BEYOND
BY DAVID FINK
When Accessories Become Necessities
Specification sheets for selecting automated valves list requirements
for the valve and actuator but often
lump all other devices together as related instrumentation or accessories.
Since the definition of an automated
valve is a flow control device that can be
remotely operated, a typical selection
hierarchy is:
1. Select the best valve for the service conditions
2. Select a suitable actuator to operate that valve
3. Provide enough information on
accessories so the valve supplier
or end user can just add them as
needed.
A comprehensive valve data sheet
may enumerate 40 to 50 line items of
information that define the scope of the
valve assembly. Since performance and
safety of that assembly are greatly
impacted if any component in the
assembly fails, the more forethought put
into the specification, the more likely
the final assembly will perform as
intended. The purpose of a specification
is not to exclude any manufacturer but
rather to include only those products the
specifier perceives as most closely meeting the equipment application requirements.
THE SPEC SHEET
If we look at all system devices as
necessities instead of accessories, we
can divide the specification sheet into
three sections.
Under Process Components all devices
that are in contact with the media
would be listed. That would include the
valve and instrumentation such as flow
meters and other process variable transmitters.
Under Force all devices that supply the motive power to operate the
valve would be listed. This would
include the actuator—pneumatic,
electric or hydraulic; hand-wheel
overrides; linkage kits/mounting
brackets; solenoid valves; filter
regulators; pressure gauges, etc.
The third column would be
headed Intelligence. This category
would include valve position sensors.
Intelligent devices interface the
operator’s commands and the
valve’s response. Without supervi-sion/control, the automated valve
will not function; therefore, the
process will not function.
Valve position sensors indicate
to the plant operator that an automated or control valve is open,
closed, or between open and
closed. A valve position sensor or
switch can take numerous forms:
inductive proximity sensor, reed
switch or mechanical switch. Sensors can be directly mounted to a
rotary pneumatic actuator, brack-et-mounted to a linear valve,
enclosed inside a switch box or an
electrically operated actuator, or
inside a valve positioner.
An inductive proximity sensor
is a non-contact device, which
means the sensor is free from
wear. A common metal target is
mechanically attached to the actuator stem of a linear or rotary
travel valve. When the target
comes into range of detecting the
sensor, an electronic switch
changes state, which confirms the
position of the valve. Actuation
speed or frequency has no effect on
inductive proximity sensors. They
are typically used in pairs, one for
The automated valve assembly shown here is for use
in a hazardous location. It consists of a process ball
valve with pneumatically operated actuator, solenoid
valve and inductive NAMUR non-incendive valve
position sensor and target. Note that the solenoid
valve is driven from the valve position sensor, which
simplifies wiring.
This automated valve assembly is made up of a
process ball valve, pneumatically operated actuator,
solenoid valve and inductive non-contact valve
position sensor and target.
