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What Causes Steam Coils to Fail?


What Causes Steam Coils to Fail?


Steam coils are a cost effective way to heat air. Costly premature failures can occur for a number of reasons, which leads to the purchase and installation of replacement steam coils. Usually this occurs at inopportune times and can lead to emergency service and overtime. Here are the five primary reasons steam coils fail:

1. Erosion

Over time, internal erosion can take place inside the coil at tubes and headers. This erosion is caused by two factors, poor steam quality and internal steam velocity.

Steam quality (or dryness fraction) is defined as the percentage of gaseous steam in a pound of steam. The more liquid it contains, the worse the quality. Some steam boiler manufacturers will guaranty steam with a quality of 99.5%. That's 99.5% steam and 0.5% liquid, usually in the form of very small droplets. Though it may leave the boiler very dry, by the time it gets to the point of use, like a steam coil, it may have degraded to 80% dry or worse. If you could see inside the piping, you might see a fog traveling down the pipe with a stream of condensate running down the bottom of the pipe. Poor planning, poor piping practices, and/or failed steam traps are typical causes for steam quality degradation.

Once these droplets enter the steam coil, they literally wear out the metal parts that are in their path, just like a sand blaster. If the coils are not designed well, the internal steam velocities can be high, making this situation worse.

We at Campbell-Sevey have developed solutions for poor steam quality and can design steam coils around acceptable internal steam velocities to lengthen coil life.

2. Corrosion

When non-condensable gasses are allowed to remain inside steam coils, oxidation and carbonic acid corrosion can occur. Carbonic acid forms when carbon dioxide dissolves in water (condensate). The cooler the condensate and/or the higher the steam pressure, the worse the effect. Localized pH's of 3 are possible, which is quite acidic and dissolves the pressure containing metals of the coil. Pin-hole leaks start to develop over time. The use of thermostatic air vents and proper piping is critical to the removal of these gasses.

3. Freezing

Why does freezing occur? Steam in the coil turns to water (condensate) as it gives up heat. Once condensate is formed it must exit the coil before the freezing air on the finned side can extract enough additional heat from the water to turn it to ice. Unfortunately, installers and designers sometimes fail to recognize the factors which cause condensate to remain in the coil.

Freezing occurs when condensate isn’t allowed to drain with freezing air temperatures blowing across the face of the coil. Proper steam coil installation planning, piping, trapping, and condensate return are critical for freeze prevention.

4. Improper Coil Selection

Not every coil is designed to work with every system. Sometimes the coil is not designed for the performance, and sometimes the coil is not designed for the duty. Conventional copper tube coils are usually adequate for low pressure steam commercial heating installations; however, industrial heating and process applications demand the most rugged possible coil construction.

Adding additional surface area beyond what is required is not always the best solution. This can lead to decreased steam pressures in the coil and possibly unnecessary cooling of condensate, causing corrosion.

For most coil replacements, Campbell-Sevey's field technicians first identify what caused the failure and then work with the design staff and our manufacturers to determine the best approach to increase the performance and life of the coil.

5. Improper Installation

If you have the right coil, but it isn't installed properly it is destined to fail. Here are several of the most common installation issues that occur:

  • The Coil is Not Pitched Toward the Return Main

Since gravity is a main force used to drain condensate, pitching the coil toward the return main can be critical factor in getting the condensate out quickly.

  • No Vacuum Breaker Installed 

Without a vacuum breaker, modulation of the steam valve causes a vacuum to form, so that the pressure in the coil is less than the pressure in the condensate main. Condensate will not drain, so the coil will begin to flood. This can cause freezing, corrosion, and/or water hammer.

  • There is Either No Drip Leg or a Drip Leg of Insufficient Height 

When the steam control valve modulates sufficiently, the only available pressure to force condensate past the strainer and trap is the static head of the condensate in the drip leg. A short drip leg may reduce the trap capacity to the extent that the trap cannot keep up with the condensate loading, so flooding takes place.

  • Insufficient Trap Capacityor Wrong Trap is Used 

Knowing that much of the time there is only head pressure to remove condensate, the steam trap orifice size selection becomes critical. It must be of sufficient size to remove all condensate generated by the coil, plus safety factor, at the pressure differential determined by the drip leg. The maximum operating pressure rating of the steam trap must be determined by the safety relief valve set pressure of the steam supplying the coil. If either of these factors is not taken into account, the wrong trap could be installed and the coil could fail.

Selecting the proper steam trap style is also important to ensure that that the coil operates at peak performance. We offer and stock many steam trap styles and will recommend the proper style for your application,

  • The Condensate Return Line Rises Above the Steam Trap Outlet

Some system designers contend that condensate will be lifted by steam pressure. While this may be true at the full load conditions, at part load conditions not enough pressure exists at the steam trap to lift condensate. The result is coil flooding with condensate.

Consult with Campbell-Sevey

For proper operation, all steam coils require additional equipment, such as control valves, steam traps, thermostatic air vents and vacuum breakers that will increase the performance and the life of the coil. When combined with the proper equipment and correct installation your coils can last for many years. At Campbell-Sevey, our experts understand the interplay among these pieces of equipment and how each affects the others. We size them all, we select them all, and we show you how they should be installed. We look at the past failures and use that information to make recommendations for improvements for the future. We look upstream and downstream of the coil, from steam supply to condensate return, to make recommendations to increase overall system efficiency and effectiveness. We understand these 5 pitfalls and how to avoid them.

Both Industrial heating and process applications demand the most rugged possible coil construction. Campbell-Sevey offers a wide selection of coil construction and materials from light duty comfort heating to heavy industrial and utility grade and we have the expertise to know when to use each type.

Consult with the steam, air and water experts at Campbell-Sevey to choose the right equipment for your steam coil applications.

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RETURN
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