Synthetic fiber plants like Nylon and Polyester use a salt bath consisting of a mixture of sodium nitrate and sodium nitrite for cleaning metering pumps, spinnerette  assemblies, valves, and fittings coming in contact with polymer melts.  These accessories are placed in a hot salt bath for a specified time and the polymer melt sticking to the surfaces is dissolved via the oxidizing nature of this bath.  After a specified time, the components are taken out and receive a final cold water wash before being placed back in service.

The incident explained here relates to an explosion that took place while the cleaning of these accessories was in progress.

Despite the best  efforts of the management, one person involved in the accident died while the other escaped with serious burn injuries to both hands. 

What Happened?

The plant had initiated an annual shutdown to its Polymerization plant.  Due to the inherent nature of the polymer melt, jacketed steam valves are provided to ensure fluidity of the melt.  One such valve was removed for cleaning and boxup before resuming polymerization as a part of the planned shutdown activities.  This large valve was lowered into the salt bath via a thick metallic wire.

As soon as the valve contacted the saltbath, there was a loud explosion followed by spraying of the hot salt bath liquid through the openings of the transparent shutters enclosing the exhaust hood.

Two people, one operator and his supervisor, were in the path of the hot solution spray.  Both sustained around 50% burns and the men were immediately transported to a nearby medical facility with a burn unit.

After a week the operator expired and the supervisor had to undergo a series of plastic surgeries to his hands for almost an year.

How It Happened

As per standard operating procedure, the piping on the jacket portion of these valves are to be blinded before cleaning.  Upon inspection of this valve after explosion, the ruptured gaskets were hanging on the flange bolts on both the sides of this valve.

What Went Wrong?

The fitter who carried out the blinding of the inlet and outlet flanges on the steam jacket had exhausted the metallic gaskets specified for blinding such jacketed valves.  To ensure that there was no time delay in handing over this valve, he put asbestos gaskets to the two flanges and sent it for cleaning.  The oversight was not corrected prior to cleaning the valve.

The Investigation and Analysis

The large jacketed valve was heated with steam at 5.5 kg/cm² (approx. 80 psig).

The trapped condensate in the jacket had not been drained fully.

With blinds on both the sides, the condensate flashed when placed in the bath.  The specific volume changed from 915 kg/m³ to 2.67  kg/m³, increasing almost 350 times resulting in an abnormal rise in pressure.  The asbestoss gaskets could not withstand such a huge rise in pressure and burst open.  This sonic boom waves carried along with it the hot saltbath liquid spraying all over the two victims.

Lesson Learned

Never compromise on design specifications even with a minor item like a gasket.  It can mean serious consequences.

Never bypass any safety procedures.   In this case,the transparent shutter was kept partially open and the hot liquid came out through this opening.

Preparation of checklists and ensuring compliance is an important activity in any shutdown.  Such checklists prepared by an operator/technician have to be verified by a supervisor and authorized by a competent authority before proceeding.

By: G. Vishwanathan (vishtech03@yahoo.co.in), a freelance consultant of energy audits, process engineering, and troubleshooting operations.  He also works as an Associate Consultant with M/S. Devki Energy Consultancy Pvt. Ltd., Baroda.  He has more than 25 years of experience in petrochemical plant operations.