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Pipes & Pipework: Why it pays to prevent the dangers of pipe vibration

Big problems can arise when pipework is not sufficiently isolated. Richard Collman explains the lessons that were learned from two recent projects.
Pipework vibration is a big potential problem in many applications. That is why, for example, it is crucial to isolate pipes from cooling towers and pumps to prevent the structure vibrating and generating noise.

Two recent projects illustrate what happens when pipework is not isolated.

A contractor installed the display cabinets, air conditioning and refrigeration plant in a small supermarket that was part of a commercial/residential development in Milton Keynes.

Following advice given on previous projects, the multi-compressor pack was mounted on steel helical compression spring isolators. The air conditioning units were mounted on rubber mounts on brackets fixed to the external wall. Immediately after the plant was switched on, a resident in an apartment above the supermarket complained of noise.

Puzzled by noise levels

The local authority's environmental health officer (EHO) visited the apartment at 02:00 hours and measured a background level of 25dB(A) and, when a large compressor began operating, a short term level 42dB(A); this was clearly audible. After two seconds the sound level dropped to about 32dB(A). In view of the efficient isolators fitted to the pack, the contract manager was puzzled, and so called Belair Research for advice.

One of Belair's consultants visited the store and apartment with the local authority's EHO. The pack was efficiently isolated, but the 41mm diameter pipe work was fixed to the soffits of the plant room, the stock room and the sales floor where it ran to the display cabinets. The hangers were isolated with strips of soft rubber.

The EHO had to serve a Noise Nuisance Notice. Initially, the EHO wanted the level reduced to inaudibility. However, Belair's consultant referred to the World Health Organisation's guidelines and BS8233:1999 and negotiated the time allowable for the works to be performed and 30dB(A), without any tones, as the design level.

As a matter of interest - using the night time five minutes reference period recommended by BS4142:1997 - as the high level was only present for two seconds, the on-time correction, 10log10 2/300 seconds, -22dB, would have reduced the level from 42dB(A) to 20dB(A) plus 5dB for intermittency = 27dB(A).

The pipework in the plant room and above the false ceiling in the stockroom and from the pack to the condenser was isolated with ACE's Flexihangers. The sound level in the apartment inclusive of the ambient noise, was reduced to 28dB(A). It was not possible to hear the plant starting, operating or stopping.

Problems can also arise from large diameter pipework from a compressor, cooling tower or boilers that are not efficiently isolated. This was exemplified at The Manchester Evening News Arena when the roof-mounted boilers and pipework induced a sound level of NR59 in the offices below the plant room. After the boilers and pipework were isolated with VertiMounts and Flexihangers, the noise level, inclusive of ambient noise, was reduced to NR37.

It is essential for the contractor to correctly support pipes, but especially vertical pipes and valves, so that isolators are able to be adjusted. Problems will not occur if the advice given in the ASHRAE or CIBSE guides regarding pipework isolation is followed.

• Examples of how to isolate pipework effectively are shown in the photographs.
27 January 2011


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