Moreover, spring supports are designed to 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. Spring Support
Why use Spring Support?
Any line operating at high temperature moves upwards/downwards (depending on the pipe configuration) due to thermal expansion. 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) is provided, capable of taking the load in all the operating and cold conditions. The spring supports provide continuous support during the expansion or contraction of the pipe. The spring support basically employs a spring element, which can get compressed or stretch out depending upon the thermal movement of pipe and the corresponding loads.
Types of Spring Supports
Depending on the loads to be accommodated and the magnitude & direction of the thermal displacement to be 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 Variation Pre-compression Length Cold Load: This 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): This 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 4. Variable effort spring VES basically 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 & this differential load is transferred to the pipe. This load is less than that would be with the rigid support. In VES, load variation is generally maintained within 25%. In VES, the loads increase with pipe movement. 5. Types of VES/CES Hanger type: In hanger type spring support, the pipe is hung from the secondary support using hanger type spring, as shown. Clevis, Hanger rod, turn-buckle, pipe clamp, etc. are some 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 6. 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 operating load within the working travel. Ensure the spring selected can accommodate the preset to operating travel within the working range. This is done by moving up & down the chart from the operating load by the amount of travel. If the spring selected cannot accommodate the movement, try a larger spring or the next travel range. Check the variation in supporting the effort for the selected spring.
Variation in supporting the effort
If this exceeds the allowable variation, choose the next travel range and go back to Step 3 above. If the variation is less than half of the allowable, then a smaller travel range may be acceptable. Choose a smaller travel range and go back to step 4. 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 1. Calculate spring rate. Spring rate = hot load x variability / movement = 0.20*307/0.5 = 122.8 lbs per inch 2. Find the spring size column on the size and series selection chart where the hot load is 307 lbs. (A size 5 series fig.82 fits the criteria) 3. Calculate the cold load. Cold load = 307 + (63 x 0.50) = 339 lbs. 4. Now check to see if the hot load and cold load fit in the working range of 5 series fig.82. If so, you have selected the proper unit. 7. Constant effort spring 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. The pipe is supported by a drop rod connected via turnbuckle to the end of the lever arm. The spring coil applies a force to the trunnion arm of the 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 pipe may move due to thermal expansion while being supported with a nominally constant force through this travel range. 8. Selection of Constant effort spring Determine the load to be supported by the hanger and the actual travel. The actual vertical movement of the pipe at the point of hanger location, refer load-travel table. The total travel for constant supports should be equal to “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 34 9. General notes & guidelines Any re-adjustment of the spring element shall be carried out only when the line is full with the fluid or its equivalent in density to balance the weight of piping and the preset load of 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 important to check whether sufficient range is available on the scale for the required movement of the pipe during operation. 10. Points to remember Design spring based on the installation load (operating load). The compactness of the units. Installation heights are designed to a minimum. During the occasional case, the pipe may move more than the operating movement. In such a case, if we choose the maximum deflection range, the spring cannot get further movement, and thus the spring fails. To overcome such a problem provides “Cushion Range” means even if the spring may get compressed in occasional cases, so choose always “MID-RANGE.” 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. For hanging spring support, the lateral movements (rod swing) should not exceed 4 degrees. For bottom type supports, where the horizontal movement of more than ½” is envisaged, Teflon-covered load pads should be specified. Always mention the hydro test load while ordering a spring. This will 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 corrosives. Does not affect the flex life of the spring. Supports are fitted with nameplates marked with the installation and operating load, support reference mark, type, and unique serial number. 11. Offshore applications Flow arm lines (Xmas tree to the production/ test manifolds). Compressors. WHRU (Waste Heat Recovery Units). 12. Spring vendors
- Carpenter and Paterson
- Carpenter and Paterson Ltd
- Bergen power
- Pipe support Ltd
Constants and Variables
There are two types of spring supports; Variable Effort Supports (VES), and Constant Effort Supports (CES).