Coil corrosion by erosion: Don’t let it wear you down

September 18, 2000
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Coil corrosion can seem to be one of life’s great mysteries. Combinations of metals in coil construction, along with wide variations in operating environments, can make the mechanisms of corrosion complex, to say the least.

When movement of a corrodent over a metal surface (i.e., through coil tubes) increases the rate of attack due to mechanical wear and corrosion, the attack is called “erosion corrosion,” and is considered a localized form of corrosion.

Water coils

Failure of water coils due to erosion of tube surfaces can many times be avoided with proper design. Conditions that cause erosion corrosion in coils typically involve fluids moving at high velocity, solids in suspension, marked turbulence, and impingement.

Erosion corrosion occurs most frequently in water coils near the inlet end of tubes or headers, along the first 2 to 6 in. of the inner surface. It may occur, however, along the whole length of the tube surface. It results in clusters of deep pits which are usually undercut on the downstream side and, in many cases, take on a horseshoe shape.

To avoid erosion corrosion, the following steps should be taken:

1. Make the proper choice of tube materials. Coils can be made from a variety of tube materials, including copper, cupro-nickel, steel, stainless steel, and aluminum. Limits for the velocity of fresh water flowing in tubes and headers made of these different materials are shown in Table 1.

Note that certain materials (copper, steel, and aluminum) can experience severe pitting when exposed to brackish-sea water and are not recommended. Velocity limits for cupro-nickel and stainless steel with brackish-sea water are the same as for fresh water.

2. Limit fluid velocities. The velocity of the fluid flowing through coil connections and tubes is limited by proper selection of connection and header sizes, and by choosing a minimum number of feeds in the coil circuiting. Table 2 shows maximum allowable water flow rates (in gpm) for various connection and header sizes, along with various materials.

Coil circuiting also must be designed to limit the flow rate per feed in order to keep fluid velocity below acceptable limits. The maximum allowable water flow rate per feed is shown in Table 3 (page 8) for various tube sizes and materials.

(Colmac’s CoilPRO engineering software automatically calculates header and tube-side velocities and prints a warning whenever the limits are exceeded.)

3. Eliminate entrained or separated air. Air bubbles in the circulating water contribute to the damaging effect of erosion corrosion. Proper purging and elimination of air from the waterside of chilled and hot water systems are essential to prevent this type of corrosion.

Air can also be drawn into the suction side of pump piping if leaks are present and pressures are lower than one atmosphere at that point in the system.

4. Eliminate sediment and contaminants. Entrained sediment and other solid contaminants accelerate erosion corrosion, and effectively lower the threshold velocities at which erosion begins to occur.

In the case of very low velocities, sediment is deposited in tubes and may contain or accumulate corrodents that initiate and sustain other types (chemical and/or electrochemical) of localized corrosion. Sediments and suspended solids must be removed by filtering the circulating water.

Steam coils

Steam can be highly corrosive in coils when it is improperly treated or misapplied. Proper steam piping and coil design are critical for good coil operation and long coil life.

Two types of erosion corrosion that can occur in steam coils are described here. Just as with water coils, steam coils can experience erosion corrosion from excessive tube-side velocity.

Also, if condensate is allowed to remain in the coil and become subcooled during idle periods or at shutdown, when live steam is again admitted and contacts the condensate, a phenomenon called “thermal shock” will take place.

1. Velocity limits — For a given steam flow rate in pounds per hour (lbm/h), steam velocity decreases as pressure increases. This is due to the increase in steam density as pressure increases.

Care must be taken to size coil connections and headers for the minimum anticipated operating steam pressure. This approach helps ensure that the coil is designed to handle the maximum velocity condition.

Colmac recommends sizing steam coil connections for velocities not exceeding 6,000 fpm. This guideline not only minimizes the chances for erosion to occur, it also reduces steam noise. Table 4 shows maximum allowable steam flow rate vs. connection size.

Steam distributing coils can experience erosion corrosion in the outer tube wall, where high-velocity steam leaves the inner tube orifices. Limiting steam loading per tube to a maximum allowable value based on velocity will mitigate this type of erosion corrosion.

Keeping these coils clear of condensate is also critical to avoiding combined erosion and thermal shock (see below) at the orifice outlets.

2. Thermal shock — When condensate is allowed to remain in a steam coil after shutdown or during idle periods, it becomes subcooled (cooled to a temperature lower than steam temperature). If live steam is then introduced into the coil and contacts the subcooled condensate, the live steam will violently recondense into the condensate.

Tiny bubbles of the live steam are injected into the condensate at the steam-condensate interface, where they collapse with tremendous destructive force. This process, called thermal shock, is very similar to cavitation in pumps, where local pressures in the pump body are allowed to fall below the vapor pressure of the water being pumped.

In the case of thermal shock, erosion corrosion occurs when the collapsing steam bubbles contact the tube surface, resulting in severe localized pitting.

Thermal shock is prevented by properly designing steam piping to thoroughly eliminate all condensate from steam coils, both during operation and after shutdown.

Summary

Erosion corrosion can be caused by a variety of factors and can appear in both water and steam coils. Properly matching materials to type of fluids used and meeting velocity requirements are important.

Correct coil connection and header sizing, along with selection of circuiting, will prevent erosion due to velocity effects.

Finally, steam piping must be designed to keep steam coils clear of condensate.

Following these design and operating guidelines should minimize the possibility of coil corrosion by erosion.

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