Flange Bolting Preload Calculation

Flanges are the main critical elements of the industrial piping.They are used for connecting pipes to equipments, valves and etc. Flanges are the main resources of leakages. When you go through the internet / literature you may see lots of flange prooblems due to the lack of engineering and workmanship in construction.

The main causes of flange leakage are:

1. Uneven Bolt Stress

2. Improper Flange Alignment

3. Improper Gasket Centering

4. Dirty or Damaged Flange Faces

5. Excessive Piping System Loads at Flange Locations

6. Thermal Shock

7. High Vibration Levels

This article aims to focus on calculating the flange preload forces and creating torque tables for critical/ non-critical flanges." With the guidance of this specification and tables, the site construction team will prepare their own method of statements, procure their lubricants, and select their torque equipment.

The torque tables are built on specific assumptions regarding bolt and nut factors. These assumptions should be verified before using a given torque table to ensure they are appropriate for the specific application.

Even if all assumptions are appropriate, results may vary depending on conditions. Many factors induce scatter in the results or increase the inherent variability in the bolting process. These include variations in the nut factor, bolt, flange, and nut condition, equipment calibration and condition, and perpendicularity of the bolt, nut, and flange.

The values in the tables are based on the equation:

 T = Torque value

F = Bolt pre-load

k = Nut Factor based on lubricant used

d = Nominal bolt diameter, (in.) 

WHAT IS NUT FACTOR?

The nut factor is not the coefficient of friction.It is an experimentally derived constant that includes the impact of friction. The nut factor is strongly correlated with the lubricant used.It shall be mandatory (according to me!) to use lubricant in bolted connections in order to guarantee required torque on the bolt as preload. If the bolt is not cleaned and lubricant is not used, you may have some additional friction forces which may decrease the preload on the bolts.

As shown, a small portion of the applied torque (roughly 10 percent) actually results in useable bolt load available that ends up applied to the gasket; a majority of that torque is consumed by friction between the threads of the stud or bolt and nut or blind hole, and the flange surface under the head of the bolt or nut. That value of useable bolt force can be increased using a more effective lubricant with a lower K factor, which reduces the overall friction effect on the assembly.

HOW TO CALCULATE TORQUE?

Normally, in the design specifications, Piping Stress Engineers assume %50-%70 of the bolt's percent yield stress for determining the recommended preload force and calculating the total torque. You can follow this basic approach. Please see the link below for an online calculator.

EZ Calc - Bolting Calculator

Here Enter & Select

• Flange Type

• Flange Class

• Flange Size

• Bolt Type (Select Stud)

• Fastener Material

• Bolt Percent Yield (%50-%70)

• K-Factor

The other values are calculated or filled automatically.

K FACTOR, WHICH VALUE SHALL BE TAKEN?

Since the K factor affects the torque value very closely, perform good research before choosing the correct value. Clearly indicate your assumptions in your calculated tables. I prefer to use K=0,16 for a well-lubricated bolted joint.

See the table below for the Nut Factor of some lubricants.

TORQUE TABLES

You can find standardised Torque Tables in ASME PCC-1, . You can check your result and assumption with the tables prepared and tabulatedin PCC-1.

ASME PCC-1 Torque Table

The assumptions & notes for the table above are:

• Bolting torque to develop 50% Bolt Yield Stress at Nut Factor, K=0.16

• This Table applies to Never-Seize paste and Fel-Pro lubricant, K=0.16.

• The above Table applies only to ASTM A320 Grade L7, A193 Grade B7, and A193 Grade B16 Stud Bolts.

• The torque values are approved for spiral wound graphite and PTFE-filled gaskets, graphite sheet gaskets of GHE and GHR types, ring joint, double-jacketed, and Camprofile gaskets with graphite and PTFE lining.

• The Final Torque Value accounts for 10% bolt relaxation.

• This Table is not acceptable for PTFE coated Stud Bolts and nuts.

AUTHORS

Erhan Yıldız, BS Mechanical Engineer, Co-Founder - KEY OIL & GAS

Ozan Gülser, MS Mechanical Engineer, Co-Founder - KEY OIL & GAS

Halil Ünal, Piping & Plant Specialist, Co-Founder - KEY OIL & GAS

Reference:

1- https://www.wermac.org/bolts/bolts_bolting-torque-tables_stud-bolts_flange-bolt-up_table3.html

2- https://www.pumpsandsystems.com/importance-using-thread-lubricants-bolted-connections

3- Guideline for bolted joint design and analysis: version 1.0, Kevin H. Brown, Charles Morrow, Samuel Durbin, and Allen Baca

4- Guidelines for Pressure Boundary Bolted Flange Joint Assembly

5- http://www.wermac.org/flanges/flanges_bolting-torque-tables_table2.html