APPS: LFBV Calibration

[The AList]

------------ Forwarded Message ------------
From: CSA
To: AUTOMATION at CONTROL.COM
Subject: Re: APPS: LFBV Calibration

"Fobbed off": to be put off by [(evasion)], from http://www.wiktionary.org.
(Had to look that one up! Thanks for the Internet, right?!)

You say the LFBV has been identified as "over traveling". Exactly what
does that mean? Does it mean the valve is more closed than it "should"
be? Does it mean that more fuel is flowing than "should" be?

We really need to know what frame size machine you are talking about
(and we're presuming this is a Frame and not an aero-derivative). If
this is a Frame machine, you should refer to the Control
Specification, Sect. 05.01, to see what the 100% stroke is supposed to
be for the LFBV; it should be stated there, and that is what should be
used.

Years ago, when the LFBVs of Frame 7E/EAs and 9Es were fitted with
LVDTs, many times they were "short-stroked", meaning they were
calibrated such that when the valve was fully closed it would read
less than 100%. If I recall correctly, Frame 7E/EAs were to be
calibrated such that 100% stroke would occur at 1.125 inches, but the
valves only had a 1.000 inch mechanical travel. I've never given that
much thought until just now, so I'm not exactly sure why that was
done.

You need to consider and remember this: The servo regulator used for
control of the LFBV is a *flow-control* loop, meaning that regardless
of the calibration of the LFBV LVDTs the valve will go to the position
that it needs to go to in order to make the flow divider feedback
equal to the liq fuel flow-rate reference. It's just that simple; the
LFBV will be operated at the position that makes the actual fuel
flow-rate, as measured by the flow divider, equal to the liquid fuel
flow-rate reference (usually signal name FQR or something similar).

The LFBV is *not* like a Gas Control Valve (GCV); most units don't
have a gas fuel flow-rate reference like there is for liquid fuel.
Rather, it is presumed that flow through the GCV is proportional to
stroke (position) (at rated speed) and that for a given stroke
(position) there is a particular flow through the GCV. Gas fuel
flow-rate is not usually fed back to the GCV servo regulator loop. The
GCV position *should be* reasonably proportional to flow through the
valve.

Liquid Fuel Flow-rate is measured, using the Liquid Fuel Flow Divider.
FSR1, the Liq. Fuel FSR, is converted to a Liq. Fuel Flow-rate
Reference, FQR, which is compared to the Liquid Fuel Flow Divider
Feedback, which is supposed to be scaled to reflect expected Liq. Fuel
Flow (meaning, in terms of calorific value, the heat content of the
fuel).

The flow divider feedback has a scaling constant, which sounds like it
may not be exactly correct. Has anyone compared the fuel analysis of
the fuel being used to that of the expected fuel analysis supplied to
GE or the packager during the requisition phase of the purchase? That
may affect or change the flow divider feedback scaling constant.

Also, you haven't told us if you have observed the LFBV position;
there is usually a scale provided to be able to monitor valve position
during operation (if we're talking about something other than a Frame
5 or Frame 61B). What is the physical valve position when the unit is
at Base Load? Is it really 99%? If so, I'd say the flow divider
feedback ain't scaled properly to begin with. Also, at Base Load, what
is the Liq Fuel Flow-rate Reference, and what is the Liq Fuel Flow
Divider Feedback?

To my mind, the LVDT calibration isn't critical. I would also suggest
that if VR4 were to open, that it would relieve so much fuel flow that
the LFBV would not be able to control fuel flow very well. It really
is just a safety relief valve, and if it opened, the unit would likely
trip on loss of flame.

If the LFBV is truly at 99% stroke as  indicated on the scale when the
unit is at Base Load, something other than LVDT feedback calibration
is much more likely wrong. If this is occurring on one machine out of
several "identical" machines on the same site, then something is
causing the LFBV of that unit to close more than the others in order
to make Base Load, something like plugged Liquid Fuel Filters (either
Low Pressure or High Pressure, or both), or a slipping Liquid Fuel
Pump Clutch, or a worn High-Pressure Liquid Fuel Pump, or clogged Liq
Fuel Forwarding Pump suction strainer. Because the LFBV should not be
anywhere near full closed at Base Load.

Now, if the Liquid Fuel characteristics have changed, this could cause
a problem like this. But, it would seem it would happen on all
machines (presuming that all machines have the same Liq. Fuel Flow
Divider feedback scaling!). So, why is this one different? That's what
enquiring minds want to know.

We really want to know how this turns out, so feel obligated to let us
know! (And I'm not talking about the situation with the Controls T.A.;
some of them are better at explaining things than others, and
sometimes they just don't have time, and sometimes they just don't
take the time. He or she might be saying to calibrate the valve per
the Control Spec. (if it says to "short-stroke" the valve) and just
isn't explaining it very well, if at all.) I'm talking about what else
might be wrong which might be causing the valve to nearly close (we
don't really know that yet, because you haven't told us the position
of the valve from the scale on the valve) to make Base Load.

So, first let us know the position of the valve from the scale when
the unit is at Base Load, and then, let us know how the problem is
ultimately resolved.

Thanks!


On May 11, 2008, SI1405 wrote:
> Liquid fuel bypass valve calibration.
> 
> The liquid fuel bypass valve on one of our machines has been identified as over travelling and the recommendation from the Controls TA on site was to adjust this out in the calibration.  ACALIB.DAT has been updated so that the Negative saturation current is now greater than 100%. Now when the valve is stroked the correct stroke lenght is achieved for 0-100%.  
> 
> My concern with this is that 100% is the closed position and now by reducing the stoke in this manner there is a possiblity that the full fuel flow may not be acheived. If the adjustment was made at the oposite end at the fully open position, it would have little or no effect on operation.
> 
> The TA has so far rejected any concerns raised and has stated that the LFBV shouldn't close fully to protect the pump.  If the this is the case what is the purpose of the relief valve (VR4).  I can't help but think i am being fobbed off.
> 
> I have data the indicates this valve was running at 99% for base load operation. 
> 
> any comments regarding this would be appreciated.
> 
> Thanks