Nd Mounts





help compensate for pipe misalignment and thermal expansion. Flexible pipe connections alone are usually not sufficient to isolate pipe transmitted vibrations but are part of an overall control strategy, which includes vibration isolation of the mechanical equipment and piping.

In high pressure hydraulic systems much of the vibration can be transmitted through the fluid so that pulse dampeners inserted in the pipe run can be helpful. These consist of a gas filled bladder, surrounding the fluid, into which the pressure pulse can expand and dissipate.

Where pipes are located in rated construction elements, closing off leaks at structural penetrations is critical to maintain the acoustical rating. Here the normal order of construction dictates the method of isolation. In concrete and steel structures, slabs are poured and then cored to accommodate pipe runs. In wood construction, piping is installed along with the framing, often preceding the pouring of any concrete fill. In both building types holes should be oversized by 1" (25 mm) more than the pipe diameter to insure that the pipe does not make direct structural contact. They are then stuffed with insulation, safing, or fire stop, and sealed. In slab construction the sealant can be a heavy mastic. With walls, the holes are covered with drywall leaving a 1/8" (3 mm) gap that is caulked. Pipe sleeves, which wrap the pipe at the penetration, are also commercially available. Details are shown in Fig. 11.17.

Electrical Connections

Where electrical connections are made to isolated equipment, the conduit must not short out the vibration isolation. If rigid conduit is used it should include a flexible section to isolate this path. The section should be long enough and slack enough that a 360° loop can be made in it.

Duct Isolation

High-pressure ductwork having a static pressure of 4" (10 cm) or greater should be isolated for a distance of 30 ft (10 m) from the fan. Ducts are suspended on spring hangers with a minimum static deflection of 0.75" (19 mm), which should be spaced 10 ft (3 m) or less apart.

Roof-mounted sheet metal ductwork, located above sensitive occupancies such as studios, should be supported on vibration isolators having a deflection equal to that of the

Figure 11.16 Vibration Isolation of Piping and Ductwork (Vibron, 1989)

Vibration Isolation of Piping Connected to Isolated Equipment

Figure 11.16 Vibration Isolation of Piping and Ductwork (Vibron, 1989)

Each of the above alternatives will result in equivalent piping isolation from a flexibly supported pump if the following suggestions are implemented.

(\) The pump inertia bases should be at least 2.5 times the unit weight.

(1) Piping isolators should have the following static deflections: Points A and B - Equal to the pump base static deflection Beyond Point B - At least O.l" for 20 ft. from point B with isolators spaced at IO ft. intervals. For critical locations tne complete pipe run should be isolated, with isolator static deflection of at least O.l" and spaced at IO ft. intervals.

(3) For pipe diameters above 5", flexible connections are necessary.

Vibration Isolation of High Pressure Ductwork

High pressure (above 4" static pressure,) duct runs should be isolated for a distance of 30 ft. from the fan. Ducts supported by hanger isolators should have a static deflection of at least 3/4" and be spaced IO ft. apart.

Figure 11.17 Pipe or Duct Penetration

The opening is oversized to allow for the penetration and covered with the same number of layers as are on the wall.

Fiberglass Board

Caulk at Opening


Figure 11.18 Forced Excitation of an Undamped Two Degree of Freedom System (Ruzicka, 1971)

Double Pendulum

Double Pendulum

Spring Mass

Spring Mass

v|/ F0 sin cj t isolated equipment to which they are attached, for the first three points of support. Beyond that point the ducts can be supported on mounts having half that deflection.

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