An Assessment of Structural Safety of Cement Mortar Lined Irrigation Water Pipeline by Hydrostatic Test

Abstract

Structural safety of cement mortar-lined water pipeline is a matter of great concern primarily due to weld defects, fabrication deficiency and assessment of pressure shared by lining. The verification of structural design of such pipeline and assessment of safety is generally carried out by conducting prototype hydrostatic test. The present paper is based on one such type of study conducted by CWPRS, Pune, for Guthpha Lift Irrigation Scheme, Nizamabad, Andhra Pradesh. The pipes of finished internal diameter varying from 1600–2200 mm have been fabricated with 12 mm thick Grade B mild steel complying with IS: 2062 specifications. The pipes have been designed further with internal lining of 15 mm thick cement mortar and external coating of 25 mm cement mortar for an internal design pressure of 6 kg/cm². In order to assess structural safety by conducting prototype hydrostatic test, two lined pipes each of 6 m length and 2200 mm internal diameter have been joined to form a single pipe of 12 m length to eliminate edge effects due to welded bulkheads at the ends. The pipeline has been supported on rigid concrete platform through three saddle supports. Electrical resistance-type strain gauges have been installed at critical locations in high-stressed zones after removing external coating of cement mortar. The pipeline has been tested up to 1.5 times of design internal pressure, i.e., 9.0 kg/cm². The allowable stress as per design has been taken as 0.66 times of yield stress with 90% weld efficiency, which worked out to be 1485 kg/cm². The hoop tensile stresses in steel pipe computed at different locations based on measured strains have been found to vary from 300 to 1000 kg/cm² at 9 kg/cm² internal pressure and thus remain well within allowable limit. The tensile stresses developed in outer cement mortar coating have been found negligible compared to that developed in the steel pipe indicating that total applied internal pressure has been shared by the steel pipe only.