Smoking Out Sewer Leaks: An Overview of Smoke Testing, an Important Part of I&I Studies

Smoke testing can be performed to determine the sources of excess inflow.

Used extensively for more than 40 years, smoke testing has
proven to be a vital ingredient of successful inflow and infiltration (I&I)
studies. It is as important now as it ever has been as growing municipalities
increase demands on aging, often deteriorating collection systems. In addition,
programs such as the U.S. Environmental Protection Agency's (EPA) new Capacity,
Management, Operations and Maintenance (CMOM) program emphasize a focus on
proactive, preventive maintenance practices. Smoke testing can aid in
documenting sources of inflow and should be part of any CMOM program.

Just as a doctor would require the aid of several instruments
to evaluate the status of a person's health, various test methods should be
used in performing a complete sanitary sewer evaluation survey (SSES). In
addition to smoke testing, these could include dyed water testing, manhole
inspection, TV inspection and flow monitoring. Specializing in sanitary sewer
evaluation surveys, Wade & Associates of Lawrence Kansas reports a
reduction of 30 to 50 percent in peak flows can be expected as a result of
implementing these types of programs.

Smoke testing is a relatively simple process that consists
of blowing smoke mixed with large volumes of air into the sanitary sewer line
usually induced through the manhole. The smoke travels the path of least
resistance and quickly shows up at sites that allow surface water inflow. Smoke
will identify broken manholes, illegal connections including roof drains, sump
pumps and yard drains, uncapped lines and even will show cracked mains and
laterals, providing there is a passageway for the smoke to travel to the
surface. Although video inspection and other techniques certainly are important
components of an I&I survey, research has shown that approximately 65
percent of all extraneous stormwater inflow enters the system from somewhere
other than the main line (see private sector diagram). Smoke testing is a
method of inspecting both the main lines and laterals. Smoke travels throughout
the system, identifying problems in all connected lines—even sections of
line that were not known to exist or thought to be independent or unconnected.
Best results are obtained during dry weather, which allows smoke better
opportunity to travel to the surface.

Necessary Equipment

engineering specifications for smoke testing identify the use of a blower,
which is able to provide 1,750 cubic feet of air per minute (cfm) However, in
today's world it seems to be the mindset that bigger is better. New smoke
blowers on the market can deliver more than 3,000 cfm. The question is: Is this
really needed? Once the manhole area is filled, the smoke only needs to travel
sections of 8- or 10-inch pipe. Moving the air very quickly is useless if the
blower does not have the static pressure to push that air/smoke through the
lines. If you have used high cfm blowers and found that smoke frequently backs
up to the surface, this may be the problem. 

There are two types of blowers available for smoke testing
sewers: squirrel cage and direct drive propeller. In general, squirrel cage
blowers usually are larger in size but can provide more static pressure in relation
to cfm. The output of the squirrel cage type usually is adjustable by
alternating pulleys and belts to meet the demands of the job. Propeller-style
blowers usually are more compact and generally offer approximately 3,200 cfm.
Other than reducing the engine throttle, the output is not adjustable since the
fan blade is attached directly to the engine shaft. If purchasing a smoke
blower, you should ask the manufacturer if the cfm and static pressure output
it is quoting is the specification of the propeller itself (uninstalled/free
air) or if it is the actual performance when installed in the blower assembly.
These two numbers can vary significantly.

Smoke Types--There
are two types of smoke currently offered for smoke testing sewers: classic
smoke candles and smoke fluids.

Smoke candles first were used for testing sewers when the
process began its popularity in 1961, and continue to be the most widely used.
They are used by simply placing a smoke candle on the fresh air intake side of
the blower. Once ignited, the exiting smoke is drawn in with the fresh air and
blown down into the manhole and throughout the system. Smoke candles are
available in various sizes that can be used singularly or in combination to
meet any need. This type of smoke is formed by a chemical reaction, creating a
smoke that contains a high content of atmospheric moisture. It is very visible
even at low concentrations and extremely effective at finding leaks.

Another available source of smoke is a smoke fluid system.
Although they just recently have been more aggressively marketed, smoke fluids
became available for sewer testing shortly after smoke candles, some 30 years
ago. They certainly can be used effectively, but it is important to understand
how they work. This system involves injecting a smoke fluid—usually a
petroleum-based product—into the hot exhaust stream of the engine where
it is heated within the muffler (or heating chamber) and exhausted into the air
intake side of the blower. One gallon of smoke fluid generally is less expensive
than 12 smoke candles. However, smoke fluids do not consistently provide the
same quality of smoke. When using smoke fluid, it is important to understand
that as fluid is injected into the heating chamber (or muffler) it immediately
begins to cool the unit. The heating chamber eventually will reach a point
where it is not hot enough to completely convert all the fluid to smoke, thus
creating thin/wet smoke. This actually can happen quickly depending on the rate
of fluid flow. If the smoke has become thin, it can be especially difficult to
see at greater distances. Blocking off sections of line usually is a good idea
with any type of smoke but becomes almost a necessity when using smoke fluid.
Some manufacturers have taken steps to address this issue and now offer better
flow control, fluid distribution and, most importantly, insulated heating
chambers to help maintain necessary temperatures.

Safety--Maybe one of
the more talked about, yet least understood aspects of smoke testing is the use
and safety of these products. As manufacturers have become more competitive,
some marketing programs and advertisements have implied danger in the use of
competing types of smoke products. Laboratory reports, scientific studies and
even Material Safety Data Sheets can be quite confusing to most people who are
not trained nor qualified to make scientific judgements on this data. Having
this information delivered to the public in the form of advertising can be
dangerous, as most people tend to believe what they read. An author of an
associated industry publication once stated, “Do not use smoke bombs, as
they give off a toxic gas.” Although the author quotes no scientific
literature to support this statement, competitive propaganda has made such
implications. It is interesting to note that the same exact statement could be
made for smoke fluids. Smoke from fluid is created in the exhaust system of the
engine, which contains carbon monoxide.

Other statements that have been made include warnings to
wear a respirator while smoke testing. While certain manufacturers have issued
this warning about competitive products, they do not qualify the statement, nor
do they mention the fact that the same thing could be said of similar products.
The fact is that a respirator should be worn whenever a person would be exposed
to any substance in quantities that exceed OSHA limits. Would smoke-testing
personnel be exposed to enough smoke to exceed these limits? Not very likely.
The bottom line on safety is that it is important to use common sense. All
smokes, candles and fluids can be used safely and effectively when used as

When planning to smoke test, it is important to develop a
proactive public notice program. Ads in local papers, door hangers, mailers and
door-to-door inquiries are recommended. It is helpful to educate the public as
to why the test is being performed and the positive benefits to the community.
In addition, it should instruct residents on what to do and who to call if
smoke should enter their homes. It also is important to notify local police and
fire departments daily as to where and when smoke testing will be taking place.

Reducing stormwater inflow into collection systems means
reduced chances of overflows, less emergency maintenance and less money spent
on treatment.       WWD

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About the author

Paul Tashian is employed by Superior Signal Co., Inc., a manufacturer of all types of smoke testing equipment and a major contributor to the original development of smoke testing practices. Also, a thanks to Wade & Associate, a company specializing in sanitary sewer evaluation surveys, for offering reference material.