cpp - Process technology for the chemical industry 02.2018

cpp ACHEMA SAFETY, SECURITY
Reverse acting rupture disc KUB clean
In-situ test to check whether the safety valve is functioning correctly
Let us take a typical installation where the safety valve faces a process
condition with a high concentration of corrosive materials,
elevated temperatures and an operating pressure close to the valve’s
set pressure. The safety valve’s limits are put to the test, leading to
poor performance below the expected levels for operational stability
and leakproofness. High maintenance costs are incurred to keep
the valve as close as possible to the original specs, along with increased
downtime for routine valve servicing and/or repairs and
higher manpower costs to handle the work.
The solution suggested by safety valve manufacturers is a valve with
a higher specification, more exotic materials with higher capex
costs and more expensive spares for maintaining the valves. In an
average petrochemical plant with several hundred safety valves, this
capex translates into a dramatic increase in valve inventory costs.
Economical solution
Rupture discs fitted upstream of the safety valve, in a material that
will isolate and withstand the process conditions, let the plant
owner install lower grade materials in the valve while still meeting
all the design requirements, though with a significant reduction in
the safety valve capex.
The costs for the rupture disc and holder are negligible compared
to the capex for an exotic or higher-specification safety valve. If account
is also taken of the lower expenditure on maintenance, the
disc and holder are free of charge. In fact, this is a win-win path
with more production uptime, less emissions, greater safety, lower
overall maintenance costs and reduced inventory.
Yet, we still see safety valves with an upstream rupture disc failing,
needing maintenance and stopping production or causing safety issues.
So what went wrong? Remember that a safety valve also has an
outlet. In many cases, this outlet is not a separate line to discharge
but is manifolded together with other parts of the plant, allowing
process gases or vapour to enter it and provoke safety valve failures.
This is something easily eliminated by a downstream rupture disc,
which prevents any process gases from entering the safety valve on
the outlet side. The rupture disc will also block any back pressure
attempting to enter the safety valve and eliminate such 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, so that operators know
instantly which valves and discs are in a green or red state.
The rupture disc manufacturer can collaborate with design and process
engineers to select the discs giving the best possible safety and
isolation performance. Unfortunately, this is still a rarity and is a
factor in the rupture disc being blamed for poor performance when
all it is actually doing is what it was designed to do, namely be the
most important safety device in the plant – the only fail-safe device
that always opens when faced with an overpressure situation.
Case history – a practical example
Rembe was approached by a refinery to help overcome serious issues
with its safety valves due to process conditions attacking the
metal structure. The requirement was to enable the plant to be operated
for three years without having to remove the safety valves for
servicing and recertification except in an emergency. When the
safety valves were removed during a scheduled shutdown, several of
them were found to have broken or cracked springs while others
were corroded to failure point. Bellows were corroded and broken.
Several safety valves were so badly affected that the disc and nozzle
were corroded solidly together and the valve was unable to open.
This raised major safety concerns that the plant could no longer
meet its three-year maintenance-free plan and it was decided to return
to the old test method of regular safety valve removal and replacement.
An analysis carried out in conjunction with Rembe showed that aggressive
process gases were present both upstream and downstream
of the valves, and that isolation with rupture discs in a suitable material
would offer the best protection. What’s more, the user would
stand a more realistic chance of meeting the three-year target, with
the ultimate goal of extending this to five years.
In addition, the use of KUB V rupture discs allows for in-situ lift
testing of the safety valves on site during the three-year period, to
ensure that they are still operational. As a further benefit, the costs
are lower because the Rembe rupture discs can be removed, inspected
and reused, and are still in serviceable condition.
www.cpp-net.com
Online search: cpp0218rembe
Hall 9.1, Booth C4
AUTHOR:
ORHAN KARAGÖZ
Business Development Director,
Rembe
44 cpp 02-2018