Pipe support

Introduction:

• Piping systems -Major Part of any hydrocarbon industry.
• Pipes as irregular space frames –usually not self supporting.
• Proper pipe support knowledge during layout stage is advantageous.
• Loads to be transmitted from pipe to supporting structures with the help of pipe supports.

Why Pipe supports are necessary ?:

To prevent…..

• Pipe stresses in excess to code allowable limits.
• Leakages in flanged joints etc.
• Excessive Line Vibrations.
• Undesirable effects of Seismic loading.
• Unintentional disengagement (lift off) of piping from it’s supports.
• Excessive pipe sag (Normally more than 10 mm for process piping and 2.5 mm for power piping).
• Exposure of elements to temperature extremes, outside their design limit.
• Undesirable line movements to protect sensitive equipment against overloading.
• Excessive loading in support itself
• To limit thermal expansion

Codes and Standards on pipe supports:

• MSS-SP-58- Establishes the material, design, and inspection criteria to be used in the manufacturing of standard pipe supports. (USA)
• MSS-SP-69- Provides recommendations for the selection and application of pipe supports. (USA)
• MSS-SP-89- Provides recommendations for the fabrication and installation of pipe supports. (USA)
• BS-3974- Specification of pipe supports 1, 2, 3. (UK)
• VGB-R-510 L- Standard supports guidelines. (Germany)
• RCC-M- Specifications for pipe supports. (France)
• MITI 501- Technical regulations (Japan)

Design and selection of pipe supports:

• The complex requirement of today’s support elements are, reliable functioning, maintenance free operation, economic and easy installations, quick delivery of components and low unit prices.
• Major Criteria governing support hardware are Support function, Magnitude of expected load, space limitations, design temp of piping system, expansion effects of piping systems, piping attachment and supporting structure material compatibility, piping insulation, suitability to environment, ease of operation, inspection and installation.

Layout considerations for Supports:

• Group pipe lines so as to minimize the number of structures needed solely to pipe supports.
• Route lines close to the possible point of supports ( i.e. grade or structure which is provided for other purposes.)
• Supports or braces to be located at or near neutral pts. (thermal null pts)
• Supports to be located as near as possible to concentrated loads such as valves, flanges, heavy actuators etc.
• Piping susceptible to vibration such as compressor connected lines to be supported independently. The use of hold down or similar supports offering resistance to motion and providing some damping capacity to be used rather than hanging type supports.
• Piping connected to top of vessel to be advantageously supported from the vessel to minimize relative movement between supports and piping.
• Always maintain the distance between supports as per project specification recommended support span table. ( it is applicable to straight run pipe length only.) When change of direction in a horizontal plane occur, it is suggested that the spacing be limited to ¾ times the standard pipe span.
• Sufficient space to be provided to facilitate support assembly installation, inspection and maintenance.

General support terminology and basic function:

• Restraint- Any device that prevents, resists, or limits the free thermal movement of piping.
• Brace- A device primarily intended to resist displacement of piping due to forces other than thermal expansion and gravity.
• Anchor- A rigid restraint providing substantially full fixation.
• Stop- A device which permits rotation but prevents translatory movements of piping.
• Guide- A device which prevents the rotation about one or more axis.
• Hold Down Support- A device which holds the pipe in position disallowing vertical upwards movement or allows only decided upward movement.
• Hanger- A support by which piping is suspended from a structure which functions by carrying piping load in tension.
• Resilient support- A support which includes one or more largely elastic members to carry pipe sustain + thermal loads at the same time allowing pipe thermal movement in the desired direction.
• Sliding support- A device which is provided below piping to take gravity loads, offering no resistance other than frictional to horizontal motion.
• Rigid support- A support providing stiffness in at least one direction.
• Damping element- A device which increases damping of a system offering high resistance against rapid displacement, caused by dynamic loading while permitting essentially free movement.

Types of Supports:

• Hanger Supports – 1) Variable Hanger 2) Constant Hanger 3) Rigid Hanger.
• Dynamically Loaded Supports – 1) Struts 2) Snubbers 3) Sway Brace 4) Energy absorbers 5) Pipe Clamps 6) Pipe whip / Hold down restraints.
• Pipe Bearing Components – 1) Pipe Saddle 2) Pipe Shoe 3) Pipe Trunnion 4) Wear Pad.
• Threaded Members – 1) welding nut 2) welded beam attachment 3) Clevis 4) Turn buckle 5) Tie rod 6) Stud bolt, nut, locknut, spring washers etc.
• Slide Bearing Plates – Teflon, Stainless steel, graphite.
• Pipe Supports for Cold Service.
• Depending on Support Position-Primary Support and Secondary Support

Pipe Support Manufacturers:

• Piping Technology and Products Ltd.
• Carpenter and Paterson Ltd.
• Lisega ltd.
• Binder Group Ltd.
• Pipe Support Group Ltd.
• Sarathi Engg Ent Pvt Ltd.
• Anvil Group etc to name a few

Support Engineering:

Minimum Data required to start supporting:

1. Piping GAD
2. Electrical and Inst cable trench/trays layouts
3. Civil and Structural drawings
4. Piping spec and line list
5. Insulation spec
6. Valve’s weights
7. Equipment connection displacements
8. Stress recommendations (Stress isometrics) and Support loads.

Pipe Span :

Typically 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.

Basis for calculation of Maximum support span:

There are three main factors which affects the support span.

• Stress
• Deflection/ sagging and
• Frequency of piping system (for two phase flow lines, reciprocating equipment connected lines, vibrating lines etc.)

Guidelines for Typical Piping System:

1. Supporting of Stress Critical Lines:

• Criteria for critical lines
• Support location by Designers
• Support type by stress engineer
• Primary attachments and secondary supports by designers
• Line stop/Guide gaps to be taken care

ii. Supporting of non critical lines:

• Sr. designer to decide support type
• Support span
• Guide span
• Concentrated loads e.g. valves, inst
• Long piping leg, stress engineer to be consulted

iii. Supporting of Insulated Pipes:

• No direct resting, pipe shoe to be provided
• Min. clearance between the insulation and the supporting structure shall be at least 50 mm.

iv. Supporting of Non- Insulated Pipes:

Directly rested except following

• Pipes with sizes larger than DN 600
• CS pipes with less than SCH 20
• SS pipes with less than SCH 10S
• The pipe require a slope
• Dissimilar material to avoid galvanic corrosion
• Pipe to be supported on pipe shoe to avoid damaging to pipe wall

v. Vertical pipes:

• Standard span chart does not apply
• Supports to be located on the upper half of the portion (i.e. above C.G. of pipe)
• Vertical guide spacing
• Clamped supports with weld-on shear lugs to avoid the pipe slipping under the clamp

Basically codes do not take account of local stresses in pipe support not like pressure containment parts. It is because normally the structural members are analyzed using beam type elements. The constant stress is special and the buckling shall be another design consideration to take care.

For example, ASME B31.3 requires stresses for pipe supporting elements in the following:

321.1.3 Stresses for Pipe Supporting Elements.

Allowable stresses for materials used for pipe supporting elements, except springs, shall be in accordance with para. 302.3.1. Longitudinal weld joint factors, Ej, however, need not be applied to the allowable stresses for welded piping components which are to be used for pipe supporting elements.

302.3.1 General. The allowable stresses defined in paras. 302.3.1(a), (b), and (c) shall be used in design calculations unless modified by other provisions of this Code.

...

(b) Shear and Bearing. Allowable stresses in shear shall be 0.80 times the basic allowable stress in tension tabulated in Table A-1 or A-2. Allowable stress in bearing shall be 1.60 times that value.

The detailed analyses using FEM performed to categorize clamp induce piping loads as primary, secondary and peak in nature, and the due considerations shall be paid to allowable stresses in accordance with the corresponding failure modes regarding pressure components and supporting devices. If bolts and nut are modeled with threads, the maximum stress is easily beyond yield at first thread when fastened. But this phenomena is allowed if the integrity is guaranteed as a whole.

In addition, the experimental performance check validates the integrity as well.

Please consult hanger manufactures what they think about the best practice for their support design in standing point of safety and cost.

Related information

WRC Bulletin No 353 - Position Paper on Nuclear Plant Pipe Supports

Pipe hanger manufactures

http://www.thomasnet.com/products/pipe-hangers-36890200-1.html

http://www.amfibi.com/us/7085-pipe_hangers_manufacturers

http://dir.indiamart.com/impcat/pipe-hangers.html

http://www.alibaba.com/showroom/pipe-hanger.html

MSS SP-58 Pipe Hangers and Supports - Materials, Design, Manufacture, Selection, Application, and Installation (ANSI-approved American National Standard)

MSS SP-69 Pipe e Hangers and Supports – Selection and Application (ANSI-approved American National Standard)

MSS SP-77 Guidelines for Pipe Support Contractual Relationships and Responsibilities of the Pipe Hanger Contractor with the Purchaser's Engineer or the Pipe Fabricator and/or Erector

MSS SP-89 Pipe Hangers and Supports - Fabrication and Installation Practices

MSS SP-90 Guidelines on Terminology for Pipe Hangers and Supports

MSS SP-127 Bracing for Piping Systems Seismic-Wind-Dynamic Design, Selection, Application

Design practice - Bare pipe on beam

Kellogg formula, beam on elastic foundation,

(2/3)S(d+b)(t^2)

Allowable span L (feet) = ----------------------

(0.644)W(R/t)^0.5

S - Hot Basic Allow. Stress (psi)

d - support beam width, inches

b - bearing width, 1.5% of pipe OD, inches

t - corroded wall thickness, plus any repad, inches

W - unit weight of pipe and contents, lb/ft

R - pipe radius, OD/2, inches

This limits the local stress due to bearing to 67% of S(hot)

Peng book, chapter 5.6, where the circumferential membrane stress in pipe is taken as

S=0.643*f*[sqrt(R)]/[t^1.5]. There "f" is the load per unit length of circumference,

f =W*L/(d+b)

So, S=0.643*[W*L/(d+b)]*[(R/t)^0.5]/t

Limiting stress to 67% of S_hot (a safety coefficient of 1.5)

Normally pad should be provided for nominal diameter larger than 10".

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