Cooling Tower Fundamentals - SPX Cooling Technologies

SECTION II
46
3. Riser Sway Braces: Wind and/or earthquake
considerations will occasionally influence specifying
engineers to call for sway braces which tie
the upper end of a foundation-cantilevered riser
to the larger tower structure, which is assumed
by the specification writer to have the greater rigidity.
This is not good practice when the riser
is of a material having a high modulus of elasticity,
such as steel. The cooling tower structure
will react quite differently from the riser under
an imposed load condition. For example, under
earthquake acceleration the riser will respond at
high frequency and low amplitude, whereas the
tower structure (of lower modulus material) will
respond at lower frequency and greater amplitude.
The result is that any connection between
the two will be attempting to transmit the seismic
response of the tower into the more rigid risers,
and damage to the endwall framing or the piping
connections may follow.
A riser brace capable of transmitting the high
loads generated by a differential response is a
costly auxiliary structure which imposes significant
loadings at anchorage points in the cold
water basin, and its utilization in such cases
should be avoided. Properly designed cooling
tower header piping will accommodate typical
horizontal and vertical movement without distress,
and the flanged joint between the riser
and the distribution header should never be
considered as a riser support. In cases of doubt,
an expansion joint or flexible coupling should be
provided between the riser and header to allow
relative movement to occur.
Risers of FRP plastic pipe have a modulus of
elasticity close to that of the tower structure and
will experience similar seismic response. Therefore,
destructive transfer of opposed loads is
unlikely, and the flexibility of the FRP pipe riser
may require lateral support at the top. In such
cases, a riser sway brace may be a desirable
solution.
4. Ancillary Systems: Provision must be made for
make-up, overflow and blowdown, and should
be made for by-pass. Since proper by-pass utilization
is covered more fully in Sections I-H and
V-F, suffice it here to say that the location of the
by-pass should be in accordance with the manufacturer’s
recommendations to prevent damage
to the structure or the fill.
The amount of make-up water required consists
of the total water losses accrued through
evaporation (Sect. I-D), drift (Sect. II-I), blowdown
(Sect. I-G-1) and system leakage. On
relatively small towers, make-up is controlled by
a mechanical float valve responding to the basin
water level. (Fig. 65) Float switches or electric
probe systems are normally used on larger towers
to open and close a make-up valve, or to
start and stop a make-up pump. (Fig. 66) These
may be located in a stilling chamber designed
to suppress the effect of normal wave action in
the basin. Make-up lines from potable sources
are usually brought to the cold water basin, installed
with their point of discharge downturned,
and sufficiently above the basin water level to
preclude contamination of the supply by the
circulating water. Non-potable water, of course,
may be connected for injection of the make-up
supply at any point in the water circuit.
Overflow lines (Fig. 50) may be sized large
enough to facilitate flush-out cleaning of a sump,
but must be large enough to handle full make-up
flow in case that device malfunctions.
Blowdown lines should be sized to handle
the maximum anticipated amount of water to be
wasted. (Sect. I-G-1) Although blowdown can
be taken from the cold water basin, it is usually
easier to control if taken from the pressure-side
(i.e. the inlet riser) piping.
Figure 65 — Float-operated mechanical make-up valve.
High water alarm
Make-up supply cut-off
Make-up demand
Low water heater
cut-off and alarm
Figure 66 — Electric probes can be used to actuate
various systems.