Where speed sensors mounted to measure speed of gas turbine and steam turbine? Why?

Where is blade path temp sensor installed? How many types of IGVs?

Thank you all

 

student,

>Where speed sensors mounted to measure speed of gas turbine
>and steam turbine? Why?

Turbine manufacturers locate them in different places for different reasons. Most commonly, the speed sensors are mounted "above" a toothed wheel (made by machining either the shaft or a "disc" that's bolted to the shaft). The teeth have different shapes for different reasons (to smooth pulses is the most common reason), but there are basically high spots and low spots that pass under the sensors which have a magnetic field and this results in a frequency output of the sensors which is converted to RPM (or, some control systems use RPS (Revolutions per Second).

Some gas turbines (usually aircraft-derivative gas turbines) have the speed sensors mounted on gearboxes driven by the turbine shaft.

So, there's really no "standard" speed sensor mounting location. Any location where a pulse output which is directly proportional to speed can be used or adapted for use. Again, most turbines (gas and steam) use toothed wheels either machined into the shaft or bolted to the shaft to produce the pulse rate which the speed sensors input to the turbine control system ("governor"). Speed control is the most important aspect of turbine operation (both speed during normal operation as well as preventing overspeed which can be very destructive). Even when the speed doesn't change very much during normal operation (such as generator-drive gas turbines producing AC (Alternating Current), speed control is still the major basis for controlling power output of gas and steam turbines (that is, increasing, decreasing or maintaining the power output during normal operation).

>Where is blade path temp sensor installed?

Some gas turbines don't monitor blade path temperatures; in fact, most monitor gas turbine exhaust temperature. And the location of blade path temperature sensors varies depending on the manufacturer's definition of "blade path temperature"--which can vary depending on the manufacturer. Measuring actual hot gas temperature as it flows through the nozzles and blades of the turbine section of a gas turbine (some manufacturers call the turbine blades "buckets") can be very difficult--because there is stratification of the gases (meaning that in the flow there can be high temperatures and low temperatures so the placement of a sensor to gather "average" temperature can be a problem, and the stratification can change with hot gas flow-rate). Also, putting a sensor in the hot combustion gas path means that if the sensors breaks off or a piece of the sensor breaks off (which can happen because of the very high temperatures of the hot gases) it will strike the nozzles/blades and cause physical damage, even catastrophic damage if one of the blades breaks and gets caught in the rotating blades. This is one of the most common reasons for only monitoring exhaust temperature, and either calculating a blade path temperature or assuming a blade path temperature based on experience (called "empirical data").

>How many types of IGVs?

Basically two: fixed (or stationary) and variable. Variable IGVs afford more control of air/fuel ratio and mass flow through the axial compressor and turbine sections at different operating points (including start-up and shutdown). Axial compressors are very unique devices and have unique operating characteristics (mostly limits of operation at various speeds and air temperatures). Sometimes these operating characteristics (limits) are counter-intuitive.

After the ability of hot gas path components to withstand high temperatures (combustor components; turbine nozzles; turbine blades (buckets); exhaust components) without failing quickly (in other words, having a long parts life before needing replacement or being prone to failure) the ability of present-day axial compressors is one of the limiting factors on the amount of power that can be produced by gas turbines. They just can't flow enough air without causing compressor damage and so the turbine output is limited as a result. New compressor designs (most with multiple stages of variable stator vanes (VSVs) are vastly improving the abilities of axial compressors to flow ever-increasing amounts of air--leading to the improved power outputs of many machines. The amount of power being produced for the same size engine these days in really phenomenal compared to machines of the same size just a decade earlier. (Again, materials technology has also contributed to the increased power outputs, also--both hot gas path materials and compressor component materials.)

Hope this helps!

 

Thank you Sir!

How many speed sensors are generally used? I understand that it is installed on generator side,is that true? what if shaft broken?

Student

 

I need to qualify my answer--I'm referring to a single-shaft combustion turbine which may be directly coupled to a generator, or may be coupled through a reduction gear assembly to change the RPM of the turbine to that required by the generator. A minimum of two speed sensors are usually installed, though some manufacturers use as many as six.

The speed sensors are usually installed on the combustion turbine shaft, since the turbine speed and generator speed are either the same (when directly coupled), or are proportional when coupled through a reduction gear assembly. An AC (Alternating Current) generator will not overspeed when synchronized to a grid with other generators should the coupling (shaft) break, but the gas turbine could overspeed if the fuel isn't reduced or cut back very quickly if the shaft/coupling were to break.

Some gas turbines which drive generators have two, or even three, shafts--only one of which is coupled to the generator.

 

Okay; I gotta ask.

To the person who gave a Thumbs-down, indicating the response was not helpful: What was not helpful about the response versus the questions that were asked?

You can post your reply as Anonymous, but I would like to understand how the response could have been better, or what was hot helpful or what might have been wrong.

Thanks!!!