Moreover, spring supports absorb/accommodate the vertical displacement of a piping system caused by temperature change (thermal expansion/contraction) or subsidence, etc. In conjunction with hanger rods and slide bearings, they also accommodate any horizontal displacement that may occur.
Spring supports are critical components because, without such support, excessive pipe stress would cause significant damage to the piping system and other components connected to that system.
For relatively small displacements (up to about 75mm), we recommend the variable spring support.
For displacements over 75mm, constant spring support is recommended.
Use of continual effort support may also be appropriate when only a slight change in load is accepted (e.g., close to sensitive equipment) or where space considerations may prevent the use of variable effort support.
Why use Spring Support?
Any line operating at high temperature moves upwards/downwards (depending on the pipe configuration) due to thermal expansion. Therefore, any rigid support provided on such a line tends to lift the pipe/down and remain inactive during operating conditions. In such a case, flexible support (springs) can load in working and cold conditions.
The spring supports help with continuous support during the expansion or contraction of the pipe. The spring support employs a spring element, which can get compressed or stretch out depending on the pipe’s thermal movement and the corresponding loads.
Types of Spring Supports
Loads accommodated and the magnitude & direction of the thermal displacement supported, spring supports are broadly classified as:
- Variable effort springs
- Constant effort springs
Some of the common terminology associated with the selection and procurement of any springs are listed below:
- Cold load
- Hot load
- Spring rate
- Spring travel
Load variation or pre-compression length cold load refers to the load on the spring hanger when the system is in standby or non-operating condition.
Hot Load: This refers to the actual load on the spring hanger during operating conditions.
Spring rate/stiffness: This refers to the spring rate, force per unit length in N/mm, kg/mm, etc., determined from flexibility analysis.
Spring travel (Installed to operating): Refers to the maximum vertical movement of the spring due to piping loads at operating conditions determined from flexibility analysis.
Load variation or variation: This refers to the allowed variations between the hot load and cold loads.
Pre-compression length: It is the initial compressing of the spring for the sustained load. Installation height = loaded length – pre-compression length.
Variable effort spring VES consists of a spring that can get compressed or expanded according to the thermal movement of the pipe. However, this movement causes an increase or decrease in supporting force depending on its stiffness. Moreover, this differential load is transferred to the pipeline.
This load is less than that would be with the rigid support. In VES, we maintain load variation maintained within 25%. In VES, the loads increase with pipe movement.
Types of VES/CES hanger type: In hanger type spring support, the pipe is hung from secondary support using hanger type spring. Clevis, hanger rod, turn-buckle, pipe clamp, etc., are other attachments associated with such support.
Hanger type VES/CES bottom support type: In the bottom support spring, the pipe is resting on the top of the spring load plate, as shown. This type of spring support is also known as ‘CAN’ type or ‘F’ type spring.
Hanger type or bottom support type is selected based on pipe layout and the space availability for mounting. Bottom support type
Selection of Variable effort spring Determines the required effort & pipe movement (up or down) from installed to operating condition. Select the smallest spring size from the vendor catalog, which has the operational load within the working travel.
Ensure the spring selected can accommodate the preset to operating travel within the dynamic range. You can do this by moving up & down the chart from the operating load by the amount of travel.
Try a giant spring or the following travel range if the spring selected cannot accommodate the movement. Check the variation in supporting the effort for the selected spring.
Variation in Supporting the Effort
If this exceeds the allowable variation, choose the following travel range and go back above. A smaller travel range may be acceptable if the variation is less than half of the permissible. Again, choose a smaller travel range and go back to step.
If the variation exceeds the allowable selection, then constant effort support is required, or the possibility of routing changes to be studied. Example: Select a variable spring for the following conditions: Hot load = 307 lbs, movement = ½” up, variability = 20% maximum.
Calculate spring rate. Spring rate = hot load x variability / movement = 0.20*307/0.5 = 122.8 lbs per inch.
Find the spring size column on the size and series selection chart where the hot load is 307 lbs (A size five series fig.82 fits the criteria).
Calculate the cold load. Cold load = 307 + (63 x 0.50) = 339 lbs.
Now check to see if the hot load and cold load fit in the working range of 5. If so, you have selected the proper unit.
Whenever load variation exceeds 25% or exceeds the specified maximum load variation percentage in a variable hanger, a Constant Effort Spring is selected. In CES, the load remains constant when the pipe moves from its cold to hot position. Thus irrespective of travel, the load remains constant over the complete range of movement. We support the pipeline by a drop rod connected via turnbuckle to the end of the lever arm.
Spring coils apply a force to the trunnion arm lever, which tends to pull the lever-arm UP against the load of the pipe. The geometry of the lever arm provides a balance btw the pipe load & spring force. Therefore, the pipeline may move due to thermal expansion during constant pressure through this travel range.
The selection of constant effort spring determines the hanger and the actual travel support of the load.
The actual vertical movement of the pipe at the point of hanger location, refer load-travel table. The total travel for constant supports should equal “actual travel” plus 1” or 20%, whichever is greater. After determining the size, considering available room for suspending the pipe and hanger will indicate whether a vertical or horizontal hanger is desirable.
After hanger size & design are determined, the type of constant support to be used depends on the suspension problem’s physical installation. Example: Total travel = 4 “ and 2750 lbs. The hanger size would be 349.
General notes & guidelines: Any re-adjustment of the spring element shall be carried out only when the line is packed with the fluid or equivalent in density to balance the piping weight and the preset load of the spring. The adjustment of the hanger-type spring element is made by rotating turnbuckle or adjustment nuts provided in the hanger rod.
During hydraulic testing, flushing, or chemical cleaning of the pipeline, the spring must be kept under locked condition or protected against overloading due to weight of testing/flushing fluid by providing temporary. After re-adjustment, it is essential to check whether sufficient range is available on the scale for the required movement of the pipe during operation.
Points to remember design spring based on the installation load (operating load). The compactness of the units. We design installation heights to a minimum. During the rare case, the pipe may move more than the functional movement. In such a case, if we choose the maximum deflection range, the spring cannot get further movement, and thus the spring fails.
The initial design itself, the spring, cannot be designed for occasional loads (e.g., Seismic, wind, etc.) & movement. Then, it may be an over-design. To overcome such a problem provides “Cushion Range” means even if the spring may get compressed in occasional cases, so choose always “MID-RANGE.” for hanging spring support, the lateral movements (rod swing) should not exceed 4 degrees for bottom-type supports, where the horizontal direction of more than ½” is envisaged.
You should specify Teflon-covered load pads. Always mention the hydro test load while ordering a spring. Try to help the spring vendor in designing the spring locking arrangement.
Standard inventory finishes: Hot-dip galvanized.
Coils come with a protective coating: Protects from a wide range of corrosive. The protective layer does not affect the flex life of the spring. Supports fitted with nameplates marked with the installation and operating load, support reference mark, type, and unique serial number.
Offshore applications flow arm lines (Xmas tree to the production/ test manifolds). Compressors WHR (Waste Heat Recovery Units).