Blow-off tower, Tower Road, Derrydrummuck, Banbridge, Co Down, BT32

Blow-off Tower, Tower Road, Derrydrummuck

This is a free-standing rubble-masonry blow-off tower built around 1905 on the Foffany-Banbridge/Portadown water conduit. It is a scaled-down version of a conventional 19th-century steam engine boiler chimney in its proportions and fabrication. One of only two such towers on this pipeline, it is a rare provincial example of this type of waterworks structure.

The tower stands in a small plot on the west side of Tower Road, just below the summit of a prominent hill with wide views over the surrounding countryside. It is constructed of rubble blackstone brought to courses, with roughly dressed quoins, a slightly advanced chamfered base finished in granite, and a moulded concrete crown. The tower is of square cross-section and stands to its full height of approximately 6 metres. According to Northern Ireland Water, both the tower and the associated reservoir remain in use.

The plot is enclosed on three sides by a post-and-wire fence and by a hedge along the roadside. On the opposite side of Tower Road stands an above-ground grass-covered water service reservoir. Both sites are surrounded by fields.

The tower lies on the pipeline running from Foffany Reservoir in the Mourne Mountains to Banbridge and Portadown. This pipeline was laid by Messrs Collen Brothers of Portadown between 1904 and 1906 for the Portadown & Banbridge Joint Water Board at a cost of £36,000. Water was drawn from the Shimna River in the north-western foothills of the Mournes and ran underground through a 9-inch diameter cast-iron pipe to the Banbridge service reservoir at Drumnahare (north-east of Loughbrickland), and thence to a terminal reservoir at Drumclogher Hill on the eastern outskirts of Portadown. The scheme was conceived by Robert H. Dorman, County Surveyor for Armagh, and John H.H. Swiney, an established water engineer. Water first flowed on 5 July 1906. Initially it was drawn directly from the river; the reservoir at Foffany was not completed until 1909. In the 1950s, this supply was augmented by water from Spelga Dam, fed from the headwaters of the Upper Bann at the Deer's Meadow above Hilltown. Spelga Dam was opened by the Portadown and Banbridge Regional Waterworks Joint Board in 1957 and required a second trunk pipe to Portadown.

The tower is first shown on the 1903–18 Ordnance Survey six-inch map and is captioned 'Blow-off Tower'. A second similar tower exists on the same pipeline at Ballyvarley, west of Banbridge. The Valuation revision book notes this plot of land for the first time in 1906 as belonging to the Portadown & Banbridge Joint Water Board.

The tower contained a vertical pipe connected to the underground pipeline. Its purpose was to relieve any excess water pressure—for instance, if the pipe became blocked further along, which might otherwise have caused the pipe to burst. It also prevented any build-up of air that would otherwise cause an air lock and stop the water flowing. In operation, a column of water would always have been present in the shaft of the tower, the pressure at its bottom equalling the hydraulic pressure in the pipe. The tower's height was such that water did not overflow from its top under normal circumstances. However, had a blockage occurred, pressure in the pipe would have built up (as water continued flowing in from the Foffany end), forcing the column of water upward until it spilled over. The tower was sited at a high point on the pipeline because pressure was least there and air rose most readily—crucial design considerations, as a much taller tower would have been needed if positioned lower, to contain the column of water present during normal operations. The 1975 Ordnance Survey map also shows a covered reservoir on the opposite side of the road, probably dating from the 1950s when Spelga Reservoir was opened.