Deflection – What Is Piping

Pipe Support Span is defined as the optimum distance between two supports so as to avoid excessive stress, sagging, or failure of the piping or pipeline system in extreme cases. We all know that while routing aboveground piping or pipeline from one part or equipment to another we have to support the pipe at some definite spans. A properly designed pipe support span helps the piping design personnel to support pipes at regular spacings, thus reducing his work for unnecessary calculations. Pipe Support Span is also known as Pipe Support Spacing. Refer to Fig. 1 that defines the pipe support span for a pipeline system. Factors on Which Pipe Support Span Depends There are various factors that influence the pipe support span. Pipe support spacing varies with changes in Pipe Material: Pipe Support spacing varies with pipe material, For non-metallic pipes, the support span is lower than metallic pipes of the same size. Even Stainless Steel pipes have lower pipe support spacing as compared to Carbon steel pipes. Nominal Diameter of Pipe & Schedule: With the increase in pipe diameter and schedule, pipe support span increases. That is the reason you will find that a 10-inch pipe has more support span as compared to a 4-inch pipe support spacing. Type of Fluid Service: Piping support span varies with fluid service; Pipes carrying liquid service have less support span as compared to pipes carrying gaseous fluids. This means with an increase in density of the flow medium pipe support spacing decreases. Type and Thickness of Insulation Material: With an increase in thickness and density of the insulation material, pipe support spacing reduces. An increase in insulation density and thickness imposes more load on the parent pipe which needs to be supported by increasing the number of supports which means pipe support span reduces. Piping Configuration (Straight pipe and Pipe with elbows): Pipe support spacing is dependent on the piping routing or geometry. A straight pipe has more support span as compared to pipes with directional changes. Because of this reason, to find out the span for piping including elbows, the straight pipe span is multiplied by a factor as shown in Fig. 2. Locations of Valves and Rigid Bodies: The presence of rigid bodies in a piping or pipeline system reduces the pipe support span. It is a general engineering practice is to provide at least one support near to the rigid-bodies like valves (Preferable to provide support on both sides of the valve). Structural Availability for support: Available structures are normally used for supporting the pipe. So pipe span chart may be reduced in those places. Also, an increase in the number of supports distributes the piping loads on supports and increases the piping stiffness. So, if a structure is available, pipe supports are usually taken from those structures. Vibrating or Pulsating lines: For vibrating or pulsating lines pipe support span is reduced to avoid vibration tendency and to increase the natural frequency of the piping system. A reduction in pipe support spacing increases the system rigidity which reduces the tendency of pipe vibration. Fluid Temperature: With an increase in fluid temperature as allowable stress value reduces, the pipe is supported nearby position and thus reducing the pipe support spacing. Fig. 1: Figure showing pipe support span Deciding Pipe Support Span Pipe Support Span Length Depends On- Bending StressDeflectionIndentationAllowable LoadsVibration Possibility and Natural Frequency of the piping system Bending Stress Bending is caused mainly due to two reasons: Uniform Weight LoadConcentrated Weight Load Uniform Weight Load Own Weight Of PipeInsulation WeightWeight of Fluid During operationWeight of hydrostatic fluid During Hydro Test Concentrated Load Weight Of Valve, Flanges,Strainer, specialty items, inline items, etc. Deflection Deflection (Δ) is defined as a relative displacement of the point from its original position. The basic piping practice to limit pipe deflection between supports to 1” or 1/2 the nominal pipe diameter, whichever is the smaller.The most important reason for limiting deflection is to make the pipe stiff enough, that is , of high enough natural frequency, to avoid large amplitude response under any slight perturbing force. As a rough rule, for average piping, a natural frequency of 4 cycles per second will be found satisfactory. The natural frequency can be calculated by Indentation Where, t=corroded Thickness of pipe Wall(mm)S=0.67Sh(N/mm^2)R=Radius of pipe (mm)d=Bearing Length(mm)b=Bearing width(mm) Allowable Load at Support Where, Pa=Allowable Load at the Support pointt=effective local thickness (pipe wall +Reinforced Pad If Any)R=outer radius of Pipeb=Bearing length of pipe (along the axis) on the support structure IF THE ACTUAL LOAD AT SUPPORT IS GREATER THAN THE ALLOWABLE LOAD GIVEN BY THE ABOVE FORMULA, A REINFORCEMENT PAD WILL BE REQUIRED. Normally project-specific Support Span is provided in tabular format for straight pipes that are known as "Pipe Support Span Chart". But for elbows or turns, the span is to be reduced by a factor as shown in the below-attached figure (Fig. 2). Readymade support spans for specific pipe diameter and thickness are available in the MSS code. For Shell group of companies' support span is provided in DEP in tabular format. Fig. 2: Factor to reduce support span depending on layout. Pipe Support Span Chart A pipe support span chart is a tabular chart giving a rough idea of supporting distance. These charts are normally mentioned in piping stress analysis project specifications. In the following image (Fig. 3) pipe support span chart from MSS SP-69 is reproduced as a sample. Fig. 3: Sample Piping Support Span Chart (Reference: MSS SP-69) Few more resources for you… Supporting of Piping Systems: Few Guidelines Supporting of Dual Insulated Piping System Purpose of Pipe Supports: A small article for beginners Piping Stress Analysis Piping Design and layout