Flexible Design of Pipeline

This paper introduces the significance of pipeline flexibility design and the factors affecting pipeline flexibility, and discusses the methods of pipeline flexibility design. Several methods to improve pipeline flexibility are proposed. Through the design example, the paper analyzes in detail the skills of improving the flexibility of the pipeline and reducing the thermal load of the pipeline by optimizing the spatial direction of the pipeline, and provides a reference for the design of similar pipelines.

1. Flexible design of pipeline

The flexibility of the pipeline reflects the ease of deformation of the pipeline, indicating the ability of the pipeline to absorb deformation due to thermal expansion, cold contraction and other displacements caused by temperature changes through its own deformation. The purpose of flexible design of pressure piping is to make the piping system flexible enough under various conditions to prevent the following from occurring due to changes in temperature, self-weight, internal pressure and external load of the pipeline or due to the restriction of the piping support and additional displacement of the piping endpoints [1,2]:

• ① Pipeline damage due to excessive pipeline stress or metal fatigue.
• ② Leakage at pipe joint connections (including elbows and tees).
• ③ The thermal expansion and contraction of the pipeline causes excessive thrust or moment, resulting in the force on the equipment orifice exceeding the equipment design standard, affecting the normal operation of the equipment.
• ④ The thrust or moment caused by thermal expansion and contraction of the pipeline is too large to cause damage to the pipeline support or rooted structural members.

1.1 Methods of pipe system flexibility improvement

1.1.1 Optimize the location and type of pipe supports

Different bracket types are different for the thermal expansion constraint of the pipe system. The rigid bracket has infinite stiffness, which limits the displacement in the direction of constraint; the flexible bracket has less stiffness, which has a certain constraint effect on the pipe and allows a certain displacement of the pipe at the same time [3]. Optimizing the type and location of the pipe support hanger to improve the flexibility of the pipeline can only improve the flexibility of the pipeline within a certain range.

1.1.2 Optimize the spatial orientation of the pipeline

In the pipeline arrangement, the equipment arrangement is generally no longer adjusted, so the position of the two ends of the pipe system is also fixed. When the pipeline is too rigid in one direction, increasing the length of the pipeline in its vertical direction can reduce the pipeline stiffness [2]. The pipeline orientation can be optimized by changing the overall pipeline orientation to “L”-shaped pipe section, “U”-shaped pipe section or “Z”-shaped in space, if the plan and space layout allow. If the pipeline elbow is set properly, the pipeline flexibility can be improved even without increasing the total length of pipeline expansion [4].

1.1.3 Selection of pipeline compensator

Pipeline compensator, also known as an expander or expansion joint, expansion joint, is mainly used to compensate for thermal expansion and contraction of the pipeline due to temperature changes. If the pipeline is not completely free to expand or contract when the temperature changes, the pipeline will generate thermal stress. Pipeline compensator is suitable for high temperature, low pressure and large diameter pipelines, but the pipeline compensator is far weaker than the process pipeline body. Therefore, for safety reasons, it is not easy to choose the compensator for high pressure and high risk media pipeline.

2. Design example analysis

2.1 The main design parameters of the pipeline

A reactor outlet to the heat exchanger inlet piping preliminary version of the plane piping diagram is shown in Figure 1.

Figure.1 Preliminary version of the plane piping diagram
Pipe material for 20 # steel, pipe diameter Ф508mm × 22mm, pipe operating temperature of 260 ℃, the design temperature of 280 ℃, the operating pressure of 3.85MPa, the design pressure of 4.15MPa, the initial additional displacement of the relevant equipment orifice is shown in Table 1.

Mouth of pipe	ΔX(mm) Operation/design	ΔY(mm) Operation/design	ΔZ(mm) Operation/design
Reactor outlet (Node number：10)	-3.8/-4.3	-5.0/-6.0	0/0
Heat exchanger inlet (Node number：460)	0/0	3.6/5.0	-3.8/-4.9

2.2 Flexible design of pipeline

According to Table 1 and Figure 1, after detailed modeling and calculation by CAESARII, the comparative deformation diagram of the pipeline in cold state and pipeline operation after warming up is shown in Figure 2 (the light color in the diagram indicates the original condition of the pipeline in cold state, and the dark part is the deformation of the pipeline in warming up operation).
From the conditions given in Table 1, the initial displacement of the reactor outlet downward is relatively large, the initial displacement of the heat exchanger inlet upward is relatively large, if the pipe mouth near the use of rigid bracket is not very suitable (bracket off the air or top of the pipeline equipment expansion of heat), which leads to excessive force on the mouth of the equipment or bracket or root parts of the damage. In these two places, spring supports should be considered to support the pipe, so that the supports can allow a certain vertical displacement of the pipe system while bearing a certain load [3], thus relaxing the restraint and reducing the force on the equipment orifice. Spring brackets were installed at the first elbow of the reactor outlet and on the horizontal pipe near the heat exchanger inlet. After the stress analysis software calculation, the primary and secondary stresses of the pipeline can meet the requirements of the standard specification; the calculated forces at each location are shown in Table 2, and the allowable forces at the equipment orifice are shown in Table 3.

Figure.2 Comparison of deformation of pipeline in cold state and pipeline operation after warming up

Node number	Fx(N) Operation/design	Fy(N) Operation/design	Fz(N) Operation/design	Mx(N.m) Operation/design	My(N.m) Operation/design	Mz(N.m) Operation/design
10	113549/126380	-22308/-22810	47565/52350	67439/75671	-73674/-87836	-135606/-149301
159	0	-16200/-16595	0	0	0	0
310	0	-28274/-27812	0	0	0	0
460	-112469/-125380	16333/16678	-47355/-53650	-59779/-64334	108317/136009	103586/109658

Fx(N)	Fy(N)	Fz(N)	Mx(N·m)	My(N·m)	Mz(N·m)
44100	30625	30625	132300	93100	93100

2.3 Calculation result analysis and optimization adjustment

From the calculation results in Table 2 compared with the allowable orifice forces in Table 3, the reactor outlet and heat exchanger inlet have corresponding forces and moments exceeding the allowable static equipment orifice loads specified in HG/T20645.5.
Figure 1 shows that the length of the pipe in the X direction is longer than 7000 + 1800 = 8800mm, the length of the pipeline Z direction 3500mm, the length of the X direction is much greater than the length of the Z direction, and the pipeline operating temperature at 260 ℃, so the pipeline in the operation of the X direction of thermal expansion displacement is larger, plus the additional displacement of the reactor outlet in the X direction is negative, and the pipeline in the X direction of expansion to Superposition (see Figure 2 in the dark pipe deformation), resulting in the reactor outlet and heat exchanger inlet X-direction thrust are exceeded, the X-direction thrust caused by the Z-direction moment exceeds the standard. See Figure 3, by increasing the length of the pipe perpendicular to the X direction (i.e., Y and Z directions) to change the pipe stress and improve the flexibility of the pipe system, while increasing the number of elbows and the length of the pipe system expansion can also achieve the purpose of improving the flexibility of the pipe system.

Figure.3 Diagram of modified scheme

2.4 Comparison of results and analysis

The adjusted model calculation results (values at design temperature) are shown in Table 4, and the thrust force in X-direction and Z-direction are decreased compared with the orifice force of the equipment in Table 2. However, the two solutions differ greatly in their effectiveness in improving the orifice force, i.e., the flexibility of the pipeline system. Scheme a shown in Fig. 3 has a significant effect. Scheme a reduces the thrust in X and Z directions, and also reduces the moment in Z direction, and when compared with the allowable orifice forces in Table 3, it can be seen that the thrust and moment in each direction are within the allowable range, achieving the purpose of flexible pipeline design. Scheme b shown in Figure 3 also reduces the thrust in the X and Z directions, but the effect is not as good as scheme a, while scheme b is basically ineffective in reducing the moment in the Z direction.

Node number	Fx(N) a/b	Fy(N) a/b	Fz(N) a/b	Mx(N·m) a/b	My(N·m) a/b	Mz(N·m) a/b
10	29616/68523	-10981/-15519	17120/23730	35172/59160	-35269/12537	-61348/-122612
460	-29616/-68523	-5900/-266	-17120/-23730	-29936/-38041	77137/98245	48590/123909

Comparing the spatial orientation of the a and b schemes, it can be found that the total length of the pipe unfolding of the two schemes is basically the same, and the position and type of pipe support are also the same, the difference is that the spatial orientation of the pipes of the two schemes is different. It can be seen that the same number of elbows and pipe length are increased, but the improvement effect on the flexibility of the pipe is very different. When optimizing the spatial orientation of the pipe, increase the length of the pipe away from the line of the fixed point of the pipe, and when increasing the length of the pipe unfolding, try not to increase the length in the direction of the long axis of the line of the fixed point of the pipe, but increase the length in the direction of the short axis, so that the spatial shape of the pipe is closer to the shape of the square, and the flexibility of the pipe with bends is approximately proportional to the cube of the length of the straight section.

3. Conclusion

The reasonable arrangement of the pipeline is essential for the safe operation of the device. In the piping design, corresponding methods can be used to improve the flexibility of the pipe, effectively reduce the heat load and ensure the safety of the pipe and equipment. In the actual design process, the feasibility, safety and economy of various methods should be considered to select the appropriate method to improve the pipeline flexibility.
Author: Hu Yanqiu

Source: China Piping Solutions Provider – Yaang Pipe Industry Co., Limited (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, and Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels, and other industries.)

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