Proper flanged joint assemblies depend on the interaction of the flanges, gasket, and bolts.  ASME PCC-1 Appendix O investigates the interaction of flange stresses, gasket stresses, and bolt stresses to determine proper bolt torque values.

Why use Metalmark Engineering for your ASME PCC-1 Appendix O assessment?

Maximum Permissible Bolt Stress for the Flange, Sfmax calculation. Metalmark Engineering uses a complete Appendix 2 flange assessment with WRC 538 to determine the Maximum Permissible Bolt Stress for the Flange needed for ASME PCC-1 Appendix O.

Target Assembly Gasket Stress, Sgt determination. Metalmark Engineering works with gasket manufacturers and does not rely on assumed default values for Target Assembly Gasket Stress, Maximum Permissible Gasket Stress, Maximum Operating Gasket Stress, and Maximum Seating Gasket Stress.

Assembly Bolt Stress. SBsel determination. Metalmark Engineering determines the Assembly Bolt Stress and assesses if the upper bolt limit controls, if the lower bolt limit controls, or if the flange limit controls before determining if gasket seating stress is achieved.

Low strength bolts don’t work well with ASME PCC-1 Appendix O. 

What is wrong with low strength bolts? The low Minimum Permissible Bolt Stress, Sbmin combined with the low Maximum Permissible Bolt Stress, Sbmax creates a nightmare scenario for engineers like me attempting to give bolt torque values to operators assembling flanges in the field. There simply isn’t a bolt torque range that will work.

The biggest issue is that low strength bolts make it difficult to achieve the clamping force required for gasket seating. This is because the Maximum Permissible Bolt Stress, Sbmax is too low to ever meet an acceptable Selected Assembly Bolt Stress, Sbsel in the (O-7) equation for achieving the Minimum Gasket Assembly Seating Stress, Sgmin-s. Gaskets that do not meet the Minimum Gasket Assembly Seating Stress, Sgmin-s are considered improperly seated and represent a major source of leaks in flanges. Please consider replacing your low strength bolts with high strength bolts at your next turnaround. 
Unsure if low strength bolts are controlling your PCC-1 Appendix O results? Metalmark Engineering loves checking your PCC-1 Appendix O calculations to determine the source of leaky flanges.

Does your Owner/Operator refuse to use bolt lubrication? 

Then make sure you are using a dry Nut Factor, K in your ASME PCC-1 Appendix O calculations. This may make a significant difference in your results as small Nut Factor, K increases may drastically increase the amount of applied torque needed to meet the Selected Assembly Bolt Stress, Sbsel. 

It is not recommended to use rusty or dry bolts for bolted flange joint assemblies because the applied torque required to meet the Selected Assembly Bolt Stress, Sbsel may vary wildly from bolt to bolt. The only recourse for engineers like me is to increase the Nut Factor, K for all bolts to ensure the average applied torque is high enough to meet the Selected Assembly Bolt Stress, Sbsel. There is a much higher risk of leaky flange gaskets if the applied torque is not high enough to account for the friction force associated with dry bolts. 

Many times Metalmark Engineering finds leaky flanges are due to poor assumptions made in their ASME PCC-1 Appendix O torque tables. Torque tables assuming incorrect Nut Factor, K values are a common source of flange leaks.

Let’s consider the Minimum Permissible Bolt Stress, Sbmin needed for the ASME PCC-1 Appendix O calculation. 

Unlike the Maximum Permissible Bolt Stress, Sbmax, which uses a percentage of the yield stress the Minimum Permissible Bolt Stress, Sbmin should be a hard floor of at least 20 ksi (140 MPa). This ensures that the bolts are preloaded enough to maintain their clamping force over time. The spring-like effects that make bolts such attractive fasteners are negated when they are not tight enough. Many times the Minimum Permissible Bolt Stress, Sbmin default values used by designers are based on 20% of the bolt yield stress, which may be TOO LOW or your application.

What considerations do I make when selecting the Minimum Permissible Bolt Stress, Sbmin? 

• How high is the Maximum Permissible Bolt Stress, Sbmax value? Anything above 65 ksi and I use 35 ksi with confidence. 

• What material are the bolts? Stainless steel bolts such as SA-193 B8 2 may have a Maximum Permissible Bolt Stress, Sbmax value as low as 35 ksi.  I like to use 20 ksi for stainless steel bolts with low Sbmax values so I have a useful range between the minimum and maximum.

• What value does the manufacturer suggest? Bolt manufacturers know more about bolts than you do. Just ask them.

Are poor assumptions being made in your torque tables? Metalmark Engineering loves reviewing torque tables to determine the source of leaky flanges.

Let’s talk about the Maximum Permissible Bolt Stress, Sbmax needed for the PCC-1 Appendix O calculation.

When performing 3rd party PCC-1 Appendix O reviews I often see a “default” value of 73.5 ksi (507 MPa) used for all bolt sizes and all bolt materials. Understand that 73.5 ksi (507 MPa) represents 70% Yield of SA-193 B7 bolts under 2.5”. Please remember to look up the yield stress to determine Maximum Permissible Bolt Stress, Sbmax values for other materials and sizes above this range as this value may be TOO HIGH for your application.

Let’s take a look at what the Maximum Permissible Bolt Stress, Sbmax, depends on.

Material:

Notice that SA-193 B8 2 stainless steel bolts above 1.25” have less than half of the ambient yield of SA-193 B7 bolts. Choosing the wrong material’s yield stress can have a big impact on your PCC-1 Appendix O results. 

Size:
Notice that the bolt size has a big influence on ambient yield stress and the yield stress may be drastically lower at larger sizes. Also notice that the size ranges given for ambient yield stresses vary by material. Looking up the ambient yield stress at the wrong size can be a determining factor on the quality of your PCC-1 Appendix O results. 

Bolts seem like the simplest part of the PCC-1 Appendix O calculation but they are often a source of bad assumptions. Unsure about your PCC-1 Appendix O results? Metalmark Engineering loves checking your PCC-1 Appendix O calculations to determine the source of leaky gaskets. 

Are you using carbon steel torque tables for stainless steel nuts and bolts?

If so, you may be in serious danger of flange leaks due to undertightened bolts. 

Please keep in mind that the Nut Factor, K for stainless steel nuts and bolts should be about 30% higher than the Nut Factor, K for the equivalent size carbon steel nuts and bolts. Using the carbon steel Nut Factor, K for stainless steel nuts and bolts may result in undertight bolts that self loosen over time. Flange leaks from loose bolts may present a major operational risk at your facility. 

Unsure where any of the values in your torque tables come from? Metalmark Engineering loves checking torque tables against the real world case to determine the source of flange leaks.

Do you reuse rusty nuts and bolts after opening a flange pair?

It’s true that a little surface rust is not going to reduce the bolt area enough to change the bolt properties in a significant way. However, surface rust greatly increases the Nut Factor K needed in the ASME PCC-1 Appendix O calculation. I do not like using large Nut Factor K values in my ASME PCC-1 Appendix O calculations so please replace your rusty nuts and bolts when opening flange pairs during your next turnaround. 

The applied torque required to meet the Selected Assembly Bolt Stress, Sbsel may vary wildly from bolt to bolt because breaking the frictional forces from rust varies from bolt to bolt. The only recourse for engineers is to increase the Nut Factor, K for all bolts to ensure the average applied torque is high enough to meet the Selected Assembly Bolt Stress, Sbsel. In a practical sense, this means a higher amount of torque is required to meet the Selected Assembly Bolt Stress, Sbsel to make up for this variation. I prefer to replace rusty nuts and bolts with new nuts and bolts because they make it easier to meet the Selected Assembly Bolt Stress, Sbsel. 

New nuts and bolts are an inexpensive way to help prevent leaky flanges.

What is ASME PCC-1 Appendix O?

It’s all about fixing your leaky flanges by taking a deeper look at the flange assembly bolt stress and ensuring the gasket is sealing properly. ASME PCC-1 Appendix O has a lot of stress values to look up (or calculate) and using “defaults” isn’t going to help you solve real world problems. It is important to know where to get some of the stress values you will need in order to properly determine bolt torque values using ASME PCC-1 Appendix O.