The Trouble with Steam Traps

As the age of our steam systems increases, and the knowledge about that technology is lost, the smaller components such as steam traps get confused and changed usually for the worse. The article below is meant to give you a brief overview of the technologies available and how they work.

To start the conversation, lets describe what a steam trap is really doing. A steam trap is not exactly doing as the name suggests, trap steam, but more trap its byproduct in the form of water or what the industry calls condensate. As the condensate accumulates inside of the body of the trap it releases it, either in bursts or at a constant rate, downstream. Now this is done in a handful of ways which can be broken into four major categories; mechanical, thermostatic, thermodynamic, and orifice traps. Each has their specific application and use but generally these groups separate out the different modes of operation. I will take this moment to mention I always default to the manufacturer of the product you are working with if there is any question if your application is not explicitly stated in their literature. Nobody wants to be in the situation of misapplication of product, especially the manufacturers, because at the end of the day if something goes wrong they are the first to be blamed. Mechanical: First we will cover mechanical traps. Mechanical traps operate through a lever-valve system that generally regulates a constant flow of condensate through the valve and downstream to your condensate receiving system. This lever is attached to a floating ball-like device (think buoy or toilet bowl float) that is naturally buoyant. The valve is directly linked to the float in some form so as condensate fills the cavity of the steam trap it raises the float and in-turn lifts the valve off of its seat which allows the condensate to flow freely downstream. This makes sense and seems to be a pretty straightforward way to relieve condensate, but requires knowledge of the system that you are putting them into. Things meant to be buoyant are only buoyant when the fluid they are designed for at the pressures they were designed for are present. If this balance is either changed through the introduction of another fluid (such as air which has a way of finding its way in to a steam system) or by pressures greater than what the device is capable of handling then the trap will either perform erratically or not at all. Most companies making this type of trap have designed tables and in some cases additional mechanisms to counteract these effects and ensure that their product operates properly for their various applications. I do have to mention one sub-category of mechanical trap that contradicts nearly everything that I have just said; bucket traps. Bucket traps, though mechanical in nature are not a regulating trap, they are a trap that relives condensate in bursts like other traps. They do have a float, but their float is not a hollow ball, but a bucket which in itself has specific properties that interact with the steam and condensate that only allow it to relieve condensate at rapid changes in the steam-water balance. Once this balance is altered the bucket lifts and relieves the condensate very rapidly. Once the condensate is gone, the steam then fills the cavity of the trap and slams shut. This can be an effective way to relieve condensate, but has to be separated from other mechanical traps because of its obvious differences. Thermostatic: The next type of trap that I will discuss is the thermostatic trap. This type contains a valve and seat like the mechanical trap series but instead of a linkage-driven relationship it has a metallic element that opens and closes relative to the temperature it is experiencing at the pressure the system is operating. In general there are two sub-types, the bellows and the bi-metallic type. The bellows type is filled and set to react within 10C of the saturation temperature of the steam (also called sub-cool) at whatever pressure the system exists at. It cares very little what that pressure is as long it is withing its built conditions. A bellows type will operate at the same capacity with respect to its pressure differential no matter the incoming pressure. Once the cavity of the trap is cooled to its designed sub-cool temperature the bellows will open, allowing the flow of condensate downstream. As the condensate leaves it is replaced with steam which allows the bellows to heat up and expand, again closing the valve. This opening and closing of the valve generally occurs in bursts quickly eliminating condensate from the system. The bi-metallic work in a similar way but instead of a traditional bellows it reacts on a difference in metals. What si meant by difference in metals is that at certain temperatures it expands and contracts opening and closing its valve. This type is meant for a particular range of pressures since it requires a certain degree of tuning to operate properly. Unlike the bellows type the range is set from the factory at which it can operate so is more particular to how and where it works. Thermodynamic: Thermodynamic series traps are next for discussion. Thermodynamic traps are far more simple than the other styles of traps discussed. Normally a thermodynamic trap consists of a seating surface and a disc that rest atop it. The entire reason that a thermodynamic trap can work is due to what is known as the Bernoulli Principle. In-short the principle states that the velocity of a fluid increases at a rate equal to a reduction in pressure and loss of potential energy. what this means for a trap is that as the condensate fills the trap cavity atop the disc the properties of the fluid change the relationship of it and the fluid that sits on the other side of the trap ie the outlet of the trap. as these conditions become similar the disc begins to lift releasing condensate downstream. As the fluid in the cavity of the trap changes back to steam as the condensate is released, the properties again change resulting in a closing of the disc onto the seating surface. This is done is a series of very fast bursts which efficiently release condensate. Like thermostatic traps, thermodynamic traps can operate on a wide range of pressures as long as a balance between inlet and outlet can be achieved in enough consistency to lift the disc. Orifice: Orifice are the last series of trap to be discussed here. These traps are by far the simplest in operation and design of all the types of traps. Orifice traps are designed to only work with a very particular range of flow. They solely operate on pressure difference between the inlet and outlet of the trap face and only with a set steam capacity relative to the amount of condensate that it is capable of seeing. These traps have to be carefully selected for each particular application that they are being put into since they have no capability of adapting to varying flow conditions or pressure conditions. If the condensate rate is higher than what the trap was sized for then condensate will back up into the system before the trap. If the condensate rate is lower than what it was designed for then your system will waste steam through the orifice. Though easy to calculate this can be the most difficult trap to use since it reliant on the proper sizing of its load conditions. If you liked this article feel free to share your comments. I plan to go into applications as well as specifics about other parts of steam systems in future articles. If you have a particular request feel free to contact me at sustainedengineeringsolutions@gmail.com.