The
maintenance tasks depend not only on the type of components
and failure mechanisms involved but also on the consequences
of the failure in terms of cost, safety and environment
impact. They also analyse whether failures are immediately
evident upon their occurrence or whether they are hidden.
A hidden failure is one, which is not evident until
the component gets a chance to perform the intended
function. Take the instance of a low oil pressure trip.
Since oil pressure is not normally low, the switch has
to be tested to confirm that it functions.
Combining
two approaches
RCM
can be characterised as a Failure Modes and Effects
Analysis (FMEA) followed by a task analysis. Similarly,
HAZOP can be characterised as a system-level FMEA directed
by guidewords. A combination of HAZOP and RCM, termed
as HAZROP was then evolved to achieve synergetic results
for better benefits. HAZROP requires a few modifications
in the composition of the traditional HAZOP team.
The
HAZROP team has to necessarily include a person having
a good knowledge of maintenance and reliability issues.
In HAZROP, all information, data and perspectives on
reliability must be included along with the process
description. Hidden failures must be identified. A task
analysis is then done using the failure modes and effects
generated in a HAZOP
In
order to link the system level deviations, the consequences,
and equipment level failure modes, generic FMEA (gFMEA)
templates for various types of common equipment in the
plant like pumps, condensers, motors, control valves,
and separators are developed. These templates are prepared
by the HAZROP team in consultation with suppliers. Once
an equipment failure is identified as a cause for a
deviation, a gFMEA is prepared for that equipment. Irrelevant
failure modes are then eliminated and an order of magnitude
of failure is applied to the failure modes.
Apart
from the process-oriented guidewords and parameters
used in HAZOP, additional parameters are used in the
HAZROP to highlight operating conditions related to
reliability problems. The guideword ‘unsteady’
is combined with parameters like flow, pressure and
temperature. Similarly, the guideword ‘during maintenance’
is used with parameters like inspection, testing and
partial shutdown requirements. New parameters like unsteady
load, more vibration, and no/less flow are also used.
Numerical
ratings are assigned based on the frequency of occurrence
of the deviation. Severity of the consequences in terms
of business, safety and environment are roughly estimated
and assigned ratings based on the magnitude. These risk
values highlight the critical equipment and the most
significant failure modes.
Task
analysis is the next vital step. The following article
provides an insight into the task analysis along with
tips to simplify HAZROP implementation.
Related reading: Hazard and Operability Studies.
Lihou, M.
HAZROP: an approach to combining HAZOP and RCM.
Post, L. R.; Hydrocarbon Processing, 05/01.
Safety Culture. Whetton, C.; Hydrocarbon Processing.
