How does spring support work?

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.

Click here to get more.

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).

If you want to learn more about power engineering topics, be sure to check our Power Engineering Fundamentals Video Course. You can also get access to downloadable engineering articles (like this one) and handbooks by joining our newsletter.

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 50&#;C. 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.

If you are looking for more details, kindly visit zhiang.

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.

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.

Spring Support

1. Why to use a 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 pipe up/down and hence remain inactive during operating conditions. In such a case a flexible support (springs) is provided which is capable of taking the load in all the operating and cold conditions.
• The spring supports provides continuous support during 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.

2. 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

3. Terminology

• 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/ Stifness

  : 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 sustained load.

• Installation Height 

  = loaded length &#; Pre-compression length

4. Variable effort spring

• • VES basically consist of a spring which can get compressed or expanded according to thermal movement of the pipe. However, this movement causes 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 maintained generally within 25%.

• In VES the loads increases 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 a support.

Bottom support type: In 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.

6. Selection of Variable effort spring

1. Determine the required effort & pipe movement (up or down) from installed to operating condition.
2. Select the smallest spring size from the vendor catalogue which has the operating load within the working travel.
3. 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.
4. If the spring selected cannot accommodate the movement try a larger spring size or the next travel range.
5. Check the variation in supporting effort for the selected spring.

6. If this exceeds the allowable variation then choose the next travel range and go back to Step 3 above.

7. 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.

8. If the variation exceeds the allowable selection then a constant effort support is required or 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 size 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, then a Constant Effort Spring is selected.
• In CES the load remains constant when the pipe moves from its cold position to hot position. Thus irrespective of travel the load remains constant over 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. The pipe may therefore 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 hanger as well as the actual travel, ie. 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, consideration of 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 upon the physical installation required by the suspension problem.
  • Example:
  • Total travel = 4 &#; and lbs

• Hanger size would be 34

9. General notes & guidelines

• • Any re-adjustment of 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 hanger type spring element is done by rotating turn buckle 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 scale for required movement of the pipe during operation.

10. Points to remember

• • Design spring based on the installation load (operating load).
  • Compactness of the units. Installation heights designed to a minimum.
  • During occasional case the pipe may move more than the operating movement. In such a case, if we choose 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 in occasional cases the spring may get compressed, so choose always &#;MID-RANGE&#;.
  • 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 degree.
  • For bottom type supports, where 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 finish: 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

If you want to learn more, please visit our website spring pipe support.

• Flow arm lines (X&#;mas tree to the production/ test manifolds).
• Compressors.
• WHRU (Waste Heat Recovery Units).

12. Spring vendors

• Sarathi
• Carpenter and paterson Ltd
• Lisega
• Anvil
• Bergen power
• Pipe support Ltd