Pipes and Fittings: No vibration is good vibration!
Richard Collman of Acoustical Control Engineers looks at the causes of vibrations and how to stop them happening
AIR handling units, chillers, cooling towers and pumps move when resiliently mounted. When a machine moves the pipework and the connections to the machine also move. As the pipework connections tend to be at the ends or tops of the machine, if the machine displacement is large, the displacement of the connections is greater.
It is also known that if the vibration isolators are 90% or more efficient, the displacement of the machine and the connections is minimised.
The efficiency of a vibration isolator is conventionally based on the ratio of the forcing frequency to the natural frequency of the vibration isolators, rather than of the system. As the majority of chillers and cooling towers are roof mounted, the deflection of the roof must be considered. Two factors which affect the efficiency of the isolation are the deflection of the roof under the imposed load and how level the roof of the building is. Despite the first factor being discussed for more than 50 years, many manufacturers still supply rubber mounts or pads with a maximum deflection of 6mm to isolate the vibration from the machines. When the floor deflects 50mm and the floor tolerance is ±20mm, then 6mm is inadequate.
This can be demonstrated by undoing some of the mounts and removing them from the machine without affecting its stability.
As a result in many installations, some rubber mounts are under loaded and some are over loaded. This ensures the theoretical deflection is not obtained. On site, the result is that the vibration generated by the machine is amplified rather than attenuated and so both noise and vibration problems are encountered.
Additional problems manifest themselves as failures of the ahu's or chiller's coil or the pipework and on occasions failure of electrical components in control panels.
These problems can be obviated by correctly selecting and fitting efficient vibration isolators. For the last 35 years Acoustical Control Engineers has worked successfully with cooling tower company, Baltimore Aircoil on many sensitive projects such as Canary Wharf, the British Library, semi conductor plants, recording and television studios and less sensitive projects such as office buildings and shopping malls.
As the roof of the podium block of the British Library deflected in excess of 60mm the acoustic consultant was rightly concerned about the cooling tower vibration isolators. However ACE designed Vertimounts to provide in excess of 100mm static deflection. Sixteen trouble-free years later, as part of the refurbishment to the cooling towers, the company provided replacement Vertimounts.
The inverter-controlled low speed fans used in cooling towers present more of a problem than the higher speed fans used in ahus.
However the ahus' fans typically operate at a higher pressure and so must still be efficiently isolated.
Pipework must also be isolated to prevent the transmission of vibration into the structure which then generates noise. When pipework is suspended on efficient vibration isolators and the points of suspension are correctly spaced, excessive movement of the pipework is prevented.
There are a number of reasons for easing vibrations from machines:
· prevent damage to plant and pipework systems;
· prevent gas losses that otherwise occur in air conditioning systems;
· prevent vibration damaging the structure of the building;
· prevent vibration from the machine causing damage to the machine and its components.
It is essential when steel helical compression spring isolators are used to ensure the design of the springs is laterally stable so they do not buckle. The isolators used to isolate vibration from pipework systems generally do not need to deflect as much as those used to isolate the primary source. It is also essential to ensure the required deflection is achieved so the correct ratio of the forcing frequency to natural frequency of the system is achieved.
When pump and motor sets have to be isolated, an additional mass is often required to reduce the displacement of the pump and the pipework system to a more acceptable level. This is achieved by using an inertia base complete with efficient isolators. As everyone knows from their school days, Force is the sum of mass and acceleration. By increasing the effective mass of the machine the acceleration is reduced. This often enables the discharge elbow from pumps to be simultaneously supported on the inertia base and efficiently isolated.
It also assists the stability of the unit to design the inertia base so that some of the mass is below the plane of the isolator's mounting brackets.
Inertia bases are often specified when very efficient isolators are all that are required to ensure a satisfactory arrangement is achieved.
Inertia bases can be designed so that the minimum amount of work - filling the base with concrete - has to be undertaken on site. When access is limited they can be designed in knock-down form for assembly on site.
ACE has worked with many London boroughs on the problem of noise generated by vibrating pipework in high-rise residential units.
These problems have been overcome by securing an inertia base to the low level pipework before the vertical risers. It must be appreciated that an inertia base will not affect airborne noise emanating from a plant room or breaking out from a pipework system.
Acoustical Control Engineers
T: 01954 718 344
1 January 2007