Indlægget Vibration test blev vist første gang den Alternative Fuel Pipe Solutions.

The static vacuum is applied during production and must then remain complete in the entirety of the operating life of the vacuum insulated piping. Many of the materials used are quite standard, but may often require specific finishes and production processes in order to ensure the quality of the vacuum. which makes it a complicated and expensive product. Further-more there are limited number of enterprises that are able to successfully complete long and complex section of vacuum insulated piping.

AF Pipes has been specialising in double wall piping and double wall pipes for many years now, and the double wall pipe products of AF Pipes have been extremely well applied, operating trouble-free for many years of service in many marine, offshore, and land based installations. One of the particular challenges in the marine and offshore segments are the vibrations associated with propeller, engine, hull, and wave movements. The AF Flex system for double wall pipes is specifically designed to handle these vibrations in the piping, and at present is the most robust way of dealing with damaging vibrations. Unfortunately vacuum insulated piping was not designed for the harsh environment, and consequently is potentially damaged when exposed to the high amplitude vibration conditions. This can result in damage to the piping modules or module connections, and subsequent failure of the supporting mechanisms of the inner and outer pipe, compromising the integrity of the vacuum itself. As previously mentioned, the static vacuum is applied at factory, so if the vacuum fails during service there is no way to recover the vacuum without replacing the failed modules. Failure of the vacuum could be critical depending on the circumstances, and level of redundancy available.

AF Flex double pipe is often used as a closed system (with closed annular space for pressure detection), so it comes naturally for AF Pipes to investigate  and recommend a reliable and cost-effective method of applying the vacuum to the AF Flex system in order that the AF Flex double pipes can also be used with the vacuum system up to certain vacuum levels, without many additional measures to be taken.  Therefore AF Pipes has designed an alternative method to producing vacuum piping, with continuous vacuum piping. This is a complete vacuum throughout the entire piping arrangement, instead of separate static vacuum modules, and the vacuum is applied on site at the end of the installation. 

The introduction of the AF Flex anchor Cryo enables this concept. The anchor is an insulated fixed piping support point between the inner and outer pipe which allows a fixing point of the inner pipe relative to the outer pipe. This method has been available for some time for non-cryogenic fluids, but now the anchor has been developed for cryogenic fluids, with an insulated fixing block between the inner and outer pipe. The use of such an anchor is particularly relevant with cryogenic fluids, due to the large thermal contractions that will occur when using standard stainless steel piping with cryogenic fluids. The purpose of the anchor is to provide fixing points in the piping arrangement whereby the inner pipe movements can be controlled around these points. This must be analysed carefully during the pipe stress analysis, and ensure that the nozzle loadings on the anchor are within the specification. The flex anchors are validated during testing, and provided with axial loading, bending and torsional moment specifications.  When used in conjunction with the AF Flex supports, which are also cryogenic insulators with specified loadings and displacements, it is therefore possible to design an entire piping arrangement for cryogenic vacuum use with the AF Flex double wall piping system.

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Indlægget Cryogenic Testing blev vist første gang den Alternative Fuel Pipe Solutions.

Double wall pipe design, part I, which was first issued in 2017 gave some very general methods for design of double wall pipes, and some of the common methods encountered at shipyards, and installation aspects for the adoption of double wall pipe installations. In 2017 the number of vessels requiring double wall pipe was very limited, as dual fuel vessels in service or on order could be counted on hands. Now we are rapidly moving into the greener future, albeit still using fuels, and in 2021 there is a significant uptake in dual fuel vessels, with nearly 50% of all newbuilds predicted to be dual fuel by 2030.  This rapid expansion of new technology has forced many smaller shipyards to offer dual fuel newbuilds if they want to compete in the future market of shipbuilding. AF Pipes has witnessed an interesting range of different methods on how to solve the double wall pipe installation, the good, the bad, and the broken. There is certainly a lack of standard solutions in double wall pipe design, and as the shipyards compete for prestigious dual-fuel projects, each shipyard typically creates a unique solution. This is desirably cheapest in first in first cost, without too much though given to the need to repair poorly executed double wall pipe installation. Which is often several times more than the initial expense of the whole double wall pipe system in the first place. Several months after the newbuilding leaves the yard, and the piping has found its natural resting place, there may well be instances where the double wall piping has shaken itself apart and is rattling around, no longer fit for purpose. Learning is typically by painful experience, and this blog post will hopefully point the reader into the right direction of the important aspects to be considered when designing the double wall pipe installations. publication of this article should help identify some of details in design of double wall pipes that need careful attention.

Expansion & Contraction

Ventilated or pressurized double wall pipes often have a significant annular space, also in the case of negative pressure, and this will lead to the condition of the outer pipe seeking thermal equilibrium with the ambient temperature, and the inner pipe seeking thermal equilibrium with the process fluid temperature. As the inner and outer pipe are thermally “independent” of each other, the thermal contraction and expansion will then need to be compared to establish the thermal displacement of the inner and outer pipe relative to one another. It is therefore  vital to design the piping according to the worst case temperature differences, and subsequent expansion and contraction cases. Typically all stress analysis software will be able to provide detailed analysis at each support point giving forces and displacements for the inner pipe and outer pipe supports. AF Pipe solutions is currently using Autopipe, from Bentley systems, as this was found to have the most user friendly approach to double wall pipe installations. The results from the software will then need to evaluate the combination of the two movements & forces to establish if these values are within the design specification of the supports. The AF Flex supports are the only inner pipe support where the support design specification is provided to the customer so that these values can be confirmed. If it is not possible to confirm if the supports are within there specification, then it is not really  possible to adopt this engineering approach to the double wall piping. For example when the support design specification is exceeded at some locations, then strategically placed fixed supports, such as the AF Flex Anchor, will be required. In some cases, and specifically with very low temperature piping, additional compensator pieces may be required in the inner pipe line to be able to “reset” piping thermal contractions. Due to the flexibility of the AF Flex supports however, these compensator points can be vastly reduced when compared to conventional designs.

Stress Analysis

According to classification requirements high pressure installations (all 2-stroke dual fuel systems, even “low pressure! 2 stroke!) requires a stress analysis of the double wall piping. This is also the case for cryogenic piping. The stress analysis provides exactly what is says, analysis of stress in the piping, however it is not particularly useful with regards to placement of the inner pipe supports. The outer pipe supports (typically U bolts) are sensitive to excessive forces due to the lack of flexibility. It is quite common to see a very stiff outer pipe support regime where the outer pipe supports are expected to take up a significant amount of force from the piping, and thereby allow the piping system to easily “pass” the pipe stress analysis code requirements. The reality of the situation is that if this is then installed in the vessel, the user will soon experience significant outer pipe movement at the support points, where it is supposedly “fixed”, as the outer pipe supports give way to the high forces inherent in the piping. If this leads to support failure, then the end result is a completely non-fixed piping, which is then able to vibrate freely until destruction.

Vibration

Large amounts of vibration are the symptom of a poorly designed double wall pipe installation on board vessels. If the piping is too flexible, then it will vibrate too much, if it is too fixed, then there will be no allowance for expansion and contraction, and the piping will work itself loose, leading to a system that is too flexible. The black art is to find the balance of a fixed-flexible solution for both the inner and outer pipe simultaneously, to provide the optimum double wall pipe design. The simplest way to deal with vibration is to accept that a small amount of vibration will occur by making a slightly flexible double wall piping system, and then install flexible supports on the inner and outer piping that absorb a small amount of vibration. This is the fundamental design principle behind the AF Flex support solution.

Indlægget Double Wall Pipe Design, part II blev vist første gang den Alternative Fuel Pipe Solutions.

We have experienced an increasing number of enquiries from customers within the recent months, for different sizes and types of double wall pipe supports, also from different industries that we have not expected to see. Consequently, we are very happy to say that the range of the AF Flex support for double wall pipe has been extended quite considerably. This is partly to accommodate the range of different sizes that have been requested, but also for somewhat unexpected applications that have arisen. As we are a relatively small firm with niche products, it is less complicated for us to develop new products and solutions extremely quickly, and then mature the product through our extensive in-house testing program.  This involvs a range of product specific tests that have evolved over time to reflect the environments in which the products are used. Which also means that should a customer have a request for a specific test, then we will be more than happy to oblige. The increased range of products does lead to the inevitable question however – “which inner pipe support type is most suitable for my application?”

You can now find a more detailed brochure for the AF Flex products, which covers the current range of types and sizes. The product brochure is available here on the website, and should be able to answer some of the questions, but in order to summarise, there are three different types of inner pipe supports in the AF Flex range:

• Standard type: Historically used in the high pressure gas supply (ME-GI) double pipe system to provide a good balance of allowable radial deflection, and stiffness. This is also typically used on standard double wall pipe sizes where the schedules are larger than for low pressure systems, and whereby the standard design allows in such case for a larger radial deflection than the compact type.
• Compact type: A further development on the standard type, using the same design concept in order to allow for smaller outer pipe sizes relative to inner pipe. Thereby reducing the weight and cost of the installation. This type is also best for the smallest pipe diameters, and can be made for outer pie sizes down to 1 “ in diameter. For sizes below this, then we would recommend that the AF Slim is considered.
• Centraliser type: Developed for larger pipe diameters, where it was not possible to economically produce either the standard or compact type. This has a high stiffness, but limited radial deflection. Due to the small bearing surface area, and high stiffness, this is a very good choice for liquid, and cryogenic liquid piping. There are in fact two different designs for the centraliser, depending on the required outer pipe dimensions. The smallest gap (where NPS difference is only 2”) gives the stiffest, but least radially flexible design, which is only really recommended for axial displacements.

Typically the pipe support types and sizes are designed to give certain stiffness specifications, preferably to ensure dry mass deflection less than one millimeter. It may be apparent from some some specific sizes, that additional measures are necessary in order to achieve this general requirement, for example more frequent placing of supports. It is rather dependent on the acceptance criteria of the piping arrangement. In some cases two to three millilmetres of static deflection (without any loading) may be deemed acceptable. The deflection must also be considered in relation to the size of the piping being considered. The double wall piping arrangement must be stiff to the extent that it is not loose but, not too stiff as to overstress piping joints, connections, or the piping itself. This is unfortunately the most common cause of piping failure. It is also important to consider additional masses of the pipe, particularly when adding liquid mass to the pipe, in which case it may be preferable to use a stiffer model of support such as the centraliser type when dealing with long straight sections. A single or pair of standard type of supports could then be mounted around the bends, in order to take the radial deflection at the bend. This gives an example of how it is also possible to mix the different types of supports in order to provide the best possible double wall pipe installation.

We have detailed AF Flex design guidelines available for the design of the double wall pipe arrangement using the AF Flex double wall pipe supports, which if complied with, will cover most installations, without the need for extensive design resource. Please do not hesitate to contact me for a copy. For critical applications it is always recommended to run a pipe stress analysis, in which case the full range of load cases can be considered. For recommendations on the load cases, please contact us and we will gladly assist in this matter. Finally, and most importantly, by instigating the inner pipe support regime based on the stress analysis, the end user will be assured of a robust double wall pipe arrangement.

Indlægget Double wall pipe support types blev vist første gang den Alternative Fuel Pipe Solutions.

Due to popular demand, the AF Pipe flexible supports are now available in most common sizes for double wall pipe from 1/2″ to 12″ inner pipe. There are now three distinct product types available to accomodate all these sizes,

which vary in design and performance. The new datasheet for the AF Flex Support is found here

“Compact” type which is used to achieve the smallest possible outer pipe, for reductions in outer pipe cost and weight:

“Standard” type which is used for larger annular space, and typically not as stiff as the compact type

“Centraliser” type, which is used for the larger dimenion pipes from 6″ inner pipe upwards, especially with heavy wet loads (liquids) where higher stiffness is required

Indlægget Now for all dimensions of double wall pipe blev vist første gang den Alternative Fuel Pipe Solutions.

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Ventilation Flow Testing

The ventilated double wall pipes, or soon to be know as AF Flex double pipe, consist of two ends open to atmosphere. At one end a ventilation fan is mounted to pull the air all the way through the annular space between the inner & outer pipe, and the double wall components that are installed in the double wall system. In order to select a suitable fan the designer typically needs to know the required flow rate, and pressure loss of the system at that flow rate. The flow rate can be calculated relatively simply from the volume of the annular space (including dead space volume from components in the piping system), and the required number of exchanges per hour, which, if not already specified, can be determined in order to achieve a high dilution factor. The pressure loss of the system can be calculated from the sum of all pressure losses within the system, which will consist of the pressure loss from the piping surfaces & bends, pressure loss from supports, and pressure loss from any other components in the double wall system. The pressure loss from the piping can be estimated by calculation, but the pressures losses from supports & components must come from empirical testing (or validated simulations), from which a pressure loss co-efficient can be deduced. For the inner pipe flex support, the test is fairly simple; install some (in this case 6) supports in a straight piece of double wall pipe with known annular cross section, block off the inner pipe, install a fan at one end, install a dP measurement just before the fan to measure vacuum against atmospheric pressure, and measure the flow speed. There’s an example here:  https://youtu.be/oMLOc_xxyCc

Do this a few times at different flow speeds and you have a well specified pressure loss versus flow rate (the flow rate can be calculated with the flow speed, and the annular cross section), which can be used to specify a pressure loss coefficient.

Durability Testing

The inner pipe support is a flexible support, and the flexibility is designed and specified with the support, in conjunction with the carefully selected material. This is described in more detail in the previous blog post. As the design, and the material, go hand in hand, then it is essential that the support is produced with the correct material to ensure premature failure does not occur. In order to test for durability, a small mock-up of a double pipe bend is made, together with a reciprocating cylinder that can run with a specified deflection, continuously, for a long period of time. The test can be seen here:  https://youtu.be/xWb1Yun6X7I

The cyclical loading simulates, within a few days, a lifetime of thermal or pressure expansions of the piping, and provides the specification for the longevity of the inner pipe flex support. As an example of a representative number of cycles, if the supports are installed in a vessel which is designed for 25 years of service, and the gas supply is operated 2 times daily (start & stop), that’s 4 cycles per day. After 25 years then that’s 36,500 cycles, for these tests, there are typically around 300,000 cycles prior to failure. It should also be noted that incorrect installation of the flex support, will reduce the liftetime by about half, and it is actually therefore that AF pipes typically specifies these supports for 150,000 cycles at max deflection.

In addition to the two tests mentioned above there are many additional tests performed on the inner pipe flex supports, including corrosion, wear, stiffnes, and vibration testing.

Indlægget Double Wall Component Testing blev vist første gang den Alternative Fuel Pipe Solutions.

The most sensitive part of a double wall pipe system is the method of support for the inner pipe. If the inner pipe is not correctly supported, then it is highly likely that at some point in the lifetime of the double wall piping system, excessive and repetitive thermal displacements of the inner pipe will eventually lead to premature failure of pipe supports. Particularly if compensation for these movements isn’t considered correctly. Therefore it is typical to include compensation mechanisms in the inner piping itself to allow for all these displacements. This has traditionally be in the form of compensator bellow welded into the piping itself, or U bends in the piping. It is also possible to offer an inner pipe with extremely low thermal expansion coefficients, in which case the piping may “take care of itself”, however this is likely to be a less cost effective option. Displacements must be considered based on the minimum and maximum design conditions of the piping, and taking into account at every point in the piping system, particularly around bends.

An inner pipe support system consisting of welded joints only to the outer pipe is very time consuming to install, and leads to a fully fixed double wall piping arrangement, that is highly likely to fail prematurely. Furthermore due to weld shrinkage during installation of the piping sections, additional stresses can be introduced onto the inner double pipe support during installation, leading to premature failure, particularly on these unflexible systems. A flexible, more common inner pipe support system uses plastic sleeves to centralise the inner pipe, allowing axial sliding, and therefore unlimited axial movements of the inner pipe relative to the outer. It must be noted however that around bends problems can initiate, whereupon an axial displacement is translated into a radial displacement. As the plastic sleeve does not allow for any radial displacement, then compressive forces will push onto the plastic sleeve, leading to premature failure or distortion of the sleeve supports themselves. Therefore, a support offering both unlimited axial displacement, and a degree of radial displacement seems to be a good. In general the more flexibility that can be offered by the structure, the more robust it will become. This of course assumes that external forms of vibration are also taken into account.

It should also be noted that such supports have existed for some time, and are readily available from AF Pipes. When delivered with well documented specifications, the “flexible” inner pipe support can prove an asset to the double wall pipe design team. These flexible inner pipe supports may also offer additional flexibility for pipe routings, often with reduced piping dimensions when compared to traditional support regimes, which reduces material and welding costs. It is important to note that such flexible inner pipe supports have to be accurately located and installed according to product specifications, and specifically installed at the correct radial angle, so they work in the correct direction relative to the radial displacements. If not installed correctly, premature failure may be expected through excessive stress, or cyclic stresses. Incorrect installation can also lead to metal to metal contact on the outer pipe, which is also undesirable.

It is also absolutely essential that the flexible supports are designed for the application, and produced with exactly the correct materials to allow for full flexibility and robustness. AF Pipes supplies accurate material certification, and ensures quality of supply throughout the production process. Just small reductions in material quality will result in premature failures of these support types, as they are designed with very specific and high quality material specifications.

The flexible inner pipe support concept has the potential to improve many of the parameters of double wall pipe design, whilst decreasing the installation complexity. The flexible inner pipe support is very simple to install and offers increased level of reliability due to the in-built compensation affects for misalignment of piping sections during installation, which is extremely common. Depending on the exact piping dimensions, and therefore inner pipe support size, it may be preferential to consider the flexible inner pipe support as an axial support, with limited radial freedom. This of course depends on the exact specification of the double pipe support chosen, but this can be considered the most robust method of designing the double wall pipe installation.

Indlægget Double wall pipe design blev vist første gang den Alternative Fuel Pipe Solutions.