Page 37 - Gi flipbook April 2018

FIGURE A: Outline indicates the geographical area supplied gas from Glenmavis FIGURE B: Digital representation of a
Mokveld FCV

FIGURE C: Image of the Mokveld valve at
Glenmavis

The travel indicator allows the and the potential implications of such
engineer on site to view the status of a failure, an emergency repair was
the valve’s position during operation, required in order to minimise any
testing or maintenance. This has possible disruption to a major supply. FIGURE D: Indication percentages used for
proven to be particularly useful during (See Figure D) the purpose of a valve travel test
a telemetry fault where gas control
could potentially have no visual What happened next? Fully Fully
confirmation of the valve’s position or We contacted Mokveld and requested 100% = Open 75% - 50% - 25% 0% = Closed
operational status. assistance from a technician who, in
The valve body contains a order to help, would have to travel
mechanically-operated valve seat, directly from their headquarters in
which serves as the primary control Gouda, Holland. FIGURE E: The Mokveld FCV being
element over the volume of gas Prior to the arrival of the technician, prepared for repair
entering the offtake pressure preparations were completed on site
reduction station. (See Figure C) ahead of the repair process. An
emergency non-routine operation
FAILURE OF A MOKVELD (NRO) was prepared; the valve was
VOLUMETRIC FLOW VALVE isolated, decommissioned, removed
Part of the Mokveld valve from the inlet pipeline and placed in a
maintenance schedule is a ‘travel test’. designated work area. (See Figure E)
This is carried out by our electrical
and instrumentation (E&I) technicians What caused the valve to fail?
in collaboration with gas control, who There were a number of factors
are responsible for monitoring our which contributed to the valve
network’s transmission telemetry and failure. Evidence of damage to the
alarm systems. valve seat was apparent, which we
The valve seat and chamber later discovered was a result of dry INVESTIGATION
capacity is calibrated and positioned grease deposits and small metal From the evidence found within the
through stages. debris passing downstream into the valve itself, we had to establish just
During a routine travel test, the valve chamber. how dry grease deposits and metal
valve reached its fully closed position I was informed by the Mokveld debris were able to escape through
at zero per cent. This was confirmed technician that, despite the valve’s the site inlet filter system and travel
by E&I, who then instructed gas operational capabilities on a high downstream.
control to start the re-opening pressure gas installation, it is simply After an investigation, two separate
process. Almost immediately, there not designed to pass any type of causes were identified that we believe
was a fault alarm indicating the valve foreign or solid debris other than its contributed to the valve failure.
was not responding and subsequently design profile of natural gas. We inspected the site inlet filters
failed to re-open. Given the criticality (See Figure F) where a large mixture of metal debris,

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