shell and tube corrosion and failure description

8/13/2019 Shell and Tube Corrosion and Failure Description

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SHELL AND TUBE CORROSION AND FAILURE DESCRIPTION

Metal Erosion: Fluid velocity in excess of themanufacturer recommendation on either the shell or

tube side of the heat exchanger will likely cause

erosion damage as metal wears from the tubingsurfaces. If any corrosion is already present on the

exchanger, the erosion is accelerated, exposing the

underlying metal to further attack without a

protective coating. A metal erosion problem most

often occurs inside the tubes, along the U bend and

near the tube entrances. Figure (A) is an example of

metal loss in a section of U bend caused by

extremely high-temperature water flashing over tosteam.

Figure A. Metal Erosion in a U-bend tube section.

Likewise, tube entrance areas often experiencesevere metal loss when a high-velocity fluid divides

among the smaller tubes upon entering the heat

exchanger. When a single stream divides into smaller

streams, turbulence results with a very high localized

velocity. It is this high velocity and turbulence that

produces a horseshoe erosion pattern at the tube

entrance.

The maximum recommended tube and entrance

velocity is a function of many variables, including

the material of the tube, fluid involved, and

temperature. Materials including steel, stainless steel,


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as well as copper-nickel typically withstand much

higher tube velocities than standard copper alone. A

pure copper tube is normally limited to less than 8

FPS; other materials can handle upwards of 10 or 11

FPS. If the fluid contains suspended solids or for

example, soft water, the velocity should be less than

7 FPS. Less typical is erosion problems on theshell side of the tubes; typically erosion in this area is

a result of impingement of wet, high-velocity gases,

including steam. To mitigate this, wet gas

impingement is controlled by designing an oversized

nozzle inlet nozzles, or baffling the inlet nozzle.

Steam or Water Hammer:Pressure spikes, surges or shock waves as a result of

a sudden and rapid acceleration or deceleration of

any liquid can cause damaging steam or water

hammer to the exchanger. Pressure surges have been

seen in levels in excess of 20,000 psi, which would

result in the complete rupture or collapse of the

tubing of a heat exchanger. As an example, drawn

copper 3/4 in. x 20 BWG tubing typically has a ratedburst pressure of 2100 psi, along with a collapse

pressure of 600 psi.

Pressure surges can be a result of an interruption in

cooling water flow, stagnant water heated with a

resulting generation of steam, or a resumption of

flow producing steam. All these processes would

likely cause a pressure surge, steam or water

hammer. Therefore, the flow of the cooling fluid

should always start prior to adding the heat load.


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Figure B. Tube Damage From Steam Hammer.

Control valves that open or close suddenly to control

fluid flow can produce water hammer. A modulatingcontrol valve is a preferable option to on-off types.

A vacuum breaker vents are a must if the process

involves a fluid that can or may condense on either

the shell or tube side. Vacuum breakers prevent

steam hammer from developing and causing damage

as a result of condensate accumulation. Figure (B) is

an example of typical tube damage caused by steam

hammer. In the example provided, condensateaccumulated in the shell rapidly, producing a high

pressure shock wave that subsequently collapsed the

tubes and caused multiple tear holes. Correctly sized

steam traps with installed return lines pitched to a

receiving container for condensate or a condensate

return pump should be installed as to prevent such

damage.

Vibration:

Excess environmental vibration from equipment

including air compressors, refrigeration machines or

other motors can cause tube failures that form as a

result of fatigue stress cracks and or erosion where

the tubes make contact with the baffles. Ideally, heatexchangers should be isolated from all forms of

vibration.

Fluid velocities that exceed 4 FPS could cause


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vibration induced damage in the tubes, often causing

baffle supports to cut into the tubes (Figure C).

Velocity-induced vibrations may also cause fatigue

failures by hardening the tubes at the contact points

between baffles or in U-Bend segments, eventually

leading to cracks and splits.

Figure C. Velocity-induced vibration along a tube.

Thermal Fatigue: Tubes, predominantly in

the U-bend sections, can fail as a result of fatigue

from accumulated stresses related to constant thermal

cycling. This problem is significantly aggravated as

the temperature difference across the U-bends

increase.

Figure D. Thermal Fatigue Failure in U-bend segment.

Figure D is an example of typical thermal fatigue.

Temperature differences caused tube flexing, which

subsequently produced a stress load that, until thematerials tensile strength was exceeded and therefore

cracked. The resulting crack most commonly runs in

radials around the tube, and may result in a complete


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failure.

Thermal Expansion: Thermal expansion

failures are commonly found in exchangers involving

exchangers; however, they may occur in most any

process in which a fluid being heated is turned offwithout a provision for absorbing the subsequent

thermal expansion.

In systems that involve steam heating, the cool down

or condensing of residual steam on the shell side

after the steam control valve closes will continue to

heat water or other such fluids on the tube side. A

resulting heat load with nowhere to go will causethermal expansion, creating pressure well in excess

of the tube, tube sheet, cast head, and component

strength. Cast heads made from iron will fail due to

lack of ductility; steel tube sheets will bow or

become distorted permanently because the material

yield point is exceeded. Figure (E) is an example of

thermal expansion and failure of a cast iron head.

Figure E. Thermal expansion failure of cast iron exchanger head.

Relief valves are often installed in the fluid being

heated to prevent a failure of this sort. Manufactures

commonly install and or advise for a means to absorb

fluid expansion.


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