Heat Exchanger

The codes for heat exchanger designs allows Engineers to minimise flange geometry and loss of containment. This saves initial costs but leads to high failure rates.

High multitudes (36-84..) of small diameter bolts complies and has adequate strength but leads to very high interactional pre-load losses when they are tightened, consequently they regularly leak.

We measure each bolt to identify the "highs and lows". Adjust the "lows" (which normally reduces the "highs") and deliver an evenly tensioned flange joint. This ALWAYS results in long lasting leak free joints.

We have provided this service to:

BP Oil, Shell, Chevron, Arco China, CSBP and many other process companies.

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Why girth joint leaks happen and how to prevent them

Why do heat exchanger girth joints have a high failure rate? Equipment designers and manufacturers always comply with the various codes. So, strictly approved verifying bodies approve designs but still they fail, why?

Well, we need to understand that the design codes are there to provide safety from explosion; maintenance is not considered in detail.

The failure modes lie in the gaps between:

  1. Heat exchanger designs and maintenance; and

  2. Heat exchanger and bolt tooling designs.

There are several contributing factors, each with it’s own solution:

  1. The codes:
    In recent years, bolting control processes have been improved by focusing on tooling and training, but that is the band aid and does look towards a root cause solution.
    We look at the shortcomings of the codes and identify steps that can be taken.

  2. The available bolt tightening methods:
    There is much debate about the various tightening methods and pre-load requirements. Providers of tooling make claims and counter-claims about their systems. The truth is that the optimum method to be used depends on the design of the joint.

  3. Manufacturing methods:
    QA/QC and inspection during the manufacturing process is stringent but not always completed at the right time.

So how can failures be eliminated?

We will present some case studies to cover each joint design scenario and provide the “best fit solution” for each set of parameters, these include:

  • Improved specifications

  • Modified inspection procedures

  • Joint design improvements

  • Higher tightening accuracy



The Purpose of this paper is twofold.

  • Understanding Heat Exchanger Custom Flange design deficiencies.

  • Why Heat Exchanger flanges can leak.

This article will cover Heat Exchanger Girth Joint Reliability Solutions. In order for us to provide some insight into why girth joint leaks happen and then prevent them from occurring, we will use over 26 years' worth of experience gained from working in improving bolted joint reliability and integrity.

We will provide a comprehensive discussion of this issue as we use our experience and authority as one of the pioneers of ultrasonic bolt stress monitoring to provide real-life (and not assumed) examples. We will look at the manufacturing process, bolt-tightening methods and systems, and the effect of the design codes and how they contribute to this problem. In particular, we will discuss the shortcomings of the codes themselves and identify steps that can be taken to prevent issues.

In this article, we aim to present some relevant case studies that will allow for greater insight into this problematic issue and allow for an in-depth discussion of each of the potential problems that can be faced. Inspection during the manufacturing process might be stringent but it is not always completed at the right time, which can have knock-on effects. Each case study we present here will cover different joint design scenarios and provide solutions for each of these issues, ensuring that future failures can be eliminated.

Solutions will focus on specifications and inspections, as well as design improvements and tightening accuracy. A clear understanding of the codes and manufacturing process is vital in ensuring joint reliability in every situation, as will be elaborated on further.

Case study

Where the codes fall down

The various codes and guidelines ensure that heat exchangers are safe to operate. The mechanical integrity of the plates, tubes, shells and flanges is assured by compliance with the design criteria. This does not ensure that leaks will not occur (as they often do).

The first weak point is the flange design, particularly for girth flanges. Design engineers use the criteria to provide the least costly solution (it's a very competitive business), which mostly results in the smallest and thinnest flanges. One way to optimize this is to use high multiples of small diameter bolts.

This complies with code but creates a maintenance nightmare. The interaction between each bolt during the tightening assembly process causes very high variations in residual bolt tension. The bolt tightening industry generally states that torque tightening is +/- 25% accurate but, with high interactions, this figure is often +25%-100% (it's very common to find at least one bolt loose after the final tightening sequence).


The available bolt tightening methods

This effect can be minimised, or even eliminated, by use of multi-point bolt tightening systems, such as Hydraulic Bolt Tensioners, if they’re available and suitable (which they often aren't).

To explain the last comment, when codes and manufacturers are combined, they fail to provide a solution. The TEMA bolt spacing and weld neck taper mean that only a small tensioner will fit. As the piston size determines the applied tension, and this is governed by the available space, wherever a tensioner is used, the pre-load that it can apply is very low. The manufacturers also rarely sell small tensioners, as the industry perception is that "small bolts don't need to be hydraulically tensioned".

Another restriction to the use of bolt tensioners is, that due to the multitude of dimensions that TEMA allows it is unlikely that a range of sizes can be harmonised.

The second method is controlled bolt torquing. Each bolt is tightened in a start pattern at 30% of the assumed torque, then at 60%, then 100% and then, again, at 100%. It is a slow process and, as our results show, highly inaccurate.

Manufacturers offer multi (four) point options but this has no effect on the inaccuracy, nor is it any quicker. Most field technicians are convinced that single point is much faster, the pump unit runs extremely slowly when its trying to charge four outlets.


Manufacturing methods

Reputation is extremely important to manufacturers: the quality is rarely in question and all fabricators strive to do a good job.

It is a requirement that stringent inspection points are completed by the manufacturers and, in many cases, an independent inspector will be required to witness and verify those inspections. The issue here is that the codes do not state at what stage in the manufacturing process these must be completed.

heat exhanger

Common Heat Exchanger Bolt Pattern, several small bolts, high bolt load interaction losses, high inaccuracy, often difficult to fit hydraulic bolt tensioners of adequate capacity.

48 Bolt HX Girth Joint graph

Results from ultrasonic bolt tension measurements. Original results adjusted to provide even force and leak free joint. The original tightening occurred in the standard “star pattern” four (4) pass (30%, 60%, 100% and 100% of calculated torque setting).

The Solutions


A clear understanding of what the codes allow can help the client to upgrade the specifications for a better long term solution. For instance, a simple statement of a maximum number of bolts on a large girth flange (> 1m) is 24, or that all bolts must be a minimum of 1 1/4in diameter. This will force each designer to increase the thickness and diameter.

Another could be "flanges to be designed to fit hydraulic bolt tensioners and retain a bolt tension must not be less than 50% of yield stress". Some manufacturers in South Korea have adopted this process. The Bolt Tensioner manufacturer also worked on reducing the tool size in order to minimise the dimensional increase of the flange.

The Bolt load design specification placed in the HX purchase order could specify:

  • That flange sealing bolts must target 50% yield stress as the operating condition, this ensure preload bolting devices do not overload the bolt with the applied load. It also ensure more bolts and

  • That bolt assembly spot faces machined surface diameter is 5 times the designed bolt diameter, this will ensure bolt tensioning devices will fit the flange adequately.

  • That QA documents of bolt load, bolt applied load and gasket type and dimensions are a Purchase Order quality deliverable and performance warranty item.


Changing from 60 x 1in to 2in bolts can reduce the number from 60 down to 16, which is both much quicker and improves accuracy significantly.

This example would result in flanges having a 5-10% thickness increase and a 10-20% increase in the outside diameter.


Bolt tightening methods

In the case where the heat exchanger is an existing unit that cannot easily be modified measured, control methods will resolve the failures.

To do this, there are several methods: ultrasonic bolt stress monitoring, compression load cells, strain gauges, gauge rod bolts etc. All of these work and used the same basic procedures:

  1. Measure the tension of each bolt

  2. Identify the lows

  3. Adjust the low ones to the target

  4. Re-measure each bolt; normally the highs will relax slightly after this

  5. Adjust the new lows to the target

  6. This will provide a joint with even bolt tension that will not leak.

BP Oil, Total, Agip, EDF, Endessa, National Power, and Shell at several refineries and power stations have successfully employed this latter method.


Manufacturing Methods

The two inspections that should be carried out at specific times are the flange sealing faces and the tube sheet sealing surface and division plate grooves.

Tube sheets will either have the heater tubes welded or expanded to them. This will, inevitably, cause distortion. In most cases, the inspection of the sealing surfaces will occur prior to this activity. Consequently, records will show that the items are compliant but in practice, they will not be.

Girth flange sealing surfaces are machined, compliant and verified before they are welded to the shell and the channel cover. This will cause distortion and it is likely that the sealing surfaces will not be compliant.

The solution is that the final machining should happen after the welding and tube expansion processes. In the case where it’s an existing unit, there are many capable in-situ machining providers who can do this work on site.

typical hydraulic tensioner assembly

Typical Hydraulic Tensioner assembly

sectional view hydraulic bolt tensioner

Sectional View of a Hydraulic Bolt Tensioner, pressure x piston area = force applied

cassette style hydraulic torque wrench

Cassette Style Hydraulic Torque Wrench, in comparison to square drive types it is safer, quicker ad induces less bending.

square drive hydraulic torque wrench

Square Drive Hydraulic Torque Wrench, very flexible as many different socket sizes can be applied. Lowest cost but highest inaccuracy.


Despite verifying bodies approving designs, girth joints still have a high failure rate and this is not often understood. Even when complying with the various codes, you can (and often do) see a failure of the joints. This high failure rate is most often due to the fact that design codes were made to provide safety from explosion but they do not take into consideration maintainability. Heat Exchanger Designs also often eliminate best practice bolt tightening.

Through our experience gained from real, and not assumed, measurements, we have shown exactly what happens to bolted joints. In order to adequately prevent leaks from occurring, it is important to consider improved specifications, modified inspection procedures, joint design improvements and higher tightening accuracy, as evidenced in the preceding case studies that were discussed in detail. Avoiding bolts below 1 1/4 inches, minimising the number of bolts used and ensuring Hub, Shell and Flange allow fitment of Hydraulic Bolt Tensioners will all drastically reduce the incidence of failure.

Providers of tooling will make claims and counter claims about their systems but, in reality, the optimum method that is needed depends on the design of the joint itself. Once the reasons for girth joint failures are understood, future leaks can be prevented. We believe the solutions that have been discussed will be able to assist with this issue and ensure that joint reliability problems can be successfully avoided in the future.

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