Easing the pressure
Chris Lee of Golder Associates Ltd in Sudbury,
Ontario, and Craig Vucinovich of Golder
Associates Inc in Ballwin, Minnesota discus
the safe, rapid and low cost depressurisation
of paste backfill pipelines
aste and cemented paste backfill pipelines are subject to blocking
for a variety of reasons including power outages, material
segregation, foreign objects or incomplete flushing. Blockages are
typically removed by opening the paste pipeline and flushing out the
majority of the unblocked line prior to dealing with the blocked portion of
the line using high pressure water.
Transportation pipelines can also be quite long and depending on
where the blockage is, the location(s) where the pipeline must be opened
could be anywhere along its length. In addition, the time available to
open the pipeline and flush the majority of the unblocked contents is
limited by the working time of the paste backfill and is typically less than
four hours.
The depressurisation of pipelines is also required for tight filling
operations where the final fill of a stope is performed with the paste
exiting the pipeline into a submerged condition. In this case, flushing and
cleaning of the pipeline to the stope is not possible and the pipeline
must be opened up at a point upstream of the pour point.
A pressurised pipeline can be extremely hazardous and release
significant stored energy when opened, however, using typical knifegate
or ball valves is not possible due to the pluggage of the dead leg with
cemented paste which may prevent pressure relief of the line when
opened.
There are now purpose built automated valves which provide a
reliable method of depressurising a pipeline however these valves are
expensive, require support services, installation and maintenance and
although there is definitely a place for these valves in key locations
within a distribution system it is our opinion that the cost of these valves
makes them an inappropriate replacement for the blasting or cleanout
tee which is typically installed at every 50 to 100 m in a paste pipeline.
There is a need in the mining industry for a convenient and
inexpensive method to safely depressurise, flush and clear these
underground backfill distribution pipelines.
P
scoop bucket to knock the pipes apart. Obviously this method was
hazardous and would never be accepted today as a safe operating
procedure.
To address the hazard of having personnel exposed to pipe
depressurisation, the industry moved towards remote blasting of pipe
couplings on a tee in order to destroy the couplings and allow the pipe to
be depressurised. In the past 20 years this has become the most
common method of backfill pipeline depressurisation. The disadvantage
of this method is that it requires an unplanned blast in the mine, special
equipment and personnel, safety and approval procedures that are often
time-consuming to the point of compromising the clearing operation.
Currently, piston valves are used for key areas such as the bottom of
major boreholes or at frequently used flush locations. However, with the
possibility of blockage occurring anywhere along pipeline systems that
can be 10 km long, the cost of placing these valves at frequent spacings
(200 or 300 m) is difficult to justify and most mines use blasting tees at
regular spacings of 100 m or less.
A novel approach to a depressurisation tee
Golder’s new depressurisation device was designed to achieve the
following:
n Inexpensive to purchase, operate and maintain.
n No explosives are required.
n The operators are not directly exposed to any hazardous discharge.
n Facilitates rapid opening of the pipeline system.
n Simple and easy to install.
n Maintenance free, and still will effectively function even after being in
service for long periods.
The Golder Depressurisation Tee consists of a relatively standard
looking branch tee that is fitted with a manually removable conical plug
that interfaces directly with the main pipeline stream, thus avoiding any
dead leg, all contained within a protective shroud to protect the operator
and prevent discharged paste from being directed at the operator. The
conical plug is safely retained by the shroud during pressure relief, and
uses a standard Victaulic coupling to lock it in place. The coupling is
fitted with special nuts that are removed by the operator whilst standing
outside of the splash g uard containment via a specially designed socket
wrench and lever arm.
This patent pending device has been successfully installed in
underground mines in Africa, Sweden and the USA. It has been used in
several pressure relief situations and its robust and explosive free
operating nature has allowed pipelines to be cleared much faster than
possible with blasting tees.
Pipeline
pressure relief:
early attempts
and current
practices
Macassa mine
implemented one of
the first paste
backfill systems in
the mid 1990s. Their
system included a
long borehole to
underground, and
experienced
blockage during
commissioning. The operators devised a plan of
loosening the pipeline clamps and then used a
Figure 1. Depressurisation tee installed in a 200 mm paste backfill line (left); Combination flush valve
and depressurisation tee installed in a 200 mm backfill line (right)
MARCH 2018 Supplement | International Mining P1