How is tower deflection controlled?

Thread Content

I. Hazards of excessive deflection in towers 1. When the ratio of the tower’s height to its inner diameter is large and the wall thickness is thin, wind loads can cause excessive deflection at the top of the tower. In the case of sieve tray towers, this leads to significant tilting of the trays, resulting in uneven liquid layers on these trays and uneven gas-liquid mass transfer; as a consequence, the efficiency of the trays decreases, affecting the quality of the product； 2. Excessive swaying of the tower causes additional tensile and bending forces at the flanges connecting to the tower body, leading to leakage ； 3. When the length-to-diameter ratio of the tower is large, wind loads may induce resonance in the tower, leading to fatigue failure ； II. Control of tower deflection: The deflection at the top of the tower is inversely proportional to the equivalent diameter of the tower body and its wall thickness. Therefore, to reduce the deflection of the tower, it is possible to increase the diameter of the tower body or its wall thickness. During the design process, keeping the deflection at the top of the tower low is beneficial for the operation and safety of the tower. However, with the same height and diameter, increasing the overall stiffness necessarily means increasing the thickness of the tower body. For tall and slender low-pressure towers, the thickness of the cylinder may increase significantly, which not only raises the cost of investment but also introduces various problems related to manufacturing and inspection. In engineering design, it is necessary to control the deflection of towers at a reasonable level, in order to ensure their proper operation and safety while keeping investment and operating costs at acceptable levels. Unfortunately, tower design standards do not specify exact values for deflection control. Various design companies and design manuals provide some guidelines for such control values, as shown in the table below:

Design Manual/Design Firm | Deflection Control Value of Tower (Y) (mm)
HG 20652-1998 | SH/T3098-2011
When DN ≤ 1000 mm, Y ≤ H/100
When 1000 mm < DN ≤ 2000 mm, Y ≤ 10H/DN
When DN > 2000 mm, Y ≤ H/200

U.S. Moss, 4th Edition of “Pressure Vessel Design Manual”: 6 inches/100 feet (equivalent to H/200)
U.S. Burke Company: H/170
U.S. Esso Corporation: H/160
Tang’s recommendation: H/160
Kellogg Company: H/200
U.S. SD Design Company: At the maximum wind speed considered in design, the deflection control value is H/200

Japanese “Handbook of Chemical Plants” (1970): Based on operational and safety considerations, the deflection control value is 6 inches/100 feet (equivalent to H/200)

A domestic design institute: When H/DN ≤ 20, Y ≤ H/480; when H/DN > 20, Y ≤ H/200
Note: H – Total height of the tower (including the base); DN – Nominal diameter of the tower. For towers with varying diameters, the weighted average of the diameters of each section is used.

III. Recommended Values for Tower Deflection
Numerous calculations have shown that for towers with a diameter greater than 2000 mm and a relatively small length-to-diameter ratio, it is easy to keep the deflection within the range of H/200. Even if the deflection is controlled at H/480, it does not require a significant increase in wall thickness. In China, especially in the chemical industry, towers with smaller diameters and larger length-to-diameter ratios are commonly used, particularly low-pressure towers with a diameter of less than 1000 mm and an H/D ratio greater than 20. In such cases, it is difficult to keep the deflection at H/100; if H/200 is to be achieved, it is necessary to increase the wall thickness of the tower to enhance its stiffness. Extensive practical experience has shown that the deflection limits recommended by standards HG 20652-1998 and SH/T3098-2011 are reasonable. For towers with a diameter of less than or equal to 1000 mm and an H/D ratio greater than 40, it is more appropriate to use steel frames to support the tower in order to limit its deflection.