The evaporation of water into a gas results in the production of steam and in order for the process of vaporization to take place, the water molecules need to be provided with an adequate amount of energy so that the bonds that hold them together (hydrogen bonds, etc.) may be severed. The term “latent heat” refers to the heat that is released when a liquid is converted into a gas. That latent heat is transmitted and used by the medium being heated and after this has been released, the steam condenses. The condensate doesn’t posses the same heating capacity as the steam and if is not quickly removed from the steam piping or heat exchangers, the efficiency of the heating system will drastically drop. (more about steam system can be found by following this link).
I believe that many of you working onboard vessels have encountered, at least once, problems with the steam heating for different systems, problems that were mostly related to the steams traps installed along the steam lines, which most of the time are found to be rusty, seized and replaced with a regular ball-valve.
Steam traps are very important parts of the steam system and they are a form of automatic valve that are designed to prevent steam from escaping while simultaneously removing condensate (also known as condensed steam) and non-condensable gases such as air. Like in any other industry, steam onboard vessels is used for either the purpose of heating or as a driving force for mechanical power. In these kinds of applications, the usage of steam traps is necessary to prevent the loss of steam.
As per ANSI/FCI 69-1-1989 the steam traps are defined as “self contained valve which automatically drains the condensate from a steam containing enclosure while remaining tight to live steam, or if necessary, allowing steam to flow at a controlled or adjusted rate. Most steam traps will also pass non-condensable gases while remaining tight to live steam.”
Using a ball-valve instead of a steam trap, it might look and sound like a “good” compromise, especially when you don’t have a spare steam trap, but because the valve opening is set to discharge a constant amount of fluid using this method, it is unable to correct for fluctuations in the load of condensate because of this arrangement. This is the method’s most significant drawback. In point of fact, the amount of condensate produced by a particular system is not a constant. When it comes to the piece of machinery in question, the load of condensate that is present during start-up is distinct from that which is present throughout normal operation. Variations in the product load also result in variations in the amount of condensate that is created as a byproduct of the process. Condensate that ought to be discharged will instead pool inside the equipment or the pipe, and the effectiveness of the heating system will suffer as a result, because the manual valve is unable to adapt to fluctuations in condensate load. On the other side, steam leakage will take place when there is a decreased condensate load, which will result in the loss of steam.
There are different types of steam traps used onboard vessel and those mainly are:
- mechanical traps: float type, inverted bucket type.
- thermostatic type traps
In contrast to other types of steam traps, which rely on either a change in temperature or a change in the velocity or phase of the steam, mechanical traps are steam traps that function according to the principle of specific gravity (more specifically, the difference between the specific gravities of water and steam). When it comes to mechanical traps, the movement of a float that rises and lowers in response to the flow of condensate is what causes the valve to open and close.
When using float traps, the amount of condensate in the trap has a direct influence on the location of the float inside the trap. The float is sensitive to the flow of condensate and adjusts its position to either open or close the valve in response. A float is typically fastened to the lever that operates the valve in designs known as lever floats. The float will become buoyant as condensate begins to enter the trap. This will cause the lever to move, which will then cause the trap valve to open. On the other hand, because the lever arm has a restricted range of motion, the head of the valve frequently remains in the direction of the flow of condensate. This might result in an additional pulling force being required to close the valve when there is a significant volume of flow.
In steam traps that use an inverted bucket, the bucket itself is coupled to a lever that controls the opening and closing of the trap valve in reaction to the movement of the bucket. The steam causes the bucket to become buoyant and rise to the surface when air or steam enters into the underside of the inverted bucket and condensate surrounds it on the outside. When placed in this position, the bucket will bring about the closing of the trap valve. A vent hole is located at the top of the bucket, and it is there so that a small bit of the vapor can be released into the top of the trap, and then it can be discharged further downstream. When vapor is released through the vent hole, condensate begins to fill the interior of the bucket. This causes the bucket to sink, which enables the lever to open the trap valve and release any condensate that has accumulated.
Mechanical traps are able to function in precise response to the flow of condensate and their performance is unaffected by the majority of the environmental influences that may affect other types of traps. This is one of the distinguishing advantages they have over thermostatic and thermodynamic steam traps.
Steam traps of the thermodynamic type are highly sought after because of their small size and adaptability across a broad pressure range. They could have a straightforward structure and be able to function in either the horizontal or the vertical orientation.
Thermodynamic disc steam traps have an operating characteristic that is cyclical and intermittent at the same time. After opening to allow the discharge of condensate for a few seconds, the valve mechanism, which is made up of a disc and seat rings, subsequently closes for a generally longer period of time until the beginning of a new discharge cycle. The differential in the forces that are exerted on the top and bottom sides of the valve disc is what’s responsible for the opening and closing action that thermodynamic disc traps exhibit. Variations in the kinetic energy and pressure energy of the typical fluids involved, which include air, condensate, and steam, are the fundamental basis for these forces. At the beginning of the operation, incoming fluids at line pressure consisting of air and/or condensate (and even sometimes steam) exert an opening force (lifting force) on the bottom of the valve disc, which causes it to raise and open. Because of this opening force, the disc is lifted off of its seat to provide room for condensate passage.
There are several reason of why a steam trap doesn’t function properly and these can be (if you want to learn more about it follow this link):
- negative pressure differential – the pressure drop across steam trap must be zero or positive in order steam to be discharged.
- steam lock – steam locking occurs when the steam is trapped between condensate and the steam trap.
- group trapping – collecting condensate from different sections of steam heated equipment with different condensate pressure into one condensate line with one steam trap.
- high backpressure
- debris or deposits
- backward installation
As is the case with any mechanical device, a steam trap, regardless of how long-lasting it may be, will at some point require either repair or replacement. When a trap is used for a longer period of time, it has more of a chance to become worn. Because of this wear, the performance of the trap will deteriorate, and it may finally become impossible to use.
Because a trap’s lifecycle can be influenced by a number of factors, such as the steam trap type, application, pressure, condensate load, piping configuration, and steam/condensate quality, determining when a trap will fail is an extremely difficult task. This is due to the fact that a trap’s lifecycle can be influenced by a number of factors. Every vessel that uses steam should implement a steam trap management program to assist prevent premature trap failure and identify failures in a timely way.
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- YouTube video credit: The Engineer Portal T.E.P; hararat
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