FLEXIBLE RUBBER CONNECTORS
An old sales technique is the flexing of a hand held rubber or metallic hose to show its flexibility. Nothing is limper than a rolled or folded canvas fire hose. However, when the hose is pressurized, it takes two men to bend and direct it. All flexible connectors are stiff under pressure, and you can only expect so much from them. They compensate for misalignment, relieve equipment flange strain, provide comparative freedom for floating equipment on isolators, and relieve the equipment of pipeline weight. Flexible connectors are a vital part of connections to equipment in seismic zones, as they will allow large equipment excursions without rupture.
Hose or rubber expansion joints reinforced with spiral wire or steel rings will not reduce pipeline vibration at rpm, but only at rpm multiplied by the number of blades in the pump wheel. Unfortunately, this frequency will continue to pulse through the fluid, and generally regenerate the pipeline vibration on the far side. In order to reduce noise and vibration at hydraulic frequency, a 2 arch molded rubber expansion joint is the correct choice as the reinforcement is much like the sidewall of an automobile tire. There is no steel wire and the expansion joint expands and contracts volumetrically at the hydraulic frequency. This dissipates energy and reduces the noise and vibration as confirmed by all of our acoustical tests. Spherical rubber connectors are preferable in general, as they are more effective high frequency vibration sound isolators and they do have the volumetric response capability not shared by metallic sections or wire reinforced hose.
Our rubber hose specification has been upgraded over the years. 38 years ago we suggested rubber flanged hoses without qualification other than pressure and temperature capability. There were buckling problems and we found that this type of hose was manufactured with an allowable elongation of as much as 7% of its length (2”(50mm) longer for a 30”(750mm) section). We changed our specification to limit elongation to 1.25%. This was so costly we recommended installation of control cables to prevent excessive elongation. Unlike control rods, control cables are quite flexible transversely, and the assembly is manufactured with bridge bearing Neoprene bushings and 0.5”(13mm) thick neoprene washers of the proper area to limit loadings to a maximum of 1,000psi. Cables are a definite improvement over rigid control rods, but they still tend to reduce the effectiveness of any rubber connector as axial capabilities are limited. Fortunately, our rubber technology moved on. Modern spherical handbuilt molded rubber expansion joints are available for less than the cost of the ineffective rubber flanged hoses and far easier to install because of their short lengths.
We developed the method of pre-extending the expansion joints by leaving a gap that is longer than the nominal face to face dimension of the expansion joint to allow for the predicted fixed elongation at a given pressure. Since there is a need for maximum volumetric expansion to eliminate noise, we are recommending twin sphere molded expansion joints in all applications.
In the past 25 years we have progressed from Neoprene to peroxide cured EPDM, because of greater temperature tolerance and resistance to ozone and sunlight. Nylon reinforcement moved on to Polyester and then to Kevlar. Nylon will fatigue and fail at temperatures between 200°F(93°C) and 250°F(121°C). We redesigned the mechanism for locking the raised face rubber flange behind the ductile iron flanges by building in a solid steel ring rather than the cable used by virtually all the other manufactures. With this combination of changed materials, we now had the safest and strongest rubber expansion joint on the market. However, there was still the possibility of manufacturing errors no matter how few. Therefore, all Safeflex connectors are factory tested to 150% of the rated pressure before shipment. (See Safeflex Development Bulletin SEJ-3 in the Hose section.)
Test bulletins 901 and 902,show a 20db reduction in vibration acceleration in the pipelines. This reduction in pipeline wall vibration, as well as the smoothing of the fluid pulsation, lowered sound pressure levels in spaces far removed from the pumps, but adjacent to the piping by 10 to 20db.
These improvements peak at blade passage frequency where the vibratory and noise problems are usually most troublesome. Therefore, the specifications now include a requirement for a pre-tested product.
This is the only specification where we urge you not to accept competitive products unless they are manufactured with Kevlar reinforced high temperature peroxide cured EPDM, and embedded rings rather than cable in the rubber raised face flanges. We would not have developed Safeflex if the other construction were safe. We and all the other manufacturers had similar failures prior to our redesign. We have not had a single failure out of the 30,000 units sold in the two years since changing to the present Safeflex design.
Even so, all expansion joints should be installed on the equipment side of the shut off valves and only in equipment rooms. It is not recommended that they are installed out in the general structure in areas over finished ceilings, etc.
Rubber expansion joints shall be peroxide cured EPDM throughout with Kevlar tire cord reinforcement. Substitutions must have certifiable equal or superior characteristics. The raised face rubber flanges must encase solid steel rings to prevent pull out. Flexible cable wire is not acceptable. Sizes 11/2” through 14”(40mm through 350mm) shall have a ductile iron external ring between the two spheres. Sizes 16” through 24” (400mm to 600mm) may be single sphere. Sizes 3/4” through 2”(19mm through 50mm) may have one sphere, bolted threaded flange assemblies and cable retention.
Minimum ratings through 14”(350mm) shall be 250psi at 170°F and 215psi at 250°F. (1.72MPa at 77°C and 1.48MPa at 121°C), 16”(400mm) through 24”(600mm) 180psi at 170°F and 150psi at 250°F. (1.24MPa at 77°C and 1.03 MPa at 121°C). Higher published rated connectors may be used where required.
Safety factors shall be a minimum of 3/1. All expansion joints must be factory tested to 150% of maximum pressure for 12 minutes before shipment.
The piping gap shall be equal to the length of the expansion joint under pressure. Control rods passing through 1/2”(13mm) thick Neoprene washer bushings large enough to take the thrust at 1000psi (0.7 kg/mm2) of surface area may be used on unanchored piping where the manufacturer determines the condition exceeds the expansion joint rating without them. Submittals shall include two test reports by independent consultants showing minimum reductions of 20 DB in vibration accelerations and 10 DB in sound pressure levels at typical blade passage frequencies on this or a similar product by the same manufacturer. All expansion joints shall be installed on the equipment side of the shut off valves. Expansion joints shall be SAFEFLEX SFDEJ, SFEJ, SFDCR or SFU and Control Rods CR as manufactured by Mason Industries, Inc.