By Tim Carey
Wood utility poles have been the backbone of electrical and communication networks since their inceptions. It goes back to Alex-ander Graham Bell attempting to lay his first telegraph line underground only to have it fail, forcing him to purchase poles and the rest is, as they say, history.
Since this ignoble beginning, wood poles have increasingly become a part of our landscape from sea to shining sea and everywhere in between. Today 150 to 300 million wood poles are reported to be in service in North America.
Wood poles are simply an accepted commodity to most of us, but as current trends call for better reliability and environmental stewardship, wood poles are being looked at by lots of different people. Engineers are looking at the design criteria in relation to alternative products. Environmentalists and lawyers are looking at the liability issues, and accountants are looking at life cycle costs.
Wood poles are a renewable resource and those that are in service can have their life extended beyond depreciation through a thorough pole maintenance program.
Pole maintenance begins when the specifications for purchasing poles are developed. How poles are handled prior to treatment is important to lengthening service life. All poles should be drilled and framed prior to treatment. Drilling prior to treatment allows preservatives additional access to wood during treatment. It also prevents decay access to the untreated interior wood that can occur with post treatment drilling. The species choice is important in relation to which preservative system can be used, size of the pole needed and the potential exposure to deterioration. Depending on the species, the preservative system should be specified. Oil borne preservatives, while offering a potential long life, are more susceptible to depletion than water borne preservatives that basically become fixed to the wood cells. Some preservatives work better with certain species. Some treated poles are easier to climb, some are more expensive, and there are always environmental issues with any preservative system. Once it is determined which pole and preservative system is wanted, verification of conformance to the specifications needs to be assured.
Conditioning poles is critical to insure that they obtain the necessary amount and depth of preservative treatment. The conditioning process also sterilizes the wood prior to treatment to stop and prevent decay. This process should be monitored to assure that, while promoting maximum treatment and sterilization potential, it does not weaken wood or retard the preservative penetration of the poles.
Poles that are well treated in the initial pressure process will last longer whether they receive supplemental treatments or not. For this reason it is critical to verify the amount of preservative retained by the poles as well as the depth that the preservative achieved within the poles. Increment cores should be taken to verify the results of treatment. The cores can be analyzed to determine the amount of a preservative in a given zone. The cores can also be visually observed to determine the depth of penetration. Chemical indicators are often applied to these cores to aid in the differentiating of heartwood from sapwood as well as the depth of penetration.
The improper storage of poles prior to and after treatment can have a significant impact on the life of a standing pole. Vegetation and water need to be kept away from storage areas to reduce the decay hazard. If a pole has a strength reducing decay problem prior to treatment due to improper storage conditions, you cannot put enough preservative into the pole to return the lost strength. After treatment, poles stored for long periods of time can change shape. Poles that are treated with oil borne preservatives and not rotated within the bins will experience preservative migration due to gravity. This migration can lead to premature decay on the side of the pole facing upward during storage, creating a climbing hazard. Poles were meant to be vertical and the sooner they are set in a vertical position the longer they will last.
Once a pole is set, it can begin to produce revenue for the system. As an asset, poles require little maintenance to remain a productive part of the power distribution system. However, we have to remember that wood is a vegetable and therefore a food source. It helps to know the enemies of wood poles and how they attack. The purpose of a wood preservative is to make a wood pole undesirable to these pests. Over time some preservatives will leave the pole through evaporation or migration, or some areas not properly treated within the pole can become exposed through checks, mechanical damage, etc. When decay is noticed it is critical to stop the damage before it forces the total replacement of the pole.
Options in Pole Maintenance
Once a pole is in line there are maintenance options that affect service life.
One option is to do nothing and let nature take its course. When this type of maintenance is chosen, there are no controls over scheduling of change outs, which can be expected to become necessary, particularly during a storm at 3:00 am. While initially this program seems less costly, this approach typically becomes the most expensive form of pole replacement.
Another option is to replace poles based on age. If this option is chosen, poles that are in good condition could be replaced while others may fail before their scheduled replacement. This option has no control over the life of the pole and is an expensive mismanagement of an asset.
Poles can be inspected and their current status reported. This type of action is the lowest level of a pole maintenance program. Every pole should be inspected prior to beginning any work that involves climbing. This option does allow you to begin to collect data on the condition of the electrical system. However, there is no way to predict how a particular pole may deteriorate if decay is observed. Once again there is no control over pole life.
Finally, poles can be actively maintained to assure maximum service life. This option includes inspection and incorporates actions to be followed based upon the results of the inspections. These actions could include leaving the pole as is, replacing it, adding some form of supplemental chemical protection and/or adding a structural enhancement. With a pole maintenance program, through low short-term costs, you gain control over the expected life of this asset.
It is up to each utility to choose which option best fits its need for optimum pole life (depending on initial preservative type) while offering their customers continued reliable service in todayÕs increasingly competitive market. If the option chosen is a pole maintenance program, there are once again a number of options to consider, each with a recommended inspection cycle and reliability.
While a visual and sound inspection should be done annually as a minimum, every lineman should perform this check prior to climbing a pole. While this is the least reliable method, it will give an indication as to the condition of the pole. If a lineman can see through the pole or bury his hammer into it he should walk away and report it.
Visual, sound, and bore inspections can be done every couple of years. This program is likely to find 40 to 60 per cent of the potential rejects, assuming that the sounding is accurate and the boring is done in the determined location.
By doing a visual, sound and bore and internally treating the structure, you get closer to 60 per cent identification of rejects through the drilling of multiple holes for treatment. Since treatment is added, the length of the time between inspection cycles can be extended to the life of the internal treatment.
Adding a shovel depth partial excavation to the visual, sound and bore, whether on one or two sides, allows greater access to the area of the pole at the ground line and just below, where decay can be most prevalent. A good inspector can find between 80 and 90 per cent of the rejects. Once again, using an internal treatment extends the length of the inspection cycle.
Excavating the pole to 18 to 24 inches deep Ñ in addition to the visual, sound and bore Ñ allows access to the pole surface to a depth below the soil oxygen level. With this level of access, a competent inspector can find 99 per cent of the potential rejects. Also, with this complete access, additional preservative can be coated onto the surface of the pole to extend the life of the pole by 8 to 15 years depending on the preservative and the local environment.
Waterborne preservatives only need a visual, sound and bore inspection. Rarely does a CCA pole need any form of remedial treatment. If it does, it will be internal, due to the level of protection offered by the preservative.
Life Extension Through Remediation
Now lets discuss how pole life is extended through remediation.
For the surface of poles, treatments fall into two main categories: Greases and wraps. Greases are preservative products that are thickened to aid application to the surface of the pole by brush, trowel, etc. adding from 7 to 12 years to the pole's life depending on formulation level of application and the local environment. These products include: copper naphthenate; sodium borate + copper naphthenate; and sodium fluoride + creosote + potassium bichromate.
Wraps are greases in a ready-to-use format attached to a moisture barrier for ease of installation. Because the wraps are an all-in-one kit, they are significantly higher in cost. Similar life extensions can be expected from these products. The preservative wraps include: copper naphthenate; sodium borate + copper naphthenate; and sodium fluoride.
For the inside of poles there are two types of treatment: those to be used in solid wood and those for use in voids in the woods. In solid wood, where a softness of the wood is noted or there is a potential for decay to develop, the preservatives are solids or diffusible greases, which move with the levels of moisture. Examples include: solid boron; sodium borate + copper naphthenate grease; and solid fluoride.
To treat voids caused by decay or insects, the liquid or grease formulations can be applied. These formulations should be applied under pressure to assure adequate coverage of the voids and channels. Examples include: boron solution; copper naphthenate solution; sodium borate + copper naphthenate grease, sodium fluoride + sodium dichromate + trisodium arsenate solution; and pentachlorophenol + petroleum solution.
In situations where surface preservatives cannot be applied, a fumigant or dispersive is used. These products move through the pole in response to moisture and temperature. Fumigants convert from liquids or solids into gases and move through the wood, providing protection from a couple of years to potentially 20 years, depending on the formulation and environment. Fumigants include: basamid; vapam; cholorpicrin; and MITC.
Dispersives are a relatively new category for use in poles. They have provided protection to smaller wood products. They are totally dependent on moisture to activate them and have a limited area of protection. They also can eventually be leached out over an unknown period of time depending on the amount and velocity of moisture. These preservatives include: boron solids + boron liquids; and sodium fluoride solids.
Structural Enhancement
The next step in pole maintenance is structural enhancement. These include:
Replacement of the pole: Metal trussing, or the driving of a steel sleeve beside the pole and then banding it to the pole to transfer the load. This is the least expensive and still most popular means of structural enhancement.
Encasement of the pole in steel or concrete: The pole is encapsulated in either of these two materials to replace the strength lost to damage. Encasement is very expensive and I believe has been abandoned.
Composites: Composites in bonded and non-bonded designs, transfer the load from the good wood below the damage to the good wood above the damage. Composite restorations often can protect from other types of damage such as fire, vehicles and yard sale signs.
Program Start-up
Now that we know what can be done, lets discuss the start-up of a program. To begin, use a pilot program of 1 per cent sample that is representative of the types of poles and environments that make up your system. Collect and analyze the data to determine what type of program, potential services, cycle length and size of annual population is best for your maintenance.
From the information gathered you should be able to determine the value a pole line maintenance program can have. Assume the original installation of an oil borne pole costs $1,000 and due to premature failure, the pole must be replaced at 32.5 years at another $1,000 (a good rate) for a cost of $2,000. If you take away the non-depreciable value of $431, you get a life cycle cost of $1,569, or $31 per year.
If that same pole had been inspected and additional preservative added in years 19, 27, 35, 43, and 51 at a cost of $20 per cycle, the life cycle cost of the pole would have been $1,100 or $22 per year. The pole maintenance program generated savings of $9 per pole per year.
Now that we know that the program has value, how is it effectively budgeted? Some initial assumptions: There are 10,000 distribution poles in the system. There will be a regular 10-year inspection cycle of 1,000 poles per year. The average cost of the pole maintenance is $20 per pole. Based on these numbers, one would project a first year maintenance budget of $20,000. However the pilot project follows experience and shows that we can expect a reject rate of up to 10 per cent (100 poles). Of these 100, it is estimated that:
- 40 poles need to be replaced at a cost of $1,800 per pole.
- 5 poles can be restored at a cost of $800 per pole.
- 55 poles can be reinforced at a cost of $300 per pole.
All three of these actions can be capitalized. However, some utilities prefer to apply the restoration and reinforcement to their annual maintenance budget.
Based on this scenario, the actual budget for the first year's program should be $112,500. In year 11, this amount reduces drastically as the expected reject rates should reduce to less than 1 per cent. It is important to correctly budget so that all poles that are found to need some form of remediation can be maintained, whether chemically and/or structurally.
Remediation Procedure
Now that we have addressed the issues of the remediation process, let's go through the procedure.While most people consider remediation to be a ground line inspection, it actually begins at the top of the pole, where the physical condition of the pole and its attachments are observed and recorded. A split top or woodpecker damage could reject the pole and stop the evaluation process at this step.
We will use a hypothetical scenario. This particular pole was of an age that required excavation per a utility's specification to allow access for sounding below ground. Dirt is piled on tarps to keep the area cleaner and make refilling the hole easier.
The pole is sounded from below ground to the height attainable by the inspector to determine if there are any voids and their approximate location. Since no voids were located, the pole is bored at ground line next to the largest check. The hole is bored at a 45 degree angle down past the center of the pole. A shell gauge device is inserted to determine the size and depth from the surface of the void, to check the condition of the wood, and to determine the thickness of the shell. The location of the void from the surface is critical in determining the remaining strength of the pole, as over 75 per cent of the pole's strength is in the outer 2 inches of the pole's circumference. The measurement of the remaining shell will also determine whether the pole can remain, be reinforced, be restored or should be replaced. The hole should then be plugged with a treated tight-fitting dowel.
The pole circumference is measured to compare with the pole class requirements and for use in the calculation of remaining shell if a void or external decay is found. The surface is then cleaned to remove loose fibers and to aid coating the surface with an external preservative. The grease is applied typically with an applicator brush. It is important that the applicator knows and follows the label directions for protecting himself, the environment and those around him when applying the different preservative systems. The preservative should be applied uniformly in thickness and in a neat manner. A moisture barrier should then be applied to hold the preservative against the pole and away from the surrounding soil. The barrier is then attached tightly to the pole. Hold the barrier in place until the soil can be returned and tamped into place. The area around the pole should be returned to its original condition. Before leaving a pole, a tag identifying the company and the preservatives applied should be placed on the pole at eye level in the direction of likely approach. A record of all observations and actions should be completed.
When surroundings prevent excavation and treatment is needed, a fumigant is often used. Holes are drilled from the ground line up in stair step pattern of sufficient diameter and length to assure the proper amount of fumigant is deposited into the pole. These holes are then filled with tight-fitting treated plugs to protect the integrity of the hole and fumigant.
The goal of any pole remediation program is to maximize the value of an electrical system, through extending pole life and increasing circuit reliability. If properly maintained, it is estimated that pole life can be extended to near the 100 year mark, which makes the choice of wood poles still a value in today's reliability based market.
Tim Carey is with Hickson Corporation. He can be reached at (770) 801-6600 or Email: tim_carey@hicksoncorp.com. ET