Questions about flange bolt torque – Do you really dare to use such a low torque for small bolts in practice?
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I would like to ask those of you in the petrochemical industry based in Sichuan: in practical operations, when working with flanges of small diameter, such as those under 10”, are torque wrenches used, and is it necessary to operate according to the torque values specified in the torque tables? What’s the effect of using torque? The reason for raising this topic is that after reviewing numerous manufacturer documents and the official instructions for so-called bolts, it was found that the torque values specified there are actually very low. As shown in the figure below, the operating pressure of my equipment is 2.7 Mpa; yet the torque specified for bolts of the M36 size is only 249 Nm. What does 249 Nm mean? It’s a value that can be achieved even by twisting with just one hand using a lever; if a hammer is used, the torque generated will be at least 500 Nm to 800 Nm. So, would you dare to apply such a tightening force as specified in the equipment diagrams? Of course, this opinion on the torque value is just my personal one; those who strictly follow the values recommended by the manufacturer when operating the equipment are also welcome to share their views. After all, the above guesses are merely my personal opinions; it’s also possible that with strict implementation, the flange will experience even stress distribution, resulting in an unexpectedly good sealing effect.
1. It is not clear here the form of the flange, the grade of the bolt material, and other related factors; therefore, it is not appropriate to evaluate whether the original design values are suitable or not. 2. Determining the bolt installation torque is related to the number of bolts – for example, in the case of a specially designed flange that exceeds standard specifications, the load required per bolt can be reduced. 3. The bolt installation torque is also dependent on the material of the bolts; lower material grades result in a lower allowable load for those bolts. 4. For standard flanges, assuming a 8.8 grade material and a lubrication coefficient of 2, the torque required for an M36 bolt is around 2500 Nm
Looking at these flange parameters, they correspond to standard flanges. The term 900# is the way in which the ASME standard is expressed. Nowadays, even the lowest-quality bolts used by manufacturers are made of 35Cr material, and such bolts have a strength grade of 5.5. Below is the data sheet from another heat exchanger manufacturer; it’s hard to understand why the torque values are so low. Is it because the quality of our bolts is too poor, or is it because the quality of the gaskets is too good?
1. After reviewing the information on the heat exchanger and performing an estimation, it can be concluded that the designed installation torque value is reasonable; this is because there are a large number of bolts, which distributes the force to be borne over each bolt, thereby reducing the force per bolt. 2. The torque values I provided in my previous response are those recommended for pipe flanges. These values assume the following conditions: 2.1 The material grade is high, at 8.8 grade; 2.3 The stress on the bolts reaches 50% of their yield limit. Slightly higher loads are suitable for environments with higher temperatures, as this helps to compensate for stress relaxation caused by temperature fluctuations. 3. Generally, considering the internal pressure load and gasket pre-tension, the bolt stress is in the range of 15–30% of the yield limit. 3.1 Assuming a material grade of 5.5, a yield strength of 50%, and a lubrication coefficient of 0.2, the torque required for an M36 bolt is approximately 880 Nm. 3.2 Assuming a material grade of 5.5, a yield strength of 20%, and a lubrication coefficient of 0.2, the torque required for an M36 bolt is approximately 350 Nm. 4 For the heat exchanger example, assuming a working pressure of 2.5, an inner diameter of 400 for the gasket, spiral-wound gaskets, a working temperature of 250 degrees, 48 M20 bolts, and a sealing pressure ratio of 69, then: 4.1. The torque required for the bolts is around 130 Nm. 4.2. The stress on the bolts is around 30% of their yield strength. 4.3. These values are roughly on the same order of magnitude as the design requirements specified. The factors that determine torque are numerous, and calculations are used to guide installation; thus, the traditional method of using hammers to apply force needs to be replaced. The above views are for reference only