When a piping system experiences large vertical movements and if the variability at the point of support exceeds 25%, it is recommended to use contant support springs. As per MSS SP 58, constant support hangers shall be used on piping systems where the deviation in supporting force must be limited to 6 percent and which cannot be accommodated by a variable spring hanger. Constant support hangers are also referred to as constant effort supports.
Why are they call Constant Spring Hangers? In a constant spring hanger the load remains constant when the pipe moves from its cold position to the hot position.
When are Constant Spring Hangers preferred? A constant spring hanger is preferred under the following conditions:
- Where thermal expansion (or contraction) of the piping is significant, typically more than 75-100 mm from it’s ambient to it’s operating condition position.
- The thermal expansion of piping system results in spring variability greater than 25%.
- The piping system is connected to critical strain sensitive equipments such as pumps, compressors and turbines.
- Space considerations prevent the use of variable spring hangers.
Principle of Operation of Constant Effort Supports
Constant spring hangers of types 54, 55 and 56 as defined in MSS SP 58 may be considered as variable spring hangers, where the variable spring force is compensated with a mechanical linkage or auxiliary springs that limits the spring force variability to 6 percent. Constant spring hangers are based on a simple mechanical principle of opposing moments. As the pipe of constant weight moves up or down, it is supported by a varying spring force acting upon a changing moment arm, to produce a constant supporting force on the pipe. The principle is depicted in figure below. As the hanger rod connected to the cam moves up or down, the spring is decompressed or compressed by the spring rod. The moment of the support load about the pivot point varies as the supported pipe moves up or down. The moment of this load about the pivot point is balanced by the spring force about the same pivot point. By selecting proper arm lengths for the external force and spring force, the load variation can be optimized.
In the above illustration it is noted that at position blue, moment of Support Force (W) about pivot point W x b1 is balanced by moment of spring force (F1) about pivot point F x a1.
Thus W x b1 = F1 x a1
Similarly at position green, moment of Support Force (W) about pivot point W x b2 is balanced by moment of spring force (F2) about pivot point F2 x a2.
Thus W x b2 = F2 x a2
The moment arms are optimized such that W = \(\large\frac{F_1 * a_1}{b_1}\) = \(\large\frac{F_2 * a_2}{b_2}\) is within 6% variability.
As per MSS SP 58 constant supports shall be provided with:
- a travel scale
- a load adjustment scale
- provisions for field load adjustment of at least ± 10%
- provisions for “hot” and “cold” settings
- limit stops to prevent over travel or release of load
The information required to select a constant spring hanger is the design (normal operating, typically hot) load and the thermal movement of the support point between the ambient and operating conditions. If any unanticipated additional hanger travel is likely to be encountered, the design thermal movement is increased by an amount to accommodate any additional travel. This additional hanger travel is typically referred to as the hanger “total travel.”
Like variable spring hangers, constant spring hangers are installed to an ambient position mark and after steady state operating conditions have reached, the travel indicator of the constant spring hanger should be at the operating position mark. If the indicator is not at the operating position mark, the hanger may require and adjustment to balance the operating movement about an average travel position. The ambient position mark and operating position marks are normally equidistant from an average travel position and the travel margin at either end of the travel indicator is typically at least 10%.
It may be noted that the travel position indicator is not an indication of the load being accommodated by the constant spring; there is a separate load indicator on the spring typically set at the manufacturer's factory based on designer input.
Selecting a Constant Spring Hanger Configuration
Step-1
Determine the Total Load to be supported by the hanger. It may be noted that the cold load and hot load shall be same for constant support.
Step-2
Determine the Total Travel = Calculated Travel (the vertical movement of the pipe at the point of hanger location) + Over Travel.
The total travel of the hanger should always be greater than the calculated travel of pipe line to allow for discrepancy between the calculated and actual travel. The recommended over travel is 25mm for travel up to 125mm and 20% of calculated travel for travel in excess of 125mm.
This should then be rounded up to the next whole 10mm increment giving the total travel thus allowing a standard support to be selected.
Problem:
Load = 10kN Calculated Travel = 90mm
Solution:
In this example, we will use catalog from Binder Group. Other manufacturers will have similar tables that can be used.
Given a calculated travel of 90mm, which is under 125mm, the amount of recommended over travel is 25mm.
Thus, the total travel = calculated travel + over travel = 90mm + 25mm = 120mm (rounded to the next 10mm increment)
In figure below, locate total travel column of 120mm and follow down column (see vertical red box) for load greater than equal to 10kN. This reads 10.41kN.
Next, move horizontally to the left under column (see horizontal red box) headed ‘Constant Support Size’. In this example the selected unit would be size 22.
