Duplex versus Galvanised Coating on a Coastal 132 kV Line
A 132 kV double-circuit line of 38 km with 190 lattice towers runs 3.5 km from the sea in a high-humidity, monsoon-heavy belt. The tender specifies a duplex coating (hot-dip galvanising plus 120 µm epoxy primer and 80 µm polyurethane topcoat) at ₹6.10 lakh per tower. The successful bidder offers a value-engineering alternative: hot-dip galvanising alone at 610 g/m² (about 86 µm) at ₹4.30 lakh per tower, claiming it satisfies the corrosion requirement. Data for a 30-year analysis period at an 8 per cent discount rate, present worth factors (P/F): year 8 = 0.5403, year 16 = 0.2919, year 20 = 0.2145, year 24 = 0.1577.
Option A (galvanising only): touch-up ₹0.60 lakh at year 8; repaint ₹1.90 lakh at year 16; repaint ₹2.60 lakh at year 24. Option B (duplex): one maintenance cycle of ₹1.10 lakh at year 20.
Every intervention needs a two-day shutdown of both circuits, valued at ₹0.45 lakh per tower per event. The line feeds a coastal industrial estate with no alternate source.
1. Compute the present worth per tower of both options including outage cost, and state the difference for the whole line on an annualised basis.
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Option A: 4.30 + (0.60 + 0.45)(0.5403) + (1.90 + 0.45)(0.2919) + (2.60 + 0.45)(0.1577) = 4.30 + 0.5673 + 0.6860 + 0.4810 = ₹6.03 lakh per tower
Option B: 6.10 + (1.10 + 0.45)(0.2145) = 6.10 + 0.3325 = ₹6.43 lakh per tower
A is cheaper by ₹0.40 lakh per tower, or ₹75.6 lakh (about ₹0.76 crore) across 190 towers. Annualised over 30 years at 8 per cent (capital recovery factor 0.0888), the saving is about ₹6.7 lakh per year - real, but small against a line of this size.
Two points of interpretation. First, the capital gap of ₹1.80 lakh per tower is largely recovered by A's later maintenance; the ranking is driven by the timing of expenditure, not by the coating quality. Second, outage cost, though only ₹0.45 lakh per event, matters disproportionately because a shutdown of both circuits on a radial feeder is a reliability event, not just an expense.
2. Give your recommendation on the bidder's alternative, and identify the assumption that could reverse it.
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Recommendation: conditional acceptance of A, since a 6 per cent present-worth margin does not justify a 30 per cent capital premium on its own - but the margin is thinner than the uncertainty in the inputs.
The reversing assumption is the discount rate. At 5 per cent, the factors become 0.6768, 0.4581, 0.3101 and 0.3769: A = 4.30 + 0.711 + 1.077 + 0.946 = ₹7.03 lakh; B = 6.10 + 0.584 = ₹6.68 lakh. The ranking reverses and B wins by ₹0.35 lakh per tower. At 12 per cent, A's advantage widens to about ₹0.95 lakh per tower. Since the discount rate is a policy parameter, not an engineering one, the utility must use its own sanctioned appraisal rate; a present worth computed at the bidder's chosen rate proves nothing. Second sensitivity: if repaint costs escalate faster than assumed because of access and shutdown charges, A's margin erodes by roughly a third.
Accept A only on the 178 towers in normal exposure. Retain duplex for the towers within about 1 km of the shoreline and at the river crossing, where access is hardest and shutdowns longest - a hybrid that captures most of the saving without betting the worst locations on a thin coating.
3. If the alternative is accepted, what verification and contractual conditions do you attach?
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- Coating verification: confirm a minimum average mass of 610 g/m² by strip weighing and magnetic thickness gauges to IS 4759/IS 2629, with the specified individual minimum, on a sampling plan witnessed by the engineer. Verify surface preparation and the galvaniser's test certificates per batch.
- Defect liability: a coating-specific warranty longer than the general defect liability period, with the contractor bearing recoating costs for early breakdown, plus a performance bank guarantee sized to one repaint cycle.
- Corrosion evidence: expose coupons at three towers and measure zinc loss annually; set an inspection at year 5 with a written decision rule for the first intervention. This converts the assumed maintenance schedule into a monitored one.
- Design for maintenance: bolted rather than welded connections on members likely to need recoating, safe access provisions, and a shutdown plan agreed with the system operator before commissioning.
- Failure consequence: specify the remedy if coating mass is deficient - rejection and re-galvanising, not a price rebate, because the line cannot be derated later.
The condition that carries the decision is the year-5 inspection trigger: without it, the present-worth case for A is only a forecast.