Hydrocarbon Processing - October 2021 - 17

Valves
C. LLOYD
REMBE GmbH Safety + Control
Protect safety valves using rupture discs
For many years, emissions were an
How can a rupture disc help improve
unavoidable consequence of industrial
development. An increased awareness of
environmental issues and subsequent legislation
are pressuring oil and gas companies
to cut greenhouse gas emissions, and
several have responded by setting reduction
targets over the coming decades.
Operators have many options when
pursuing emissions reductions. The focus
here is the impact of various safety
devices on this target. The first point of
consideration should be the safety valves
in use. Valves are an obvious place to
start-no valve is 100% leak-tight, and
their effectiveness decreases each time an
activation occurs and the valve re-seats.
Within the design and construction of
a new plant, specifying a valve with a
lower leak rate is a fairly simple solution.
However, existing plants face substantial
investments to replace older designs with
newer technologies. In most cases, this is
not a viable economical solution.
While there have been significant increases
in their capabilities, safety valves
are still not the ideal product when considering
future net-zero targets (e.g., it
is difficult to meet the exacting requirements
of legislators). An alternative solution
is needed.
Although rupture discs have been
in use for many decades, they are often
considered only as secondary relief when
safety valve failure may occur. A lack of
understanding persists among engineers
in industry, and a number of myths surround
the use of rupture discs.
Rupture discs. A rupture disc is a nonreclosing
device and must be completely
replaced after an activation. Nuisance
downtime leads many operators to consider
rupture discs as problematic; however,
if a disc is rupturing frequently, a
problem with the process is likely. Many
operators still do not recognize that a
disc performing correctly is the solution
rather than the problem.
safety valve performance? Rupture
discs are 100% leak-tight, and installing
a rupture disc in front of a safety valve
provides double protection and helps
meet emissions requirements. Leakage
through the safety valve in normal operation
is eliminated, and where overpressure
activation exists, the valve
re-seats
to seal the process once the
pressure is vented.
The belief that this arrangement increases
project cost has been proven to
be false. In fact, the opposite is true: a
correctly engineered rupture disc will
help lower operating costs and increase
plant uptime.
In processes with a high concentration
of corrosive media, increased temperatures
and an operating pressure close to
the safety valve set pressure, safety valves
are pushed to their limits, resulting in
poor performance, high maintenance
costs to keep the valve as close to original
specifications as possible, increased
downtime for routine valve servicing
and/or repairs, and higher manpower
costs to cover the work scopes.
Solutions provided by some safety
valve manufacturers include a higher
specification valve, more exotic materials
with higher CAPEX costs, and an
increased cost of spares to maintain the
valves. If a typical petrochemical plant
with several hundred safety valves is considered,
the CAPEX can be significant.
A rupture disc (FIG. 1) fitted upstream
of the safety valve completely isolates
and protects the valve from the process,
reducing maintenance requirements.
Reducing CAPEX costs is also possible
by sourcing a rupture disc and holder in
an exotic material, and a standard safety
valve. The costs of a disc and holder are
usually significantly lower than sourcing
a high-specification safety valve that is
compatible with the process media.
Protecting safety valves with rupture
discs has become increasingly common
FIG. 1. A rupture disc for isolating safety
valves.
in recent years across several industries.
However, many operators fail to fully
protect the safety valve by also isolating
the valve from potential corrosion issues
at the outlet of the valve.
In many cases, the valve outlet is not a
separate discharge line, but is connected
to other parts of the plant via a manifold
that allows process gases/vapor to enter
the outlet of the valve. If the process media
can damage the valve via the inlet, this
is also the case downstream.
A rupture disc can be used to isolate
the safety valve outlet and prevent any
contact with the process media. The rupture
disc will also block any back pressure
from entering the safety valve and remove
those concerns during valve selection.
With burst sensors installed both upstream
and downstream, rupture discs
can be monitored and connected back
to the control room for system reporting
across the plant. Operators will know
instantly which valves and discs are in a
green or red state.
A myth surrounding rupture discs
is that they can leak. Installing a disc as
the primary safety device (i.e., without
a safety valve behind it) can be a concern
for operators looking to reduce
emissions. The majority of leakages via
rupture discs are caused by corrosion or
damage during installation by mishandling
or incorrect torquing.
Hydrocarbon Processing | OCTOBER 2021 17
Hydrocarbon Processing - October 2021

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