Blow-off tower

The Department's description

Exterior description and setting

A free-standing blow-off tower situated in a small plot of ground on the west side of Tower Road, just below the summit of a prominent hill commanding wide views of the surrounding countryside.

The tower is of rubble blackstone brought to courses, with roughly dressed quoins, slightly advanced chamfered base (with granite chamfering), and moulded concrete crown. It is of square cross-section and stands to its full height of c.6m.

According to N.I. Water, both the tower and reservoir are both still in use.

Setting

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

Historical information

This tower is on the line of a water pipe 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 at the behest of the Portadown & Banbridge Joint Water Board and at a cost of £36,000. The water was drawn from the Shimna River, in the north-western foothills of the Mournes, and ran underground along a 9-inch diameter cast-iron pipe to the Banbridge service reservoir at Drumnahare (NE of Loughbrickland), whence it continued onwards 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.

The water first flowed on 5 July 1906. Initially, it was drawn directly from the river as it was not until 1909 that the reservoir at Foffany was completed.

In the 1950s, this supply was augmented by another from Spelga Dam, fed from the headwaters of the Upper Bann at the Deer’s Meadow above Hilltown. The new dam was opened by the Portadown and Banbridge Regional Waterworks Joint Board in 1957 and also entailed a second trunk pipe to Portadown.

The tower under review is first shown on the 1903-18 OS six-inch map; it is captioned ‘Blow-off Tower’. There is a second such tower on the same pipeline at Ballyvarley, west of Banbridge (grid J0940 4509; HB17/03/060). The Valuation revision book entry for 1906 also notes this plot of land for the first time as belonging to the Portadown & Banbridge Joint Water Board.

The tower contained a vertical pipe up from the underground pipe. Its purpose was to relieve any excess water pressure if, say, the pipe became blocked further along and which might otherwise have caused the pipe to burst. It also prevented any build-up of air which would otherwise have caused an air lock in the pipe and stopped the water flowing.

In operation, there would always have been a column of water in the shaft of the tower. The pressure at the bottom of this column of water would have been equal to the hydraulic pressure of water in the pipe. The height of the tower was such that the water did not overflow from its top in normal circumstances. However, had there been a blockage, the pressure in the pipe would have built up (as the water was still flowing in from the Foffany end), thus forcing up the column of water until it eventually spilled out.

The tower was located at a high point on the pipeline as this is where the water pressure was least and also where the air was best released (air will always rise to the highest point in a pipe). If sited lower down along the pipeline, a much taller tower would have been needed to contain the column of water present during everyday operations.

The 1975 OS map also shows a covered reservoir on the opposite side of the road. This probably dates from the 1950s, when Spelga Reservoir was opened.

References – Primary sources:
1. PRONI OS/6/3/34/4. Fourth edition OS six-inch map, Co Down sheet 34 (1903-18).
2. PRONI LA/51/29/B/14. Document relating to objections by Lords Roden and Annesley to proposed water abstraction scheme at Foffany, 1903.
3. PRONI VAL/12/B/16/14D, p.27. Valuation revision book entry for 1906.
4. PRONI OS/11/238/5/1. First edition OS 1:2500 IG map sheet 2380-05 (1975).

References - Secondary sources:
1. Portadown and Banbridge Regional Waterworks Joint Board, ‘A brief history Portadown and Banbridge Regional Waterworks Joint Board’, in official programme to accompany opening of Spelga Dam and Fofannybane Filtration Works, 30 June 1960.
2. S. C. Lutton, ‘Portadown and Banbridge Regional Waterworks - Foffany Catchment Area’, in Review - Journal of the Craigavon Historical Society, vol.1 (1), 1969.
3. Information on tower’s operation from Mr John Collins, Information Manager Unit, Northern Ireland Water (email of 9 Dec 2011).

Criteria for listing: architectural interest

A. Style
B. Proportion
C. Ornamentation
D. Plan Form
J. Setting

Criteria for listing: historic interest

X. Local Interest
Z. Rarity

Evaluation

A free-standing rubble-masonry blow-off tower of c.1905 on the Foffany-Banbridge/Portadown water conduit. This tower is scaled-down version of a conventional 19th century steam engine boiler chimney in terms of its proportions and fabrication. It was sited at a high point on the pipeline to enable any build-up of water pressure and/or trapped air to be released. It is one of two such towers (the other is HB17/03/060) on this pipeline and is a rare provincial example of this type of waterworks structure.