Usually coiled type springs are used in variable type support. During compression, the resistance of a coil to a load changes, so these types of supports are named as ‘variables’. Actually pipe work under change in pressure & temperature results in vertical displacements. To accommodate/ absorb such vertical displacements, this type of support is recommended. They are generally used at points where stress due to transfer load does not have an adverse effect on the overall stress of the pipe work. A careful study is required for the piping system having a large flexibility because deflection is likely to occur even when there is only a small stress variation due to transfer load. Variable Spring Support has a wide range of application and it is important to select the proper one for a specific purpose. There are different designs for variable supports and so TECHNO INDUSTRY provides particular nomenclature to identify them. As many as seven (A to G) different types of variable spring support are obtained at present and they are given below:
This type of support is suitable where there is sufficient installing space. It is mainly provided with a threaded bushing in the top plate, in order for a simple rod attachment for the upper connection.
This type of support is mainly used where installing space is limited. It is generally furnished with one or two lugs welded to the top plate of the casing.
This type of support is suitable where spring box/casing. For type ‘D’ a rod adjustment is permitted from the top. For type ‘E’ a rod adjustment is permitted from either above or below the spring.
This type of support is designed to support the piping upward, i.e. from below, directly from the base/ floor or supporting steel. An adjustment in height is available by turning the load column and for fastening four holes is drilled at the base plate.
This type of support is generally formed by welding two standard variable spring supports back to back at the ends of a pair of channels.
The standard range of variable spring supports are obtained in three main categories, with the travel ranges are the following:
•VS1 TRAVEL RANGE: 0- 40 MM.
•VS2 TRAVEL RANGE: 0-80 MM.
•VS3 TRAVEL RANGE: 0-160 MM.
When a load is applied to the variable spring support, the responds of it depends on the coils of the spring. The most important part of the variable spring hanger support is the spring. So outmost care is provided during the selection of a particular spring. A spring is mainly designated by its spring rate, which is expressed in units of kg/mm. Coil size/Rod size of a spring is determined by the applied load & the no of coils is determined by the travel range / range of movement. As such a spring is placed in single & in series depending on the type of load & travel range.
A good deal of calculation is needed prior to the selection of the standard variable spring support in order to ensure that the connecting equipment and pipe work can withstand the large load changes that occur during pipe movement. Generally load variation is limit to the maximum of 25 %( according to MSS SP-58). So it is quiet natural to select variable spring hanger support on this basis. Therefore during the selection of variable spring hanger support care must be taken to ensure that sufficient travel is available in the spring assembly to permit free travel of the pipe / vessel within the required pipe work movement range.
In order to select a variable spring hanger support, first of all you will have to select the type of support depending on the location of support, load, and available movement. Naturally you must know the actual load which the spring is suppose to be support, also the magnitude & direction of the pipe line movement. The next step for the selection of variable spring hanger support is to calculate the Cold Load. Cold Load is mainly calculated on the basis of the formulas given below:
Here the sign (+) indicates the upward movement of the pipeline, and the sign (-) indicates the downward movement of the pipeline.
After being calculate the Cold Load, the next most important selection criteria is to know the variation of the load i.e. % load change. Variability is required to select the size and figure number. Variability is defined as the percentage of change in the supporting force between the Hot (Operating) load and Cold (Installed) load calculated as shown:
VARIABILTY (V) = (COLD LOAD – HOT LOAD) X 100 %
V = MOVEMENT X SPRING RATE X 100 %