From a paradigm perspective, any
deviation in the performance curve that
triggers an alarm is certainly worth
spending time to interrogate. Even if the
user is uncertain whether the problem
involves any component/package, or a
complete valve, actuator, solenoid or
valve monitor, simply preordering a
package to be on site could save hours
and even days of downtime—not to mention unnecessary expediting fees. Conversely, the data may allow the expert
(end user or vendor technician) to determine that the behavior is linear, which
would simply require an adjustment
and/or replacement of the packing without having to remove the complete automated package.
The valve equipment industry has
responded to the needs of batch processing operations by developing truly intelligent valve controllers for discrete applications. These versatile,
microprocessor-based devices are
designed to meet a wide range of
rotary/linear valve actuation requirements, including simple on/off functions,
hazardous operating environments and
emergency shutdown (ESD) systems with
partial stroke testing (PST).
From a non-contact sensor for precise
position feedback, to a fully encapsulated
electronics module providing detailed
information regarding the performance
of the pneumatic AVP, the new discrete
controllers have intelligence, speed and
communication options previously unattainable in the on/off valve market. They
incorporate advanced solutions for integrated valve asset management and PST
functionality—not to mention
local/remote configuration and auto-calibration capabilities.
The new generation of intelligent
control monitors provides robust diagnostics and data acquisition capabilities
allowing plants to verify the performance of automated discrete valves in
real-time, thus reducing the potential
for catastrophic failure. Unlike off-line
signatures, users can capture the variables needed to develop a pressure-pro-filing baseline dynamically—while in
service—thus eliminating the need to
remove valves from service in order to
diagnose a performance issue. These
controllers use a “pass/fail score card”
technique to predict and prevent actuated valve failures.
The latest discrete controllers are
designed with integrated pressure sen-
Figure 2. Dynamic (Process) versus Function (Nil-Process) Signatures
Dynamic – Signature developed when the automated packaged is commissioned under normal process conditions.
Performance – Signature developed prior to commissioning the automated package, valve/actuator supplier, instrument shop, pre-commission
We see in the Open chart there is an increase in the differential pressure at the break. On the function test plot, it is time 0.095s and dynamic
time 0.0115s. This can be due to the required increase in torque to move the valve against the media in the line. Note the break time increases but
the travel time is very similar.
In the Close chart it takes longer to close and again it requires more differential pressure to put the valve into the seat at the end of the stroke.
A Last (In-Process) Maintenance Signature Function will capture every discrete output command for opening and closing the valve assembly. It
will then be logged as a Last Maintenance Signature and compared to the Baseline Signature. This is where a Dynamic Baseline Signature Function
provides much more accurate data than a Performance Baseline Signature Function. If we were to compare a maintenance signature to the
performance baseline signature, we would already be concerned as the graphs would clearly indicate a deviation from the baseline, but is this
deviation a result of normal process dynamic or impending equipment failure? The objective is to compare true dynamics during commissioning
and break-in so that maintenance signatures are not compared to those that were captured without process.
If the maintenance signature was compared to a dynamic signature, our baseline data already includes dynamic process conditions so any