Seismic vs Proximity Probe
Seismic vs Proximity Probe
(OP)
Hi,
We just procured gas turbine. Wonder whether is there any particular standard on using seismic or proximity probe for the alarm/tripping and protection of the machine.
Someone told me that using proximity probe is not that common for tripping the GT? Is it true
Thanks
norzul
We just procured gas turbine. Wonder whether is there any particular standard on using seismic or proximity probe for the alarm/tripping and protection of the machine.
Someone told me that using proximity probe is not that common for tripping the GT? Is it true
Thanks
norzul





RE: Seismic vs Proximity Probe
RE: Seismic vs Proximity Probe
In our case, the vendor had proposed that the seismic probe will be located on the compressor side casing of the GTG. This is for tripping (safeguarding) purpose. And the proximity probe (X-Y) at each radial bearing for radial vibration monitoring and thrust bearing to monitor the axial displacement.
Is this a common setup for the GTG?
Our company technical stds require that shutdown protection shall also be provided as well for the bearing (radial & thrust). So, what the supplier intend to provide is a clear deviation to our company std...
Just checking whether the deviation is genuinely technical or due to some commercial reasons...
RE: Seismic vs Proximity Probe
You could review the make and model of the prox probe equipment and see if relay contacts are provided that can be used.
I would think the desired display from prox probes would be in V since the desired D for tripping would be a function of speed.
Another item to know would be if the prox probes are absolute or relative. since relative measures gap between journal and bearing, it is possible for some bearing designs to vibrate in phase with journal and relative reading be very low for a severe condition. (a function of bearing support stiffness) Absolute usally have an acceleramoter included.
I would also expect the thrust prox probes are for actual position (not axial vibration) monitoring to see a thrust bearing failure (wear) has occurred.
RE: Seismic vs Proximity Probe
RE: Seismic vs Proximity Probe
Velocity pickup detectors have flat response of amplitude as a function of frequency. Alarm setting is unchanged regardless of speed. Diagnostics is limited due to being very directional. If you change its direction of pickup the value of the same force will change.
Acceleromters mounted on the casing pick up the spectrum of problems transmitted from the shaft to the casing and offer velocity regardless of frequency. Used to id hi-freq problems ie blade flutter, surge, etc..
RE: Seismic vs Proximity Probe
I'd personally go for vibration monitoring through accellerometers
RE: Seismic vs Proximity Probe
If a gearbox is present and fitted with journal type bearings, then both accelerometers (to measure high freuqneices associated with gear mesh frequencies) and proximity probes might be fitted
RE: Seismic vs Proximity Probe
There are 2 primary types of vibration probes or transducers:-
1) Contact or Seismic Type - normally attach on the casing or bearing housing i.e. external
a) Velocity Probes
b) Acceleration Probes (Accelerometer)
2) Non-Contact or Proximity Type (eddy current) - located inside the bearing housing i.e. internal
Yes I do agree with TPL, the choice of probes largely dependent on the type of bearing (i.e hydrodynamic, roller, ball) fitted on the machine. However, it is also depending on the supplier. In our case, GE/NP who is the supplier for our GT, strongly recommend to use the seismic - accelerometer probe on the compressor side casing for protection/tripping. It is their standard practice for Frame 5 GTG (industrial). In addition, for monitoring purpose only they also have 2 proximity probes (X-Y) on each radial & thrust bearing (Bently Nevada 3500).
BTW, Bently Nevada is part of GE now.
API 616 (for gas turbine) specifies proximity probes for radial & axial shaft vibration monitoring. It also specifies casing vibration (seismic) for monitoring. The protection or tripping based on proximity probes is stated as optional not mandatory...
API 612 (for steam turbine) specifies proximity probes for radial & axial shaft vibration monitoring and protection.
Thanks
norzul
RE: Seismic vs Proximity Probe
what you mention is and has been the GE standard for decades.
seismic probes are used for monitoring and protection (trip function)
proximity probes are used for monitoring and diagnostic (Bently Nevada ADRE)
The main reason GE uses the seismic probes for protection is that it allows for redundancy:
e.g. brg#1 has two probes that individually detect vibrations in the vertical plane
while the proximity probes require the composition of the X and Y probe and redundancy at the position is not possible.
lately the use of newer versions of the control systems allowed for the use of the proximity probes for protection (trip function), but reserved for the high end of the product line, i.e. F and H class units... unless so specified by the owner.
HTH
saludos.
a.