What are piping supports?
Piping systems must be able to cope with the internal and external forces applied to them without the process fluid being discharged/leaked from the system. To enable this to occur, piping supports are used. This article discusses common piping support designs, types, how piping supports work, the purpose of piping supports, and common piping support problems.
Variable Piping Support Hanger
What are piping supports used for?
Piping supports are used for:
- Supporting the weight of a piping section.
- Catering for thermal expansion.
- Catering for shock loads (water hammer and hydraulic thrust).
- Catering for seismic activity (earthquakes etc.).
- Catering for wind loads (where applicable).
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How do spring piping supports work?
The general working principle relating to spring hangers involves Hooke’s law, which was created by the English scientist Robert Hooke. Material deformation is measured in terms of elasticity and plasticity.
- Elasticity – when a material deforms due to applied stress and returns back to its original form when the stress is removed.
- Plasticity - when a material deforms due to applied stress and does not return back to its original form when the stress is removed.
Within a material’s elasticity range, the amount of deformation in relation to the amount of stress applied, is described by Hooke’s law. The relationship between stress and deformation is better described as the relationship between stress and strain. Strain is expressed as a ratio comparing the change in length to the original length; this ratio is unitless i.e. is not restricted to metric, imperial, or any other form of distance measurement.
Hooke’s Law
By knowing four variables, it is possible to know how far a material will extend once a given pressure is applied. The equation is:
This simple linear relationship between the stress (force) and the strain (elongation) was formulated using the following notation.
P = force producing extension of bar (lbf for imperial units, Newton metres for metric units)
= length of bar (inches imperial, or millimetre, centimetre, or metre for metric)
A = cross-sectional area of bar (inches squared imperial, or millimetre, centimetre, or metre squared metric)
δ = total elongation of bar (inches imperial, or millimetre, centimetre, or metre for metric)
E = elastic constant of the material, called the Modulus of Elasticity, or Young's Modulus (lbf/in.2 imperial, or Pascals (Pa) metric)
Note that one pascal is equal to one Newton per square metre.
The quantity E, the ratio of the unit stress to the unit strain, is a material’s modulus of elasticity when in tension or compression and is often called Young's Modulus.
Thus, knowing the above equation and the associated four variables, allows engineers to calculate how long a spring will extend once a given force is applied. This means that engineers can also calculate how far a spring in a spring hanger will extend once a given force/load is applied.
IMPORTANT
Hooke’s law is only relevant for calculations within a material’s range of elasticity! Once a load exceeds a material’s range of elasticity, it enters the range of plasticity, which leads to permanent deformation of the material. Hooke’s law does not hold true within a material’s range of plasticity.
What is the difference between primary and secondary supports?
A primary support is directly fixed to the piping system parts and/or components, whereas a secondary support is connected to the primary support only. A piping system is typically classified as any item that supports, or is attached to, the system. Thus, secondary supports are usually considered part of a piping system.
Primary and Secondary Pipe Supports
Piping Support Classifications
There are various ways to categorise piping supports, although one of the most common is by construction design. Piping supports may be rigid, elastic, or adjustable.
Rigid Construction
Rigid piping supports are fixed to the system via welding or clamping. This type of piping support is very common, has a simple design, and does not flex or adjust once installed. Welded shoes, clamp shoes, support brackets, valve holders, and trunnions, are all types of rigid piping support.
Elastic Construction
Elastic piping supports cater for movement of the piping system. High temperature systems will always use elastic supports to cater for thermal expansion within the system. There are two main elastic type supports, these are the variable and constant types (discussed later in this article).
Adjustable Construction
Adjustable piping supports are similar to rigid piping supports, but they allow for some adjustment when being installed. Adjustment usually involves being able to rotate the support to align it with the pipe, and/or increase or decrease its height marginally to better support the pipe.
Expansion Joints and Expansion Loops
Expansion joints and expansion loops are also used to cater for thermal expansion within piping systems. Maintenance requirements associated with expansion loops are far less than that of expansion joints. Expansion joints tend to crack or fracture over time, especially when exposed to harsh environments e.g. coastal regions with a salty corrosive environment. For this reason, piping loops have become a more popular choice with piping system designers in recent years.
Piping Loop
What causes thermal expansion in piping systems?
Thermal expansion in piping material occurs as the temperature of the material increases. As materials are heated, they expand due to the kinetic energy they absorb from the heat. When a material is cooled, the material contracts/shrinks.
Piping that operates within a wide temperature range e.g. steam systems, will expand and contract as the temperature increases and decreases respectively. The amount of expansion or contraction depends upon the type of pipe material, length of the pipe, and the minimum and maximum temperature range (delta T).
Pipe Thermal Expansion
Example
A 50-metre length of pipe experiences a temperature increase of 50⁰C. The resultant effect upon the pipe’s length will be:
- 175mm elongation if manufactured from PVC.
- 40mm elongation if manufactured from copper.
- 25mm elongation if manufactured from stainless steel.
If piping supports do not cater for this expansion, the expansion will create a resultant force which acts upon any fixed item to which the piping system is attached. The force will far exceed any restraint upon the piping system and thus damage to the piping system and surrounding area is likely to occur.
Spring Supports
Spring supports use springs to cater for thermal expansion within a piping system and are a type of elastic support. A spring support contains a spring that is compressed when loaded. When not loaded, the spring returns/expands back to its original shape.
Springs may be installed individually, or in series (stacked on top of each other). Movement is categorised in terms of direction (up and down) and magnitude (distance of movement).
Each spring has a certain number of coils (number of turns), and these coils are constructed from ‘wire’. Coils are wound to have varying diameters depending upon their design and purpose; the difference between each coil is measured by the pitch. A large diameter coil is termed ‘loose’, whilst a smaller diameter coil is termed ‘tight’. Wires are typically manufactured to standard wire diameters using standard approved materials.
Spring Nomenclature
How To Adjust Spring Stiffness
Adjusting the thickness of a spring’s wire, or the tightness of a spring’s coil, will adjust its stiffness (resistance to geometric change when loaded). For example, a thick wire spring will require more force to compress than a thin wire spring. Likewise, a ‘tighter’ wound spring will require more force to compress than a ‘looser’ wound spring. Changing a wire’s material will change its stiffness also, because the density and structure of materials vary.
Spring Stiffness
Spring Support Types
There are two types of spring support, the variable and constant types. Both types can be further classified as either under or over pipe supports. Under pipe supports support piping from beneath, whilst over pipe supports support piping from above. Hanger supports are a type of over pipe support. Bottom supports are a type of under pipe support.
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Under Piping Support (bottom support)
Constant Spring Support
The constant spring support is also known as the ‘constant effort spring support’. This type of support is designed so that no matter what load is exerted (from the piping system) on the support, the supporting load remains constant. The supporting load also does not vary irrespective of the pipe’s position (providing the pipe is within the support’s designed working range).
Constant spring supports are more expensive than variable spring supports and are also unable to self-adjust the load; these are the two main reasons why they are not as common as variable spring supports. Another disadvantage is that the lead-time (delivery time) is longer compared to variable spring supports; this is a problem if a spare is required quickly.
Variable Spring Support
As a spring is compressed, its resistance to further compression increases, which is why the term ‘variable spring support’ is often used to describe this type of support (the supporting force varies). Variable spring supports do not offer constant support, but are favoured because they are cheaper than their constant support counterparts. As a rule, no more than 25% of the maximum working load (usually the load when the piping system is in the operational or ‘hot’ condition) should be transferred to a variable spring support.