By Nwaoha Chikezie
A steam trap is a device attached to the lower portion of a steam filled line or vessel which pass condensate but will not allow the escape of steam. It automatically controls the condensate removal, air and carbon dioxide from a piping system with minimal steam loss. The hot condensate removal is very necessary to prevent water hammer, which is capable of damaging or misaligning piping instruments.
It is also very desirable to avoid air in the steam system as any volume of air consumes part of the volume that the system would otherwise occupy. Apart from that, the temperature of air/steam mixture normally falls below that of pure steam. Again, it has been prove that air is an insulator and clings to pipe and equipment surfaces resulting to slow and uneven heat transfer.
Traps are used on steam mains, headers, separators, and purifiers, where they remove water formed as a result of unavoidable condensation or carry-over from the boilers. They are also used on all kinds of steam heating equipment in which the steam gives up heat and is converted to condensate. Coils are used in heating buildings in water heaters, and in a wide range of industrial processing equipment are included in the classification.
Whether a trap is used to keep condensate from accumulating in a steam line or to discharge water from a steam-heated machine, its operation is important. If it leaks, steam will be wasted; if it fails to operate, water will accumulate. A satisfactory trap installation must pass all the water that flows to it without discharging steam, and must not be rendered inoperative by particles of dirt or by accumulation of air, and must be rugged in construction with few moving parts, so that it will remain operative with a minimum of attention (see Fig. 1).
The presence of carbon dioxide reduces heat transfer because the steam pushes it to the walls of heat transfer surfaces. Secondly, carbon dioxide also dissolves in the condensate to form carbonic acid, which may corrode piping and equipment.
A strainer is usually installed in the line ahead of the trap, to prevent sediments from stopping up the trap orifice. When selected in the correct pressure and provided with strainers these traps will give satisfactory service (see Fig. 2).
Traps are termed "non-return" when the condensate is discharged into a receiver or heater rather than directly to the boiler. A "return" trap delivers the condensate directly to the boiler. Return traps are located above the boiler, and when filled with water a valve automatically opens and admits steam at boiler pressure. This equalizes the pressure and the water flows into the boiler as a result of hydrostatic head caused by the elevation of the trap. These traps are sometimes used in connection with low pressure heating boilers.
There are different types of steam trap used in the upstream and downstream industry. Depending on the suitability of the process, these different types of traps are employed. Typical examples are as follows:
• Inverted bucket trap
• Impulse steam trap
• balanced pressure thermostatic trap
• Float actuated trap
• Bimetallic trap
Bimetallic Trap
Like in Port Harcourt Refining Company Ltd, Nigeria, the most commonly used steam trap is the bimetallic trap. This trap is small and lightweight and provides maximum discharge of non-condensables. It can withstand freezing, water hammer, high pressure and super heated steam. It discharges condensate well below steam temperature to reduce flash steam.
The bimetallic trap consists of a strip of two dissimilar metals with different coefficient heat of expansion joined together. When cold the bimetallic strip is flat, allowing the valve to open. When steam enters the chamber, the strip is heated and takes an oval shape, causing the valve to close. It is always used in constant steam main and tracing lines.
Inverted Bucket Trap
Several principles are employed in the operation of this trap. The water and steam enters at the bottom and flow upward into the inverted bucket. As long as the bucket contains steam, it is buoyed up in same way that an inverted empty bucket is buoyed up in water. While in this position, the valve is closed and there is no discharge of water or steam from the trap. As water enters the bucket it displaces the steam and the bucket looses its buoyancy and drops, causing the valve to open. After the water has been discharged, the bucket again fills with steam, the buoyancy is restored and the valve closes. A small vent at the top of the bucket allows air to escape, thus preventing it from interfering with trap position.
Thermostatic Trap
The release of air is controlled by a thermostatic vent, which opens when the temperature decreases and allows the air to be discharged from the trap. Under normal operating conditions with steam in the trap, the vent remains closed. The valve seats and disks are made of stainless steel or other alloys to resist corrosion and wear. This trap is intended for use in removing condensate from steam systems. It has a thermovent valve which prevents air from entering the trap operation. When temperature of the trap decreases.
Float-Actuated Float
May be adapted to a wide range of operating conditions. When water fills the body of the trap, the float rises thereby opening the valve. After the water has been discharged, the float drops closing the valve and preventing the escape of steam.
Impulse Steam Trap
This operates on the on the principle that hot water under pressure tends to flash into vapor when the pressure is reduced. When the hot water is flowing to the trap the pressure in the control chamber is reduced, causing the valve to rise from its seat and discharge water. As steam enters the trap, the pressure in the control chamber is increased, causing the valve to close.
Common Problems Associated with Steam Trap
Air Binding
When traps are connected by a long length of small diameter horizontal pipe, condensate holds up in the steam space and cannot flow to the trap. To prevent air binding, the piping to the traps should be a larger diameter pipe and short length, which allows a higher flow rate. Another method is to put a vent valve at a high point in the system.
Dirt
Steam condensate often contains particles of scale and corrosion products that can erode trap valves. Some of these particles are large enough to plug the discharge valve or jam it open. To prevent this, a strainer must be installed upstream of each trap.
Improper Sizing
Traps are typically specified several times larger than required using a safety factor to calculate the trap capacity. A trap that is undersized will cause condensate to interfere with the heat transfer efficiency. Therefore, traps having too much excess capacity waste money act sluggishly and generate high backpressure that may significantly reduce the life of the trap
Steam Leakage
This occurs when the valve seat of a steam trap is subjected to erosion and corrosion. When the seat is damaged, the valve will not sit properly and the trap may leak live steam. Steam leakages can be managed by installing a Steam Lock Evaluator (SLE).
Steam Locking
When steam trap is installed in a similar way to the case of air binding, steam locking can also result. In this case the steam may prevent the condensate from reaching the trap. Condensate cannot get to the trap unless it can displace the steam. To prevent steam locking, it is necessary to put the trap as close as possible to the equipment or the line to be drained. If the trap is just below the equipment or line, a balance pipe may be installed between the two to serve as a vent and prevent steam locking. Traps can also be fitted with a steam-lock release valve.
Water Hammer
Condensate lying at the bottom of a steam line causes water hammer when the steam system is in service. The steam in the pipe travels at very high velocity producing waves as it passes over this condensate. As the condensate quantity increases, the high velocity steam pushes the condensate and creates a dangerous sludge when it change direction
This is referred to as water hammer. When the high velocity condensate comes to a halt, the kinetic energy is converted to pressure and this sudden pressure increases to destroy the trap mechanism. To prevent water hammer, ensure that low points are drained adequately before commissioning the steam.
Trouble Shooting
The first step to trouble shoot a steam trap problem is to make sure it is (trap) installed properly. The table below provides guidance on trouble shooting three common problems.
About the Author:
Nwaoha Chikezie has worked as an operator (student trainee) with the Port-Harcourt Refining Company (PHRC), Nigeria, with nine months of success. His varied experience includes: refining, gas production and processing, wastewater treatment and tank farm management. He is currently a final year student at the Federal University of Technology Owerri, Nigeria, majoring in petroleum engineering. He is a student member of SPE, ASME, AICHE, IMechE, ASTM, IGEM and Nigerian Gas Association (NGA).
He is also the current coordinator of IMechE FUTO chapter. He can be contacted via email at: [email protected], or via telephone at +234-703-135-3749.
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