Learning From Case Histories

[Pages:19]Chemical Process Safety: Learning from Case Histories

by

Roy E. Sanders PPG Industries P.O. Box 1000 Lake Charles, LA 70602 e-mail: rsanders@

Prepared for Presentation at:

Mary Kay O'Connor Process Safety Symposium Beyond Regulatory Compliance-Making Safety Second Nature

Texas A& M University

October 26, 1999

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Chemical Process Safety: Learning from Case Histories with a Focus on Incidents Involving Water & Steam

Water and steam are nearly ever present in most chemical processes. Their physical properties are generally well understood. Periodic process perils are sometimes related to the infamous deeds of water and steam. This paper will review basics on the hazards of water and steam within the chemical and petroleum refinery industries. To the youthful, this chapter can be instructive with eye-opening reality of fundamentals. To more seasoned individuals these case histories can serve as a reminder of the potential hazards of water and steam using vivid examples which were costly in disappointments, dollars, professional reputation and injuries.

This paper is taken from Chemical Process Safety: Learning from Case Histories, Chapter 3 by Roy E. Sanders, Butterworth-Heinemann, Boston, 1999. ISBN 0-75067022-3. This material is copied verbatim (except for very minor editorial needs and the removal of most photos) with the kind permission of the Butterworth-Heinemann Publishing Company.

Some years ago, Standard Oil Company (Indiana) published a 70 page booklet entitled Hazards of Water in Refinery Process Equipment. (Reference 1) This booklet appears to be for the bible for hands-on refinery employees and it provides easy-to-understand fundamentals on the hazards. The booklet concludes with these thoughts: "The hazard of water in process units has been present throughout the entire history of petroleum refining. Many persons have been injured or killed because of the uncontrolled mixing of water and hot oil or the heating of bottled-in liquids with no provisions for pressure relief. Stills have ruptured, vessels have exploded violently and exchangers have been blown apart. . . A knowledge of how water reacts, where it can be expected and how to eliminate or control it will make your unit safer and your job easier."

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Amoco Oil Company - Refining and Engineering now publishes a 2 inch thick hard bound book entitled Process Safety Booklets. (Reference 2) I highly recommend this book. This Amoco publication is supported with numerous sketches and photos, and incorporates nine previously published, practical booklets: Hazards of Water; Hazards of Air; Safe Furnace Firing; Hazards of Steam; Safe Handling of Lights Ends; and four other topics. It is economically priced at $ 50 per copy. Copies can be obtained from Mr. Matt L. Smorch, (312) 856 - 7232, or via the mail from Amoco Oil Company, Process Safety Coordinator, Mail Code 1204B, 200 East Randolph Dr., Chicago, IL 60601

Even before refineries, about 100 years ago, poorly designed, constructed, maintained and operated boilers along with water's evil twin - steam were responsible for thousands of boiler explosions. Between 1885 and 1895 there were on average over 200 boiler explosions a year. During the next decade, which spanned the turn of the century, things got worse. (Reference 3)

In the ten year period from 1895 to 1905, there were a reported 3,612 US boiler explosions, an average of one a day. The human suffering was worse. Over 7,600 individuals (or on average 2 people a day) were killed between 1895 and 1905 from boiler explosions. The American Society of Mechanical Engineers (ASME) introduced their first boiler code in 1915 and other major codes followed during the next 11 years. (Reference 3) Over the next half century as technology improved and regulations took effect, US boiler explosions tapered off are now considered a rarity. Equipment damages resulting from problems with water and steam still periodically occur.

The fundamentals of the sometimes destructive nature of water and steam within a chemical plant or petroleum refinery will be discussed and illustrated via case histories. Seven sketches of incidents will be reviewed. An improperly conducted hydrotest creates $35,000 more unanticipated repairs. An 82 foot (25 m) tall distillation column is accidentally filled with water and while the column is quickly drained, it suddenly collapses and topples. A 73 % caustic soda tank is partially filled with water to clean it

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and a steam/caustic mix erupts from the vessel. Steam is improperly is applied to a chlorine cylinder and an ammonia cylinder in separate incidents. A brand new refinery coker is being steamed out in a pre-start up activity and as the steam condenses, the vessel catastrophically collapses. A tragic explosion and fire occurred in a fluid catalytic cracking unit during a startup after a turnaround when superheated oil is allowed into a vessel which had accidentally accumulated water.

A Hydrotest Goes Awry On a summer evening in 1996, a Gulf Coast chemical plant had just rejuvenated a recovery tank and was performing the hydrotest when things went wrong. The poorly executed test added $ 35,000 to the repair costs. Destructive hydraulic forces of mild mannered water were displayed in a classic style. There was embarrassment, but no one was injured. In the previous three weeks this tank had received repairs consisting of a new floor and new walls in the lower section.

The metallic victim was a vertical API 620 tank that is 12 feet (3.66 m) in diameter and 20 feet (6.1 m) high with a 16,900 gallon (64,200 L) capacity. The flat-bottom, domeshaped roof vessel was designed for a 12 psig (83 kPa gauge) maximum allowable working pressure rating. Environmental regulations required a hydrotest at the conclusion of the extensive repairs to ensure fitness for service.

While preparing for the test, two mechanics blinded a six inch (15 cm) pressure relief flanged top mounted nozzle. The craftsman also capped two smaller nozzles on the tank roof with flanges. These smaller nozzles normally were equipped with a pressure gage and a utility hose coupling.

The mechanics connected a fire water hose (1 1/2" diameter and 50 feet long) to the tank to fill it quickly and effectively from a hydrant. The normal firewater system pressure was 135 psig (932 kPa gauge). Next, they opened two 1/2" pipe vents on top for air bleeds. They opened the hydrant, as a strong rain shower began, The crew walked away from the job temporarily.

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When the maintenance crew returned to the job site a short time later, they observed a water stream forcefully squirting from the two 1/2" vents. They noticed this normally vertical tank leaning at a slight angle. The bottom bulged upward at the bottom-to-shell joint as much 8 inches (20.3 cm) above the foundation. Anchor nuts ripped through the anchor chairs, and catwalks attached to the tank roof had torn away from welded clips.

(Four vivid photos of damage to this tank can be found in the Chemical Process Safety book.)

In the past at this section of the plant, chemical process operators (not maintenance mechanics) handled the tank filling step in the hydrotest process. Typically, operators would drape a fire hose into an oversize roof nozzle and fill the tank from the nearest hydrant. In this case, it was reported the two mechanics asked their maintenance supervisor if they could connect the hose to a flanged lower valved nozzle on the tank. The supervisor remembers requesting the mechanics to roll a blind flange at the top for venting purposes if they used the lower nozzle for filling. There must have been poor communication. Obviously the mechanics failed to understand the dynamics of the filling operation, when they chose to open only the two 1/2" top vents.

Two 1/2 inch diameter pipe openings provided only 0.46 square inches of relieving area. These openings were sufficient for the air being expelled, but not for the water that would follow. A Loss Prevention Engineer calculated the flow of fire water from a 135 psig supply through 50 feet of 1 1/2" hose and into the low-pressure tank. During the initial filling as air was being expelled an effective orifice area of only 0.09 square inches was required in order to keep the system pressure below the maximum allowable vessel rating of 12 psig (83 kPa gauge). Once the air was displaced and water was flowing out the tank a relieving area of 2.5 square inches was required. The required relief area was about five times the area of the openings provided.

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