Volume 4 fuel cells and hydrogen technology 4 05 – hydrogen storage compressed gas

Volume 4 fuel cells and hydrogen technology 4 05 – hydrogen storage compressed gas Volume 4 fuel cells and hydrogen technology 4 05 – hydrogen storage compressed gas Volume 4 fuel cells and hydrogen technology 4 05 – hydrogen storage compressed gas Volume 4 fuel cells and hydrogen technology 4 05 – hydrogen storage compressed gas Volume 4 fuel cells and hydrogen technology 4 05 – hydrogen storage compressed gas

D Nash, University of Strathclyde, Glasgow, UK

D Aklil, E Johnson, R Gazey, and V Ortisi, Pure Energy Center, Unst, Shetland Isles, UK

© 2012 Elsevier Ltd All rights reserved

4.05.3.2.1 Composite vessels for hydrogen storage

4.05.4 Codes and Standards and Best Practices

4.05.4.1 Storage Tanks

4.05.4.1.1 Gas storage systems and cylinders best practice

4.05.4.2 Connectors – Joints and Fittings for Hydrogen

4.05.4.4.4 Gas contamination monitoring

4.05.4.5 Codes and Standards

4.05.4.6 Case Studies

4.05.4.6.1 Designing a hydrogen storage tank

4.05.4.6.2 The Pure Project case study

References

4.05.1 Introduction

Storage of compressed gas presents significant challenges over the storage of liquids due to the compressible nature of the medium Hydrogen storage, in particular, presents a number of technical challenges for hydrogen generation and production, stationary storage sites, transportable storage, and hydrogen refueling stations Compressed hydrogen gas can be stored in high-pressure tanks with pressures up to 700 bar (70 MPa) In addition, hydrogen can be cryogenically cooled to –253 °C in insulated tanks within a pressure range of between 6 and 350 bar (35 MPa) It can also be stored in advanced materials, either within the structure or on the surface of the material or in a chemical compound form which will generate hydrogen when undergoing some release reaction Hydrogen has a very high energy content by weight (about 3 times that of gasoline fuel), but it has a very low energy content by volume (liquid hydrogen is about 4 times less energy dense than gasoline) This makes hydrogen a challenge to store, especially when transportable storage is required for use in a vehicle situation

Hydrogen is colorless, odorless, tasteless, nontoxic, and nonpoisonous Although it is noncorrosive, it has the potential to affect the metallurgy of some materials especially when welded This can result in hydrogen embrittlement and lead to inherent weaknesses in some storage systems Although natural gas and propane are also odorless, industrial manufacturers incorporate sulfur-based additives to enhance their detection on leakage Currently, such additives are not used with hydrogen because of separation and dispersion issues These additives have been known to contaminate fuel cells and other storage systems and can lead

to compromised structural integrity

Hydrogen is over 50 times lighter than gasoline vapor and 14 times lighter than air The impact of this is that if it is released in an open environment, it will typically rise and disperse rapidly This is a significant safety advantage in an outside environment While there are risks associated with the storage of dangerous mediums, hydrogen, like petroleum, gasoline, or natural gas, is a fuel that must be handled properly It can be used as safely as other common fuels when simple guidelines are followed

and workmanship, inspection, quality control, and testing ‘ISO 16528 Part 1’ defines a pressure vessel as a housing designed and built to contain gases or liquids under pressure, and the Pressure Equipment Directive (PED) defines a pressure vessel as a housing, and its direct attachments, designed and built to contain fluids or gases under pressure Storage vessels for compressed hydrogen gas are usually designed, constructed, and maintained in accordance with applicable codes and standards, for example, the ASME VIII Boiler and Pressure Vessel Code for structural design and the NFPA2 Code for Hydrogen Storage for testing and operation

4.05.3 Theory and Principles of Design

A pressure vessel consists of various elements welded together Certain elements (such as flat ends) may be attached by bolting These could, in theory, be of any shape In practice, they are nearly always cylindrical This is because a cylinder is a very efficient shape for the containment of pressure Cylinders with relatively thin walls are subject only to tensile stresses when under internal pressure A spherical shape is even more efficient than a cylinder in terms of the thickness required to withstand a particular pressure, but spherical vessels are difficult to make and provide an awkward shape for purposes other than storage They are used mostly for storing liquefied gases under pressure To complete the enclosure, the cylindrical shell must be fitted with ends or heads Nozzles are used for getting the fluid or gas into and out of the vessel, for instruments and valves, and for venting and draining the vessel A typical nozzle would consist of a short length of pipe with a flange welded to one end and the other end welded into a hole cut in the vessel This is called a set-in nozzle A set-on nozzle would be welded onto the outside of the vessel This type would normally be used for small nozzles on thick-walled vessels

Horizontal vessels are nearly always supported on two saddles Typical dimensions of saddles for vessels of various sizes are given in

‘BS 5276: Part 2 – Specification for saddle supports for horizontal cylindrical pressure vessels’ Wrapper plates are often used to reduce the local stresses in the shell at the support Small, vertical vessels are often supported on legs – usually three or four, but sometimes more Larger vessels are usually supported on skirts Conical skirts are used for tall, slender vessels to give a larger base ring diameter Sometimes, it is necessary for the supports to be attached part way up the shell – such as for vessels supported in a structure In this case, brackets are used – typically two or four brackets will be fitted In the case of heavy vessels, a continuous ring would be used

‘ISO 16528-1 clause 7.3.1’ states that pressure vessels shall be designed for loadings appropriate to their intended use, including loadings induced by reasonably foreseeable operating conditions and external events ‘ISO 16528-1 clause 6.2’ lists the common failure modes that are generally considered in the design codes, and they are classified as short-term, long-term, and cyclic-type failures ‘ISO 16528-1 Annex A’ gives a brief description of these failure modes for guidance

‘Short-term failure modes’ are those due to the application of noncyclic loads that lead to immediate failure:

• brittle fracture;

• ductile failures – crack formation, ductile tearing due to excessive local strains, gross plastic deformation, and plastic instability (bursting);

• excessive deformations leading to leakage at joints or other loss of function;

• elastic or elastic–plastic instability (buckling)

‘Long-term failure modes’ are those due to the application of noncyclic loads that lead to delayed failure:

• creep rupture;

• creep – excessive deformations at mechanical joints or excessive deformations resulting in unacceptable transfer of load;

• creep instability;

• erosion, corrosion;

• environmentally assisted cracking, for example, stress corrosion cracking, hydrogen-induced cracking, and so on

‘Cyclic failure modes’ are those due to the application of cyclic loads that lead to delayed failure:

• progressive plastic deformation;

• alternating plasticity;

• fatigue under elastic strains (medium- and high-cycle fatigue) or under elastic–plastic strains (low-cycle fatigue);

• environmentally assisted fatigue, for example, stress corrosion cracking or hydrogen-induced cracking

4.05.3.1 Steel Vessels

4.05.3.1.1 Materials

When considering the most appropriate material for construction, be it the main pressure-retaining shell or valves and seals, consideration must be given to the possible deterioration of properties when exposed to hydrogen at the intended operating conditions The mechanical properties of metals, including steels, aluminum and aluminum alloys, titanium and titanium alloys, and nickel and nickel alloys, are detrimentally affected by hydrogen Exposure of metals to hydrogen can lead to embrittlement, cracking, and/or significant losses in tensile strength, ductility, and fracture toughness This can result in premature failure in load-carrying components

4.05.3.1.2 Design for pressure loading

The main loading, which all vessels will be subjected to in their lifetime, is that of ‘internal pressure’ Thin shells under this loading are normally analyzed by membrane stress analysis Thick-walled pressure vessels are normally analyzed by using the Lamé equations Design equations based on this analysis are given in ‘ASME VIII Division 1, Appendix 1’

The cylindrical shell is the most frequently used geometrical shape in pressure vessel design The stresses in a closed-end cylindrical shell under internal pressure can be found from the conditions of static equilibrium and by evaluating the governing hoop stress The design equations are based on thin shell theory

pr

σθ ¼ t Rearranging this equation to give the required thickness e for a shell of inside diameter Di and with design stress f gives the following equation as used in British Code PD 5500:

pDi

e ¼ 2f −p

In practice, the chosen minimum wall thickness for the design must take into account mill tolerance and corrosion allowance

The American ASME VIII Division 1 code suggests the following approach For vessels with a known inside radius R, the minimum required thickness t for pressure loading is the ‘greater’ of the thicknesses obtained from clause UG-27(c)(1) or UG-27(c) (2) These relate to the circumferential and longitudinal stresses, respectively For vessels with a known outside radius Ro, the minimum required thickness t is obtained from Appendix 1, clauses 1-1(a)(1)

The allowable stress S is obtained from ASME II, Part D, Table 1A or 1B at the design temperature The joint efficiency, which introduces additional thickness to compensate for differences in weld configuration, E, must be the appropriate value from clause UW-12 for the joint being considered

For ‘circumferential stress’ (longitudinal joints), when the thickness does not exceed half the inside radius or P does not exceed 0.385SE

R 0:6tþ ¼ Ro − 0:4t For ‘longitudinal stress’ (circumferential joints), when the thickness does not exceed half the inside radius or P does not exceed 1.25SE

PR

t ¼ 2SE þ 0:4P

For circumferential joints, the MAWP is given by

2SEt

P ¼ R − 0:4t Note that the circumferential stress formulae will govern unless the circumferential joint efficiency is less than half the longitudinal joint efficiency, or if there are other loadings that increase the longitudinal stress

For ‘thick cylindrical shells’, where the limitations on thickness or pressure given in clause UG–27(c)(1) or UG-27(c)(2) are exceeded, the equations in Appendixes 1 and 2 must be used The results are very close to those obtained using the Lamé equations (see any standard text on Mechanics of Materials)

For ‘circumferential stress’ (longitudinal joints), when the thickness exceeds half the inside radius or P exceeds 0.385SE

Note that the above equations for t and P may be used in lieu of those given in UG-27(c)

For ‘longitudinal stress’ (circumferential joints), when the thickness exceeds half the inside radius or P exceeds 1.25SE

Once the basic cylindrical shell is defined, heads must be added to close the pressure envelope Heads can be spherical, elliptical,

or torispherical in form They can be designed using a spherical shell calculation with suitable modifications to allow for changes in geometry from the true sphere The design equations are again based on thin shell theory

pDi

e ¼ 4f − p

In PD 5500, the equations are approximately based on Lamé equations and incorporate a safety factor, which means that the pressure term has a multiplier These equations have the following form:

o 4f þ 0:8p The equations in ASME VIII Division 1 are given in clause UG-27 and Appendix 1-1 When the thickness does not exceed 0.356R or

P does not exceed 0.665SE, the minimum required thickness t for pressure loading is obtained from clause UG-27(d) for vessels with a known inside radius R or from Appendix 1, clauses 1-1(a)(2) for vessels with a known outside radius Ro

2SE − 0:2P ¼2SE þ 0:8P The MAWP is given by

R þ 0:2t ¼Ro− 0:8t For ‘thick spherical shells’, where the limitations on thickness or pressure given in clause UG–27(d) are exceeded, the equations in Appendixes 1–3 must be used The results are very close to those obtained using the Lamé equations

When the thickness exceeds 0.356R or P exceeds 0.665SE

4.05.3.1.3 Dished ends

When considering pressure containment, the ideal shape or form for the shell is spherical Therefore, when designing end closures, a hemisphere would be the obvious choice, especially if the vessels were subjected to a high internal pressure However, fabrication of hemispherical ends (and indeed, spherical vessels) is expensive, normally using a labor-intensive cap and petal method The most commonly used closures for pressure vessels are torispherical and ellipsoidal dished ends Ellipsoidal ends are usually specified as 2:1 (the ratio of major to minor axes), but other ratios may also be used (1.8:1 is commonly used in some European countries)

4.05.3.1.4 Nozzles and openings

Openings are required in pressure vessels to provide access to the vessel shell Although most openings provide a means for the contents to enter and exit the shell, access can also be required as part of the process or service inspection Openings can take the form of nozzles, sight glasses, handholes, and manways as well as even larger openings such as entry holes for large mechanical devices Openings are normally circular or partially elliptical, although some sight glasses can produce rectangular openings When an opening is present in a vessel, it produces enhanced stresses around the hole due to the discontinuity and it is therefore

a potential weakness Material can be added around the hole to recover the strength of the vessel When performing nozzle calculations, the basic objective is to select and provide suitable reinforcing to ensure adequate strength for the design loadings, which may comprise the pressure loading plus some additional mechanical loading

The traditional way of performing nozzle reinforcing calculations for an opening or nozzle is to provide material near the hole in excess of the required thickness for pressure loading It has been found from experience that for a section through the shell at the center of the opening, the cross-sectional area of the additional material must be at least equal to the area removed by cutting the hole in a shell of minimum thickness required for pressure This design approach, known as the ‘area-replacement method’, is used

by ASME VIII Division 1 and by a number of European codes

In the area-replacement method, a section of the shell within a specified ‘limit of reinforcement’ is considered The increased stress due to the opening is assumed to be uniformly distributed across the area of the shell plus the area of any additional reinforcing element within the limit of reinforcement, including the nozzle neck up to a specified distance from the outside of the shell To prevent this increased stress from exceeding the allowable limits, the total cross-sectional area available must not be less than the cross-sectional area of the ‘unpierced’ shell within the limits of reinforcement, multiplied by the ratio of the stress in the unpierced shell to the allowable stress The area available is the cross-sectional area of the shell (excluding the area of the opening) plus the nozzle and additional reinforcing element (such as a pad) within the limits of reinforcement

4.05.3.2 Composite Vessels

Composite materials have characteristics that are often very different from those of more conventional engineering materials As such, composite materials are becoming useful in a great number of industrial applications For example, they are used in the chemical industry, where pipes, tanks, pressure vessels, and storage vessels are being manufactured from fiber-reinforced compo­sites Often, it has been the case that these materials offer direct replacement of traditional metallic materials

Composite materials commonly used within the hydrogen industry are carbon or glass fiber-reinforced plastic (CFRP or GRP), where both weight and corrosion resistance are influential factors Composite pressure vessels are generally lightweight, being one-fifth the weight of steel and half the weight of aluminum

Composite vessels (GRP) having near-isotropic properties can be constructed by suspending a chopped strand mat (CSM) fiber matrix in a suitable polymer resin Orthotropic properties are normal for a laminated construction (An orthotropic material has properties that are different in three mutually perpendicular directions at a point in the body and it has three mutually perpendi­cular planes of symmetry Thus, the properties are a function of orientation at a point in the body.) When considering glass reinforcement, the matrix constituents can also comprise directional filament winding (FW) or woven roving (WR) produced from a weave of long fibers The properties of a composite material can thus be tailored to suit the intended application, by varying laminate thickness and the orientation and constituents of the individual lamina

Although vertical GRP vessels are widely used, the vessels considered here for hydrogen are principally designed for horizontal application Generally, horizontal vessels are employed where there is a restriction in height or when there is modest operating pressure Traditionally, horizontal, cylindrical vessels are supported by two supports located symmetrically about the mid-span of the vessel These systems have proved to be very efficient in the support of the traditional metallic vessels However, when the vessel is fabricated from GRP, the manufacturing processes often produce outer surface irregularities For large GRP vessels, twin supports, symmetrically placed, are also preferred, thus avoiding the transference of load, which occurs if differential settlement takes place in a multiple-support system Composite vessels are designed based on the allowable failure strain rather than the stress-based limitation as found in metallic vessels, which is typically two-thirds of the elastic yield strength of the material For GRP systems, a 2000 microstrain limit is applied, which is increased to 2500 for exceptional loads and test conditions For CFRP systems, this is much higher, set at 4000 microstrain and usually in the compressive failure mode

4.05.3.2.1 Composite vessels for hydrogen storage

Modern high-pressure hydrogen storage tanks can be significantly more complex in their design Tanks can be made up of several layers, each performing a specific function in the overall integrity of the system A multilayer sandwich style vessel may have an impact-resistant outer shell that provides resistance to damage and impact In addition, the domed end of the vessel can have a foam covering, again for impact protection Thereafter, a carbon fiber composite shell supports the main structural pressure loading, and a high-molecular-weight polymer lining can be added to serve as a gas permeation barrier The system is completed with the addition

of a nozzle, providing access for an in-tank regulator which measures the pressure and temperature via a sensor

Although these vessels can be and are being made at present, there remain a number of technical challenges that need to be addressed before large volumes of low-cost equipment can be employed in the industry From a cost basis, the carbon fiber accounts for 40–80% of the total cost of a CFRP tank Development of a low-cost carbon fiber will facilitate greater use and deployability The use of improved sensor technology to provide ‘intelligent’ structures will lead to improved weight efficiency and costs The design burst criteria can therefore be reduced by 25% by reducing the burst ratio factor from 2.35 to 1.8 In addition, reducing the temperature even further can increase the energy density of the fuel

4.05.4 Codes and Standards and Best Practices

All hydrogen pressurized vessels, components, devices, apparatus, and/or systems should always be designed, manufactured, installed, commissioned, operated, and maintained (at regular intervals) as certified in accordance with local and international applicable codes and standards This section provides a short review of a number of codes and standards for compressed hydrogen

tanks and connectors including joints and fittings, as well as a discussion on some of the best practices After reading this section, the reader shall be able to define some of the most common codes and standards for hydrogen storage, connectors, and piping as well as the most frequent practices including some basic safety requirements

4.05.4.1 Storage Tanks

There are many issues that need particular attention when designing a hydrogen storage system One of the most common and well-recognized issues is ‘hydrogen embrittlement’ Hydrogen embrittlement is the process by which various metals become brittle and fracture following exposure to hydrogen Nowadays, many codes and standards exist to reduce the embrittlement issue and other well-documented problems when storing hydrogen These codes and standards can be divided into the following three categories:

• stationary storage systems codes and standards based on PED;

• mobile storage systems codes and standards based on Transport Pressure Equipment Directive (TPED);

• liquid hydrogen codes and standards

The above three categories of codes and standards agree that the selection of appropriate materials is key to the successful design and manufacture of long-lasting hydrogen storage tanks (with sometimes low to no embrittlement potential) To select the most appropriate material for use, the designer of a pressurized hydrogen storage tank, and associated piping systems, must focus on hydrogen interaction with materials used It is also important to understand that certain techniques used for finalizing the surface of

a hydrogen tank can result in hydrogen moving into the crystalline structure of the materials Hence, these techniques can accelerate deterioration of the material through the phenomenon of hydrogen embrittlement

The most common materials used for designing and manufacturing hydrogen storage tanks include copper, copper alloys, aluminum alloys, and the well-known stainless steels (the 316 type) A combination of aluminum and carbon fiber has recently been used It is important to note that nickel and nickel alloys must not be utilized due to their high hydrogen embrittlement potentials Similarly, cast iron-type piping and storage mechanism must not be used with hydrogen

4.05.4.1.1 Gas storage systems and cylinders best practice

Good engineering practice dictates that a good project is one that has been well documented This is also true for hydrogen storage mechanisms whereby documentation for each cylinder installed in the field should consist of a short description of the cylinder, the main list of drawings, and, most importantly, the most recent inspection results with the responsible person’s name and a contact phone number for emergency

It is of critical importance that any hydrogen system must have a naming plate displaying ‘hydrogen gas pressure cylinders in use’

or a similar inscription such as ‘pure hydrogen’ to

1 allow anyone not involved in the installed hydrogen storage system to know that there is high-pressure gas available in the vicinity,

2 promote safety,

3 remove any potential confusion during operation and an emergency

Figure 1 illustrates a set of hydrogen cylinders with a printed inscription on the cylinder

The display of the nameplate is very significant during any emergency situation where the firemen or any other emergency services will be able to define the risks and dangers by seeing the plate A common best practice is to display on-site a set of signs showing the following:

• A clear emergency phone number

Figure 2 shows a typical example of a sign that needs to be displayed on a hydrogen site

Any container that is designed and manufactured to contain pressurized gas must be marked with its corresponding code and standard In other words, when you want to manufacture a pressurized container, you must first select your code and standard Then you manufacture the container strictly following the code and standard Finally you must display the code and standard on the container There are two common methods to display the code and standard; either by stamping the code and standard on the cylinder itself or a nameplate is attached to it Figure 3 illustrates a sample of a cylinder nameplate summarizing its corresponding code and standard with other information The cylinders must be tagged with their certificate(s) for use and any special instruction Documentation should also be understood by the end user, and if necessary, the end user shall be appropriately trained To avoid any confusion, cylinders must have permanent stamped inscriptions (stamped into the cylinder or tank body) or plates

Figure 1 An example of an inscription on cylinders

Figure 2 An example of a sign displayed on a hydrogen site

permanently fastened In addition, when the cylinders are empty, they should be labeled so Figure 4 shows a cylinder with its permanent inscription The cylinder is used for calibration purposes and its content is hydrogen in air

Stationary storage systems must be installed on noncombustible supports, and if paint is used, this should comply with fire-retardant requirements Any hydrogen cylinders and associated storage system must be located outside in a well-ventilated area at a safe distance from any structures Country-tailored authorized safe distance should be checked, as this will be dependent on storage pressures, volumes, and nearby building and structure types

If hydrogen cylinders are used indoors (not recommended under normal circumstances), then one must follow a full risk assessment procedure and ensure that implementation is in accordance with local applicable standards For example, one shall use the Dangerous Substances & Explosive Atmospheres Regulations 2002 in the United Kingdom

When using compressed gas cylinders (such as the B-type or K-type hydrogen cylinders), these cylinders must be secured and stored vertically (using a restraint to avoid the cylinders being knocked over) in a well-ventilated area, preferably outdoors The area should be cool (say in a shaded location) A shaded and cool area will avoid an increase in the internal pressure of the cylinder under excessive heat exposure It is commonly known that when hydrogen cylinders are exposed to heat, such as with direct exposure to sunshine rays, the hydrogen gas will expand and the internal pressure of the cylinder will increase Figure 5 illustrates cylinders being secured vertically

Figure 3 A hydrogen cylinder marked with its code and standard

Figure 4 A permanent stamped inscription on a cylinder

Figure 5 Cylinders secured vertically and an example of cylinders not secured

Figure 6 A fencing system for storage of cylinders

One should aim to locate the cylinders in a restricted access area (using appropriate fencing and security systems) far away from any emergency or normal exits Figure 6 shows an example of a fencing system used for hydrogen (red tank) and nitrogen storage (gray bottle)

The area where the cylinders are located must be free of any combustible materials One should also avoid installing hydrogen cylinders nearby corrosive substances, highly salted environments (if at all possible), or highly wet surroundings to maintain the integrity of the cylinder to its maximum Installing cylinders in a dry area without corrosive substance will reduce early corrosion and premature rust Cylinders must be protected from rolling, dragging, and/or dropping The preferred method for moving single cylinders is to use two-wheeler hand-type trailers (Figure 7)

4.05.4.2 Connectors – Joints and Fittings for Hydrogen

There are many different standards for joints and fittings for hydrogen In essence, these standards together highlight the following best practices:

1 All joints and fittings must be checked for suitability for hydrogen usage prior to installation in the field and specifically dimensioned and selected for the particular operating conditions Hydrogen fittings mostly used by the industry are the stainless steel typ 316 Figure 8 illustrates a compression fitting

2 Joints and fittings have inherent hydrogen leaking potentials if not fixed appropriately Thus, codes and standards always prefer

to highlight welding pipes at joints point as the favored option instead of using mechanical compression joints Properly welded joints can provide a superior safety margin to prevent hydrogen leaks when compared with other mechanical joints Figure 9

illustrates a welded joint, whereas Figure 10 shows hydrogen fitting joints

3 Joints and fittings (such as compression/flange/screw type and others) other than welded-type joints are generally accepted by codes and standards In the case that nonwelded joints and fittings are used, then appropriate and demonstrable procedures must be put in place to ascertain and guarantee that leak testing is performed regularly When nonwelded joints are used, one must ensure that at installation time each individual joint and fitting has been installed accurately Means such as regular training should also be provided for correct hydrogen leak testing Supervised inspection must be performed prior to an installation being launched Figure 11 illustrates two bad joints after leak testing This is shown by the bubble The leak testing was carried out

Figure 7 A two-wheeler trailer for moving cylinders

Figure 8 A 316 SS compression fitting

Figure 9 A hydrogen welded joint

using a soapy substance, hence the bubble The joints are of threaded type, which have a higher incidence of hydrogen leakage as described below

4 For more than obvious reasons, the fewer the mechanical fittings and joints, the better the hydrogen system The fundamental nature of hydrogen, being the lightest gas, means that the fewer the joints, the less potential leakage will be available Also, the fewer the joints, the lower the maintenance cost for testing

Figure 10 Hydrogen threaded fitting joints

Figure 11 An example of bad joints

Figure 12 A crack on a hydrogen fitting

5 One shall not wrongly assume that a joint or fitting is tight Each individual joint or fitting shall be treated suspiciously as loose during installation and testing, reducing considerably the risk of failure This is a common error when one becomes acquainted with a system; hence, the level of vigilance drops Continuous training and warning of potential bad practice as well as good procedure shall be put in place to reduce the likelihood of leaks Overtightening is also a common issue with installations and can be the cause of cracks on joints and fittings Figure 12 illustrates such a crack on a fitting The cracking is very minor and can