desen pentru incendiu cladiri f inalte
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Terminal VelocityD epending upon the rate of flow from the branch drain
into the stack, the type of stack fitting, the diam eter of the
stack, and the flow dow n the stack from upper levels, the
discharge from the branch m ay or m ay not entirely fill the
cross-section of the stack at the point of entry. As soon as
the w ater enters the stack it is im m ediately accelerated at
the rate of 82.2 feet/second/second by the force of gravity,
and in a very short distance form s a sheet around the innerw all of the pipe. It can be sim ply described as a hollow
cylinder of w ater. This sheet of w ater w ith a core of air in
the center continues to accelerate until the frictional force
exerted by the pipe w all on the falling sheet of w ater
equals the gravitational force. The frictional force varies as
the square of the velocity, and thus resistance to flow is
very rapidly increased. From the point w here frictional
force equals gravitational force, the sheet of w ater w ill con-
tinue to fall at a velocity w hich rem ains practically un-
changed. This ultim ate vertical velocity is called term inal
velocity,and the distance in w hich this m axim um veloci-
ty is achieved is called the term inal length.
F.M . D aw son and A.A. Kalinske in Report on H ydraulics
and Pneum atics of Plum bing D rainage System s(State
U niversity of Iow a, Studies in Engineering B ulletin 10,
1987) and R.S. W yly and H .N . Eaton in Capacities of
Plum bing Stacks in B uildings(N ational Bureau of
Standards Building M aterials and Structures Report BM 132,
1952) have investigated term inal velocity and derived a
w orkable form ula by treating the sheet of w ater as a solid
hollow cylinder sliding dow n the inside w all of the pipe.
The form ulas developed for term inal velocity and term inal
length, w ithout going through the com plicated calculus in-
volved, are
V t= 3.0 (q/d)2/5
Lt = 0.052 V2t
w here: V t = term inal velocity in stack, fps
Lt = term inal length below point of flow entry, ft
q = quantity rate of flow , gpm
d = diam eter of stack, in.
Applying the form ulas for various size stacks, it is found
that term inal velocity is achieved at approxim ately 10 to 15
fps and this velocity is achieved w ithin 10 to 15 feet of fall
from point of entry. The im portance of this research is that
it conclusively destroys the m yth that w ater falling in a stack
from a great height w ill destroy the fitting at the base of the
stack. The velocity at the base of a 100-story stack is only
slightly and insignificantly greater than the velocity at the
base of a three-story stack! There is no scientific reason for
lim iting the height of a soil or w aste stack of any size and
the stacks can be run straight dow n, w ithout offsets, for 1000
feet or m ore w ith the utm ost confidence. So-called velocity
breaksare absolutely unw arranted and, in fact, could cause
excessive pneum atic pressure fluctuations in the stack.
Stack OffsetsM ost engineers are aw are of the problem w hich exists
w henever a stack offsets at an angle greater than 45 de-
grees. At the point of offset, flow enters the horizontal drain
at a relatively high velocity w hen com pared to the velocity
of flow in a horizontal drain under uniform flow conditions
at full or half-full flow . W hen the w ater reaches the bend
at the offset it is turned at right angles to its original flow ,
and for a few pipe diam eters dow nstream it w ill continue
to flow at relatively high velocity along the low er part ofthe horizontal pipe.
Since the slope of the horizontal piping is not adequate
to m aintain the velocity of flow that existed w hen the w a-
ter reached the offset, the velocity of flow in the horizontal
drain slow ly decreases w ith a corresponding increase in the
depth of flow until a critical point is reached w here the
depth of flow suddenly and sharply increases. This increase
in depth is often great enough to com pletely fill the cross
sectional area of the pipe. This sudden rise in depth is
called the hydraulic jum p(Figure 1). The critical distance
at w hich the hydraulic jum p m ay occur varies. It is de-
pendent upon the entrance velocity, depth of w ater w hichm ay already exist in the horizontal drain w hen the new
flow is introduced, roughness of the pipe, diam eter of the
pipe and the slope. The distance varies from im m ediately
at the stack fitting up to ten tim es the diam eter of stack
dow nstream : Less jum p occurs if the horizontal drain is
larger in size than the stack. Increasing the slope of the hor-
izontal drain w ill also m inim ize the jum p. After the hy-
draulic jum p occurs and fills the drain, the pipe tends to
High-Rise DWV and Storm SystemsAlfred Steele, PE CIPE
O ne of the oldest and m ost persistent m yths in the plum bing profession is the belief that extrem ely high velocities de-
velop in the stacks of high-rise buildings. The plum bing engineer is invariably asked how he is going to provide for these
velocities at the base of the stack. H ow is he going to prevent the base fitting from being blow n out or broken? N o spe-
cial provisions are required for trem endous velocities, and no special precautions are required to protect the base fitting.
Excessive velocities just do not occur!
54 Plumbing Systems & Design Sept/O ct 2003
C o n t i n u i n g E d u c a t i o n
Repri n ted fr om Al fr ed Steele (1984), Advanced Plum bing Technology, Elmhu rst, IL: Constru ction In du stry Press.
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Sept/O ct 2003 Plumbing Systems & Design 55
flow full, w ith large bubbles of air m oving along the top of
the pipe w ith the w ater. Surging flow conditions w ill exist
until the frictional resistance of the pipe retards the veloci-
ty to that of uniform flow conditions. Any offset of the
stack, at any floor of the building, greater than 45 degreescan cause hydraulic jum p.
W hen the hydraulic jum p occurs, and proper venting has
not been provided, trem endous pneum atic pressures are
built up in the area behind the jum p. There have been cas-
es w here this excess pressure (greater than a one-inch col-
um n of w ater) has extended 40 feet up the stack. It m ust
be stressed that this excess pressure occurs only w hen ad-
equate venting has not been provided. U nder no circum -
stances should the fixtures on the floor directly above an
offset connect to the stack before the offset. These fixtures
should be piped and connected to the horizontal offset
m ore than ten stack diam eters dow nstream or preferablyconnected to the vertical at least tw o feet below the hori-
zontal offset (Figure 2).
Sizing Offset StacksM any high rise buildings decrease the floor areas at cer-
tain specified heights. To accom m odate the decreased ar-
eas, fixture layouts are changed and stacks m ust be offset
to new locations. There is an acceptable m ethod of sizing
offset stacks w hich can result in substantial econom ies.
Figure 3illustrates a typical offset stack. The procedure for
sizing is as follow s:
1. Size the portion above the offset as for a regular stack
based upon the total num ber of fixture units abovethe offset.
2. Size the horizontal offset as for a building drain.
3.The portion of the stack below the offset shall be at
least the size of the offset or based upon the total
num ber of fixture units on the entire stack (both
above and below the offset), w hichever is the larger.
Expansion and ContractionExpansion and contraction of stacks is another question
alw ays brought up for consideration. There has not been
an actual docum ented instance of stack failure due to ex-
pansion or contraction of a stack caused by a variation in
tem perature. The flow of w ater is not constant enough tokeep it in contact w ith the pipe long enough for the trans-
fer of necessary heat to affect the pipe. O f far greater im -
portance and danger in a high-rise building, and it is a
problem w hich is often overlooked, is the shrinkage in
height of the low er stories of the building. As the gross
w eight of the upper structure is added, foreshortening at
the low er floors has been noted to be as m uch as tw o inch-
es in a sixty story building. Soluble gaskets installed in the
caulked joints have obviated this problem . The structural
engineer should alw ays be consulted to determ ine the ex-
tent of foreshortening to be expected.
Suds PressureThe prevalent use of high-sudsing detergents in w ashing
m achines, dishw ashers, laundry trays and kitchen sinks has
created serious problem s in all residential buildings and es-
pecially in high-rise buildings. U ntil m anufacturers are
forced to m arket only detergents w ithout sudsing charac-
teristics the plum bing engineer m ust understand and cope
w ith the dangers created in the sanitary system by the pres-
ence of suds. (An interesting sidelight: suds, in and of
them selves, do not enhance the cleaning ability of soaps or
detergents in any w ay).
Figure 1. Hydraulic Jump at Offset Figure 2. Piping for Fixtures Directly Above Offset
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C o n t in u in g E d u c a t io n : H ig h - Rise D W V a n d S t o r m S y st e m s
56 Plumbing Systems & Design Sept/O ct 2003
W hen the flow of w astes from upper floors contains deter-
gents, the suds-producing ingredients are vigorously m ixed
w ith the w ater and air in the stack as the w aste flow s dow n
the stack and further m ixing action occurs as other branch
w aste discharges m eet this flow . These suds flow dow n the
stack and settle in the low er sections of the drainage system
and at any offsets greater than 45 degrees in the stack.
Investigation has show n that w here sudsing w astes are pres-
ent the sanitary and vent stacks are laden w ith suds, and this
condition w as found to exist for extended periods of tim e.
Liquid w astes are heavier than suds and easily flow
through the suds-loaded drainage piping w ithout carrying
the suds along w ith the flow . Everyone is aw are of the dif-
ficulty of flushing the suds out of a sink. The w ater sim ply
flow s through the suds and out the drain, leaving the m a-
jor portion of the suds behind. The sam e action occurs in
the low er sections of the drainage system except for one
im portant difference: air, as w ell as w ater, is now flow ing
in the piping. This air, w hich is carried dow n w ith the
w aste discharge, com presses the suds and forces them to
m ove through any available path of relief. The relief path
m ay be the building drain, any branches connected to the
building drain, the vent stack, branch vents, individual
vents or com binations of any of the foregoing. A path of re-
lief m ay not alw ays be available, or could be cut off or re-
stricted by the hydraulic jum p, or a path m ay just be inad-
equate due to location or size. If one or m ore of these con-
ditions exist, excessively high suds pressure can develop
and blow the seals of traps w ith the accom panying ap-
pearance of suds in fixtures.
H igh suds pressure zones occur at every change in di-
rection, vertically or horizontally, w hich is greater than 45
degrees. W here vent stack base connections, relief vents,
branch vents or individual vents serve as the relief path for
the high suds pressure, they are usually found to be inad-
equate in size w ith resultant suds conditions appearing at
the fixtures. The vent pipe sizing tables in practically every
code are calculated on the basis of air flow capacity and do
not in any w ay provide for the m ore dem anding flow of
suds. Sizes w hich are based on these code tables are inad-
equate to accom m odate suds flow and thus are incapable
of providing adequate suds pressure relief.
Suds are m uch heavier than air and consequently do notflow w ith the sam e ease. They produce a m uch greater fric-
tion head loss for the sam e rate of flow . The density of old
or regenerated suds varies from 2 pounds per cubic foot to
a high of 19 pounds per cubic foot, dependent upon the
detergent used. For equal rates of flow and pressure loss,
the vent pipe diam eter for suds relief flow m ust be from 20
to 80 per cent greater than for air flow .
W henever a soil or w aste stack receives w astes from
w ashing m achines, dishw ashers, laundry trays, kitchen
Figure 3. Sizing of Stack with an Offset Figure 4. Suds Pressure Zones
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Sept/O ct 2003 Plumbing Systems & Design 57
sinks or other fixtures w here sudsing detergents are used,
the drainage and vent piping for the low er floor fixtures or
for the fixtures above offsets m ust be arranged to avoid
connection to any zone w here suds pressure exists.
Suds pressure zones exist in the follow ing areas:
1. At a soil or w aste stack offset greater than 45 degrees:
40 stack diam eters upw ard and 10 stack diam eters hori-
zontally from the base fitting for the upper stack sec-
tion. A pressure zone also exists 40 stack diam eters. up-
stream from the top fitting of the low er stack section.
2.At the base of a soil or w aste stack: The suds pres-
sure zone extends 40 stack diam eters upw ard from
the base fitting.
3.In the horizontal drain from the base of a stack: The
suds pressure zone extends 10 stack diam eters from
the base fitting, and w here an offset greater than
45 degrees in the horizontal occurs, the
pressure zones extend 40 stack diam e-
ters upstream and 10 diam eters dow n-
stream from the offset fitting.
4. In a vent stack connected to a suds
pressure zone: The suds pressure zone
exists from the vent stack base connec-
tion upw ard to the level of the suds
pressure zone in the soil or w aste stack.
Figure 4 illustrates all the above zones.
Vent SystemThe design of vent stacks for high-rise
buildings conform s to the design criteria
established for any building w ith slight ad-
ditional precautions. It is im portant to un-
derstand that the sole purpose of a vent
stack is to relieve excessive pressure fluc-
tuations in the soil or w aste stack it serves.
Just as the flow of w ater obeys all the law s
of hydraulics so does the flow of air obeyall the gas law s. The length of run and size
of pipe should be designed to m aintain
pressure fluctuations in the sanitary system
w ithin lim its to m aintain a m axim um pres-
sure variation of plus or m inus one-inch
colum n of w ater at fixture traps.
W hen w ater is flow ing in the sanitary
system , pressures in the drainage and vent
stacks of a m ulti-story building are con-
stantly fluctuating. The vent stack connec-
tion at the base of the drainage stack and
the branch vent connections to the branchdrains cannot alw ays elim inate these fluc-
tuations. For reasons w hich have not as yet
been determ ined, excessive pressure fluc-
tuations occur in stacks w hich have m ore
than ten branch intervals. It then becom es
extrem ely im portant to balance pressures
throughout the drainage stack by m eans of relief vents lo-
cated at various intervals. D rainage stacks in buildings hav-
ing m ore than ten branch intervals should be provided w ith
a relief vent at each tenth interval, counting from the top-
m ost branch dow nw ard. The low er end of the relief vent
should connect to the soil or w aste stack below the
drainage branch connection and the upper end should
connect to the vent stack at least three feet above floor lev-
el.Figure 5 illustrates m ethods of providing relief vents at
stack offsets and Figure 6 illustrates relief vents for stacks
having m ore than 10 branch intervals.
The vent-sizing tables in m any existing codes do not list
the size of pipe required for the exceptionally long lengths
of run encountered in high-rise construction. U tilization of
the follow ing form ula w ill result in the m axim um perm issi-
ble length.
Figure 5. Venting at Stack Offsets
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Sept/O ct 2003 Plumbing Systems & Design 59
An expansion joint or offset should alw ays be provided
at the connection to roof drains. This is required to prevent
pipe expansion from raising the roof drain and destroying
the integrity of the w aterproofing of the roof. Storm w ater
piping is probably subjected to the m ost frequent m ove-m ent of any plum bing system , but not necessarily the m ax-
im um expansion. The m ovem ent is due to the frequently
changing difference in the outside tem perature relative to
the inside tem perature. In w inter, m elting snow and ice at
low tem peratures flow into the drain
w here the piping is surrounded by an
am bient tem perature of 70 degrees or
higher.
Low tem perature liquid flow in the
storm w ater piping w ill cause condensa-
tion to form on the outside of the piping
in the building. It is therefore advisableto insulate all storm w ater offsets to pre-
vent condensation from staining ceilings.
The storm w ater system for high-rise
construction is usually adequately cov-
ered by code, but there are m any
codes w hich do not cover, and there-
by by om ission do not perm it the in-
stallation of controlled-flow roof
drainage. It is w ell w orthw hile dis-
cussing the use of controlled-flow roof
drainage w ith the authorities if it is not
covered by their code. Controlled-flow
roof drainage has advantages to rec-
om m end it, in lieu of conventional
roof drainage, in m ost applications. It
is especially advantageous for high-
rise construction. The higher the build-
ing, the m ore econom ical its use be-
com es. Econom y is of prim e im por-
tance to the builder, but of even far
m ore im portance than the econom ies
realized, controlled-flow roof drainage
is one of the best w ays to com bat w a-
ter pollution and flooding during
heavy rainfalls. It is the authors con-sidered opinion that every m unicipali-
ty should m ake it m andatory to use
controlled-flow roof drainage w here
com bined public sew ers are utilized.
H ere is an ideal and practical m ethod
of fighting pollution w hich does not
cost the com m unity one red cent!
D uring heavy storm s, sew age treat-
m ent plants cannot handle the increased flow s and conse-
quently trem endous quantities of untreated raw sew age are
dum ped into the nations stream s, lakes or oceans. W here
separate public storm and sanitary sew ers are available, itis still desirable to use controlled-flow as a m eans of alle-
viating flooding. By lim iting the quantity of flow into the
storm sew ers during heavy storm s, the sew ers are better
able to handle the runoff from other areas.
Figure 7 . Typical Wet Stack
Table 1. Fixture Units for Wet Stacks