VSG Riser Guides
The hanger, mounting and anchor discussions on the previous pages include explanations of the various commodities and the reasons for their design. We have called for deflections in the first four supports to be equal to that of the equipment as these locations often require capacities that are as great as the equipment loadings and the frequencies and amplitudes are virtually the same as well. It would be completely inconsistent to reduce these deflections immediately. After four hangers or approximately 30 feet(9m) the reduction in amplitude makes it feasible to use a more economical hanger.
(Note: Earlier specifications said three hangers. To move 30’(9m) from equipment normally requires four as the first supports the riser.)
Some specifications limit the application of hangers to the “equipment room,” “50 feet(15m) from equipment,” etc., or some other formula based on a multiple of the diameter. They all try to reach out to a mythical location where the piping becomes “Quiet”. While it is usually true that the more severe vibration will occur close to the equipment, the rest of the pipeline is likely to remain troublesome. We have done expensive corrective work on pipe supports 20 stories away from the pumps and there is no way to predetermine if or where pipelines can be installed with solid supports and not risk noise transmission, because of pipe or building resonance. Therefore, we are continuing to suggest complete pipeline isolation as hangers are relatively inexpensive and worthwhile insurance. Rubber expansion joints minimize the noise problem at the source, but as explained in the “Explanations” Specifications O and P, flex connectors do little or nothing to reduce vibration at RPM.
The specification calls for specific deflections for piping suspended from occupied spaces if the machine room is in the basement. The reason is the selection guide calls for minimum floor isolation in basements because there is normally nothing below them that is critical in any way or the basement may be on grade. If the hangers were to mimic these deflections, there would be a problem, because the piping may be suspended from ceilings under sensitive areas.
The first four pipe hangers in the main lines near the mechanical equipment shall be as described in specification Type G. Hangers supporting piping 2”(50mm) and larger in all other locations throughout the building shall be isolated by hangers as described in specification F. Floor supported piping shall rest on isolators as described in specification D. Heat exchangers and expansion tanks are considered part of the piping run. The first four isolators from the isolated equipment shall have the same static deflection as specified for the mountings under the connected equipment. If piping is connected to equipment located in basements and hangs from ceilings under occupied spaces the first four hangers shall have 0.75”(19mm) deflection for pipe sizes up to and including 3”(75mm), 11/2” (40mm) deflection for pipe sizes over 3”(75mm) and up to and including 6”(150mm), and 21/2” (65mm) deflection there-after. Where piping connects to mechanical equipment install specification O expansion joints or specification P stainless hoses if O is not suitable for the service. All piping passing through the equipment walls, floors or ceilings shall be protected against sound leakage by means of an acoustical seal, as described in Specification Q.