(122a) Analysis of Refinery Equipment to Improve Unit Start-Ups and Shut Downs

Analysis of Refinery Equipment to Improve Unit
Start-ups and Shut Downs

Gerald W. Wilks, Senior Engineer

Lemont Refinery, CITGO Petroleum Corporation

Abstract

In the Oil Industry much of
the equipment operates at high enough temperatures to avoid the risk of brittle
fracture during typical operations. However, there are start-up and shut‑down
periods that subject pressure vessels and piping to combinations of stress and
low temperature that could lead to brittle fracture.  In the past, the risk of equipment failure
led to the development of start-up and shut-down procedures that sometimes
require long delays. To minimize the time required for unit start-up and shut‑down,
brittle fracture analysis of refinery equipment is being conducted today to determine
the acceptable operating temperatures over the range of pressures.  The guidelines in API 579 are usually used
for these analyses. The results of these analyses are the generation of graphs called
?safe pressurization curves?.  Operations
that keep the operating conditions above these curves can be used to shorten start-up
and shut-down time periods thereby saving refineries money.  There is also a risk of brittle fracture
during auto-refrigeration events associated with pressure changes in light hydrocarbon
systems, and brittle fracture analyses can also deal with auto‑refrigeration
risks. This paper provides background information on brittle fracture analysis
along with several examples of brittle fracture analysis of refinery
equipment.  Examples that show the
effects of past process conditions on the results of brittle fracture analysis
are also reviewed.   

Keywords:  Fracture,
brittle, pressure vessel, piping, refinery, hydrogen, hydrotesting, auto-refrigeration,
carbon steel, low alloy steel.