Analysis of Flange Fastening Construction in Saudi Aramco Station Project

With the development of construction technology and process technology in oil and gas chemical industry, flange bolt connection has gradually become the most important connection form because of its convenience, quickness and stability. The pipe fitting connection of Saudi FFP project is only in the form of welding and flange bolt connection, and the valves of 2 inches and above adopt flange bolt connection. Therefore, flange management is particularly important in station construction, which will directly affect the success of production. Based on the construction experience of 7 stations of FFP project in Saudi Arabia, this paper discusses and analyzes the flange fastening construction.

Introduction

Saudi Aramco is the major operator of oil and gas resources in Saudi Arabia, with more than 130 oil and gas fields, and its oil and gas field surface construction and oil and gas pipeline construction market is very broad. With the implementation of “One Belt, One Road” strategy, the market of oil and gas field construction for Chinese enterprises in Saudi Arabia will also expand rapidly.
Flange bolt connection is one of the widely used connection forms in petrochemical industry [1]. Currently, in Saudi Aramco’s station expansion and new construction, flange connection form is preferred instead of welding form, which increases the flexibility of process connection to a certain extent and, at the same time, increases the quantity and difficulty of flange management. In addition, the quality of flange connection construction directly affects the safety of operation, and is one of the most important construction processes in the whole construction.
Flange bolted connections are widely used in Saudi Aramco’s station construction. In the gas transmission station of Saudi Aramco’s Harad project, flange connections are used for valves of 2 inch and above 2 inch. In Saudi Arabia FFP project, for example, the seven remoteheader stations in the south involve 14 types of flange sizes from 2 inches to 48 inches, 58 types of bolts, nearly 5,000 sets of flange fastening and 45,000 bolts of various sizes. Therefore, flange management is particularly important in station construction, which will directly affect the success of commissioning.

Overview

Flange is mainly used for the connection between pipe ends or equipment, and two flange pieces are connected with gaskets. Flange management refers to the general term for activities such as storage, welding assembly, bolt tightening, site inspection, tagging and data preparation of flanges, and the whole process management within the scope of prefabrication, installation and construction, commissioning and operation phases [2].
At present, there are two main international standards for pipe flanges, namely the European pipe flange system represented by DIN (including the former Soviet Union) in Germany and the American pipe flange system represented by ANSI/ASME pipe flanges in the United States. In addition, JIS pipe flanges are also available in Japan. The station construction and design of Saudi Aramco mainly adopts the American ASME standard (ASME B16.5 and ASME B16.47).
Flange fastening (bolt fastening) is the use of suitable tools, the use of bolt tension will be flange and gasket surface together, so as to play a sealing role, the pipe or pipe fittings connected, the common methods are torque fastening method and tension fastening method. This project adopts the torque fastening method, and the main fastening tools include conventional wrench, torque wrench and hydraulic torque wrench.

Determination of torque

Tension of the bolt

1) Definition of bolt tension
The steel used to manufacture the bolt and nut has an inherent “elasticity”, i.e. it can be stretched between two points, while the tension applied to the bolt acts as a clamping force to keep the bolt assembly together without exceeding its “yield strength limit”.
By twisting the nut, a holding force is generated between the two nuts and the member, and according to the physical principle of equal action and reaction forces, it is known that the bolt is subjected to a tensile force equal to the holding force, and the tension we often refer to is the tensile force on the bolt rod (see Figure 1).
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Figure1. Diagram of tension
2) Determination of tension
In order to make the bolt just fasten the fastener (clamping) together, without loosening, the bolt must have the ability to withstand sufficient tension. According to Hooke’s law, the bolt has a certain modulus of elasticity E, and the stress σ and strain ε are proportional: σ = Εε.
Since the tension F and stress σ are proportional, the tension F must not cause the stress σ of the bolt to exceed the yield limit, and considering the safety factor and experience, the tension is generally taken to be 40% to 70% of the elastic limit.
3) Hazards of unreasonable tension
Tension is too small: A threaded fastener that is not tight enough may loosen and separate, resulting in “shear” between the mating parts, which may cut the bolt in half. Loose fasteners may also cause further mechanical loosening of surrounding mechanical parts, resulting in unwanted vibration and wear.
Too much tension: When tightened too much, the fastener may damage the bolted component, causing the bolt and/or nut to deform, resulting in the fastener exceeding its elastic limit and losing tension.

Torque of the bolt

1) Definition of bolt torque
According to the physical definition, torque is the tendency of an acting force to rotate an object around a rotation axis or pivot point, and its magnitude is equal to the product of the force and the force arm: M = F x L.
In the case of a bolt, torque is defined as the rotational or torsional force applied to the nut or bolt head, and is the product of two measurements (i.e., force and distance) (see Figure 2).
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Figure2. Diagram of torque
2) How to apply torque
The two most common methods of applying torque to a threaded fastener are
a. Using a manual or electric type torque wrench to apply torque to a threaded fastener (see Figure 3).
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Figure3. Diagram of torque applying
b. The hydraulic tensioning device of a bolt tensioner applies direct tension to the fastener (see Figure 4).
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Figure4. Diagram of tensioner applying
3) Principles of torque determination
The torque of the bolt mainly depends on two aspects, one is the yield strength limit of the bolt itself, in general the design torque value of the bolt only reaches 40% to 70% of the strength limit; on the other hand, the bearing capacity of the gasket, too much torque will make the gasket broken, too little torque will make the residual stress of the gasket after compression not reach the sealing ratio pressure, resulting in the leakage of the sealing surface [1]. Therefore, the torque of the bolt is the most important issue in the construction process.
According to the construction experience and construction specification, the torque of high-strength bolts used in the process installation of this project should meet the requirements of Table 1.
Table1. The list of high-strength bolt torque

Flange diameter

Grade

Number / size of flange bolts per group

Torque value

(inch)

(class)

个/inch

(lbs .ft)

2

1500

8 × 7/8″

243

4

900

8 × 1 1/8″

480

1500

8 × 1 1/4″

740

6

900

12 × 1 1/8″

480

1500

12 × 1 1/3″

1000

8

900

12 × 1 3/8″

900

10

900

16 × 1 3/8″

900

12

900

20 × 1 3/8″

900

16

900

20 × 1 5/8″

1680

20

900

20 × 2″

3200

24

900

20 × 2 1/2″

5734

30

900

20 × 3″

8880

36

900

20 × 3 1/2″

14,220

48

900

24 × 4″

21,330

Tool selection

At present, the tools commonly used in flange fastening construction are basically divided into 3 categories.

  • 1) Ordinary wrenches.
  • 2) Torque wrenches (divided into manual torque wrenches and hydraulic torque wrenches).
  • 3) Controlled hydraulic bolt tensioner.

The design pressure of process pipelines in pipeline stations is generally high, the strength of flanges and bolts is relatively high, and the requirements for torque are relatively strict, so the use of ordinary wrenches cannot meet the construction requirements, while the controlled hydraulic bolt tensioner is less efficient, and the construction is limited by the length of the screw, therefore, torque wrenches are generally used for bolt tightening.
According to the experience of hydraulic test pressure and test run, as well as the requirements of bolt torque value, the general principles of torque wrench selection are as follows.

  • 1) When the bolt size is not larger than 1 inch, a manual torque wrench can be used for bolt tightening.
  • 2) When the bolt size is between 1 inch and 13/8 inches, a manual torque wrench can be used for the first two tightening and a hydraulic wrench for the last two.
  • 3) When the bolt size is larger than 13/8 inches and less than 4 inches, the hydraulic wrench must be used for tightening.

Flange fastening construction steps

Preparation for flange fastening construction

  • 1) Before the flange fastening, it is necessary to check the flange and pipe type, batch number, material and grade carefully to ensure that the site installation meets the requirements of drawings and design.
  • 2) Pay attention to distinguish the torque value that different gaskets can withstand, the torque value of alloy steel gaskets and insulating gaskets are very different, prevent the torque value is too large and damage the gasket, resulting in seal failure.
  • 3) Check that the flange surface and gasket are free of impurities such as oil and water, and that the gasket is free of scratches, grooves, burrs and deformations.
  • 4) Check the grade and size of the bolts for damage, corrosion, rust, burrs, and suitability and compatibility.
  • 5) Check the thread engagement of the bolts, and do not use repaired bolts.

Selection of lubricant

During the construction process, it is usually necessary to apply lubricant (molybdenum disulfide) to the bolts and nuts to reduce the coefficient of friction. The choice of lubricant brand directly affects the size of the torque, and also affects the quality and work efficiency of the bolt tightening. In Saudi Aramco project, the bolts with anticorrosive paint are used. According to the construction experience, when the bolt diameter is less than 2 inches, the bolts with anticorrosive paint do not need to be coated with lubricant, and the friction coefficient is calculated by 0.11 when the torque is calculated, and when the bolt diameter is greater than 2 inches, auxiliary lubricant is needed for tightening.

Installation of spacers and bolts

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Figure5. The deviation of flange in stallation

Bolt tightening

In order to ensure that the entire flange surface is evenly stressed during the fastening process, the “cross line” method is generally used to fasten the flange diagonally (see Figure 6).
The tightening process does not allow a single tightening in place, but needs to be carried out in stages [2].

  • First pass: application of 30% torque force.
  • Second pass: application of 60% of the torque force.
  • Third pass: application of 100% torque force.
  • Fourth time: Using 100% torque, tighten all bolts one by one in a clockwise direction to verify that the required torque is applied to each bolt uniformly and that the deviation of the final torque value from the set torque value must be less than 5%.

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Figure6. The sequence of flange tightening

Recording and marking

After the bolts are tightened, they should be recorded and marked in time.

  • 1) According to the quality record form, carefully fill in the date, person, torque value and other information of flange tightening.
  • 2) Use “10” mark or “1” mark on each bolt.
  • 3) Quality inspection personnel on site will review and check the flange surface, mark the date of inspection, whether it is qualified, and sign.

Common quality problems

In the flange fastening construction, there are mainly the following common quality problems.
1) The flange surface is not parallel and the stress is too large
An important cause of the problem is the large dimensional deviation of the process piping installation, resulting in excessive clearance or non-parallelism of the two flange faces. Therefore, before process piping installation, the dimensions should be carefully checked, and when flanges are welded to the process pipe, avoid excessive welding deformation.
2) Bolt tightening is not carried out in the prescribed order, resulting in an overall unbalanced force on the flange surface.
The cause of the problem is that the operator is not trained properly. Therefore, when the flange is tightened, training should be conducted first, and all personnel must be licensed to work, and at the same time, the process supervision should be increased.
3) Failure to apply lubricant as required, or lubricant does not meet the requirements, resulting in damage to the threads of the bolts.
The solution to this problem is mainly to purchase qualified lubricant in time and train the field personnel, especially for bolts over 2 inches, which must be coated with lubricant according to the regulations.
4) The tightening torque is too large or too small
The main reason for this problem is that the hydraulic wrench leaks oil or the readout is not allowed and needs to be recalibrated. Therefore, the tightening tool must be calibrated in time within the calibration cycle, and if oil leakage is found, timely maintenance is carried out, and “sick” use is strictly prohibited.

Summary

With the development of construction technology and process technology in the oil and gas chemical industry, more and more construction and production activities using flange connection form, the flange connection construction, the most core construction steps is the flange fastening. Therefore, understanding the principle of flange bolt torque, choosing the right bolt, determining the torque and selecting the right fastening tool can extend the process life, reduce the risk of leakage, improve construction efficiency and maximize cost savings.

Authors: Liang Liguo *, Bian Chen, Yang Ming, Jing Guofang

Source: China Flanges Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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, 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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References:

  • [1] Meng Fanliang. Calculation of tightening torque of high pressure flange bolts [J]. Petrochemical Equipment,2014(z1):18-19.
  • [2] Ma ZJ, Yang Q, Wu Xue. An introduction to quality control of foreign flange management[J]. Petrochemical design,2017,34(2):59-61.

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