by The requirement of piping supports are as below follows :
• To carry the weight of the pipe, fittings, valves with / without insulation, with operating / test fluid.
• To provide adequate stiffness to the piping against external loads such as wind load, ice, snow, seismic load etc.
• To avoid overstressing of the piping material.
• To avoid sagging of pipe which creates draining problem.
• To control the thermal expansion / contraction in desired manner
• To withstand and dampen vibration produced by connected equipment such as pump, compressor etc.
Type of supports,
Rest Support – Avoid deflection due to weight
Guide Support – Control lateral movement due to pressure and temperature
Axial Stop Support – Stop axial movement due to thermal expansion
Hold down Support – Control pipe lift-up due to pressure and temperature
Fixed Support – Control deflection , lateral and axial movements
Anchor Support – Control Pipe from all movement and rotation
Standard Pipe Supports
. Pipe shoe
. Guide
. Stop
. Fixed
. Anchor
. Clamped Support
. Dummy/Trunnion
. Hanger Rod
. Spring
Standard Frames for Pipe Supports
. Cantilever
. Braced Cantilever
. L-Frame
. T-post
. Goal Post
Type of Slide Plate
. PTFE Slide plate
. Copper Slide plate
Type of vessel cleat support
. Light Bracket
. Heavy Bracket
. Reinforcing plate – Single
. Reinforcing plate – Double
Type of insert plates
. Insert plate for Cantilever Beam
. Insert plate for L-Bracket
. Insert plate for Braced Cantilever
Pipe Span
Piping is supported at regular intervals on steel supports embedded in concrete foundation or directly on steel structure. The distance between supports is the span. Normally we calculate the support span considering pipe with one end simply supported and other end is fixed.
Calculation of Max. span:
There are four main factors which affects the support span.
- Stress
- Deflection
- Free draining
- Natural Frequency
Pipe Span based on Stress :
The stresses are acting through the pipe wall at supports due to sustained load.
1. Pressure stress
2. Bending stress due to weight
3. Local stresses at the point of support attachment
Pipe Span Chart
Types of Supports
Hanger/Support
To sustain the dead weight of the piping system
. Rigid Hanger
. Spring Hanger (Variable/Constant)
. Shoes
. Trunnions
Restraint
To restrict the movement due to thermal/dynamic loading
. Anchor
. Guide
. Directional Anchor
. U clamps
. Struts
Vibration Absorbers
To restrict the movement due to vibration caused by wind , earthquake ,fluid flow
. Snubbers
. Sway Brace
. Hold down
Hangers / Supports
Hangers / Supports are used to carry the dead weight of the piping system along with its contents. The primary difference between a hanger and a support is that the hanger sustains the dead weight in tension, while the support sustains it in compression. Consequently, hanger supports the piping system from top, while supports are placed below the piping. The type of support / hanger assemblies from which the piping engineer selects the appropriate kind of support are generally classified as follows:
1. Rigid Hangers
2. Shoes
3. Trunnion
4. Flexible Hangers (Variable, Constant)
Hangers /Supports – Rigid Hanger
Rigid Hangers:
Rigid hangers are normally used at locations where no vertical movement of the pipe occurs. The design considerations for a rigid hanger are pipe temperature for selection of pipe clamp material and the load for the selection of components suitable for the pipe weight involved.
Although these hangers prevent vertical movement of the piping system in many cases, they allow little horizontal movement at the point of their attachment such that the inclination from the vertical direction is limited to 4°. The compressive force imposed on the hanger due to thermal expansion of piping needs to be controlled to avoid buckling of the hanger rod.
The main advantage of rigid hangers over Shoe/Trunnion is that it does not provide any frictional resistance for thermal growth. However, it is expensive and it does not dampen the vibration.
Constant (Counter) Weight Hanger:
In this type of support, weight of the piping & contents (bearing load) is balanced by a counter weight. To reduce the size of counter weight, it is often the practice to apply the principles of pulley or lever & fulcrum mechanism as shown in the Figure.
When the piping system moves up / down due to thermal growth, the balancing force on the piping remains constant. Hence this is advantageous over Rigid hanger where lift off is restricted.
E.g Supporting of Radiant Header in Fired Heaters
However, these hangers require the entire support to be larger since either the counterweight should be heavier or the pulley / lever & fulcrum should be larger requiring more space as the bearing load increases.
Hangers / Supports – Shoes
Shoes:
Shoes are commonly used for carrying weight of the piping & contents where the system is insulated to prevent heat loss.
Shoes have the advantage of occupying less space than the rigid hangers. However, they provide friction against the horizontal movement which may be reduced by the use of rollers or Slide Plate Assemblies.
While using shoes, care has to be taken that pipe is not lifting off from rest position due to thermal expansion in which case, supporting function is not achieved at all.
It is recommended to check the local stress at support location for piping system having higher corrosion allowance (>1/8”) or wall thickness lesser than STD wt while using single web shoes.
Hangers / Supports – Trunnions
Trunnions:
Trunnion / Dummy supports are generally used in vertical run as well as when the clearance between the BOP and Grade is higher. Dummy supports are suitable for supporting a piping system from an Elbow
While using dummy supports, care has to be taken that pipe is not lifting off from rest position due to thermal expansion in which case, supporting function is not achieved at all.
It is recommended to check the local stress at support location as well as strength of the dummy pipe.
Hangers / Supports – Flexible Hangers
Flexible Hanger:
When the piping system moves up or down due to thermal expansion, conventional rigid support is not feasible, as it may not take load at all or it may act as a pivot. Under such condition, it is recommended to use flexible support such as spring hangers. These spring hangers sustain the primary load by their resistance against compression while allowing for free thermal expansion.
Flexible hangers are of two types:
1. Variable Spring Hangers
2. Constant Spring Hangers
Variable Spring Hangers:
This is used to support the piping subjected to vertical movement where constant supports are not required. The inherent characteristic of a variable spring hanger is that its supporting force varies with the spring deflection as explained here.
Hangers / Supports – Flexible Hangers – Variability
The pipe weight is balanced by the spring resistance which is equal to the spring rate multiplied by the amount of compression the spring undergoes due to the pipe weight. During thermal expansion, the vertical movement of the pipe tends to reduce the compression on spring, thereby reducing its supporting force on the pipe also. Since the pipe weight is same during any condition, cold or operating, the variation in supporting force results in pipe weight transfer to the adjacent supports and / or equipment nozzles and consequently additional stresses in the system. When variable spring hangers are used, the effect of this variation must be considered. To quantify this variation, a factor called Variability Factor is introduced which is defined as,
Variability = (Hot load – Cold load)/Hot load * 100
Accepted practice is to limit the variability upto 25% for critical system applications. Variable spring hangers are recommended for general use on non-critical piping systems and where vertical movement is of small magnitude on critical systems.
The main advantage due to pre-compression of the spring hangers is that the head room required at installed condition for the spring hanger will be less.
Below arrangement is used when the head room clearance available is more
This arrangement is normally used when the head room clearance available is not higher.
Identical to Type B except that Lug (steel attachment) will take lesser load than that of Type B
Normally used whether Head room clearance available is too less to accommodate can. Load adjustment is done from the top with the help of adjusting nut
Identical to type “D” except that adjustment is done from below using turn buckle but also can be done from above the piping
Designed to support piping from a base below or from the bottom. The base flange being fastened to the floor or to the supporting beams. This is not to be used for excessive horizontal movement.
Made up of two standard spring units welded to the end of a single or pair of channels. Suitable for excess loads or low head room clearance. Each spring is carrying one half of the total load.
Hanger / Support – Constant Effort Springs
Constant effort hangers provide constant supporting force in both cold and operating conditions. When piping stresses and reactions are known to be close to allowable, most economical type of flexible support is constant support hanger. For critical, high temperature piping, at hanger location where the vertical movement of piping is high or the variation in support force is to be controlled within 6%, constant effort hangers are recommended. .
Restraints
Anchor: An anchor is a mechanical connection (welded and/or bolted) between a pipe (or exchanger, etc.) and a structure. The structure must be strong enough so that it cannot bend excessively under large forces. Anchors do not allow the movement or rotation of support points and completely fix the line in its place. Equipment nozzles are also Anchors.
Directional Anchor: DA stops movement parallel to the center-line of the pipe, but permits sideways pipe motion.
Guides: A guide stops sideways movement of a pipe, but permits movement parallel to the pipe’s centerline.
U – Clamps: It is acting as guide / anchor as per the installation. Normally used in small bore piping.
Restraints – Sway Struts
Sway Struts are used to restrain movement of piping in one direction while providing for movement due to thermal expansion or contraction in another direction.
Features:
– Preferred where conventional Guide/DA are not feasible due to non availability of steel structure.
– Preferred when effect of friction is to be minimized in the system.
– Effective under either tensile or compressive force.
– Provides 2” – 4” field adjustment on either side at the time of installation
– Allows angular motion or misalignment of ± 5°.
Vibration Absorbers
Besides thermal expansion, pipe in a plant may suffer from vibrations caused by various sources such as:
1. Mechanical vibrations transmitted through pumps, compressors, turbines and rotating and reciprocating machinery.
2. Impact and vibrations due to the sudden opening / closing of valves.
3. Surging of compressed gas or two phase fluid.
4. Periodic wind forces against the outdoor pipe lines.
5. Earthquakes.
The best and fundamental way to resolve the problems associated with vibrations is to find out the sources of vibration in the particular pipe and then to remove them away or reduce them acceptable levels. There are various kinds of supports in use in the industry for mitigating the effects of vibrations like Sway Braces, Vibration Dampers, Snubbers.
Vibration Absorbers – Snubbers
The requirement of a snubber is to provide little resistance to the movement associated with the thermal expansion of the piping, while providing stiff supporting reaction against the vibratory motion associated with dynamic loading such as earthquake load. Snubber goes to restraint mode based on a particular velocity or acceleration of motion depending on its design concept and is known as activation level.
Typically Snubbers are located where:
1. Large concentrated loads such as valves, large components, flanges and other components that are not anchored.
2. Changes in piping direction
3. Within long vertical risers
4. On long straight pipe runs
Snubbers are basically of two types:
1. Hydraulic Snubber
2. Mechanical Snubber
Vibration Absorbers – Hydraulic Snubbers
Hydraulic snubber consists of a Double acting cylinder with piston assembly (A), Flow control devices (B) & Reservoir (C). The flow control device permits the fluid to flow at a predetermined rate.
During normal thermal growth, (piston movement <= 2 mm/sec), the valves (flow control devices) are kept open by helical coil spring, thereby allowing free flow of fluid from one side of the cylinder to the other side. This term is called “FREE FLOW CONDITION”.
During sudden vibration (piston movement > 2 mm/sec), the fluid pressure on the valve plate which is higher than the spring stiffness closes the valve. Thus fluid flow is stopped and movement is blocked. This term is called “LOCKED CONDITION”.
The Reservoir is fitted with a spring which applies internal pressure inside the cylinder. The positive internal pressure ensures that no air can be drawn into hydraulic circuit during operation.
Features:
Allows angular motion or misalignment of ± 5°.
Limitations:
The unit is not effective against low amplitude, high frequency movement
Vibration Absorbers – Mechanical Snubbers
In general, Snubbers are required to restrain undesirable displacement of piping systems or components when they are about to oscillate due to seismic or other types of dynamic loading, while, to allow their free movement during the thermal displacement mode. Mechanical snubber is designed to provide such essential two – way functions by means of a rotary inertia mass, which inherently generates a resistance force against a quick input movement, in combination with a ball screw assembly, which transforms reciprocal linear motions to rotary motions.
Design principle of Mechanical Snubber is a utilization of an inertia mass, which generates different magnitude of resistance force against the input load in proportion to its acceleration level.
Namely, M α = F (where: M = Mass, α = acceleration, F = force generated)
Low input acceleration generates a negligible level of resistance force allowing the free movement of piping, while high acceleration assumes sufficient resistance force to completely restrain the vibration of piping.
The one end of Mechanical Snubber is connected with a structural member through extension attachments, and the other end is pin-connected to the piping or component. It is also a dominant design feature in Mechanical Snubber that an efficient built – in braking mechanism enhances the function of the inertia mass reducing its size, while developing a clear acceleration threshold in its performance.
1. When dynamic (vibration) or gradual (Thermal) displacements are developed in the piping system or component, they are instantaneously transferred to the snubber unit, and give telescoping motions to be ball nut.
2. The nut, when pushed or pulled, causes rotary motions of the ball screw shaft.
3. The rotary motions of ball screw shaft give integral motions to the inertia mass as it is solidly assembled with the shaft. So far as the input displacement is relatively slow, the inertia mass rotates without assuming any significant inertial force, thus allows the displacement as if the snubber does not exist.
4. When a quick displacement takes place, the mass functions to prevent the screw shaft from rotation because of its inherent inertial force, thus suppressing the dynamic displacement of the system.
Advantage
Mechanical snubbers are preferable over Hydraulic snubbers wherein make up for leakage of oil, periodical change of oil are required.
SWAY BRACE
Application
• Controlling vibration
• Absorbing shock loading
• Guiding or restraining the movement of pipe resulting from thermal expansion
• Bracing a pipeline against sway ·
Selection Criteria
The amount of force needed to control piping should be in proportion to the mass, amplitude of movement, and nature of disturbing forces acting on the pipe.
Working Principle:
The unit comprises a pre – loaded spring, which can be extended or compressed in order to give increasing resistance in both directions. This increase in resistance overcomes the dynamic forces generated by the equipment thereby damping vibrations, opposing sway and absorbing shock forces.
Installation:
Connect coupling to pipe attachment and turn coupling so that spring is compressed in direction opposite to and by approximate amount of piping thermal movement. When correct tension adjustments are completed, the brace exerts no force on the pipe in its operating position.
Final adjustment should be made with the pipe in its hot or operating position. Turn the coupling until both spring plates are in contact with the end plates of the Sway Brace. This is called neutral position.
Under shutdown conditions, the brace allows the pipe to assume its cold position. It exerts a nominal cold strain force equal to the pre-load force plus the amount of travel from the hot to cold position, times the spring scale of the particular size of the Vibration Control and Sway Brace.
Support Symbols
