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Tuesday, December 18, 2018
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Understanding Control Specification for Gas Turbine

In order to understand the control system of GE-design heavy duty gas turbine, I'm starting to learn how to read Control Specification. Can someone please explain how RPM is converted to counts in the following control specification sheet? And what values Mark VI will use for calculation in various algorithms?

Moderator's note: scroll down to see table.

`_________________________________________________________________________________02.01.03		SPEED SIGNAL: HP TURBINE (PERCENT)              		ALGORITHM: PR_MEAS             TNH              ELEM SH: 42MTHE TNH_PR SIGNAL IS RESCALED TO PERCENT OF RUNNING SPEED AT LINE FREQUENCY,THE GOVERNOR SPEED, TNH.5728, B15                         (ADJUST TNKH2 FOR 26214, B15 AT LINE FREQUENCY)SPEED           %SPEED               COUNTS          CONSTANT NAME5106*             100              26214, B15               TNKH26381              125              32767, B15               TNKH3                                  26214*(2**TNKH3) 9000*NOTE: TNKH2= ---------------------              RATED TURBINE SPEED                 (WHERE: SPEED = RPM)___________________________________________________________________________________CONSTANT	K	VALUE		UNITS		DESCRIPTIONTNKH2		K	11551		CNT 15		TNKH3		K	-2		CNT 15`

1 out of 1 members thought this post was helpful...

SB,

The section you provided seems to be from a very early Mark IV Control Specification. The Mark IV used fixed point math, and the documentation was very cryptic and could be confusing. Fixed point math requires the use of shifts to keep the binary maths and the decimal maths "together."

I found it best to ignore the "extra" binary scaling information and shifts. Since we can't "see" what's going on in the binary maths (which is what the Mark IV actually uses) unless you are trying to write new code/rungs which use maths it's best to just presume the Mark IV microprocessors are doing the binary maths correctly (which it always does).

Use your preferred Internet search engine to look for information on fixed point binary maths if you want to pursue the matter further. It would consume a small text book to try to explain it here, and I'm sure there are better (and worse) explanations on the World Wide Web.

CSA,

Thank you very much!

I'll also prefer ignoring the "extra" maths.

Hello Sir,

Can you please tell whats is "CNT15" and "B15"?

1 out of 1 members thought this post was helpful...

sabtain,

The Mark IV uses fixed-point mathematics. Not floating-point point maths we are most accustomed.

Because the Mark IV was an early digital control system thr designers decided to describe the binary information for many control signal values, because troubleshooting without that information is very difficult.

The really great news is that the Mark IV and Mark V (which also used fixed-point maths) we're both very well designed and didn't (don't) need the level of troubleshooting they designers anticipated.

You can find all sorts of information on the World Wide Web about fixed-point maths. There is also a very good description of fixed-point maths in the Mark V Application Manual, GEH-6195, which can be downloaded from many sites on the World Wide Web.

Hope this helps.