Gasket blowout is the most catastrophic of gasket failures. It usually occurs with no discernible warning and causes a sudden and significant release of internal pressure, usually accompanied with loud popping sound or whistle. Depending on the process being contained and/or the amount of stored energy at the time, the result can be fatal.
Part One of this two-part Sealing Sense series brings attention to the balance of forces present at the potential moment of gasket blowout and provides guidance on how to protect against this type of gasket failure. Equations are provided to assist with evaluating these forces and results are presented to show that to protect against blowout, clamping force can be significantly more important than gasket tensile strength.
For the purpose of this article, Figure 1 identifies the general condition of potential blowout. It shows an axial “cut” across the thickness of the gasket and the three general forces affecting blowout. Equation 1 provides the force balance in the radial direction. The potential condition of blowout is taken when the sum of these forces is equally balanced. That is, the forces holding the gasket in place and together is equal to the outward force trying to dislodge or damage it.
Equation 1: FGten + FClamping = FBlowout
These are defined as follows:
FGten = Force supplied by the (resisting) tensile strength of gasket
FClamping = Frictional force on gasket from combined axial forces
FBlowout = Outward radial force trying to separate gasket from flanges
These three forces give a general descriptive condition of blowout, but do little to identify the components upon which they are dependent. Equation 2 provides greater detail to draw attention to these components.
Equation 2: (FGten) + (FBtot) - (FGrelax) ± (Fhyd) ± ( Fother) = FBlowout
FClamping has been subdivided into four components. These components are defined as follows:
FBtot = The total initial force on the gasket created from tightening the fasteners to a prescribed target torque
FGrelax = Loss of compressive gasket stress because of time, temperature and load effects
FHyd = Force resulting from hydraulic load being contained
FOther = General collection of other effects that reduce or increase the clamping force
Fhyd and FOther, though normally negative, can be positive and help prevent blowout.
Table 1 lists common conditions that should be considered when evaluating each force. The reader is cautioned that this is only a partial listing. The intent is to bring attention to the type and number of factors that can affect the force balance.
Equation 3 provides a further redefinition and rearrangement of terms that allow the potential blowout pressure to be evaluated directly. Example values are supplied for a 2-inch, Class 150, ASME B16.21 ring gasket.
The added terms are defined as follows:
σtg = Tensile strength of gasket, 2,700 pound-force per square inch (lbf/in2)
Ablowout = 2 x gasket width x gasket thickness in compressed condition, in2
μ = Friction factor, gasket on flange face, 0.2
FBtotal = Total bolt load at original tightening torque, 32,914 pound force (lbf)
σgRF = Loss of gasket stress to time, temperature and loading history, 2,050 lbf/in2
Ag = Axial area of gasket, 5.84 in2
Di = Inner diameter of gasket, 2.38 inches
t = Thickness of gasket in compressed condition (0.0625 inch)
FOther = as above, 10,000 lbf
PBO = Blowout pressure, lbf/in2
Analysis of Blowout Factors
A way to trial different values to inspect their potential effect on PBO is available. It will be used to test the general belief that gasket tensile strength is always a major factor in preventing gasket blowout. To investigate, two series of calculations are evaluated to the resulting value of PBO.
The first series is performed by holding all values fixed, except σtg. Its value is progressively reduced in 5 percent increments. In the second series, all values are held fixed except, FOther. This shows the potential effect of an axial piping load or axial flange misalignment that tends to separate the flanges. Its value is progressively increased in 5 percent increments. The results are plotted in Figure 2.
The plot of gasket tensile strength is relatively flat. This shows PBO to have a low sensitivity to changes in gasket tensile strength. On the other hand, PBO shows a significant sensitivity to the effect of the change in axial piping load. During the range considered, the overall change in PBO for tensile strength and the piping load are approximately 4 percent and 41 percent, respectively. In fact, a 5 percent drop in clamping force produces larger drop in blowout pressure than a 50 percent drop in tensile strength. Clearly, the clamping force can play a much larger role in protecting against blowout. This is equally true for any effect that reduces the clamping force.
In this example, the total value of clamping force was adjusted by considering a single factor such as piping load or flange misalignment. In reality, the ultimate value of clamping force will be the result of the combined effects of several factors. Unless the value of gasket tensile strength is very high, the clamping force will be most responsible for protecting against blowout.
A review of Table 1 shows a significant number of conditions that can negatively impact blowout pressure. Developing values for these factors can often be a challenge. Next month, Part Two of this Sealing Sense series will investigate two strategies to help make this challenge manageable.