By SAUPA, edited by Leon Louw
South African Utility Pole Association’s (SAUPA’s) role in the promotion development and maintenance of the wooden pole as the preferred utility pole for electrification and communication.
Over the past 15 years SAUPA has been proactive in working with end-users all over Africa to promote suitable wooden pole treatment standards, the correct drying and treatment as well as the handling of wooden poles to ensure the poles purchased and used by utility companies last the required 35- 40 years. Eskom can be credited for the positive contribution they have brought to the utility pole industry in South Africa specifically.
Currently, the Eskom specifications are drawn up to achieve their objective of less than 3% failure at 10 years and, with a suitable wooden pole maintenance programme, the poles last 35 to 40 years.
SAUPA has been working with utility companies all over Africa to find ways to improve the quality of the supply of wooden poles. Several critical factors have been identified that need to be addressed to achieve the objective of the wooden poles lasting the required 40 years.
Strength requirements of each species
The biggest challenge in African specifications is the lack of testing and data on the fibre strength of the different species grown in Africa. It is vital in the design and purchasing of wooden poles that different specifications specify test methods for determining the bending strength and modulus of elasticity in bending.
Significance and Use
Tests of wood poles are made to determine:
- Data for use in establishing ultimate fibre stress;
- Data upon which to base economical line design;
- Data on the strength properties of different species in pole sizes;
- Data as to the influence of defects on the strength properties of poles; and
- Data for correlating the strength properties of full-size poles.
Unfortunately, the only specification in Africa that has been updated is SANS 754 Eucalyptus poles, cross-arms and spacers for power distribution and communications systems.
All wooden pole suppliers need to have test facilities and equipment available at the plant to prove the fibre strength and MOE of poles being supplied to utility companies.
Drying requirements before treatment
Wooden poles need to dry before treatment for the following reasons:
To achieve maximum penetration and retentions in the available sapwood;
Suppress any checking after treatment to avoid untreated heartwood being exposed while in use.
Most wooden poles in Southern and East Africa are from the Eucalyptus species. Generally, the pine timber in Africa is fast growing and does not have the strength requirements for transmission poles.
Eucalyptus timber has a smaller sapwood zone than pine timber. The result of the bigger heartwood zone results in the timber drying slowly.
It has been identified that post-impregnating splitting of poles treated with CCA and to a certain extent creosote is the biggest cause of early pole failure due to untreated timber being exposed to termites and fungus which results in decay at ground level. The cause of post-impregnating splitting is due to the heartwood portion not being dried down to a sufficient level to prevent any further splitting.
Eskom identifies need for kilns
Research done in South Africa on air seasoned poles indicate very high mc% in the heartwood even after the 4 to 6 weeks’ drying period. Only some drying has taken place in the sapwood, whereas further away from the pole surface, the mc% is like what one would expect in a freshly felled tree.
The sapwood dries rapidly and can dry after 12 to 20 weeks. In certain sizes the moisture has fallen to 30% and below. In contrast, very little moisture movement and loss has taken place at a depth of 20-60mm in the heartwood. The moisture can still be around 50% at 60mm. If the poles are air seasoned to the correct moisture in the heartwood the poles will need to be stacked for between 4 to 12 months depending on the diameter of the pole.
Air seasoning times are dependent on climate conditions. The advantages of air seasoning include the need for minimal equipment and the absence of direct effects on wood properties. These aspects are offset by some negative attributes. First, air-seasoning requires large areas of land and requires that a company hold onto untreated stock for long periods.
This increases material costs. By far the largest negative aspect of air-seasoning is the risk of decay or insect attack. Freshly harvested trees remain at risk of fungal and insect attack from the time they are cut until the wood dries below 20% moisture content.
Removing the bark reduces the risk of insect attack, but also exposes large amounts of stored sugar to fungal attack. Field trials suggest that poles should not be air seasoned for more than 3 to 6 months and that nearly every air-seasoned pole will have some decay fungi after air-seasoning. Prolonged air-seasoning of wooden poles can result in poles with sharply reduced bending properties (MOE).
Kiln drying usually involves exposing wood to elevated temperatures while controlling both the air flow and relative humidity. Heating at lower humidity will tend to dry poles more rapidly.
While this reduces drying times, it also increases the risk of developing extreme checking. Therefore, it is important that the customer specification stipulates poles are not dried at temperatures or humidities that can result in extreme checking.
The primary drawback to kiln drying is the costs associated with the equipment, but the benefits of kiln drying include the ability to rapidly process poles, thereby reducing pole holding costs as well as the risk of fungal damage associated with air-seasoning. The heating also eliminates any fungi that might have been present at the time of drying.
Retention and treatment requirements
- Effect of moisture retention The amount of preservative that can be contained in treated wood is in a large part, dependent upon the available amount of void space in the wood cell cavities. Under pressure treatment, preservative solution being forced into timber seeks cell cavities and pit membrane orifices as primary avenues of penetration.
Available void space for solution penetration is reduced progressively by increased moisture content until cell cavities are water-filled and flow of solution into otherwise permeable wood becomes impossible.
Opinions of operators in pressure-treating plants vary regarding what is optimum moisture content at which wood should be treated. Moisture content values near the fibre-saturation point (28%) are usually satisfactory. Wood becomes easier to treat as its moisture content is reduced.
- Treating plant facility requirements All treating plants need strict guidelines on the whole treatment cycle.
- Quality Control The treating plant needs to appoint an employee, trained in quality control procedures, who shall be designated as plant quality control supervisor and who will be responsible for conformance of treated products to the standards and specifications and for the accuracy of equipment used to monitor the treating processes.
- Wood Moisture Content Products before treatment must meet moisture content requirements.
- Record Keeping: Records must be kept for each processed lot or charge showing the material description, volume, and moisture before treatment. Records shall also be kept to show compliance with requirement for penetration, and for retention as measured by either gauge or assay in accordance with the applicable customer specification.
- Sampling for penetration and retention Treating plants shall perform sampling and inspection of all treated products.
- Preservative Plant Quality Control shall determine that the preservative used in each treatment conforms to the requirements for the preservative specified.
- Recording of treatment process
- Treating equipment shall have separate and visible temperature, pressure, and vacuum indicators with separate sensors for checking the accuracy of the recording equipment. Work tanks shall be equipped with a temperature indicator. Treating cylinders shall be equipped with a time and temperature recorder and with an indicating thermometer.
- Treatment The treatment shall conform to the customer specification for the commodity and type of treatment specified. Plant Quality Control shall maintain records on the cycles of treatment and the tank gauge readings and temperatures.
Careful observation during treatment shall be made of temperatures and pressures, and their duration, to make certain that minimum retentions are achieved.
- Results of treatment:
- Following treatment, an examination shall be made of the charge for any mechanical or treatment damage.
- Plant Quality Control shall bore the treated material in each and every charge to determine conformance with the penetration specified. Quality Control shall also determine, for every charge by the method specified, that the preservative retention conforms to the specification.
- Determination by the assay method, is the most accurate method and is recommended by most countries.
Role of third-party certification
The inspection of a batch of poles is not sufficient. It is important for the customers and the industry that all poles supplied are through a certification mark scheme.
Key benefits of third-party certification body:
- Ensure the quality and quantity of the product;
- Ensure the marking and product as per buyer specification; and
- Ensure the material is genuine and as per specified standard.
A certification body is a third party and an independent inspection agency which provides facilities in the different type of field without discriminatory decision. In the wooden pole market in Africa third party inspections can play a very important role in ensuring the wooden pole remains the preferred transmission and telecoms utility pole over the concrete and steel poles.
Improved product quality
One of the most important benefits of third-party inspection for suppliers is the quality improvement that often results. Sometimes inspection reveals a product defect that is easily preventable. Regardless of whether the defect is major, minor, or critical, knowing about it before shipping helps the supplier address the root cause to correct the underlying problem.