Page 407 - Boiler_Operators_Handbook,_Second_Edition

392 Boiler Operator’s Handbook

The thermo-mechanical system uses a short pivot Shrink and Swell
at the end of the tube which consists of a lever point at A simple single loop control system like the one
the end of the tube and a pivot attached to the two steel covered at the beginning of this section will satisfy the
channels on either side of the tube. The lever connected requirements of most heating boilers and commercial
to the control valve moves as much as six inches from and industrial loads with fairly constant steam de-
its end with a very small change in the length of the mands. If, however, the steam requirements change
sensing tube. As the tube expands the lever is pulled significantly the control will actually operate in the
down by the weight to open the valve. wrong direction due to shrink and swell. Shrink and
The expanding water in the jacket of the hydrau- swell are terms we use to describe what happens when
lic version acts on a diaphragm (Swartout) or bellows the boiler load changes and feedwater addition chang-
(Bailey) on the control valve, opening it. As the water es.
rises in the tube as a result of adding water the tube or When the boiler is generating steam some of the
water jacket shrinks. The shrinking tube pulls the valve volume below the water surface has to consist of steam
closed on the mechanical system. A spring pushing bubbles. The amount that is bubbles depends on the
against the bellows or diaphragm of the hydraulic sys- load, the volume of the boiler below the water line in
tem closes the valve as the water in the jacket shrinks. proportion to the capacity, the surface area of the water
Both systems will stabilize to maintain a constant line, and the operating pressure. Many boilers, mostly
water level but they do not respond rapidly to level fire-tube boilers, contain so much water in proportion
changes and always open the valve fully as the boiler to steaming capacity that the percentage of volume oc-
cools down so you have to manually close off the water cupied by steam is small and the shrink and swell are
and manually control level on boilers equipped with not noticeable.
these systems until the boiler is at operating pressure. On the other hand, a low pressure water-tube
The Copes-Vulcan system (by the way, we’ve al- boiler is most likely to show the most dramatic change
ways called them Copes valves, failing to give Vulcan because the steam density is low (volume of steam
any credit) has another system with a feature to aid in per pound is high). When a sudden increase in load
response to changes in load. The control valve is fitted occurs the steam pressure in the boiler drops and the
with a diaphragm connected to the feedwater valve steam bubbles in the boiler water expand. Also a small
with sensing lines to the steam header at either side percentage of the water flashes to steam adding to the
of an orifice. Increasing steam flow produces a higher number of bubbles. The result is an increase in the
pressure drop across the orifice which produces a high- water level which we call “swell” because the water
er differential pressure on the valve diaphragm to force level increases with no water being added to the boiler.
it further open. The lever of the thermo-mechanical A single element level control will react to the swell
tube is fitted with a chain extension that runs over a by closing down on the feedwater valve, the opposite
sprocket on the valve to the weight. The sprocket is of what is needed because more water is required for
connected to the valve stem like a rack and pinion to the larger load. Closing of the feedwater valve reduces
aid or restrict the diaphragm action for final water level the heat requirement for raising the temperature of the
control. This provides something comparable to two- feedwater so more heat is used to make steam (and
element control, which I’ll get to. more bubbles) simply make the water swell more.
Experience and modern controls and instruments When the opposite occurs and the load decreases
have convinced me that I would never want to use suddenly, pressure increases, the bubbles are com-
one of those thermo-hydraulic or therm-mechanical pressed, the water in the boiler is not up to the new
control valves again. I tell people that have them not saturated condition so some of the steam condenses
to buy spares and replace them when they need repair. to heat it up. The water in the boiler shrinks and the
They are not the easiest things to work with, they don’t level drops. A single element control senses the drop in
control the level when the boiler is cold and they’re water level and opens the control valve to increase the
relatively expensive. Now that level transmitters and flow of feedwater. The additional feedwater requires
controllers are so inexpensive the cost of those older heat to warm it to saturation condition so some more
designs can’t justify their existence. They were fantas- of the steam is condensed to collapse more bubbles.
tic controls years ago but new controls can do so much Increasing the water flow is not required because the
more. steam flow decreased, but the shrinking water level
makes the control open the feedwater valve more.