer.

Their working principle is based on area difference of the floating disc at closed condition. At normal condition, where there is only condensate available, the condensate push up the floating disk and pass to the discharge. Upon introducing steam, the steam moves across the disk and enters the flash chamber. Due to the positive pressure, the disk falls down and blocks the way. The back (upper) surface area of the disk is exposed to more amount of steam. Therefore, downward force outweighs upward force and the disk remains closed. As soon as condensate enters the main chamber again, it makes the trapped steam cool down and condense. Therefore, the disk would pup up open again and the remaining condensate will be discharged.

Thermostatic steam traps are capable of operating at continuous applications such as tracing and process. They can handle large amount of air. The main advantage is that at start-up, the trap is in open status, allowing air and condensate to be removed from the system easily.

Their working principle is based on thermal expansion and contraction of a fluid-field thermal element called bellows. When the steam is in contact with the bellows, the inner fluid vaporizes and expands, causing the valve to block the discharge by touching the valve seat. On the other hand, when the bellows are exposed to condensate or air, they contract due to the lower temperature and open the valve allowing the fluid to be removed.

Float & thermostatic (F&T) traps are mechanical types of steam traps. They are the mostly used steam traps in industry. They are usually installed at the outlet condensate line of those heat exchangers which utilize steam as their hot source. Float & thermostatic steam traps are the number one choice for drain line of process equipment. Apart from condensate’s main discharge orifice, they contain a thermostatic air vent in order to allow the discharge of the non-condensable components out of the system. Therefore, they are preferable in start-up situation, where a vast amount of air needs to be discharged.

Their working principle is based upon the density difference between floating ball and condensate. There is a floating element that connected to a valve plug. When the condensate is collected in the chamber, the liquid level rises the float causing the valve to open and discharge the liquid. In addition, when air enters the chamber, the thermostatic vent contracts and open the valve allowing the air to vent out. On the contrary, if the liquid level drops, the float goes down and close the valve. Meanwhile by introducing steam the thermostatic valve expands because of higher temperature and block the discharge of steam.

Similar to thermodynamic traps, inverted bucket steam traps are mechanical intermittent traps with cyclic on/off characteristic. They are suitable for removing condensate from steam headers. Due to their poor air handling capability, they are not used in process applications. The main limitation is that they have to contain a certain initial amount of water, known as prime, in order for the bucket to float. Without this prime, the trap fails to function correctly and would discharge steam accidently. Inverted bucket steam traps shall not be used in applications with regular pressure change. Because the prime may flash when the pressure falls down.

Their working principle is based upon the buoyancy force acting on a floating bucket. When there is only condensate available, the bucket goes down causing the valve to open and allow the condensate to discharge. There is also a small bleed hole on the topside wall of the bucket. This hole allows the air to pass through and collect at the top section of the chamber. Finally, this air is discharge alongside with the condensate. However, large amount of air will lift the bucket and temporarily lock the system. On the other hand, when the steam enters the trap it collects under the bucket and rise it due to the buoyance force. Therefore, the valve is closed. As the trapped steam gradually condenses or is bled through the bleed hole, the bucket fills with condensate, sink to the bottom of the chamber again, and opens the discharge valve.

In order to size the steam trap’s orifice, two important parameters have to be reported:

1. Condensate flowrate
2. Pressure drop across the orifice

Condensate flowrate is determined based on the estimated amount of steam converted to liquid at normal operation. However, since the system temperature at start-up is not high enough, the condensate flowrate is expected to be more than normal condition. Thus, it is always recommended to consider a safety factor for the calculated condensate flowrate. As a rule of thumb, this safety factor is equal to 2. Therefore, the reported flowrate shall be 200% of calculated flowrate at normal operation.

For reporting the design pressure drop, it is important to consider the downstream piping and elevation change. As an example, if the inlet pressure to the steam trap is 4 barg, and the condensate is planned to collect in an atmospheric storage tank with a liquid level of 5 m (0.5 bar), and the pipe’s pressure loss to the tank is 1 bar, the reported pressure drop is equal to 4 – 1 – 0.5 = 2.5 bar.

• Steam traps shall be installed at the drip pockets (lowest elevation of pipes) in order to function properly.
• Steam trap’s installing manifold is like the control valves with a double block and bleed configuration. There are two block valves at both sides, one drain at inlet and a bypass line for maintenance.

Steam traps are used to remove condensate from steam lines. There are various types of steam traps available in industry. Each type has specific characteristics and applications. Steam traps are design based on maximum condensate flowrate and pressure drop. In this article we reviewed different types of steam traps and their design procedures. We also provided some guidance on selecting the proper type of steam traps.