Page 417 - Boiler_Operators_Handbook,_Second_Edition

402 Boiler Operator’s Handbook

through the burner reduces mixing of air and fuel. As mance at different loads. Loss of linearity associated
the lower firing rates are approached the excess air may with repairs, rebuilding, and even other maintenance
go as high as 100% and, due to damper leakage, can go functions can suddenly create different operating char-
even higher. acteristics for any system. To understand why let’s use a
The principle concern with the jackshaft control principle that’s discussed in the Chapter on controls and
system itself is linkage slipping. It’s not uncommon for the graph in Figure 11-31.
one of the linkage connections to come loose. I know one You have to accept that the only thing you control
plant that chose to solve that problem by welding all the in a boiler plant, refrigeration plant, and any process
linkage only to discover that the heat from the welding is flow. When a system load changes the flows must
distorted the linkage and they had to replace it to restore change for it to be in balance and it’s important that a
the adjustments. change in flow is proportional to a change in other flows
Other tricks including drilling the links and insert- or controlling parameters. The most desirable relation-
ing tapered pins didn’t work either; they weakened the ship between flow and any control or instrument signal
shaft and linkage which subsequently broke. The best that’s used to control that flow is described by a straight
solution to loose connections on jackshaft linkage was line drawn on a graph between zero and 100% of the
provided by technicians at the Louisiana Army Ammuni- flow and controlling parameter. It was made apparent to
tion Depot outside Shrevesport. They stopped at the auto me on a job years ago and I’ve been preaching the gospel
supply store every fall to buy a different colored can of of linearity ever since.
automotive spray paint and, after making their adjust- At that time we were converting two boilers from
ments, sprayed all the connections with that paint. Any firing oil to gas / oil firing at the Central Avenue Heat-
change in position was immediately apparent because ing Plant in Baltimore. One boiler had been finished and
the paint was cracked or a different color was showing. we were waiting for a two week check-out period to
That doesn’t mean they won’t slip, only that you’ll complete on the first boiler before starting work on the
know it if they do. Judicious use of lock-tight or, prefer- second. Repeated calls from the plant regarding slow
ably, star washers to prevent them coming loose is also a control response were resolved by a control adjustment
wise thing to do. that produced more calls regarding erratic and swinging
A less common problem, but one you have to be operation. Adjusting the controls to respond properly at
aware of, is that linkage rods can be bent to change their one load would produce slow or swinging operation at
length and the relative position of the controls. That another load. Investigation revealed the problem was
won’t be evident with the paint trick described above. due to the control of the boiler air flow which had a
Some arrangements make this a difficult situation to characteristic similar to curve A in Figure 11-31. At low
spot because the rods are bent to begin with so they can loads a change in control signal (in response to a change
clear some obstruction on the burner. If you have any of in steam pressure) resulted in a significant change in air
those rods the best thing to do is mark their angle on a flow. At high loads a large change in control signal was
cardboard template and keep it for reference. required to achieve a similar change in air flow and, as
Another problem, typical with firetube boilers, analysis of the curve will reveal, inadequate change in air
is the linkage gets disconnected when the boiler is flow compared to lower loads. Steam pressure controls
opened for inspection or cleaning. The wise operator respond to a change in steam pressure by increasing or
scratches match marks at all the connections before decreasing the control signal output in a consistent man-
breaking them to open the boiler. That way the linkage ner and the controls don’t know that the controlled flow
can be put back (almost) precisely where it was. Fresh (air flow in this example) produces a varying change in
paint after matching the scratches will restore confi- flow. When the controlled flow changes vary consider-
dence in the settings too. ably depending on load the controls don’t know it and
can’t respond properly to it. The best performance will
always be achieved when a change in controller output
ESTABLISHING LINEARITY produces a consistently relative change in controlled
flow; a 10% change in controller output should produce
Okay, it’s not a word in the dictionary, that’s be- a 10% change in the controlled flow.
cause I created it to describe what’s necessary with any Adjustment of the linkage achieved performance
system to operate properly at all loads. If systems are similar to curve C which is close enough to eliminate
not linear you will experience problems with perfor- most of the variation in performance and that resolved