Bolted Flange FEA for a Heat Exchanger

Project overview

A bolted flange is only as good as two things: its strength under load and its ability to stay sealed. On a hairpin heat exchanger, the flange has to clamp tight enough to seal against the internal pressure while keeping its stresses within the limits of the ASME pressure vessel code.

We ran a static structural finite element analysis (FEA) of the joint in Ansys, with each part in its real ASME material, and analysed it the way it is assembled and used: first the bolts are tightened to a heavy preload of about 1,350 kN, then, with that preload locked in, an internal pressure of 2.07 MPa represents operation. The bolt-up and pressure stresses combine, and it is the combination that decides whether the flange holds and seals.

Objectives

Capture the true bolt-up condition by applying the full bolt preload before any pressure.

Add the 2.07 MPa internal pressure on top of the locked preload, and find the peak stress and deformation in the flange, pipe and bolts.

Check the contact pressure across the flange faces, which is what keeps the joint leak-tight.

Provide linearised stresses, so the results are judged against the ASME code rather than on the raw peak alone.

How we set up the flange FEA

Step 01

Assembly with contact

The full joint was modelled with contact between the mating parts, so the flanges could separate, slide or press together like the real joint.

Step 02

Bolt pretension

The studs were modelled with pretension elements, so the clamp-up and the way the flanges press together are represented properly.

Step 03

Step 1: bolt-up

A bolt pretension of about 1,350 kN applied to the studs to clamp the flanges together.

Step 04

Step 2: operation

The preload locked and an internal pressure of 2.07 MPa applied to the wetted surfaces.

Step 05

Results

Total deformation, von Mises stress, linearised stresses along classification paths, flange contact pressure and the working load in the bolts.

Materials and loads of the bolted flange FEA
Item	Value	Notes
Flange	SA-105	Forged carbon steel
Pipe	SA-106 Grade B	Carbon steel pipe
Studs	SA-193 Grade B7M	Alloy steel bolting
Load step 1	1,350 kN preload	Bolt-up, before any pressure
Load step 2	2.07 MPa pressure	Operation, with the preload locked
Contact	Flange faces, bolted interfaces	Free to separate, slide or press together

Results: strength, deformation and sealing

With the preload and the 2.07 MPa pressure both applied, the joint behaved well:

The peak von Mises stress reached about 235 MPa in a small, localised zone at a geometric transition, while the average stress across the parts stayed near 30 MPa. A local peak like this is expected where the geometry changes sharply, which is why the ASME assessment uses the linearised membrane and bending stresses.

The maximum total deformation was about 0.53 mm with the joint fully preloaded and pressurised, and most of the assembly moved far less.

Keeping the deformation, and with it the flange rotation, low matters in its own right: too much rotation opens a gap at the seal and starts a leak.

Total deformation plot of a bolted flange joint after bolt-up in Ansys, maximum 0.24 mm — Total deformation after bolt-up (load step 1): up to about 0.24 mm, greatest at the flange rim.

The low overall stress and the small deflection show that the flange, pipe and studs all have healthy strength margins at the rated pressure. For sealing, the contact pressure on the flange faces shows a firm, continuous band right around the joint, even after the internal pressure tries to push the flanges apart: the signature of a joint that will stay leak-tight. The bolt preload was clearly doing its job, without relying on a hand calculation.

Evidence for the ASME sign-off

The study answered both of the client's questions from one model built the way the joint is bolted up and run: the stresses and linearised results are the evidence of strength for the ASME sign-off, and the contact pressure is the evidence of sealing. We explain the code approach in our guide to pressure vessel FEA to ASME Section VIII Division 2.

Frequently asked questions

Why apply the bolt preload before the pressure?

Because that is how the joint is really assembled and used. The preload sets up most of the clamping stress, and the pressure stresses add to it; solving bolt-up first and operation second separates what each does and shows their combination, which is what decides strength and sealing.

Is a 235 MPa peak stress a problem?

Not on its own. It is a small, localised peak at a geometric transition, while the average stress was near 30 MPa. ASME assessments use linearised membrane and bending stresses rather than raw peaks, and the joint showed healthy margins on that basis.

How was sealing checked?

From the contact pressure on the flange faces. A firm, continuous band of contact pressure stayed right around the joint after the 2.07 MPa pressure was applied, showing the preload keeps the joint leak-tight.

Which materials were modelled?

SA-105 forged carbon steel for the flange, SA-106 Grade B for the pipe and SA-193 Grade B7M alloy steel for the studs, each with its own strength properties.

FEA of a Hairpin Heat Exchanger Bolted Flange Joint for Strength and Sealing

Project overview

Objectives