Fertilizer plants employ heat exchangers for a host of critical roles, from process gas heating and cooling to steam generation and boiler water heating.
Each of these processes places significant demand on the equipment with elevated temperatures, high pressures and cyclic operations being the routine rather than the exception.
Shell and tube heat exchangers are a common design option for fertilizer applications due to their simplicity, relatively low design costs and ease of maintenance. Shell and tube exchangers consist of a series of tubes encased in an outer vessel or shell. Movement of two fluids with different temperatures separated by the tubes and shell transfers heat between the two liquids. These exchangers are robust and designed for applications where pressures exceed 435 PsiG (30 BarG) and temperatures are greater than 500°F.
Efficiency challenges
Optimizing the operating efficiency of a heat exchanger requires a balance of maximizing throughput and minimizing operating costs. Each component of the heat exchanger, from the materials used, to the number of tubes to the flow rates of fluids in the system, are designed to maximize the efficiency of heat transfer between the two liquids.
For example, hot fluids moving through the tubes are usually not uniformly distributed, with some tubes experiencing more elevated temperatures. Uneven heating and cooling of tubes gives rise to variable thermal strain, which can lead to excessive stress on end plates that keep the tube bank in position.
In addition, fouling of tube walls in the form of scale and other deposits lower the unit's overall heat transfer efficiency. If left unchecked, fouling commonly leads to leaks caused by aggressive corrosion.
Common plugging solutions fall short
Friction fit tapered plugs are a common plugging option designed to isolate a section of a tube during a repair or replacement. However, these plugs present several onsite reliability risks and dangers, including their lack of a pressure rating that results in an unknown safety factor. If the plugs are expelled during shell-side pressure testing, or from tubes that have not been properly vented prior to plugging, they can become projectiles that pose serious safety concerns for personnel and surrounding equipment.
Friction fit plugs are installed with unregulated or controlled force, that can overstress or damage tube sheet joints. This uncontrolled force can also result in costly repairs when plugs are installed in tube sheets with epoxy-coated liners.
Friction fit plugs do not conform to ASME PCC-2 recommended tube plugging repair methods for operations exceeding 200 PsiG (14 BarG). In high-pressure service, these plugs must be welded in place, which raises the risk of leaking due to stress cracking. Welded plugs also typically require time-consuming pre-heat and post-weld heat treatment; even then, the quality of the weld is difficult to assure. Installing welded plugs also comes with significant safety concerns when the heat exchanger contains toxic, explosive and/or corrosive residual contaminants.
A lower-risk solution
Curtiss-Wright EST Group manufactures its Pop-A-Plug Tube Plugs to deliver a permanent, cost-effective solution that eliminates safety and reliability concerns that friction fit tapered plugs bring. The plugs meet ASME PCC-2 Article 3.12 recommendations for the inspection and repair of shell and tube heat exchangers, and are pressure rated up to 7,000 PsiG (483 BarG).
Simple hydraulic installation provides a safe method for sealing heat exchanger tubes with superior stability. As the ram pulls the tapered pin through the ring, it expands into the tube to create a helium leak-tight seal to 1 x 10-10 cc/sec. When proper force is reached, the breakaway pops.
Pop-A-Plug Tube Plugs' weld-free installation protects against damage to tube sheet ligaments and the adjacent tube sheet joints. Plug material is always matched to the host tube to prevent any undesirable galvanic interaction, maintaining a leak-tight seal under extreme thermal and pressure cycling.
For more information, visit cw-estgroup.com/pap or call (215) 721-1100 or (800) 355-7044.
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Safer HX tube repair solution for critical-use heat exchangers - BIC Magazine
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