Vent Systems

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VentSystems

Continuing Education rom Plumbing Systems & DesignKenneth G.Wentink, PE, CPD, and Robert D. Jackson

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Flow o air is the primary consideration in the design o a venting system or the ventilation o the piping and protectiono the xture trap seals o a sanitary drainage system. Since airis o such primary importance, it is essential that the plumbingengineer be amiliar with certain physical characteristics that

are pertinent to its behavior in a plumbing system.Density o any substance is its mass per unit volume. Te den-sity o air is its weight in pounds per cubic oot o volume. Tedensity o air is a ected by temperature, moisture content, andpressure. Te density o standard air is taken at atmosphericpressure and 68.4F. It is equal to 0.075 lbm/ t3. With a rise intemperature, the density o air decreases and with a lowering o temperature its density increases. Te moisture content o air inthe plumbing system has a negligible e ect on its density andcan be disregarded in all calculations. Pressure has an appre-ciable e ect; the higher the pressure the greater the density, andthe lower the pressure the less the density.

Speci c Weight o a uid is not an absolute property, but

depends upon the local gravitational eld (gravitational accel-eration on earth is g=32.2 t/sec2) and the properties o the uiditsel . Commonly called density when concerning gravita-tional orce, the numerical value o speci c weight (lb / t3) isequal to density (lbm/ t 3).

Elasticity is the ability o a substance to assume its originalcharacteristics a ter the removal o a orce that has been applied. Air is a per ectly elastic substance. From the scienti c de nitiono elasticity it becomes clear that a rubber band is really a very inelastic material. I a weight is suspended rom a rubber bandand le t or a ew hours, then the weight is removed, the rubberband will spring back, but de nitely not to its original length. I a

orce is applied to air, the orce can be applied or days or years,

and when it is removed, the air will return exactly to its originalcondition. Air is compressible. Tere is an increase in pressure when air

is compressed. In the plumbing system, only an extremely smallchange in pressure can be tolerated. For a pressure o 1 in. o water column (0.036 psi), the volume o air will be compressedby 1 400 o its original volume. Assuming an original volume o 400 t3 o air at atmospheric pressure and the application o apressure o 1 in. o water column, the air will be compressed by 400 1 400 = 1 t3. It is obvious that a comparatively small changein volume can very easily cause the accepted design limitationo 1 in. o pressure to be exceeded with the consequent dangero destroying the trap seals. Te vent piping must be designed to

permit the air to ow reely without compression or expansionexcept or the small amount necessary to overcome riction.

Static HeadStatic head is the pressure exerted at any point by the weight o the substance above that point. Te pressure can be stated in

eet o the substance, i.e., when the substance is water the statichead is 100 t o water, or i the substance is air, 100 t o air. oconvert rom eet o head to pounds- orce per square inch:

p = h and h = 144p144 where

p = Pressure, lb ./in2 = Speci c weight o substance, lb / t3

h = Static head, tPneumatic effectS in Sanitary SyStemS As water ows in contact with air in vertical or horizontal pipinthere is riction between the air and water. Te rictional e ecauses the air to be dragged along with the water and at practically the same velocity as the water. When the cross-sectionarea o the water occupying the pipe is suddenly increasesuch as at the hydraulic jump or where a branch discharges intothe stack, the air passage is constricted. Tis constriction actsexactly the same as a stoppage or a blockage to the ow o Tis causes a buildup o pressure, the highest pressure occurring at the constriction and diminishing upstream. It is or threason that excessive pressure usually develops at the low

oors o a building and at o sets o the stack. It is importanalways be aware that protection rom the entry o sewer gasea orded by the 2-in. trap seal, and the design o plumbing sytems must be such as to maintain pressure variations within in. column o water.

rate of flow from outletSTe velocity at which air ows out o an outlet to the atmospher(at the roo terminal o a stack) is due to the total pressure avable in the vent pipe at the outlet. Tis pressure is the ow pressure, which is equal to the static pressure less the pressure lost i

riction. Te maximum rate o discharge in practice is expresseas:

qD= cD q1where

qD = Actual quantity o discharge, gpmq1 = Ideal quantity o discharge, gpmcD = Coefcient o discharge

Utilizing the ormula q = AV and substituting,

qD= cD (2.448 dO2 V 1)where

dO = Outlet diameter, in.V 1 = Ideal velocity, ps

Velocity is equal to 2gh, where g = acceleration due to gravand h = height (or head) o air column.

qD = cD (2.448 dO2 2gh)

= cD (19.65 dO2 h)

Using 0.67 as an acceptable coefcient o discharge, per Equatio6-1,

Vent Systems

Reprinted fromEngineered Plumbing Design II, Chapter 8: Vent Systems, by A. Calvin Laws, PE, CPD. American Society of Plumbing Engineers.

Plumbing Systems & Design JANUARY/FEBRUARY 2007 PSDMAGA

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qD = 13.17dO2 h

Static PreSSure of airhe design criterion o maintaining pneumatic pressureuctuations within l in. o water column is constantly stressed

throughout this book. It should prove interesting to state thispressure in terms o an equivalent column o air. Te ormula orany substance is, per Equation 1:

P = h144then

(6)

P = W h W = A h A144 144where

W = Speci c weight o water, lb / t3h W = Static head o water, t

A = Speci c weight o air, lb / t3h A = Static head o air, t

ransposing and using 1 in. o water column,

h A= W h W = (62.408) (

1 12) A 0.07512 (at 70 F.)

h A

= 69.23 t o air column

A column o air 69.23 t will exert the same pressure as a columno water 1 in. high. Stated another way, a static head o 1 in. o water will support a column o air 69.23 t high.Te rate o discharge rom a vent outlet can now be determined when the pressure at the outlet is 1 in. o water or 69.23 t o air.

qD = 13.17 dO2 h= 13.17 dO2 (69.23)= (13.17) (8.32) dO2= 109.57 dO2

he gallons per minute (cubiceet per minute) discharge rateor various diameters o vent pipe

at a ow pressure o 1 in. o watercolumn is given in able 1.

friction Head loSS When air ows in a pipe there isa pressure loss which occurs dueto the riction between the air andpipe wall. Tis loss o pressure canbe expressed by the Darcy ormula:

(7)

h = L V 2

D2 gwhereh = riction head loss, t. o air column

= coefcient o rictionL = length o pipe, t.D = diameter o pipe, t.V = velocity o air, t/secg = gravitational acceleration 32.2 t./sec2

air flow in StackSTe complete venting o a sanitary drainage system is very complicated as evidenced by the variety o vents employed.Tere are so many variables that produce positive and negative

pneumatic pressure uctuations that it is not easible to prepartables o vent sizing or each particular design. Recognizing tauthorities base the ormulation o venting tables or vent staand horizontal branches on the worst conditions that may rea-sonably be expected. o determine the maximum lengths andminimum diameters or vent stacks it would be valuable review the conditions o ow in the drainage stack. At maximum design ow, the water ows down the stack asheet o water occupying 7 24 o the cross-sectional area o stack. Te remaining 17 24 is occupied by a core o air. As water alls down the stack, it exerts a rictional drag on the co air and as this air is dragged down it must be replaced by equivalent quantity o air so as not to develop negative pressurin excess o -1 in. o water. Tis is accomplished by extendithe soil stack through the roo so that air may enter the stack replenish the air being pulled down the stack. Tis is why stackmust be extended ull size through the roo and also why stacks may not be reduced in size even though the load is less othe upper portions o the stack than it is at the lower portion Any restriction in the size be ore terminating at the atmosphe would cause violent pressure uctuations.

As the water ows down the stack and enters the horizontdrain there is a severe restriction to the ow o air as the hydralic jump occurs. Te air is compressed and pressure buildupmay become very high. A vent stack is provided in this areahigh pressure to relieve the pressure by providing an avenue the ow o air. Obviously, the vent stack must be large enouto permit the maximum quantity o air dragged down the drainage stack to discharge through it and to the atmosphere withouexceeding 1 in. o water uctuation Te rate o air discharthat must be accommodated or various sizes o drainage stac

owing at design capacity is tabulated in able 2.

air flow in Horizontal drainSIt is assumed that the drainage branch ows hal ull at desconditions and the air in the upper hal o the pipe is owingthe same velocity and capacity. able 3 tabulates these value

or various slopes o drain.

Table 2 Air Required by Attendant Vent Stacks(Drainage Stack Flowing7 24 Full)

Diameter of Drainage

Stack, inches Water Flow, gpm Air Flow, gpm2 23.5 57.1 (7.6)3 70.0 170.1 (22.7)4 145.0 352.4 (47.1)5 270.0 656.1 (87.7)6 435.0 1057.1 (141.3)8 920.0 2235.6 (298.9)10 1650.0 4009.5 (536)12 2650.0 6439.5 (860.8)

Table 1 Discharge Rates o Air(1 Inch Water Pressure)

Outlet Diam,dOinches

Air Discharge,qDgpm (c m)

2 438.3 (58.6)2 684.8 (91.5)3 986.1 (131.8)4 1753.0 (234.3)5 2739.0 (366.1)

Table 3 Rate o Air In Horizontal DrainsDiameter of Drain,

inchesSlope,

inches per foot

Rate of Flow,

gpm (cfm)1 6.0 (.80)2 8.8 (1.2)2 15.5 (2.1)3 25.5 (3.4)4 1 8 38.0 (5.1)5 1 8 69.0 (9.2)6 1 8 112.0 (15)8 1 8 240.0 (32.1)

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PermiSSible lengtH of Vent PiPeTe maximum length o vent piping, or any particu-lar size with a pressure drop o 1 in. o water, is estab-lished by computing the pressure loss or various rateso ow in vents o various diameters. Combining Dar-cys pipe riction ormula (Equation 7) and the ow

ormula, and converting the terms o the equations tounits generally used in plumbing:

h = L V 2

D 2g (8)q = 2.448 d2 V

(9)

V = q2.448d2Substituting V in the Darcy equation,

h = Lq2

(d 12)(64.4)(2.448)2(d)4Solving or L,

(10)

L = h d5

= 2226 d5

0.013109 q2 q2

whereL = Length o pipe, td = Diameter o pipe, in.

= Coefcient o rictionq = Quantity rate o ow, gpm

graVity circulationTe principle o gravity circulation o air is utilizedto keep the entire sanitary system ree o oul odorsand the growth o slime and ungi. Te circulation isinduced by the di erence in head (pressure) betweenoutdoor air and the air in the vent piping. Tis di er-ence o head is due to the di erence in temperature,and thus the di erence in density, o each and theheight o the air column in the vent piping. Te coolair, being more dense, tends to displace the less denseair o the system and circulation o the air is induced.Te ormula is

(11)H = 0.1925 ( O - 1) HS

whereH = Natural dra t pressure, in. o water

O = Speci c weight o outside air, lb / t3 1 = Speci c weight o air in pipe, lb / t3

HS= Height o air column or stack, tUnder conditions o natural dra t, the rate o ow will be just

great enough to overcome losses due to riction.Vent StackSEvery drainage stack should be extended ull size through theroo . Te pipe rom the topmost drainage branch connectionthrough the roo to atmosphere is called the vent extension.Te vent extension provides the air that is dragged down thestack and also provides means or the gravity circulation o airthroughout the system. Vent extensions may be connected withthe vent stack be ore extending through the roo or may be con-nected together with other vent extensions or vent stacks ina vent header and the header extended through the roo as asingle pipe.

Every drainage stack should have an attendant vent stack. Tepurpose o installing a vent stack is to prevent the developmeo excessive pressures in the lower regions o the drainage stby relieving the air as rapidly as it is carried down by the dcharge o the drainage stack. Te most e ective location or th vent stack is below all drainage branch connections and preerably at the top o the horizontal drain immediately adjacento the stack base tting. It is at this location that pressure is

its maximum and the danger o closure due to ouling is atminimum. Figure 1 illustrates acceptable methods o vent staconnections.

Te vent stack should extend undiminished in size throughthe roo or connect with the vent extension o the drainage staat least 6 in. above the over ow o the highest xture or connto a vent header.

Vent terminalS Vent terminals should not be located within 10 t o any do window, or ventilation intake unless they are extended at least

t above such openings. erminals should be at least 6 in. aboroo level and at least 5 t above when the roo is used or

Figure 1 Various Vent Stack Connections


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purposes. When it is impractical to extend the vent through theroo , it is permissible to terminate through a wall, but the ter-minal must turn down and be covered with a wire screen. Teterminal should never be located beneath a building overhang.

fixture traP VentSTe water seal o all xture traps should be protected againstsiphonage or blowout by the proper installation o a ventingsystem. When drainage stacks are provided with an adequatesupply o air at the terminal and an adequate vent stack is pro-

vided to relieve excess pressures at the base o the drainagestack, the only additional vent protection required to prevent water seal loss in xture traps is that necessary to prevent sel -si-phonage when the xture discharges and to relieve excessivepneumatic e ects in the branch drains when other xtures dis-charge into the branch. Some municipalities require that every

xture trap be individually vented, but most localities permitalternate methods such as1. Wet venting2. Stack venting3. Circuit and loop venting4. Combination waste and vent venting

diStance of Vent from traPTe most comprehensive investigations o conditions under which xture traps will be sa e rom sel -siphonage have beenconducted by the National Bureau o Standards in the UnitedStates and by the Building Research Station in England. Te rec-ommended maximum distances o a vent rom the weir o thetrap to the vent connection are tabulated in able 4.

As illustrated in Figure 2, the vent pipe opening, except or water closets and similar xtures, must never be below the weir o the xture trap. A

xture drain that slopesmore than one pipe diam-eter between vent openingand trap weir has a greatertendency to sel -siphonthe trap seal than a xturedrain installed at a slopeo not more than one pipediameter.

Sel -Siphonage o Fix-ture raps, National Bureau o Standards Building Materials andStructures Report BMS 126 (1951), prepared by John L. Frenchand Herbert N. Eaton, is a very thorough study o sel -siphonage

Some o the conclusions drawn by French and Eaton as a resuo their investigations are very illuminating and are quotherewith:1. Increasing the diameter o the outlet ori ce o a lavatory

rom 11 8 in. to 1 in. increases the trap seal loss greatly, rquently more than 100%, owing to the increased dischargerate.

2. Flat-bottomed xtures cause smaller trap seal losses thando round xtures, owing to the greater trail discharge romthe ormer.

3. With a 1-in. xture trap and drain, an 1in. by 20-in. lava-tory gave greater trap-seal losses than did a 20-in. by 24-inlavatory, presumably owing to the greater trail discharge othe latter. When a 1 in. trap and drain were used, no par-ticular di erence was noted in the trap seal losses caused bthe two lavatories.

4. Te elimination o the over ow in lavatories will increasethe trap seal losses substantially.

5. Te e ect on trap seal losses o varying the vertical distancrom the xture to the trap rom 6 in. to 12 in. appears to

negligible.

6. For a given rate o discharge rom a lavatory, decreasing diameter o the drain will increase trap seal losses.

7. An increase in slope or a decrease in diameter o the xtudrain will tend to cause increased losses due to sel -siphonage, and these two dimensions are ully as important as thlength o xture drain in causing sel -siphonage.

8. rap seal losses are usually much greater when a long-turnstack tting is used than when a short-turn or straight-tee

tting is used. No signi cant di erence between the behavior o short-turn and straight-tee ttings was observed.Tus, since it is known that a long-turn tting is more e ec-tive in introducing water rom a horizontal branch into the

stack than is either the short-turn or straight-tee tting, thecharacteristics o these ttings are contradictory in theserespects. Te tting that is most advantageous rom thestandpoint o introducing the water into the stack is theleast advantageous rom the standpoint o sel -siphonage

9. rap seal losses are increased i the internal diameter o a P-trap is less than that o the xture drain. Tus, i weare to prevent excessive trap seal losses or a P-trap due tosel -siphonage, we should use a trap having a airly largeinternal diameter. Furthermore, siphonage o the trap dueto pressure reductions caused by the discharge o other xtures on the system can be rendered less harm ul by usinga trap with a large depth o seal. While increasing the depto seal may lead to greater trap seal losses, it also results ingreater remaining trap seal than i a trap with a shallow sea were used.

10. Te test results on the sel -siphonage o water closets haveindicated that the unvented length o drain or these xturneed not be limited because o sel -siphonage.

11. Standardization o the dimensions o xture traps andespecially o lavatory traps, with regard to internal diametand depth o trap seal is highly desirable. Minor restrictionon these dimensions can lead to substantially increasedlengths o xture drains.

Figure 2 Vent Pipe Opening

Table 4 Distance o Vent rom Fixture Traps

Size o Fixture Drain inches

Maximum Distanceo Vent to Trap

inches1 301 422 603 724 120



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12. Standardization o the hydraulic characteristics o xtures isdesirable, at least or lavatories, sinks, and combination x-tures. Substantially increased permissible unvented lengthso xture drains can be obtained or a moderate decrease inthe discharge rates o the xtures.

13. Increase in depth o trap seal above the 2-in. minimumcommonly permitted by codes will make it possible toincrease appreciably the maximum permissible unventedlengths o xture drains.

Tese conclusions clearly illustrate various approaches in thee ort to make plumbing systems less costly without a ectingefciency. Te proper design o xtures and xture drain linesand limiting the maximum discharge rates o aucet-controlled

xtures could result in longer unvented lengths o drains.

VariouS metHodS of fixture traP VentingFigure 3 illustrates various xture trap vents and their propernomenclature. When venting one trap the vent is called an individual or back vent. I xtures are back to back or side by sideand one vent is used or the two traps, the vent is a common vent. Any connection rom the vent stack is a branch vent.

All vent piping should be graded to drain back to the drainagepiping by gravity. Te vent should be taken o above the cen-terline o the drainpipe and rise vertically or at an angle o not

more than 45 rom the vertical. Te horizontal run o the veshould be at least 6 in. above the over ow level o the xt(See Figure 4.)

relief VentSPressures in the drainage and vent stacks o a multistory builing are constantly uctuating. Te vent stack connection at thebase o the drainage stack and the branch vent connections tthe branch drains cannot always eliminate these uctuationsIt then becomes extremely important to balance pressurethroughout the drainage stack by means o relie vents locaat various intervals. Te uctuations in pressure may be causedby the simultaneous discharge o branches on various separated oors. Drainage stacks in buildings having more than tebranch intervals should be provided with a relie vent at eatenth interval, counting rom the topmost branch downwardTe lower end o the relie vent should connect to the drainagstack below the drainage branch connection and the upper endshould connect to the vent stack at least 3 t above the oor lev(See Figure 5.)

Relie vents are required where a drainage stack o sets at angle o more than 45 to the vertical. Such o sets are subjto high pneumatic pressure increases and extreme surging ow

Figure 3 Various Fixture Trap Vents

Figure 4 Horizontal Run o Vent

Figure 5 Venting or Stacks Having More Than 10 Branch Intervals




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conditions. Te methods o installing relie vents are illustratedin Figure 6.

continuouS Venting A system o individual or common vents or every trap is calledcontinuous venting. Every xture trap is provided with a vent.It is the most expensive system but provides positive protectiono all trap seals.

wet Venting A wet vent is a vent that vents a particular xture and at thesame time serves as a waste to receive the discharge rom other

xtures. Te objective o using wet vents is to minimize the ventpiping required by employing one pipe to serve two unctions.Tere are three undamental rules to ollow when utilizing a wet vent: At top oor:1. No more than 1 FU is discharged into a 1-in. wet vent nor

more than 4 FU into a 2-in. wet vent.2. Length o drain does not exceed maximum permissible

distance between trap and vent.3. Branch connects to the stack at the water closet

connection level or below. (See Figure 7.) At lower oors:

Te rules are the same except that the water closets mustbe vented and the wet vent must be 2 in. minimum. Waterclosets below the top story need not be individually ventei a 2-in. wet vented waste pipe connects directly to theupper hal o the horizontal water closet drain at an anglno greater than 45 rom the angle o ow. (See Figure 8

Stack venting nds its general application in one- amhomes and the top oor o multistory buildings. (See Figureand 9.)

combination waSte and Vent VentingCombination waste and vent venting is used primarily venting oor drains and laboratory and work tables. Te drain-

age piping is oversized at least two sizes larger than requireor draining purposes only and the drainage branch and stacshould be provided with vent piping. Tis type o venting employed when it is impractical to employ the other methods.

circuit and looP VentingTere has developed a tendency to call all circuit venting bythe name applicable to a special installation o circuit venting.circuit vent is a branch vent that serves two or more oor outl

xtures, except blowout water closets, and extends rom in ro the last xture connection on the horizontal drain to the vestack. A loop vent is the same, except that it is employed the topmost oor serving xtures and is connected to the ven

Figure 8 Wet Venting Below Top Floor

Figure 7 Wet Venting at Top FloorFigure 6 Venting at Stack Ofsets



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extension o the drainage stack instead o to the vent stack.(See Figure 10.) When wall outlet xtures are connected to thebranch drain serving the oor outlet xtures, the ormer mustbe provided with individual vents that can connect to the circuit vent or loop vent.

common VentS Where two xtures are connected to a vertical branch at thesame level, a common vent may be employed. When one o the

xtures connects at a di erent level than the other, observe theollowing procedure. I xture drains are the same size, increase

the vertical drain one size. I xture drains are o di erent sizes,connect the smaller above the larger connection and maintainthe vertical size up to the top connection.

SudS PreSSureTe prevalent use o high-sudsing detergents in washingmachines, dishwashers, laundry trays, and kitchen sinks has cre-ated serious problems in all residential buildings and especially in high-rise buildings. Until manu acturers are orced to marketonly detergents without sudsing characteristics, the plumbingengineer must understand and cope with the dangers created

in the sanitary system by the presence o suds. (An interestisidelight is that suds, in and o themselves, do not enhance thcleaning ability o soaps or detergents in any way.)

When the ow o wastes rom upper oors contains degents, the sudsingredients are vigorously mixed with the watand air in the stack as the waste ows down the stack and urthmixing action occurs as other branch waste discharges meet thi

ow. Tese suds ow down the stack and settle in the lower sections drainage system and at any o sets greater than 45 degreein the stack. Investigation has shown that when sudsing wasteare present, the sanitary and vent stacks are laden with suds andthis condition was ound to exist or extended periods o tim

Liquid wastes are heavier than suds and easily ow througthe suds-loaded drainage piping without carrying the suds along with the ow. Everyone is aware o the difculty o ushing suds out o a sink. Te water simply ows through the suds anout the drain, leaving the major portion o the suds behind. Tesame action occurs in the lower sections o the drainage systeexcept or one important di erenceair, as well as water, is n

owing in the piping. Tis air, which is carried down with th waste discharge, compresses the suds and orces them to movthrough any available path o relie . Te relie path may be tbuilding drain, any branches connected to the building drainthe vent stack, branch vents, individual vents or combinationo the oregoing. A path o relie may not always be availacould be cut o or restricted by the hydraulic jump, or a patmay just be inadequate because o location or size. I onemore o these conditions exist, excessively high suds pressucan develop and blow the seals o traps with the accompanyinappearance o suds in xtures.

High suds pressure zones occur at every change in direction vertically or horizontally, that is greater than 45. Where vestack base connections, relie vents, branch vents, or individu vents serve as the relie path or the high suds pressure, t

are usually ound to be inadequate in size with resulta

suds conditions appearing at the xtures. Te vent pipesizing tables in practically every code are calculated on thbasis o air ow capacity and do not in any way prov

or the more demanding ow o suds. Sizes that are baon these code tables are inadequate to accommodate suds

ow and thus are incapable o providing adequate supressure relie .

Suds are much heavier than air and consequently dnot ow with the same ease. Tey produce a much greater

riction head loss or the same rate o ow. Te densityold or regenerated suds varies rom 2 lb/ t3 to a high19 lb/ t3, depending upon the detergent used. For equarates o ow and pressure loss, the vent pipe diameter

suds relie ow must be rom 20 to 80% greater than ow. Whenever a soil or waste stack receives wash

machines, dishwashers, laundry trays, kitchen sinks, oother xtures where sudsing detergents are used, thdrainage and vent piping or the lower- oor xtures or the xtures above o sets must be arranged to avoid connection to any zone where suds pressure exists.Suds pressure zones exist in the ollowing areas:1. At a soil or waste stao set greater than 45: 40 stack diameters upward and 10stack diameters horizontally rom the base tting or th

Figure 10 Circuit and Loop Venting

Figure 9 Stack Vented Unit




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upper stack section. A pressure zone also exists 40 stack diameters upstream rom the top tting o the lower stack section.

2. At the base o a soil or waste stack: Te suds pressure zoneextends 40 stack diameters upward rom the base tting.

3. In the horizontal drain rom the base o a stack: Tesuds pressure zone extends 10 stack diameters rom thebase tting, and where an o set greater than 45 in thehorizontal occurs, the pressure zones extend 40 stack diameters upstream and 10 diameters downstream romthe o set tting.

4. In a vent stack connected to a suds pressure zone: Te sudspressure zone exists rom the vent stack base connectionupward to the level o the suds pressure zone in the soil or waste stack.

Figure 11 illustrates all the above zones.

VaPor VentS (local VentS) Years ago water closets and urinals were equipped with con-nections or venting the xture to the outdoors to eliminate oulodors. Fixture design has been improved so that these vents areno longer required. Te use o vapor vents is now applied tosterilizing equipment and bedpan washers. Tis application isalso rapidly disappearing as new methods o condensing the

oul vapors are being built into the equipment. When a vapor vent is used, it must be isolated rom the sanitary venting system.Te base o a vapor vent stack should terminate in a trap, to prevent the escape o vapors, and spill to a trapped, vented, and

water-supplied receptacle. Te stack should extend throughthe roo .

An individual vapor vent drip can be connected througan air gap to the inlet o the trap serving the xture. Vap vents or bedpan washers and bedpan sterilizers must nconnect with the vapor vents o other xtures.Sizing o the vapor vent stack may be by empirical methoor the rational approach may be used. Te minimum size othe stack should be 1 in.

ejector and SumP VentSEjectors, other than the pneumatic type, operate at atmospheric pressure and receive drainage discharge under gravity ow conditions. An ejector is installed when the level

xture discharge is below the level o the public sewer. Iconvenient to view an ejector system as being exactly simlar to the gravity sanitary system and all o the requiremen

or the proper design o the sanitary system are applicabTus, the air required to be conveyed by the vent piping ithe same as the maximum rate at which sewage enters or ipumped out o the receiver.

Te ejector vent can be determined by re erence to Equation 10:

L = 2226 ( d2 )q2and using able 1, which gives air discharge in gpm or vous pipe diameters. It has been ound in practice that 3 in. adequate except or extremely large installations.

froSt cloSure Where the danger o rost closure o vent terminals is pent, the minimum size o the vent stack or vent extensi

through the roo should be 3 in. When a vent stack must increased in size going through the roo , the increase should bmade inside the building at least 1 t below the roo .

Te National Bureau o Standards has investigated the problem o rost closure both theoretically and experimentally. It wdemonstrated that a 3-in. vent terminal roze up solidly at -30only over an extended period o time. Closure occurs at the rao 1 in. or every 24 hr. that the temperature remains at -30

It can be seen that rost closure presents a real problem onin the ar northern regions. Te problem is serious in Canadaand they have devised various methods o overcoming it:1. Vent terminal to extend only 1 in. or 2 in. above the roo .

Te more pipe exposed to the atmosphere, the greaterthe problem. Snow covering the vent terminal has provento cause no trouble. Te snow is porous enough or thepassage o air and melts rather rapidly at the outlet.

2. Enlargement o the stack below the roo . Te increaseddiameter decreases the chance o complete closure andthe stream o air tends to ow through the enlargedportion without touching the walls o the enlarged pipe.

3. Install cap ashing at the terminal and counter ashing toleave an air space rom the heated building.

Frost closure depends upon the: (1) outside temperature, (2temperature and humidity o inside air, (3) wind velocity, (length o exposed pipe, (5) diameter o exposed pipe, and velocity o air ow. Tere is very little danger o rost clounless the outside temperature alls below -10F and remai

Figure 11 Suds Pressure Zones



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there or several days. It has been ound that i rost closure doesoccur, siphonage o traps is reduced or prevented by connectingthe drainage and vent stacks together be ore extending throughthe roo . An analysis o air ow under these conditions will con- vince the plumbing engineer o its validity, as it can be seen thatair orced into the vent stack at its base will be introduced intothe soil stack at the top connection.

teStS of Plumbing SyStemSTe complete storm and sanitary system should be subjected to

a water test and proven watertight upon completion o the roughpiping installation and prior to covering or concealment. Tetest pressure should be a minimum o a 10- t column o waterexcept or the topmost 10 t o pipe. Te test pressure shouldnever exceed a maximum o a 100- t column o water. Any greaterpressure will cause the test plugs used to seal temporarily openpiping in the system to blow. I the system is higher than 100 t,test tees may be installed at appropriate heights so as to test thebuilding in sections. Very rarely in practice are more than sevenstories tested at one time.

I it is not possible to per orm a water test, an air test is accept-able. Te air test shall be made by attaching an air compressortesting apparatus to any suitable opening, and, a ter closing all

other inlets and outlets to the system, orcing air into the systemuntil there is a uni orm gage pressure o 5 psi (34.5 kPa) or apressure sufcient to balance a column o mercury 10 in. (254mm) in height. Te pressure shall be held without introductiono additional air or a period o at least 15 min.

Upon completion o the sanitary system and a ter all xturesare installed with traps lled with water, the system should besubjected to an additional test and proved gastight.

An alternate test is the smoke test. Te smoke test is per ormedby introducing pungent, thick smoke produced by smoke bombsor smoke machines. When smoke appears at the roo terminals,each terminal is sealed and a smoke pressure o 1-in. column o water is maintained to prove the system gastight. Tis test is notpractical and is seldom used.

Another alternate test is the peppermint vapor test. At least 2oz. o oil o peppermint are introduced into each roo terminaland vaporized by immediately pouring 10 qt o boiling waterdown the stack. Te terminals are promptly sealed. Oil o pepper-mint and any person coming in contact or handling the oil mustbe excluded rom the interior o the building or the duration o the test. Leakages will be detected by the peppermint odor at thesource. However, it is very difcult to pinpoint the leak by thismethod. Tis test is not practical and is seldom used.

10 Plumbing Systems & Design JANUARY/FEBRUARY 2007 PSDMAGA



11/13JANUARY/FEBRUARY 2007 Plumbing Systems & Design


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CONTINUING EDUCAT

P S D

1 3 7

Continuing Education rom Plumbing Systems & DesignKenneth G.Wentink, PE, CPD, and Robert D. Jackson

CE QuestionsVent Sy stems (PSD 137)

About This Issues Article The January/February 2007 continuing education article is

Vent Systems, Chapter 8 o Engineered Plumbing Design II by ACalvin Laws, PE, CPD.

Flow o air is the primary consideration in the design o a

venting system or the ventilation o the piping and protectiono the ixture trap seals o a sanitary drainage system. Since airis o such primary importance, it is essential that the plumbingengineer be amiliar with certain physical characteristics thatare pertinent to its behavior in a plumbing system. This chapterexplains these undamentals that are vital to the design o avent system. It also covers vent stacks, the various types o vents and venting, the e ects o suds pressure, rost closure,and vent system pressure tests.

You may locate this article at www.psdmagazine.org. Readthe article, complete the ollowing exam, and submit youranswer sheet to the ASPE o ice to potentially receive 0.1 CEU.

Do you nd it difcult to obtain continuing education units (CEUs)?Trough this special section in every issue o PS&D, ASPE can help you accumulate the CEUs required or maintaining your Certi ed inPlumbing Design (CPD) status.

Now Online!Te technical article you must read to complete the exam is located

at www.psdmagazine.org. Te ollowing exam and application ormalso may be downloaded rom the Web site. Reading the article andcompleting the orm will allow you to apply to ASPE or CEU credit.For most people, this process will require approximately one hour. I you earn a grade o 90 percent or higher on the test, you will be noti edthat you have logged 0.1 CEU, which can be applied toward the CPDrenewal requirement or numerous regulatory-agency CE programs.(Please note that it is your responsibility to determine the acceptancepolicy o a particular agency.) CEU in ormation will be kept on le atthe ASPE ofce or three years.

Note: In determining your answers to the CE questions, use only the materialpresented in the corresponding continuing education article. Using in ormation

rom other materials may result in a wrong answer.

1. A wet vent is ___________.a. not allowed by most codesb. a vent that vents a particular xture and at the same

time serves as a waste vent to receive the dischargerom other xtures

c. not allowed to serve water closetsd. a system o individual or common vents or every trap

. The vent piping must be designed to permit the air to___________.a. fow reelyb. enter the piping network c. exit the piping network d. be compressed

. Suds, in and o themselves, ___________.a. do not enhance the cleaning ability o soaps or

detergents in any wayb. are only a problem i they are allowed to accumulate in

large numbersc. require wet venting to rinse the suds out o the drain

piped. none o the above

. The most expensive venting system is a ___________system.

a. wet vent, b. continuous vent, c. circuit vent, d. loop vent. Vapor vents ___________.

a. must be isolated rom the sanitary venting systemb. may be connected through an air gap to the trap

serving the xturec. must not be connected to the vapor vents o other

types o equipmentd. all o the above

. To compensate or suds density, the vent pipe or sudsrelie fow must ___________.a. not be depended on or suds protectionb. connect to each xture trap

c. connect 10 pipe diameters rom any high pressure sudszone

d. be 20 percent to 80 percent larger in diameter. What is the primary consideration in the design o a

venting system?a. removing odors rom the sewer systemb. correctly sizing the vent piping to match the size o the

waste pipingc. the fow o air in the vent systemd. none o the above

. For a given rate o discharge rom a lavatory, decreasingthe diameter o the drain will ___________.a. increase the water discharge velocity rom the xtureb. decrease the water discharge velocity rom the xturec. increase trap seal lossesd. prevent trap seal losses

. Smoke tests and peppermint air tests ___________.a. can detect the location o a leak in a vent systemb. are no longer allowed by OSHA and the EPAc. are not practical and seldom usedd. may not be used where rost closure is expected

10. A column o air . eet high exerts the same pressureas a column o water ___________ high.

a. 1 inch, b. 10 inches, c. 100 inches, d. 1,000 inches11. A branch vent interval is ___________.

a. determined by the foor-to-foor height o the buildingb. dependent on the arrangement o the ttings

connecting at each foorc. at least 8 eet between branchesd. o no importance in modern plumbing systems

1 . The maximum distance o a vent to a -inch-diametertrap is ___________.a. 30 inches, b. 42 inches, c. 60 inches, d. 72 inches

1 Plumbing Systems & Design JANUARY/FEBRUARY 2007 PSDMAGA


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Plumbing Systems & Design Continuing Education Application Form This orm is valid up to one year rom date o publication. The PS&DContinuing Education program is approved by ASPE or up

to one contact hour (0.1 CEU) o credit per article. Participants who ear a passing score (90 percent) on the CE questions will receivea letter or certi cation within 30 days o ASPEs receipt o the application orm. (No special certi cates will be issued.) Participantswho ail and wish to retake the test should resubmit the orm along with an additional ee (i required).

1. Photocopy this orm or download it rom www.psdmagazine.org.2. Print or type your name and address. Be sure to place your ASPE membership number in the appropriate space.3. Answer the multiple-choice continuing education (CE) questions based on the corresponding article ound on

.ps . and the appraisal questions on this orm.4. Submit this orm with payment ($35 or nonmembers o ASPE) i required by check or money order made payable to ASPE or

credit card via mail (ASPE Education Credit, 8614 W. Catalpa Avenue, Suite 1007, Chicago, IL 60656) or ax (773-695-9007).

Please print or type; this in ormation will be used to process your credits.

Name __________________________________________________________________________________________________

Title ______________________________________________ ASPE Membership No. ___________________________________

Organization ____________________________________________________________________________________________

Billing Address ___________________________________________________________________________________________

City _______________________________________ State/Province _______________________ Zip ____________________Country ___________________________________________ E-mail ________________________________________________

Daytime telephone _________________________________ Fax __________________________________________________

PS&D Continuing Education Answer SheetVent Systems (PSD 137)

Questions appear on page 12. Circle the answer to each question.Q 1. A B C DQ . A B C DQ . A B C DQ . A B C D

Q . A B C DQ . A B C DQ . A B C DQ . A B C DQ . A B C DQ 10. A B C DQ 11. A B C DQ 1 . A B C D

Appraisal QuestionsVent Systems (PSD 137)

1. Was the material new in ormation or you? Yes No2. Was the material presented clearly? Yes No3. Was the material adequately covered? Yes No4. Did the content help you achieve the stated objectives? Yes No5. Did the CE questions help you identi y speci c ways to

the article? Yes No6. How much time did you need to complete the CE ofering

article and answer the post-test questions)? ____________

I am applying for the following continuing education credits:

I certi y that I have read the article indicated above.

Signature

Expiration date: Continuing education credit will be givenor this examination through January 1, 00 .

Applications received a ter that date will not be processed.

ASPE Member NonmemberEach examination: $25 Each examination: $35Limited Time: No Cost to ASPE Member Payment: Personal Check (payable to ASPE) $ ___________

Business or government check $ ___________ DiscoverCard VISA MasterCard AMEX$ ___________

If rebilling of a credit card charge is necessary, a $25 processing fee willASPE is hereby authorized to charge my CE examination fee to my cred

Account Number Expiration date

Signature Cardholders name (Please print)

Vent Systems

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