Metrix Basic Machinery Monitoring Methodology
Metrix wants to provide early warning of a machine problem, so you can take action, and perform
machinery diagnostics when you need to.
Purpose
This application note is intended to shed light on why more and more customers are
choosing a transmitter vibration monitoring methodology in combination with
their PLC (Programmable Logic Controller) or DCS (Distributed Control System)
or SCADA (Supervisory Control and Data Acquisition) system for machine
monitoring in lieu of a permanently rack based Vibration Monitoring System
(VMS).
Questions Answered by the Application Note
Why would you need a dedicated Vibration Monitoring System if the existing Control System can
monitor vibration parameters along with all the process parameters?
Since you absolutely need the Control System, why not just monitor and protect the
machine, the process, personnel and the community with the Control System?
Why pay twice for protecting the machinery asset when the Control System can provide adequate
vibration protection with the right sensors?
Introduction
When you ask operators at various industrial facilities (e.g. power plants, refineries,
chemical plants, pipelines, water and wastewater facilities, etc.) they will
tell you that most (+98%) of their machinery is working properly. The machinery
is properly mounted, aligned, balanced, lubricated, operated and in most cases
maintained to ensure it continues to provide its designed purpose.
In the past, Metrix
would advocate a sensor solution with a vibration monitoring system (VMS) on
critical, production limiting, rotating or reciprocating machinery, and in some
cases, we still would. However, things have changed in the machinery
monitoring world in the past 20 years.
The argument used to be that the PLC (Programmable Logic Controller) or DCS (Distributed
Control System) did not have the scan rate necessary for machinery protection
for vibration and position sensors. That is simply no longer the case, even
though the scan speed is a key differentiator between DCSs and PLCs, either can
now be adequately used for machine protection. PLCs are designed to meet the
needs of applications that require scan rates of ten milliseconds or less (0.01
seconds or less). This allows them to accurately control motors and drives
running at high speeds, and this is 10 times faster than many VMSs, certainly
adequate for radial vibration and thrust monitoring. However, DCSs and SCADA
systems do not need to be this quick because they control systems rather than
individual devices. A DCSs and SCADA systems regulatory control loops generally
scan in the 100 to 500 millisecond range (0.1 to 0.5 second range),
which is usually very adequate for monitoring changes in radial vibration, and
can be adequate for thrust monitoring. When you consider most vibration, and
thrust monitoring, has a three (3) second time delay, or more, the scan rates
of 0.1 to 0.5 seconds are certainly adequate (sources: Emerson - Delta V
Distributed Control System, White Paper, Oct 2016; Siemens – DCS or PLC? Seven
Questions to Help You Select the Best Solution, White Paper, 2007).
For non-critical rotating or reciprocating machinery, which marginally affects
plant production, Metrix would advocate a transmitter based solution for
monitoring and protection, or depending upon the asset an electronic or
mechanical switch for protection. We are starting to see more and more
customers choosing vibration transmitters on critical rotating and
reciprocating machinery. This includes those machines with fluid film bearings
(also known as journal or sleeve type bearings) that utilize proximity probes.
Customers are changing from vibration monitoring systems to transmitter / PLC / DCS / SCADA
systems for their vibration monitoring needs for three reasons: Cost, ease of
implementation, and exception based machinery diagnostics. The customers that
are choosing the vibration transmitter route, even while using proximity
probes, is because the machines that they are monitoring rarely fail, and they
just need early warning if there is a change in vibration levels. If they see
a change in the vibration trend using the output of the transmitter, they then
use their portable diagnostic equipment to diagnose the possible problem.
Let’s investigate why customers are changing their vibration monitoring
strategy to transmitters in lieu of sensors and vibration monitors.
Vibration Monitoring Shift
The vibration
monitoring solution should depend upon the cost of Unscheduled Downtime.
Unscheduled Downtime caused by a problem coming from a rotating or
reciprocating machine is the costliest downtime for a plant. The plant loses
money from lost profits (Lost Opportunity Cost = loss of profitable output),
expedited repair, material and labor costs, standby labor costs, quality
issues, customer trust / delivery issues, and most importantly personnel safety
concerns. The sensory solution employed on a machinery asset depends upon the
probability of an unscheduled downtime event. If the asset downtime does not
present a risk to personnel, the facility or community, then the vibration
monitoring solution is minimal (see Metrix Application Note “Monitoring
Methodology” for more detail).
The change from a vibration monitoring system to the transmitter / PLC / DCS / SCADA solution
is partly due to the improved scanning speed and what is already being
monitored by the Control System.
Question: Of
the shutdown parameters listed below (if monitored), how many are usually found
in a Vibration Monitoring System (VMS), and how many are in the Control System?
Motor | VMS | PLC |
Low lube oil pressure | No | Yes |
High bearing temperature | Yes | Yes |
High lube oil temperature | Yes | Yes |
High current | No | Yes |
High voltage | No | Yes |
Low voltage | No | Yes |
High radial vibration | Yes | Yes |
Abnormal thrust position | Yes | Yes |
| | |
Pump | VMS | PLC |
Low lube oil pressure | No | Yes |
High bearing temperature | Yes | Yes |
High lube oil temperature | Yes | Yes |
High / Low discharge pressure | No | Yes |
Low flow | No | Yes |
Low Suction Pressure | No | Yes |
High radial vibration | Yes | Yes |
Abnormal thrust position | Yes | Yes |
The point made by the question above is, why would you need a Vibration Monitoring System
if the Control System can monitor the same parameters? Since you absolutely
need the Control System, why not just monitor and protect the machine, the
process, personnel and the community with the Control System? Why pay twice
for protecting the machinery asset when the Control System can provide adequate
vibration protection?
The argument for a VMS is two fold; 1) Discrete vibration parameters can be simultaneously
monitored along with the direct vibration, and 2) the plant asset can be connected
to a Condition Monitoring System (CMS), possibly accessible via an Intranet. These
two reasons especially apply to machinery assets, usually with fluid film
bearings, that are using proximity probes as their means of machinery vibration
protection.
Discrete vibration parameters like gap voltage, 1X amplitude and phase, 2X amplitude and
phase, nX amplitude and phase have value, but the time and effort to set these
parameters up, and maintain them, to make them meaningful, is not justified.
In most cases these parameters are present in the VMS, but are not used. Only
the direct amplitude is used and it is fed to the Control System, like a
vibration transmitter output. Only the direct amplitude from a rack based
system is used for a vibration shutdown signal, just like a vibration
transmitter output. The means of getting the discrete vibration parameters into
the Control System is not justified, unless the parameters are maintained.
Since, in most cases, they are not maintained, the expense for the VMS as an
addition to the Control System is not justified.
An online CMS
is very convenient for those machinery assets where in which the cost of
Unscheduled Downtime is very high – we call these assets Critical Assets (without
these assets the plant does not operate). Keep in mind, it is highly unlikely
the plant has the human resource capacity to perform diagnostics on machines
that have shown no change in vibration behavior. This is evident from the fact
that no action is taken on the discrete vibration parameters discussed above.
Why should the plant spend hours looking at vibration plots (Dynamic Data),
when no change in vibration behavior has been noticed? They shouldn’t. Consider
the actual sequence of events at a facility when the VMS detects a vibration
problem, and a CMS is available:
- VMS detects a significant change in vibration that is below the shutdown.
- Expert in
Machinery Diagnostics (often one person associated with the facility,
maybe, and usually not one of the operators on shift) gets online, if
possible, or goes to the plant to diagnose the change in vibration.
- The Expert
goes through the Machinery Diagnostic process looking at the Dynamic Data plots
available, using the installed and dedicated CMS, and possibly makes a
recommendation to solve the problem, live with the vibration change or
shutdown the unit.
How is this
different from when a vibration transmitter reports a problem via the Control
System? The difference is the time it takes for the Expert to go to the plant
to connect the portable diagnostic gear to the BNC’s of the proximity probe
vibration, position and phase transmitters of the asset. This delay time may
or may not be significant to the plant, when you consider the portable
diagnostic gear can be used on any asset at the plant, not just the critical
assets covered by a dedicated CMS, the delay time in conducting diagnostics may
not be significant. With some portable diagnostic gear the operators can be
trained to connect to the BNC’s of the transmitters and collect the data and
send it to the Expert remotely. Since the Dynamic Data can be gathered with
portable diagnostic gear from the vibration transmitter’s BNC the expense for
the VMS in addition to the Control System is justified only when the delay time
for gathering diagnostic information is economically significant.
Conclusion
Metrix wants to provide early warning of a machine problem, so you can perform machinery
diagnostics when you need too.