Analysis of current state and integrity evaluation of the pipeline at hydro power plant 'Pirot'

Hidroelektrana Pirot, koja je izgrađena 1990 godine, je akumulaciona elektrana derivacionog tipa, koja se sastoji od dva nadzemna vertikalna agregata sa Francis turbinama nominalne snage 41,5 MW, proizvedene u Češkoj Republici, tunela i ukopanog cevovoda ukupne dužine 2030 m i prečnika od Ø 3000 mm do Ø 3500 mm. Cevovod debljine lima 22 mm je izrađen od čelika kvaliteta S275J2G3 (stara oznaka Č.0462). Maksimalni pritisak u njemu ispred turbinskog zatvarača je 2,5 MP. Cevovod je projektovan i izgrađen bez ankernih blokova na prelomima, što pretstavlja retkost u svetu. Na cevovodu se u kontinuitetu od završetka izgradnje i puštanja u rad sprovode geodetska merenja kada je cevovod prazan i neopterećen hidrostatickim pritiskom i kada je cevovod pun. Pregledom i analizom dobijenih podataka o pomeranjima duz trase cevovoda utvrđeno je da od 2003-2004 godine postoje znatno veće razlike u pomeranjima za pun i prazan cevovod, nego u prethodnom periodu. Te razlike se prvenstveno odnose na tangencijalna pomeranja za temena oznake 6, 7 i 8, koja se u odnosu na period do 2002 godine kreću od 3 mm za teme 8 do 5 mm za teme 6. Pored geodetskih merenja u kontinuitetu se, sa unutrašnje strane cevovoda, sprovode i merenja promena horizontalnih i vertikalnih prečnika. Iz tih podataka se moze videti da u periodu 2003-2004 godine postoji i značajan trend povečanja horizontalnih, kao i manje izraženo smanjenje vertikalnih prečnika. U radu je izvršena analiza trenutnog stanja i ocena integriteta cevovoda kao celine na osnovu rezultata ispitivanja vitalnog sučeonog zavarenog spoja metodama bez razaranja u zoni preloma na stacionaži - dužini cevovoda 1+263 m (vizuelno ispitivanje, ispitivanje magnetnim česticama, ispitivanje penetrantima, ispitivanje ultrazvukom, radiografsko ispitivanje, metalografsko ispitivanje metodom replika).Hydro power plant Pirot, which was built in 1990, is an accumulation-derivative power plant, which consists of 2 above-ground vertical hydroelectric generating sets that contain Francis turbines with nominal power of 41,5 MW, manufactured in Czech Republic, a tunnel and a sunken pipeline with overall length of 2.030 m and diameter that ranges from 3.000 to 3.500 m. Pipes have been made of S275J2G3 steel. Pipe wall is 22 mm thick. Maximum pressure of 2.5 MPa occurs in front of the turbine cover. Pipeline has been designed and built without anchor blocks at curvatures, which is a rarity elsewhere. Geodetic measurements have been conducted permanently from the day the assembly was finished and pipeline was put into service, both when pipeline is empty and unloaded by hydrostatic pressure and when it is full. Analysis of obtained data regarding the movements along the pipeline route showed that from year 2003 there are significantly higher differences in movements comparing the situations when the pipeline is full and when it is empty in comparison with the previous period. Those differences primarily refer to tangential movements of vertices marked with numbers 6, 7 and 8, which, compared to the period until year 2002, are in the range from 3 mm for vertex 8 to 5 mm for vertex 6. Apart from geodetic measurements, the measurement of pipe diameter in 2 directions is also being carried out permanently. Those data show that from year 2003 the diameter in horizontal direction started to increase significantly, while at the same time the diameter in vertical direction started to decrease less significantly. This paper contains the analysis of current state and integrity evaluation of the pipeline as a whole on the basis of results of non-destructive tests performed on the vital butt-welded joint in the curvature area at chainage 1+263 m (visual testing, magnetic particle testing, penetrant testing, ultrasonic testing, radiographic testing, metallographic replication testing