Challenge

Customer installed three generator sets directly to structural slab, utilising only the integral Rubber Mountings provided by the Generator Set manufacturer.  Subsequent acoustic testing showed that

“The PPV limit of 5mm/s is selected as a value that is deemed to be achievable at this stage. BS 5228- 2, Code of practice for noise and vibration control on construction and open sites, has been selected and the most appropriate benchmark for target values due to the intermittent nature of the running of the generators. Tables B.1 and B.2 of BS 5228-2, define a PPV value under 5 mm/s should be tolerable for the likely operation duration of the generator and be below the value where cosmetic damage is expected. It should be noted that this value will still be noticeable and above standard limits for a working environment”.

The actual measured PPV values in the three orthogonal directions were up to 23.7 mm/s, well in excess of the 5 mm/s benchmark

Solution

Christie & Grey provided our unique Open Spring and Damper solution providing 50 mm nominal deflection.  In designing this solution we did not ignore the existing Rubber mountings, instead we used them in our solution, checking and confirming the expected performance using our bespoke rigid body analysis programme ensuring all twelve degrees of freedom did not create any magnification and ensuring all mountings both Spring and Rubber worked together in harmony.

The highest measurement of 23.7 mm/s was reduced to 2.7 mm/s and the new highest measurement seen was 5 mm/s, exactly on the benchmark value.