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Primary flange forces – Part 1

Primary flange forces and gasket stress

The article is the rst in a two-part series that takes a somewhat global perspective on the subject of ange forces, and the role they play in creating and maintaining the successful sealing of bolted ange connections (BFC).
Successful sealing of gaskets in a bolted ange assembly requires the gasket to remain within a particular range of stress. Its upper bound ensures the mechanical integrity of the gasket is not exceeded. The lower bound ensures su cient sealing stress to provide the intended level of tightness. Because the forces that create the gasket stress must pass through the anges, successful sealing is equally dependent on the balance of forces on the anges. If the force on the anges is too low, not enough compression on the gasket results. If it’s too high, damage to the anges, excessive rotation (bending) of the ange, or both can result, often resulting in leakage. Part 1 reveals how gasket stress is typically distributed between mating anges, and identi es some of the most common ange forces to be considered in deriving a suitable bolt load for the connection. Part 2 will discuss the role these forces play and what can be done to control them.

Gasket stress distribution

Ultimately the leak-free reliability of a BFC depends on the state of stress in the gasket. It’s often convenient to simply divide the total bolt load at bolt-up by the area of the gasket to judge the state of stress on the gasket. This simple ratio provides an average value. In actuality, the state of stress at any particular location in a gasket is di erent than this value, sometimes by a large margin. Fundamental to understanding how ange forces a ect gasket stress is to understand how gasket stress is distributed over the sealing surface of the ange. To help visualize how the ange and gasket face interact with one another, see Figure 1. The graphic is the result of a nite element analysis. The abscissa is the value of gasket stress, and the ordinate is the circumferential location in a 90° quadrant of the gasket face, of a four bolt ange. The dashed line represents the radial line through the center of a bolt. The plot shows the value of gasket stress at the outer diameter, middle and inner diameter of the gasket face. In this particular

instance the ange is an ASME, A 182 Gr 316L, 3” diameter, Class 150 welding neck ange. Results though, will be similar for other BFC with ‘ oating’ gaskets.

The bolt load is based on an average gasket stress of 5,000 psi, and is evenly distributed on all bolts. Note that even under these ideal conditions, the gasket stress varies both circumferentially and radially. This is due to the rotation of the anges and the near juxtaposition of gasket material to the bolt. As the outer edges of the anges are pinched together the gasket stress at its outer diameter is increased. The opposite e ect is seen at the inner diameter of the gasket. Controlling ange stresses is clearly important to controlling gasket stresses. Following are typical ange forces to consider in determining a gasket solution.

 

Bolt-up force

This is the force intended when the BFC is initially tightening. Most commonly the value of force for a given bolt is given by this simple, empirical equality.

Fbu = T ⋅ 1000/(k d nom)                        SI Units       (Equation1)

Where, F bu = Bolt force from torque, N

T = Torque, Nm

k = Nut factor, dimensionless

dnom = Nominal bolt diameter, mm

Fbu = T ⋅ 12/(k d nom) U.S. Customary Units (Equation 2) Where, F bu = Bolt force from torque, lbf.

T = Torque, ft.lb.

  1. = Nut factor, dimensionless dnom = Nominal bolt diameter, in.

The total force on the ange from bolt-up becomes the force of a given bolt times the number of bolts. Note that this presumes that other forces are not resisting the bolt-up force.

FTbu = Fb ⋅ Bno.                                    (Equation3)

Where, F Tbu = Total force from bolt-up, N (lbf.)

Bno. = Number of bolts in    ange

This is the primary ange force to which all other forces are either added or subtracted. It must be su cient in value to withstand the e ects of all conditions that will change it, while still being able to maintain a leak-tight value of stress on the gasket. The importance of ensuring that the intended bolt-up load is imparted to the anges cannot be over stated. Engineering solution can provide a value for successful sealing of the anges, but this success can only be realized when this load is accurately and evenly developed at the time of the installation.

Hydraulic force

After the initial bolt-up, the anges are subjected to operating pressure and temperature. The pressure, if internal will create a force opposite to that of bolt-up. If external, the force is additive to the bolt load. In either case, its value is approximated by multiplying the pressure by the internal area de ned by the inner diameter of the gasket.

Fhyd = P ⋅ Ahyd                                    (Equation 4)

Where, F hyd = Hydraulic force on ange N (lbf)

P = Pressure contained. ( +) if internal, (−) if external, MPa (lbf./in. 2)

Ahyd = Pressure area contained by gasket, mm 2 (in. 2)

In the simplest of approximations the average operating gasket stress would be the result of the di erence, or sum of the bolt-up force and the hydraulic load. This seldom occurs because other ange forces are usually in action.

Piping induced forces - alignment

Piping induced forces are often introduced into a ange pair by less than idea t-up of anges. Fit-up problems occur as a result of misalignment of anges. In the case of good ange alignment, bolts can easily be inserted by hand. Excessive misalignment of ange pairs though, requires the connected piping to be pulled or pushed into place so the bolts can be installed. The force(s) required to align the pipe are resisted by the bolts during t-up. These forces may create axial, shear, torsional or bending loads on the anges, individually, or all four simultaneously. When these forces are not excessive, they can be compensated for by additional bolt load.

Piping induced forces – expansion and contraction

Expansion or contraction of attached pipe and equipment will also introduce forces on the anges. They’re created after the initial installation and when

the temperature of the attached piping and equipment changes to its operating temperature. Determining the extent of these forces requires a piping exibility analysis which essentially reviews the piping layout of a system, and determines the resulting forces from restrained piping movement. Speci c guidance on identifying limits is beyond the scope of this article. It’s important to realize though, that the need for such an analysis and the respective limits of the e ects on piping are called out in applicable engineering Codes. Typically a limiting value of force is allowed during the design of the attached piping system and the robustness of the ange design is su cient to successfully withstand it. Troubles can arise though, if there‘s a change to the piping design and its e ect is not taken into consideration.

Piping induced forces – environmental e ects

Last in the consideration of ange forces are those created from environmental e ects. These include such things as wind, earthquake and snow. Again, like expansion and contraction forces, these forces are normally included in the design of the piping system if necessary, and are rarely included in specifying the value of bolt load to successfully seal a gasket.

Part 1 conclusion

Gasket stresses are the direct result of ange forces. Successful BFC sealing requires these forces to remain with a range of predictable and controlled values. Both excessive values, and insu ciently low values can lead to connection leakage. In Part 2 we discuss how to target the correct value of ange force, and two of the most important safe guards to controlling these forces.

Part 2 will be published in the May issue of Valve World.

The European Sealing Association (ESA) has produced this article as a guide towards Best Available Techniques for sealing systems and devices. These articles are published on a regular basis, as part of their commitment to users, contractors and OEM’s, to help to nd the best solutions for sealing challenges and to achieve maximum, safe performance during the lifetime of the seal. The ESA is the voice of the uid sealing industry in Europe, collaborating closely with the Fluid Sealing Association (FSA) of the USA. Together, they form the key global source of technical knowledge and guidance on sealing technology, which is the basis for these articles. For more information, please visit www.europeansealing.com

By ESA member R. Wacker