TTRXB Command

Good morning Dear Sir.
We have PG 5371PA Mark VIe.
I have read few threads on TTRXB very keenly. TTRX is minimum of Isothermal, TTRXP, or TTRXS.
One thing is clear that TTRXB Command is used to calculate FSRT.
I Would like to know that if TTRXB is going to MODULATION of IGV or not.

If you've read the threads, you know that TTRXB (contrary to popular, false belief) DOES NOT stand for Base Load Exhaust Temperature Reference. It stands for Biased Exhaust Temperature Reference--and the bias is usually added only for two-shaft machines and special applications (of which there are few for singe-shaft machines).

The following snippet was taken from a Frame 5 with DLN-I combustors exhausting into an HRSG which uses the TTRXV6 block, but the basics of the TTRXVn blocks are all the same.


In the snippet above, there are two possible bias signals which can be applied to TTRXB. One is a speed bias (which is controlled by the spdbias2 logic input at 7, which is LFALSE--which is always a logic "0" so the speed bias is never applied to TTRXB.) In this block is there is a second bias signal (CSRGVTXB--IGV Exhaust Temperature Control Bias) that comes from the CSRGVV3 block--BUT it is only applied to TTRXB when L83GVBP is a logic "1":


Here is the rung for L83GVBP:


L83GVBP (IGV Exhaust Temperature Control Bias Permissive) is a logic "1" when the IGVs are NOT in manual control, when L14HS is a logic "1" and when Peak Load is not selected--so, basically the IGV Exhaust Temperature Control Bias is applied to TTRXB anytime the unit is above 14HS (TNH greater than 95% speed) and the unit is not on Peak Load and the IGVs are in automatic control (which is the normal mode of operation, especially when the unit has DLN combustors).

These were taken from a Mark V CSP, but the algorithmic blocks are pretty much the same for a Mark VIe, just in a slightly different display format.

TTRXB then goes to this block, FSRTV3:


where TTXM is subtracted from TTRXB and multiplied by a gain and a time-modified value of FSR is added to it and that sum is fed to a CLAMP which ensures the output can never be below FSKTMIN or above FSRMAX to become FSRT, which is then fed to the FSR minimum value selector block to become FSR (if it's the minimum input to the selector) to control the amount of fuel flowing to the combustors.

So, TTRXB does NOTHING to the IGV control--but TTRXB IS affected by IGV Exhaust Temperature Control when L83GVBP is a logic "1".

Hope this helps!

If you are having a problem, you can state the situation here and we can try and help. Again, TTRXB is NOT the Base Load Exhaust Temperature Control reference--that's FSRT. When no biases are applied to TTRX (which is the calculated value of Exhaust Temperature Control) then TTRXB is EQUAL to TTRX, which then becomes FSRT. ONLY when a bias is applied (in the examples above it would be either a speed bias (TNH) or an IGV Exhaust Temperature Control bias (CSRGVTXB) that is added to TTRX making TTRXB NOT EQUAL to TTRX. TTRXB is the (possibly) biased Exhaust Temperature Reference that becomes FSRT--the actual Exhaust Temperature Control FSR.

(One final note: In the CSP I took the snippets from, the Longname description for CSRGVTXB is: "Fuel Temp Bias (2-shaft)", yet another example of incorrect Longnames which can't be fully trusted and must always be verified (which can only be done by following signals to their origins to see what they actually do).