EROSION

EROSION - CORROSION OF SS316L TRAYS IN FCC's SOUR WATER STRIPPER - A CASE STUDY

Authors: Saad Al-Dhafiri, Deepak Rawtal & Laxma Reddy, Kuwait National Petroleum Company

Source: CORROSION 2010, March 14 - 18, 2010 , San Antonio, TX

Keywords FCC Sour water stripper, Erosion - corrosion, Ammonium bi-sulfide, Root cause analysis, SS316L trays

Preview ABSTRACT

FCC Sour Water Treatment Unit (with Side Reflux type) having design capacity of 22.7m³/hr. was commissioned in 1987. Ever since the increase in throughput of sour water in 1998, the Stripper experienced repeated failure of top trays made of SS316L in year 2004 (after in service for 17 years) and subsequently in 2008 & 2009. This paper deals with the failure investigation and root cause analysis of repeated failure of top three trays of Sour water stripper(V-95-101). Optical Microscopy & Scanning Electron Microscopy (SEM) studies of failed sample were carried out. Root cause analysis has indicated that erosion - corrosion due to Ammonium bi-sulfide (NH4HS) in the presence of hostile environment of chloride & cyanide with increase in throughput are the contributing factors for the failure.

Remedial measures such as minor modification to the cold reflux piping, up gradation of metallurgy and increase in thickness of affected trays are also discussed in the paper.

INTRODUCTION

In sour water stripper service use of Austenitic Stainless Steel is very well proven material provided the plant is operated within the design limits. Operating the unit beyond design causes not only plant upsets and off spec products creating problems to environment but also reduces service life of material used. Pitting, stress corrosion cracking, sulfide stress cracking and crevice corrosion are the most common forms of corrosion experienced with austenitic stainless steels. General corrosion is not normally a concern. However, severe thinning is experienced on SS 316L of FCC Sour water stripper. This paper describes the investigation carried out to determine the cause of thinning of trays and outlines the remedial measures taken to tackle the problem.

PLANT DESCRIPTION

The main purpose of sour water stripper is to remove undesirable volatile components such as H2S, NH3 from process water so that the treated water can be disposed off in an environmentally acceptable condition. The FCC sour water is different from the sour water from the other units in the sense that this water contains some amount of phenols and cyanides. As a result, the treated water from this unit is not considered safe for reuse as is done in the case of other units. FCC Sour Water Treatment Unit having design capacity of 22.7m³/hr was commissioned in 1987. FCC was revamped in 1997 to increase the capacity but its SWT unit was not warranted for any hardware modifications to handle excess sour water (from 22.7 to 27.3 m³/hr ) due to available excess design capacity of Sour water stripper.

The FCC SWT consists of a sour water tank, a surge drum, a stripper column and related heat exchangers and coolers. The feed (sour water) from FCC is received in a feed surge drum. The water is then sent to a tank where it is de-oiled. The de-oiled sour water is heated in the Feed vs. Stripper bottom heat exchanger before being pumped to the sour water stripper. Stripper overhead temperature is maintained at 80 oC to avoid de-sublimation of ammonium bisulfide salts.