Vibration sensors--on any type of rotating equipment--fall into three basic types, depending on the equipment the sensors are installed on and the speed of the equipment and they desired type of measurement. There are velocity vibration sensors, distance vibration sensors and acceleration vibration sensors.
A "seismic" vibration sensor is another name for a velocity vibration sensor. Velocity vibration sensors are usually attached to some part of the machine which is very close to the rotating shaft and measures the velocity of the part the sensor is attached to. On GE-design Frame 9E heavy duty gas turbines, velocity (or "seismic") vibration sensors (or pick-ups) are usually mounted on the upper half of the journal bearing housing (often called the "bearing cap").
Acceleration vibration sensors (often called accelerometers) are also usually attached to some part of the machine in close proximity to the rotating shaft. Acceleration vibration sensors (pick-ups) are usually used on machines with a lighter frame and rotor--such as aircraft-derivative gas turbines which rotate at much higher speeds than single-shaft heavy duty gas turbines.
So, neither velocity- nor acceleration vibration sensors (pick-ups) measure actual shaft vibration, but rather measure the movement of the part of the machine the sensor is attached to. Using velocity vibration sensors to measure vibration of a heavy duty gas turbine is fine--since the shaft (of a single-shaft heavy duty gas turbine, which the Frame 9E is) has a LOT of mass (is very large and heavy--particularly the axial compressor portion of the shaft) and when it vibrates it will have a large affect on the bearing housings, where the seismic (velocity) sensors (pick-ups) are mounted.
Distance vibration sensors, or proximity vibration sensors, also sometimes called displacement vibration sensors, are mounted such that they measure the actual distance of the shaft from the face of the sensor. They require a second device to measure the speed of the rotating shaft to actually determine the magnitude of the movement of the shaft when it's vibrating. They are usually mounted near the bearing housing as the bearing housing is usually a very stable part of the machine.
For decades GE and packagers of GE-design heavy duty gas turbines only used seismic (velocity) vibration sensors. Even when distance or proximity vibration sensors became available they continued to use seismic vibration sensors for the protection of the machine (to trip the machine on high vibration as detected by the seismic (velocity) vibration sensors). Many purchasers of GE-design heavy duty gas turbines started requesting, then demanding, that distance (proximity) vibration sensors be installed on new machines they were ordering. GE, reluctantly, started providing these sensors (usually manufactured by Bently-Nevada Corporation) in addition to the seismic (velocity) sensors on many new machines. BUT, still GE steadfastly continued to use only the seismic (velocity) sensors for protection (tripping). (For a few years GE was installing proximity, or distance, vibration sensors on every machine they produced--because in the event of an increase in vibration during the warranty period when the machine was new they could connect special equipment to the proximity sensors to use in determining what was causing the high vibration and how to mitigate it. BUT, the GE turbine control systems could not be directly connected to the proximity vibration sensors for many years.)
Eventually, GE purchased and assimilated Bently-Nevada and started installing proximity vibration sensors on almost every heavy duty gas turbine they produced. One benefit of using distance, or proximity, vibration sensors is that when high vibration is detected the proximity sensors can, with the addition of special equipment, be used to determine what is causing the increase in vibration or how to place weights to reduce the vibration. GE then did start using the proximity vibration sensors for protection (tripping). And, once they owned Bently-Nevada, they were able to design printed circuit cards and modules which could be directly connected to the proximity vibration sensors also.
Which type of sensor is better? That depends on the type of information you want from the sensor. For basic machine protection, the seismic vibration sensors are fine and adequate. But, they don't provide any real useful information about the nature of the vibration or how to mitigate the vibration. That's where proximity vibration sensors excel--providing information about the nature of the vibration and how to mitigate it.
Again, accelerometers are used primarily on machines with lighter frames and rotors and which run at higher speeds--but they are not measuring actual shaft vibration or movement (displacement)--they are measuring the movement of the part of the machine the accelerometer is attached to--which is caused by the vibration of the rotating shaft.
Hope this helps! Here's a pretty useful explanation of how displacement (distance; proximity) vibration sensors work:
(Unfortunately, although it mentions the three basic types/classifications of vibration sensors, it only really details how distance, or proximity or displacement, sensors work.
Finally, people ask me quite frequently why the velocity vibration sensors on a GE-design heavy duty gas turbine are called "seismic" vibration sensors (pick-ups). Seismic shaking or a seismic event is usually associated with an earthquake or volcano eruption, where the ground moves pretty violently. When mounted on bearing caps velocity vibration sensors usually require a pretty large amount of vibration to actually detect shaft vibration--especially in the alarm and trip regions. When the shaft is vibrating that much, one can usually feel it in their feet when standing on the grating or even the ground next to the turbine--hence, a "seismic" shaking or event! Isn't the English language wonderful? (Some manufacturers of velocity vibration sensors refer to their products as seismic sensors (pick-ups); others don't. It's a common term, but it actually seems to be falling out of usage--except for GE-design machines. GE had a philosophy of, "If it ain't broke--don't fix it!" for many years (decades). Which was quite good for people who worked on or with GE machines because they stuck with consistent equipment and terms (even if the terms weren't exactly perfectly descriptive). But, things change, so do tried and true philosophies and practices.... Change, for change's sake, isn't always a good thing. But, the assimilation of Bently-Nevada into the GE "family" of companies almost forced GE to start equipping every gas (and steam) turbine and generator (and centrifugal compressor) with B-N vibration (and axial position) sensors (profit margin is their most important product, after all).
That is probably a little more information than you, hussam295, were looking for, but I write as much for other readers (now and in the future) as for the original poster.
Hussam 295Sorry I have no answer.am just like you
I want to learn more on keyphasor and radial vibration sensor 1X & 2 X
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