Sylvie Willis
When it comes to diesel fuel, there are a variety of fittings and hoses to consider. The type of fitting used depends on the specific application and environment. For example, brass fittings are commonly used for diesel applications, but bronze or stainless steel fittings are also options. It is important to avoid galvanised materials and copper lines, as they can cause issues with diesel fuel. Additionally, the location of the fuel line, such as above or below deck, will determine the grade of the fuel line. There are four grades of fuel lines: A1, A2, B1, and B2, with A1 offering the highest fire resistance and B-type fuel hoses being more permeable, making them suitable for less volatile fuels like diesel.
| Characteristics | Values |
|---|---|
| Thread type | NPT, NPTF, UNF, AN |
| Thread sealant | Must be chemically resistant to diesel |
| Hose clamps | Marine-grade stainless steel |
| Hose type | A1, A2, B1, B2 (A1 for below deck) |
| Pipe material | Brass, stainless steel, bronze, reinforced plastic, rubber |
| Pipe fittings | Must not be made of galvanized material |
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What You'll Learn
Thread types: NPT, NPTF, UNF, AN
Thread types are an important consideration when dealing with diesel fuel fittings, as they ensure optimum compatibility and functionality. Here is some information on four thread types: NPT, NPTF, UNF, and AN.
NPT (National Pipe Thread)
The NPT thread type is defined by the ANSI/ASME standard B1.20.1. It has a taper rate of 1 inch of diameter in 16 inches of length (3/4 inch per foot or 62.5 millimetres per metre). NPT threads form a seal when torqued, as the flanks of the threads compress against each other. This design means that NPT fittings should be burr-free and lubricated with a lubricant or thread seal tape. Creating a leak-free NPT connection involves the use of thread sealant tapes.
NPTF (National Pipe Thread Fuel)
NPTF, also known as Dryseal American National Standard Taper Pipe Thread, is defined by ASME B1.20.3. It is designed to provide a more leak-free seal compared to NPT, without the need for PTFE tape or other sealant compounds. NPTF threads have the same basic shape as NPT threads but with adjusted crest and root heights, eliminating the spiral leakage path. NPTF threads have two classes: Class 1 and Class 2.
UNF (Unified National Fine Thread)
UNF threads are a type of Unified Thread Standard, along with UNC (Unified National Coarse Thread). Unified threads come in three different classes, with varying tolerances to accommodate applications requiring different levels of fit accuracy and thread element precision. UNF threads are parallel and have a flank angle of 60 degrees.
AN Threads
AN threads are commonly found on pneumatic components such as air cylinders, valves, and air preparation units. They are available in different port thread types, including NPT, PT, and G port threads. Choosing the correct AN thread type is essential for compatibility with specific equipment.
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Materials: brass, stainless steel, bronze, copper, rubber
When it comes to diesel fuel fittings, there are a variety of materials to choose from, each with its own advantages and considerations. Here is an overview of the commonly mentioned materials: brass, stainless steel, bronze, copper, and rubber.
Brass is a commonly used material for diesel fuel fittings. It is known for its durability and compatibility with diesel fuel. Brass fittings are often chosen for their longevity and resistance to corrosion. Some sources mention that brass is suitable for low-pressure areas of the fuel system, while it may not be recommended for high-pressure components. Brass is generally considered a safe option for diesel fuel tanks and connections, provided that there is no seawater exposure.
Stainless steel is another option for diesel fuel fittings. It is known for its strength and resistance to corrosion. Stainless steel fittings may be more resistant to fatigue cracking compared to brass, making them a durable choice. However, some users have mentioned that stainless steel fittings can still be susceptible to corrosion over time, which was a reason for replacement.
Bronze is also mentioned as a material option for diesel fuel fittings, particularly in the context of boats. Bronze fittings may offer similar benefits to stainless steel in terms of durability and corrosion resistance. However, some users question the necessity of using bronze fittings, as brass is generally considered compatible with diesel fuel.
Copper is specifically mentioned as a material to avoid for diesel fuel fittings. There are suggestions that copper may have issues with biodiesel, potentially degrading the fuel. Additionally, copper is not favoured due to its tendency to work-harden, which can make it less durable compared to other materials.
Rubber, specifically fire-resistant rubber hoses, are mentioned as a favourable option for diesel fuel applications. Rubber hoses are MCA-approved and are known for their flexibility and impact resistance. They are considered a safe option and are less susceptible to work-hardening compared to metals like copper.
It is important to consult manufacturer guidelines, industry standards, and professional advice when selecting fitting materials for diesel fuel applications, as specific recommendations may vary depending on the context and requirements of the system.
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Hose grades: A1, A2, B1, B2
When selecting a fuel hose type, it is important to first consider where the fuel line will be located. This could be above or below deck. Above deck means in the open, such as an outboard engine connected to an above-deck tank. Below deck includes not only areas below the deck but also hoses running through confined compartments.
There are four grades of fuel line: A1, A2, B1, and B2. A1 fuel hose should be used below deck on the high-pressure side (after the pump). It offers the highest fire resistance and the least permeability. A2 has lower fire resistance and a higher permeation rate.
B-type fuel hose is more permeable and used for less volatile fuels like diesel. B1 has higher fire resistance than B2. B1 is designed to have fuel in the hose at all times and can be used in both above and below deck. B2 is a fuel vent hose without a fire-resistant cover and is not designed to have fuel in the hose at all times.
The ""A" designation means the hose can be used in enclosed spaces, while "B" means it should only be used in open, above-deck spaces. The numerical suffix indicates whether the hose is designed to have fuel in it all the time ("1") or not ("2").
To summarise, A1 is the best choice for below-deck applications due to its high fire resistance and low permeability. B1 is suitable for diesel engines and can be used in both above and below-deck locations. B2 should be avoided for diesel fuel as it is not designed to be in constant contact with fuel.
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Hose types: cotton over braided, smooth, multi-fuel, nylon, clear braid
When it comes to choosing the right hose for diesel fuel, it's important to consider the specific application and quality standards to ensure safety and durability. Here's an overview of the different hose types mentioned:
Cotton Over Braided Hose
Cotton over braided hoses are designed to be compatible with unleaded petrol and diesel fuel types. They are known for their durability and ability to withstand high pressure. This type of hose meets the DIN 73379 Type B standard, ensuring its suitability for diesel fuel applications.
Smooth Hose
While not explicitly mentioned in the context of diesel fuel, smooth hoses are generally designed for applications that require minimal flow resistance and a reduced risk of blockage. They have a consistent inner diameter, ensuring smooth fluid flow.
Multi-Fuel Hose
Multi-fuel hoses, as the name suggests, are designed to be compatible with various fuel types, including diesel, gasoline, and fuel oil. They are manufactured to supply oil-based products and are used in a range of marine and fuel extraction applications. Multi-fuel hoses are rated for specific applications using a combination of letters and numbers to maintain safety and quality standards.
Nylon Hose
Nylon hoses are known for their durability and flexibility. While nylon hoses are available for fuel applications, it is important to ensure they are specifically designed for diesel fuel. Nylon hoses can provide resistance to oil-based products and are often used in automotive and industrial settings.
Clear Braid Hose
Clear braid hoses are often sought for diesel fuel applications, but there are some concerns about their compatibility and long-term durability. Clear PVC hoses are sometimes used for short-term checks and troubleshooting, but they may not be suitable for permanent diesel fuel line applications. Aircraft supply stores may carry clear hoses designed for diesel-powered model aircraft, but these are typically small in diameter.
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Pipe types: straight thread, tapered thread
When selecting pipe fittings for diesel fuel, it is important to understand the different types of pipe threads: straight thread and tapered thread.
Straight thread pipe fittings, also known as National Pipe Straight (NPS), have a constant diameter along the pipe length and do not taper. NPS is a standard used for straight pipe threads and is designated NPSC (National Pipe Straight Coupling). Straight threads are commonly used in applications where a sealed fit is not required, such as mechanical and structural connections, couplings, and flanges. They are also used in sanitary fittings as they do not require physical tightening tools that can cause damage to the fittings. NPS threads have specific dimensions, with 1 inch of thread equal to 25.4 mm, and they meet the B1.20.2 standard for general-purpose pipe threads.
To create a tight seal with NPS threads, a gasket or an O-ring is typically used. Unlike tapered threads, straight threads do not provide a sealing function on their own. NPS threads are commonly used in conjunction with NPT threads, but over time their tightening can wear off due to the tape used with NPT threads. Therefore, the application of straight threads may vary depending on the specific circumstances.
Tapered thread pipe fittings, on the other hand, are commonly used for sealing pipes and fittings as they create stronger and leak-resistant connections. The two most common standards for tapered pipe threads are NPT (National Pipe Taper) and BSPT (British Standard Pipe Taper). NPT is the most commonly used tapered pipe thread in the United States and Canada and is defined by ANSI/ASME standard B1.20.1. The taper rate for all NPT threads is 1/16 (3/4 inch per foot), and the commonly used sizes range from 1/8 inch to 2 inches.
Tapered threads help make better seals as the male and female threads compress and wedge themselves together. However, sealants are often used with tapered threads to fill any voids between the threads and create a completely leak-free connection. Tapered threads are primarily used to carry liquids and gases, ensuring gas-tight and liquid-tight connections.
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Frequently asked questions
There are two types of thread forms: Straight Thread and Tapered Thread. The former has threads that run parallel and the latter has threads that are angled at 2°.
Brass, stainless steel, bronze, copper, and galvanized metal fittings are used for diesel fuel.
Fire-resistant rubber hoses of an MCA-approved standard are better than metal as they are more durable. Cotton over braided and smooth fuel hoses are also suitable for diesel fuel.
When choosing a sealant, look for its chemical resistance to diesel fuel or oil running through the line. Form-a-gasket No. 3 is resistant to solvent and gasoline.
AN, an abbreviation for Army-Navy or Air Force-Navy Aeronautical Standard, designates the outer diameter of the hose or tubing in 1/16" increments.
