Page 264 - Pipeline Risk Management Manual Ideas, Techniques, and Resources

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instrumentation, ability to stop flows, and abilities to mobilize trations) at all points on the system. Odorant levels are often
and execute loss-minimizing reactions. confirmed by tests using human subjects who have not been
Spreading and accumulation effects also determine conse- desensitized to the odor. When new piping is placed in serv-
quences for spilled liquids. Depending on the receptor, damages ice, attention should be given to possible odorant absorption
from a water system might be greater from spill accumulation by the pipe wall. “Over-odorizing” for a period of time is
(deeper flood waters) or from surface flow rates (erosion effects sometimes used to ensure adequate odorization. When gas
or force of flowing water). A distinction between the two scenar- flows change, odorant injection levels must be changed
ios could be made in a risk model. Slope and land-use factors appropriately. Testing should verify odorization at the new
leading to an estimate of relative resistance to surface flow flow rates. Odorant removal (de-odorization) possibilities
would logically be included in the evaluation. should be minimized, even as gas permeates through soil or
water.
Gas odorization The role that a given gas odorization effort plays as a conse-
quence reducer can then be scored as follows:
As a special leak detection and early warning system for most
natural gas distribution systems, gas odorization warrants further High-reliability odorization-consequence reduction
discussion. An important component of the leak impact from nat- A modem or well-maintained, well-designed system exists.
ural gas distribution systems is the use of odorization. Methane There is no evidence of system failures or inadequacies of
has very little odor detectable to humans. Natural gas that is any kind. Extra steps (above regulatory minimums) are taken
mostly methane will therefore be odorless unless an artificial to ensure system functioning. A consistent, naturally occur-
odorant is introduced. It is common practice to inject an odorant ring odor that allows early detection of a hazardous gas can
at such levels that gas will be detected at levels far below the lower fall into this category if the odor is indeed a reliable,
flammable limit of the gas in air--often one-fiflh of the flamma- omnipresent factor.
ble limit, meaning that accumulations of 5 times the detection Odorization-No point change
level are required before fire or explosion is possible. This allows This is the neutral or default value. Where an odorization
early warning of a gas pipe leak anywhere in the system or in a system exists and is minimally maintained (by minimum
customer’s building and reduces the threat of human injury. regulatory standards, perhaps) but the evaluator does not
Gas odorization can be a more powerful leak detection feel that enough extra steps have been taken to make this a
mechanism than many other techniques discussed. While it can high-reliability system, no change to the population score is
be argued that many leak survey methods detect gas leaks at made.
very low levels, proper gas odorization has the undeniable ben- Questionable odorization system4onsequence increases
efits of alerting the right people (those in most danger) at the A system exists; however, the evaluator has concerns over its
right time. reliability or effectiveness. Inadequate record keeping, inad-
equate maintenance, lack of knowledge among system oper-
Odorization svstem design ators, and inadequate inspections would all indicate this
condition. A history of odorization system failures would be
Aspects of optimum system design include selection of the even stronger evidence.
proper odorant chemical, the proper dosage to ensure early No odorization effortdonsequence increases
detection, the proper equipment to inject the chemical, the Despite its use in similar systems, the assessed distribution
proper injection location(s), and the ability to vary injection system does not use odorization and hence, potential conse-
rates to compensate for varied gas flows. Ideally, the odorant quences are higher, compared to otherwise equivalent
will be persistent enough to maintain required concentrations systems.
in the gas even after leakage through soil, water, and other
anticipated leak paths. The optimum design will consider gas Receptors
flow rates and odorant absorption in some pipe materials (new
steels) to ensure that gas at any point in the distribution piping For our purposes, the term receptor refers to any creature,
is properly odorized. structure, land area, etc., that could “receive” damage from a
pipeline failure. The intent of a risk assessment is to capture
System operation/maintenance vulnerabilities of various receptors, at least in a relative way as
discussed in Chapter 7. This vulnerability, coupled with other
Odorant injection equipment is best inspected and maintained aspects of the spill scenario, will show locations of greater
according to well-defined, thorough procedures. Trained per- potential consequences.
sonnel should oversee system operation and maintenance. Receptors at risk from most distribution systems include the
Inspections should be designed to ensure that proper detection following:
levels are seen at all points on the piping network. Provisions
are needed to quickly detect and correct any odorization equip- 0 Snfety--consequences involving human health issues:
ment malfunctions. Population density
0 Permanent population
Perfirmanre 0 Transitoryioccasional population
0 Special population (restricted mobility)
Evidence should confirm that odorant concentration is effec- 0 Collateral safety
tive (provides early warning to potentially hazardous concen- 0 Contamination.