Valves and actuators
D
epressurising systems are used to prevent overpressure by
releasing vessel pressure to the flare system in emergency
conditions. Depressuring can be done by using conventional
pressure relief valves, but in the case of fire for instance, the strength
of the vessel reduces at higher temperatures and a relief valve may
not protect the vessel from rupturing. In such cases, depressurising
systems may be used to reduce the risk of equipment integrity loss.
Similarly, a temperature increase in a metallic vessel can occur in an
exothermic or runaway process reaction.
In an emergency, a depressurising system reduces the pressure of
the system typically to 50% of the design level within approximately
15 minutes. Many process fluids chill to low temperatures during
pressure reduction, which needs to be considered in depressurising
system design. A process plant or unit may be equipped with two
kinds of depressurising systems: a slow system and a fast system.
The slow system can also be called an operational depressurising
system, and the fast one as an emergency depressurising system. An
operational depressurising system is intended for process control or
other operational reasons. It is meant for use as the first means of con-
trolling the process, before the fast system is taken into consideration.
A slow depressurising system is in most cases used in parallel
with a fast emergency depressurising system. It should be closed
when high-rate emergency depressurising valves are operated, due
to the limitation set by flare capacity. Rotary control valves have
shown their superiority in operational depressurising applications by
offering a solution featuring wide rangeability together with control
accuracy and tightness. Product losses and emissions through the
stem are eliminated by reliable rotary spring-loaded packing, and
emissions passing through valve trim are minimised by long-lasting
tight shutoff valve designs. The valve is normally equipped with a
solenoid valve and a positioner as a result of the dual function of the
valve. An emergency depressurising system is used to quickly evacu-
ate the system in emergency situations. It can be initiated manually
by the operator or automatically by process safety system. General
requirements or practices for emergency depressurising valves are
very similar to those for slow depressurising valves, but matching
trim capacity accurately to process conditions and noise reduction ca-
pability together with higher safety integrity are commonly required.
Actuators should be single acting, spring-to-open types because the
valve must open in the case of air failure. Valve-related instrumenta-
tion is typically more complex compared with slow depressurising
valves; an instrument air buffer vessel may be required, redundant
solenoids with voting logic may be used to meet high reliability and
availability targets.
Emergency depressurising valves require careful sizing and selec-
tion. In addition to the initial process condition, a depressurising time
may need to be modelled to verify the valve’s performance during
depressurising. To match process requirements accurately, flow
capacity of the trim should always be verified by means of testing or
calculation. Neles metal-seated trunnion mounted ball valves with
Q-trim have been successfully used in emergency valve applications
for various projects.
High-integrity pressure protection system valves
Over-pressure protection may be carried out by pressure relief valves
or by depressurising systems, as considered in the previous chapter.
A different approach to protecting piping and complete processes
against overpressure can be provided by introducing High Integrity
Pressure Protection Systems (HIPPS). HIPPS is a type of safety in-
strumented system (SIS), which is designed and built according to
safety standards such as IEC 61508 [1] and IEC 61511 [2]. HIPPS is an
independently instrumented system, which has typically higher safety
integrity than normal shutdown systems. Basically, HIPPS systems
act as an isolation barrier between high and low pressure sections of
piping or process equipment by shutting down HIPPS valves before
overpressure. Therefore, lower pressure class piping and equipment
can be used on the downstream side of HIPPS system. This obviously
reduces costs. In addition to the commercial aspects, HIPPS provide
technical and environmental benefits by improving the availability
and reliability of the plant, and minimising flare loads. The main
disadvantage of HIPPS compared with pressure relief devices is its
greater complexity.
The HIPPS system consists of initiators (pressure transmitters),
a logic solver (eg safety PLC) and final elements (shutdown valves).
Emergency shutdown ball valves are commonly used in HIPPS ap-
plications. Ball valves shown have been evaluated by a third party as
being suitable for HIPPS applications up to SIL 4 using a double chan-
nel solution. The main advantage of the ball valve over other valve
types is reliable operation combined with negligible pressure loss
when the valve is in its normal position (fully open). Well-designed
ball valves with rigid seats made of the right materials can handle
fast stroking times and high pressures.
High-end intelligent
emergency valve applications
By J Kirmanen, Metso
How intelligence solutions can be utilised and what kind of added value they bring - this article introduces examples of high-end emergency
valve applications.
T
ake note
• Modern plants are required to run for longer times and more efficiently.
• Safety requirements are, rightly, becoming more critical than in the past.
• Intelligent valves, able to provide diagnostic information and safe control,
offer an important solution to the new challenges.
Electricity+Control
July ‘13
32
