The composition of the mounting pad should follow standard practice for the required loading. Common specifications call for 2500-3000 psi (176-211 kg/cm2) concrete reinforced with eight-gauge wire mesh or number 6 reinforcing bars on 12-inch (30-cm) centers. The total weight of the mounting pad should be at least equal to the weight of the generator set (a density of 150 lbs. per cubic foot [68.4 kg per 0.03 m3] for concrete can be used for this calculation). Suggested concrete mixture by volume is 1:2:3 parts of cement, sand, and aggregate. A layer of 8-10 inches (21-26 cm) of sand or gravel should surround the pad for proper support and isolation of a pad located at or below grade. All generator sets should be anchored to concrete with bolts buried in the surface of the pad. Expansion-type anchors are not acceptable.
Note: Always refer to the dimensional drawings for the generator and accessories when considering placement of conduits and fuel lines. Dimensions are provided on these drawings for rough-ins and stub-ups of electrical and fuel connections.
3.4 Vibration Isolation
Mounting bases for generator sets 30kW and larger are typically made from “I” or “C” section fabricated steel with a width of 2-3 inches per channel. Length varies with the size of the unit, resulting in a static load on the mounting base of 10-25 psi (0.703--1.758 kg/cm2) if the total bottom surface of the channel is in contact with the mounting base. All generator sets should have vibration isolation between the engine-generator and skid or mounting base. See Figure 3-5. This may consist of neoprene or combination spring and neoprene isolators between the engine-generator and skid, or spring-type mounts between the skid and mounting pad. An advantage of factory-installed mounts between the engine-generator and the skid is that engine-generator alignment is not affected by stress induced in handling, shipment, or mounting on an uneven surface. A less rigid skid may be used, reducing weight and installation time. All connections between the generator and mounting base such as conduits, fuel lines, exhaust piping, etc., must have flexible sections to prevent breakage and isolate vibration to the generator set.
Generator sets 350 kW and larger are usually mounted directly to a structural steel base. For these units, the manufacturer’s recommended vibration isolators should be installed between the base and mounting pad. Because of the reduced mounting surface area of these individual mounts, the static load on the mounting surface will increase to the range of 50--100 psi (3.515--7.03 kg/cm2).
The vibration isolation efficiency of neoprene pad-type mounts is approximately 90%. When installed at or below grade, this degree of isolation will prevent transmission of objectionable vibration to the surrounding structure. Spring-type isolators can be expected to provide isolation efficiency of 98%. This type of mount may be desirable for above-grade installations.
In some critical applications where the generator set is installed above grade or where earthquake-proof mounts are specified, it may be necessary to install spring-type vibration isolators under the generator set mounting base. Check state and local codes for such requirements. Accessory vibration mounts should be installed at the locations of the standard predrilled mounting holes.
| 1 |
| 3 |
| 2 |
| TP-5700-3 |
1. | To engine-generator |
2. | Skid cross member |
3. | Neoprene vibration isolator |
Figure 3-5 Neoprene-Type Integral Vibration Isolators
