Flange joint sealing—Installation bolt load and its determination principles

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In accordance with the design codes for pressure vessels or pipelines, it is possible to determine the bolt load required to ensure the strength and safety of the flange joint; this load is known as the \"design bolt load.\" However, for various reasons, if the flange joint is installed using this bolt load, it may not meet the designed sealing requirements, and leakage could occur even prematurely. Therefore, the amount of preload required to install the flange joint on-site (this load is referred to as the “installation bolt load”) becomes one of the key factors determining whether an effective seal can be achieved. This lecture will answer the following two questions: What is the “installation bolt load”? What are the principles for determining the “installation bolt load”? (1) What is the “installation bolt load”? As discussed in Lecture 30, in the flange design specified for pressure vessel or pipeline design, the two design parameters m and y are used. The bolt loads under tension and compression conditions are calculated based on the minimum pre-tension stress of the gasket (y) and the minimum operating stress of the gasket (mp), and the maximum value among these two calculations is taken to determine the required minimum total cross-sectional area of the bolts. The actual cross-sectional area of the bolts selected must be at least this value, thereby allowing the determination of the required bolt size and quantity. Based on this, the bolt load necessary to verify the strength of the flange itself can be determined; this load is referred to as the “design bolt load”. Device users and installation units often treat the designed bolt load as the preload bolt load for installation, whereas the actual installation bolt load (i.e., the “installation bolt load”) is greater than the designed bolt load. This is because, first of all, the installed pressure-bearing vessels or pipe flanges need to pass a pressure test, and the typical hydrostatic test pressure is 1.25 or 1.5 times the design pressure; therefore, the bolts must be tightened to meet the sealing requirements of the hydrostatic test, which results in the bolt load being higher than the design bolt load. Secondly, the standardization of flange design does not take into account every component of the flange system, including bolt loosening, flange deformation, and gasket creep relaxation. All these factors result in the actual operating stress on the gasket being lower than the designed operating stress. Furthermore, in the actual installation of flange joints, factors such as the elastic interaction of bolts, inaccuracies in flange assembly, and different installation tools and methods – not to mention the fact that torque control is generally not used – can also result in the loading on the installation bolts being lower than the designed value. Clearly, whether the loading on the installation bolts is too low or too high, it leads to either too low or too high stress levels, as well as uneven stress distribution in the gaskets, which has adverse effects on the sealing performance of the flange joint. It also affects issues such as bolt corrosion, vibration, fatigue, and nut loosening. Whether it is a non-standard flange designed in accordance with specifications or a standard flange designed without following such specifications, no values for the installation bolt loads are provided, to distinguish them from the design bolt loads resulting from flange design per specifications. Surveys show that 85% of flange joint leaks are caused by insufficient loading of the mounting bolts, as shown in Figure 1. Therefore, in order to meet the requirements for the tightness of bolted flange joints, and particularly to ensure the necessary preload on the bolts to achieve adequate sealing, that is, to determine the bolt installation load, this becomes one of the key issues for ensuring effective sealing and leak control in bolted flange joints. (2) Principles for determining the \"installation bolt load\" As mentioned above, the flange code design does not provide a method for calculating the installation bolt load. Therefore, in engineering practice, when installing flange joints, 100 failed gaskets are randomly selected for cause analysis as shown in Figure 1. The installation bolt load is determined based on the experience of those who use the gaskets or the recommendations provided by the gasket manufacturers; a certain percentage of the yield strength of the selected bolt material (at the installation temperature) is taken as the bolt stress during installation (also known as the “target bolt stress”). The bolt load required for installation is then calculated by multiplying this target bolt stress by the cross-sectional area of the bolt shank – this is the most direct method for determining the installation bolt load. The basic principle of this method is that the bolt load must be used to pre-tighten the gaskets to an adequate stress level; during operation, sufficient stress should remain in the gaskets to ensure that the flange joint meets the desired degree of tightness. At the same time, it is necessary to prevent any possible mechanical damage to the flanges, bolts, and gaskets under conditions such as installation and operation. The main objective of this method is to determine the minimum mounting bolt load and the maximum mounting bolt load. The minimum installation bolt load should not be set too low; firstly, it is necessary to ensure that there is sufficient sealing stress on the gaskets during operation, enough to compensate for any relaxation in the actual pre-tightening bolt load caused by vibrations, shocks, pressure and temperature cycles, as well as errors in tightening tools and methods. On the other hand, the load on the mounting bolts should not be too low; otherwise, it will result in inaccurate actual bolt loads due to the torque applied when tightening the bolts, and it will also have an adverse effect on the fatigue life of the bolts. Moreover, the maximum loading on the mounting bolts should not be set too high, as this could cause mechanical damage to the flanges, bolts, and gaskets, as well as increase susceptibility to stress corrosion cracking. Therefore, the main control principles are as follows: a. For bolts, the minimum bolt installation load should be sufficient to compensate for the relaxation loss of the flange joint; that is, the stress in the installed bolts at the minimum installation load should be at least between 20% and 40% of the bolt’s yield strength (at the installation temperature) ； The stress in the mounting bolts under the maximum mounting bolt load (also referred to as the \"target stress for bolt mounting\") should be as high as possible; it is usually 40% to 70% of the yield strength of the bolt material at the installation temperature, but not exceeding 90% of the yield strength of the bolt material. b. For gaskets, the minimum mounting bolt load ensures that the operating stress on the gasket is greater than its minimum operating stress ； The gasket stress under the maximum installation bolt load shall not exceed the maximum crushing stress of the gasket. The minimum operating stress and maximum crushing stress of gaskets shall be determined by standard test methods, such as EN13555. c. For flanges, the internal (installation) stress in the flange under the maximum bolt installation load shall not exceed the maximum allowable stress of the flange material nor the allowable rotation angle of the flange; the flange installation stress and rotation angle are to be estimated and safety assessed using relevant calculation methods. For the specific methods for determining the load on mounting bolts, readers may refer to GB/T 38343-2019 \"Technical Specifications for Flange Joint Installation\".