Valves and actuators
Valve requirements are basically assessed by safety standards
and applications. Depressuring or pressure protection standards,
such as API 521, do not indicate additional safety requirements
for HIPPS valves on top of IEC safety standards. However, many
times high reliability and availability are targeted in HIPPS appli-
cations due to consequences in failure on demand and the high
cost of a spurious trip.
This typically leads to redundant solutions in the valve sub-
system as well as with the instrumentation of single valves.
Depending on the application, HIPPS valves may contain partial-
stroke devices and/or solenoid(s) and additional instrumentation,
such as limit switches and quick exhaust valves or volume boost-
ers, to meet stroking-time requirements.
Burner valves
Economic and environmental issues are major challenges
nowadays. In industrial thermal processing equipment, low fuel
consumption, safety and reliability have become vital. Reliable
and safe fuel-line shut-off together with accurate fuel control is
essential to meet industry requirements.
A typical burner fuel line consists of two emergency shut-
down valves (ESDV) and a flow control valve. In cases where the
combined use of a control valve for safety and control service is
accepted, voting 1oo2 using an ESDV and control valve can be
used. Safety standard IEC 61511 [2] allows the use of a control
valve for safety applications in cases where failure of the control
valve does not compromise the safety instrumented function of
the safety system. In such a case, the flow control valve would
be equipped with a solenoid, which is used to provide the safety
function.
A positioner controlling the valve’s operation in normal control
service shall be connected to the basic process control system and
shall not prevent the valve’s ability to provide the safety function.
The dual role of the control valve may result in additional require-
ments, such as tight shut-off and fire safety, beyond standard flow
control service. In general, opinions vary as to whether a control
valve could be used for both safety and control.
But in practice, the only reasonable way to use a control valve
in a safety system is in configurations where the control valve is
used as a redundant element, and in those cases the positioner
is not part of the safety system and should not be considered in
safety calculations.
In order to meet these critical challenges, Metso has developed
ESD burner valves that fulfil EN161 / EN ISO 23553-1 [3] as stipu-
lated by the EN746-2 safety requirements for combustion and
A
bbreviations
fuel handling systems. The automatic shut-off valves are designed
not only to meet the EN161 and EN23553-1 [1] (formerly EN264)
requirements, but also to provide exceptional protection against fire
and explosive hazards during the operation of gas- and oil-burning
equipment. When a safety function is required, these burner valves
close within 1 second to isolate the gas or oil flow. The units consist
of a soft-seated ball valve with an actuator, solenoid pilot valve or
TÜV SIL3 certified partial-stroke device and safety valve Controller
Neles ValvGuard.
The unique, flexible-lip Xtreme seat design makes soft-seated
valves bubble-tight and outstandingly durable, even in applications
with a high degree of thermal cycling. The ball valves have success-
fully passed TÜV-type approval tests according to EN161/23553-1 [3]
Class A.
What is intelligence reliability in emergency valves?
Intelligence in emergency valves is not just putting the intelligent
partial-stroke device or intelligent solenoids on top of the valve for
safety action. By contrast, intelligence in emergency valves is com-
bining correctly selected and reliable valves with adequate safety
factors, together with the added value provided by intelligent valve
controllers such as Neles ValvGuard, to achieve the full benefit of
intelligence. Even the most inherently reliable valve may not perform
well if it is selected for an application for which it is not suitable. A
simple example could be the systematic failure caused by an unsuit-
able seat design for a polymer application that causes valve jamming
due to a medium build-up between the seat and the valve body. Such
a systematic failure cannot be considered in reliability data.
Instead of this, selecting the valve design correctly with adequate
actuator safety factors for the application is the starting point for a
successful solution in emergency service. Hence, know-how of valve
challenges together with systems, safety standards and applications
is an important factor in the emergency valve solution. The target
should be a technically as well as commercially ‘fit-for-purpose’
solution without over- or under-engineering.
Ball valves would provide additional benefit by offering a possibil-
ity to do partial stroking within the dead angle without flow passage
through the valve in fail open cases such as emergency depressuris-
ing. Intelligent partial-stroke testing could be automatically initiated
through the DCS or SIS system. With the inbuilt diagnostic capabilities
of the intelligent safety valve controller, the performance of the ball
valve in depressurising or blow-down applications would be tested
with high diagnostic coverage during process uptime without ad-
ditional isolation valves. Improved average probability of failure on
demand brings additional flexibility for proof test intervals andmay in-
DCS – Distributed Control System
ESDV – Emergency Shutdown Valve
HIPPS – High Integrity Pressure Protection System
IEC – International and Electrotechnical Commission
PFD – Probability of Failure on Demand
PLC – Programmable Logic Controller
SIF – Safety Instrumented Function
SIS – Safety Instrumented System
33
July ‘13
Electricity+Control
