MSS SP 58 defines variable spring hanger as a spring coil device which produces a varying supporting effect while permitting vertical pipe movement. Variable Spring Hangers are provided in piping systems to support the weight of the pipe and contents while accommodating the thermal expansion or contraction of the piping normally up to about 50-75mm and in exceptionally rare cases up to about 125 mm.
The normal spring hanger design practice is to support the weight of the pipe and contents (the design load) in the operating (normally hot or cold) condition. The spring force will vary as the piping system expands or contracts from it’s ambient condition to the operating condition and vice versa. For example, a steam system will contract from its operating condition when shut down whereas cryogenic piping will expand from it's operating condition when the system is shut down. The variation in spring force due to thermal expansion or contraction of piping system would normally cause an additional displacement stress-range to be added to the thermal expansion stress-range.
Selection of an appropriate spring support involves selecting the correct hanger type, series and size, and depends on several factors which includes:
- the piping arrangement
- the location of support structure with respect to the pipe
- headroom or bottom clearances available
- the thermal movement of pipe
- the load to be supported under operating conditions
It is important to understand the following definitions before proceeding to select the hanger type, series and size.
Hot Load
This is the working load or operating loading of the support in the “Hot” condition i.e when the pipe undergoes thermal movement from the cold condition to the hot or operating condition. The pipe is presumed to be in the cold condition when the plant or the line is not operating (however, note that for cryogenic piping the pipe will be in hot condition when the plant is not operating). As the high temperature fluids start flowing, the pipe gets heated and starts expanding causing thermal movement. The condition of the piping system after the expected thermal movement has taken place is known as the hot condition. It is assumed that in the hot condition the loads at the pipe supports are balanced, i.e. the downward force exerted by the pipe together with the weight of its content, self-weight of pipes, fittings, flanges & valves is equal & opposite to the upward reaction of the spring.
Cold Load
Cold Load is the load which the spring hanger will exert on the piping system in the installation or cold condition. Since the cold load is normally unbalanced with respect to the piping weight loads, there is a resultant unbalanced force acting on the piping system at this location in the cold condition. A distinction may be made here between the theoretical cold load and the actual cold load. All springs are calibrated and pre- set to the pipe cold load before shipment. The spring is positively locked against UP or DOWN movement until the piping system is ready to be operated. It is important to remove the travel stops before the line is put in operation. In the locked position, the spring hanger behaves as a rigid hanger.
Thermal Movement
Thermal movement is the vertical displacement of the pipe work after flow of hot fluids through it from the cold condition to the hot condition. The pipe movement is denoted as “-” or “DOWN” and “+” or “UP” depending on whether the pipe is moving up along the Y axis from the cold set position or moving down from the cold set position.
Maximum Load Variation
This is the variation in load imposed on the piping system when moving from the cold condition to the hot condition. This is usually expressed as a percentage of the hot load. In MSS SP 58, this is referred to as variability and is defined as the Load Variation of a Variable Spring Hanger divided by the Hot Load expressed as a percentage.
Spring Variability = \(\large\frac{Difference\;between\;Hot\;Load\;and\;Cold\;Load}{Calculated\;Hot\;Load}\) *100
or
Spring Variability = \(\large\frac{Pipe\;movement \;* \;Spring \;Rate}{Calculated\;Hot\;Load}\) *100
Depending on the layout, the pipe may be suspended by the spring hanger or resting on top of the spring support. Any thermal movement of pipe will force the spring to expand or compress causing either a decrease or an increase in load. This differential change in load is known as Load Variation and is expressed as a percentage of Hot Load. MSS-SP58 specifies maximum variability of 25% for variable spring hangers. However the stress engineer may at times limit the load variation for certain critical supports below the 25% value to limit the nozzle loads on rotating equipments such as turbine or compressor nozzle.
When installed new, normally prior to operation, the spring hanger should be adjusted until the load indicator moves to the ambient (normally cold) position mark (often a white dot or triangle). After operation for a period of time observation of the spring hanger should disclose the load indicator at the operating position mark (often a red dot or triangle). If the indicator is not, the hanger should be readjusted to the operating position.
Selecting the Variable Spring Hanger Configuration
Following are some of the configurations available with spring hanger suppliers:
Type-A is provided with a hanger rod at the top and is recommended when the assembly lengths i.e. length from bottom of supporting beam to centerline of the pipe are large. They may also be used to avoid interference of the spring housing with any other facility that is in close proximity with the overhead supporting structure.
Type-B and Type-C are furnished with one or two lug plates welded to the top of the spring housing to permit bolted assembly.
Type-D and Type-E may be used when the spring hanger is required to rest on the top of structure rather than being suspended from the bottom of structure. This arrangement can be beneficial if there is limited clearance between the pipe and overhead structure to accommodate the height of spring housing.
Type-F is a base mounted type of support which is fixed to the grade or structure and the pipe rests on the top of the spring support flange. They can be used to support pipes or base elbows running close to grade or close to underneath structure. It is not recommended to rest the pipe directly on the hanger flange. The pipe must be provided with shoes or a suitable flat surface which will provide sufficient area to rest on the hanger flange. If any significant horizontal pipe movement is expected, an antifriction pad may be provided to reduce the coefficient of friction and reduce the horizontal frictional loads imposed on the spring hanger flange.
Type-G comprises of two springs with trapeze type of arrangement for supporting horizontal pipe runs. In sizing a Type G hanger, it must be noted that each spring unit carries half of the total pipe load. If the pipe is not centred within the assembly, the load distribution on the two springs will not be equal. In such cases, the loads on each spring must be calculated and correctly sized springs should be used for each side. It is not recommended to offset the pipe supports from the center in Type-G spring support.
Example of Variable Spring Hanger Selection
Problem:
Design weight load or Hot Load = 11000 N
Design movement from Ambient to Operating = -20 mm (DOWN)
Recommended maximum variability = 25%
Solution:
For this particular example we will use Piping Technology and Products Inc. Catalog. Other spring hanger manufacturers have similar tables that can be used.
In the table, the load of 11000 N is available under hanger size 120 and 130. The red lines at the top and bottom indicate the recommended working range of the spring. On the left side of the chart we can see five different series of springs with different travel ranges. PTP-1 has the lowest travel range and PTP-8 has the largest travel range. It is also noted that the PTP-1 series with smallest travel range has the largest spring constant and PTP-8 with largest travel range has the smallest spring constant.
Let us calculate the variability for the two extreme spring types for hanger size 120:
Select series PTP-1 which has a spring rate of 157.61 N/mm for hanger size of 120
Spring Variability = \(\large\frac{Pipe\;movement \;* \;Spring \;Rate}{Hot Load}\) *100
For series PTP-1, Variability % = \(\large\frac{20mm\;*\;157.61N/mm}{11000N}\)*100 = 28.66%
Cold Load = Hot Load - (Pipe movement * Spring Rate).... for DOWN pipe movement
Cold Load = 11000 - 20*157.61 = 7848 N
The variability in this case exceeds the maximum recommended variability and hence is not acceptable.
Select series PTP-8 which has a spring rate of 19.78 N/mm for hanger size of 120
Spring Variability = \(\large\frac{Pipe\;movement \;* \;Spring \;Rate}{Hot Load}\) *100
For series PTP-1, Variability % = \(\large\frac{20mm\;*\;19.78N/mm}{11000N}\)*100 = 3.6%
Cold Load = Hot Load - (Pipe movement * Spring Rate).... for DOWN pipe movement
Cold Load = 11000 - 20*19.78 = 10604 N
At first glance, we might be tempted to use spring series PTP-8 since it results in a very low variability. However this approach will result in large size of spring support and hence higher cost of support.
From engineering fundamentals we know that when the springs are connected in series the resultant spring rate is lower than the spring rate of individual springs. Manufacturers use springs in series to lower the spring rate. For example PTP-8 uses four standard springs in series whereas PTP-1 uses one short spring.
In our example, since PTP-1 is not considered suitable due to variability exceeding 25%, let us use the next spring series PTP-2 wich is manufactured from one standard spring (not short spring).
Select series PTP-2 which has a spring rate of 78.81 N/mm for hanger size of 120
Spring Variability = \(\large\frac{Pipe\;movement \;* \;Spring \;Rate}{Hot Load}\) *100
For series PTP-2, Variability % = \(\large\frac{20mm\;*\;78.81N/mm}{11000N}\)*100 = 14.32%
Cold Load = Hot Load - (Pipe movement * Spring Rate).... for DOWN pipe movement
Cold Load = 11000 - 20*78.81 = 9424 N
The cold load is located within the recommended working range of the spring.
The variability of 14.42% falls within the acceptable range. Thus the selected spring would be hanger size 120, series PTP-2 with a preset load of 9424 N.
For an economical and fit for purpose design, the engineer should preferably select the smallest spring size possible that will accomodate the design movement. However, there will be instances when a lower spring constant is desirable to accomodate allowable nozzle loads and the smallest spring size may not be the best choice. In all cases, the engineer should also check the feasibility of installing the spring hanger in the availabe space. This will include selecting the configuration of variable hanger depending on how it is attached to the supporting structure.