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8/13/2019 Shell - TAMU Case Study Handouts3

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Shell

Texas A&M Case Study

Root Cause Analysis applied toa Cooling Water Tower Failure

20 October, 2003


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Typical Cooling Water TowerAs-built

48

42

30

Hot-waterdistribution pipe

Fan Air

Air

Cold-waterBasin

Hot-waterdistributiondeck


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Cooling Water Tower Introduction

Who needs a cooling water tower?

The chemical and refining industry use many cooling water towers. You may also encounter cooling water towers inpower industry as well as some public buildings. While each one is unique, they share many of the properties welldiscuss today.

Why use a cooling water tower?

The cooling water tower is used to turn warm water into cool water. That cool water is pumped throughout therefinery/chemical plant and used to cool hot streams. The water will become warm (sometimes hot). One coulddischarge the warm water into a lake or river (it is clean water that was not contaminated by process chemicals), butthat would be expensive (it costs money to make the water clean in the first place). Instead, we take the warm waterand cool it in a cooling water tower. Then, we re-use the water.

How does it work?Warm water is delivered to the tower by distribution pipes. In the previous schematic, two pipes are shown in lightgreen. The warm water is splashed down through the tower like a hard rain storm. Air flows through the tower andcools the water droplets. The cool water droplets collect at the bottom in the cold-water basin. From the cold-waterbasin, water is pumped back to the refinery/chemical plant to cool process streams.

To get a good air flow through the tower, there are big fans on top that pull air through the sides of the tower. Thesefans can be stopped or started to allow just the right temperature in the water. A computer is often used to start orstop the fans and control the water temperature (the program can be sophisticated enough that each fan is run thesame amount of time so that no one fan wears out prematurely).


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Wetted Areacontainingsplash bars

Hot-waterDistributionPipe

DriftEliminator

Fan Cylinder

Hot-water

Deck

Plenum

Diagonal Brace

Cold-water basin

Fan

Column

Splice block

Ventpipe

Girt

Louvers onOutside walls


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Cooling Water Tower Features

The cooling water tower is typically made of wood (sometimes plastic or metal). From top to bottom, the tower mayexceed 60 feet tall. The tower may be 60 feed wide and as much as 500 feet long. The structure is braced to withstandwind (typically designed to withstand hurricane-force winds). The water pipe, water, and wet wood weigh quite a lot(equivalent of two Suburbans stacked on some columns there may be 1000 columns each 50 feet tall!), so theres a lot ofmaterial in the tower to make it strong.

The original water distribution pipes were made of wood! They actually looked like a very long wooden barrel with metalbands that held the wood together. Later, the wooden pipe was replaced with a plastic one. A vent pipe on the hot-waterdistribution pipe allows air bubbles to exit the pipe.

Hot water from the process flows to the distribution pipe. Water from the hot-water distribution pipe flows on to the hot-water deck. Holes in the hot-water deck allow water to fall (like rain) through the wetted area. Splash bars in the wettedarea slow the flow of water so it has time to cool before collecting in the cold-water basin.

Air is pulled through the sides to cool the water. A drift eliminator separates the wetted area from the plenum. This drifteliminator coalesces water and reduces the amount of water lost by unwanted spray.

The weight of the tower is held by columns. Since there are no boards long-enough to make 50 foot long columns, 18

boards are joined with splice blocks to make a column. Girts (horizontal boards) and diagonal braces keep the columnsstraight up and down which maximizes the load-carrying capacity. Louvers on the outside of the tower re-direct splashingwater into the basin.


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Whats technically possible .

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