Stainless steel return U bend

What’s a pipe bend?

Bend on process

A pipe bend is the generic term for what is called in piping as an “offset” – a change in direction of the piping. A bend is usually meant to mean nothing more than that there is a “bend”– a change in direction of the piping (usually for some specific reason) – but it lacks specific, engineering definition as to direction and degree. Bends are usually custom-made (using a bending machine) on site and suited for a specific need.

Pipe bends typically have a minimum bending radius of 1.5 times pipe radius (R). If this bending radius is less than 1.5R, it is called Elbow. Reference to any international / industry standard need to be traced. 1.5, 3 and 4.5 R are the most common bending radii in industry.

A pipe bend typically flows smoother since there are not irregular surfaces on the inside of the pipe, nor does the fluid have to change direction abruptly.

The most basic difference of them is the elbow relatively short than bend, R = 1D to 2 D is elbow More than 2D is bend. In the production process, cold bends can use Bending Machine to bend by ready-made straight bend. One-time completed also don’t need second corrosion. But elbow need manufacturers make to order, to do anti-corrosion, order cycle is long. Elbow price is higher than bend. But cost performance is much higher than bend. It is well-known that bend do not have anticorrosive processing is easy damaged, but the price is cheap so are  used very much in some demand which not very high engineering.

Pipe bends are used in many industries, including chemical, oil and gas, electric, metallurgy and shipbuilding. Pipe bends function as structural passageways to facilitate the transfer of substances, such as water and fuel. Some have a short radius while others have a long radius. Long radius bends give less frictional resistance and allow for less pressure drop when compared to short radius bends. When selecting the type of pipe bend, it’s important to choose one that is compatible to the application.

The Radii of Bend Pipes

Bend pipes come in all types of radii, including 3D bends, 4D bends, 5D bends, 8D bends and 10D bends. The radius in 5D bends is five times the nominal diameter. With a 10-inch diameter pipe, the radius of the centerline of the bend would be 50 inches. Pipe bending for the specified radii is both art and science. And with today’s modern machines and advanced software, pipe bending is highly precise. With the right machine, lubrication, tooling and material, achieving the perfect bend is a sure thing.

The Differences Between Bends and Elbows

Although the words bends and elbows are often used as synonyms, there are some differences. Bend is a term for any offset of direction in the piping while elbow is an engineering term. Elbows have limitations to angle, bend radius and size. Most angles are either 45 degrees or 90 degrees. All other offsets are specifically pipe bends. And while elbows have sharp corners, bends never do. The most basic different between a bend and an elbow is the radius of curvature. Bends have a radius more than twice the diameter, and elbows have a radius of curvature between one and two times the size of the pipe’s diameter. All elbows are bends, but not all bends are elbows.

Induction Bending for Creating 5D Bends

Induction bending is one of the means of bending pipes for 5D and other bends. Local heating, using high-frequency induced electrical power, is applied. An induction coil is placed around the pipe and heats a circumferential area of the pipe at a temperature between 850 to 1100 degrees Celsius. When the right temperature is reached, the pipe moves through an induction coil while an arm applies the bending force. There are many benefits to induction bending. It allows for large radii for smooth flow of fluid, reduces the number of welds in a system and fabricates bends quickly. With faster production, efficiency is ramped up. It’s also a clean process, as no lubrication is needed, and water is recycled.

Ram-Type Bending

Ram-type bending is an old and effective bending method for bending pipes, including 4D bends and 5D bends. This method is often used in muffler shops. It’s also one of the least expensive ways to bend pipe. A hydraulically driven ram forces the pipe against pivot blocks or rollers. Often, a ram tool is used to produce a concave surface and to prevent stretching on the exterior of the bend. The only downside is that ram-type bending is not as controllable as other methods.

Roll Bending

Roll bending is commonly used for pipes in the construction industry. Rolls are positioned vertically or horizontally to produce very large radii. The pinch-style roll bender is one of the machine types used for roll bending. A tube feeds between the lower and upper roll to produce the wanted bend angle. Some applications may require an additional roll to guide the tube outward when the coil is being formed.

Mandrel Bending

The mandrel bending pipe method is effective when the least amount of deformation is desired. The pipe is supported with a mandrel support to bend the pipe. The pipe is drawn through a counter bending die for further bending. This method of pipe bending is used in the manufacture of heat exchanger tubing, dairy tubing and exhausts like turbocharger and custom made ones. This method produces a non-deformed diameter every time.

Rotary Draw Bending

Rotary draw bending is often used for bending pipe when a constant diameter and good finish are desired. The pipe is drawn through a stationary counter-bending die onto a fixed radius former die. It’s used for roll cages, stock car chassis and other types of pipes.

Centerline Radius, Outside Diameter and Wall Thickness

Centerline radius, outside diameter and wall thickness are important variables with bending pipe methods. Plus, every pipe schedule has a nominal wall thickness. And since wall thickness can vary, any variations need to be accounted for. Other bending variables include the neutral line, outside bend radius and inside bend radius. Pipes experience spring back after bending. The harder the pipe and small the bend’s centerline radius, the more spring back, which results in radial growth. Copper pipes have less radial growth than steel pipe due to its less spring back. In pipe bending, consistency, size and quality of the weld seam matter for centerline radius, outside diameter and wall thickness. When these are right, the perfect bend can be created.

Material Used for 5D Bends, 6D Bends and Others

Common materials used for pipes and pipe bends include carbon, stainless steel, titanium, aluminum and cast iron. There are also plastic pipes of various grades. It’s important to select the right material for the right application, including the pipe fittings. Pipe fitting materials include aluminum, brass, bronze, cast iron, vitrified clay and many others. Pipe fittings must satisfy many criteria. The materials must be compatible with the fluids being transported, different pressure levels and fluctuating temperatures. Materials should also conform to certain standards like DIN, ASME, NPT and BSP. Surface finish is also important, and pipe fittings should have a corrosion-resistant property.

In some fields, the schedule 40 and schedule 80 are used for pipes. However, there are many other schedules due to increased pressure demands. The schedule of a pipe refers to its pressure rating. The higher the schedule, the higher pressure it can have. As the schedule increases, the wall thickness increases. When choosing pipe bends, it is critical to consider the schedule of the pipe. Schedule 80 is often used for heavy-duty while schedule 40 is often used for light-duty. Be sure to choose the appropriate schedule for the pipe bend, and it will be a perfect match.

Figure. Flow in a standard, long-radius bend is illustrated here, with typical flow patterns, wear points and reacceleration zone shown

Bend geometry

Pipe bends can take a variety of different geometries, which can have a significant influence on particle impact angle. Basic long-radius bends are the most commonly used because they provide the most gradual change in direction for solids, and because the angle of impact on the pipe wall is relatively small, which helps to minimize the risk of attrition or erosion.

Common-radius bends are made by bending standard tubes or pipes (Figure). The radius of curvature, RB, may range from 1 to 24 times the tube diameter, D. Common-radius bends can be loosely classified as follows: Elbow (RB /D = 1 to 2.5); Short radius RB /D = 3 to 7; Long-radius (RB /D = 8 to 14; Long sweep (RB /D = 15 to 24).

Pressure drop associated with bends

As particle impacts occur, particularly against bends, there will be a significant reduction in particle velocity. These particles will then have to be re-accelerated back to their terminal velocity, which will add significantly to the pressure drop — and hence, energy loss — for the conveying system. This is particularly true after short-radius bends.

The pressure drop in a bend depends on the ratio of bend radius to pipe diameter, the gas velocity, Ug, and the internal roughness, k, of the pipe. When a two-phase, gas-solid suspension undergoes a directional change in a pipeline, the bend naturally acts as a segregator of the two phases. Centrifugal forces act on the particles, concentrating them near the outer wall of the bend. Friction coefficients within the bend will be different than those in the adjacent straight sections.

Size

The bend is used to change the direction of run of pipe.it advantage is can matach long distance transition requirements,so it is commonly that bends dimension according to customer design.

Constants for Pipe Bends:

Formula:L = R x BL = Length of pipe requiredR = Radius of bendB = Constant from table used to find “L”L =30 x 1.5705 =47.115 in.or 47-1/8”

Standards accord to：

• ASME B16.9
• ASME B16.28
• ANSI/ASME B16.25
• MSS SP-97

Pressure: SCH5 to SCH160

Bending radius(R): R=3D, 5D, 7D and 12D
Bending angle (θ):15°, 30°, 45°, 60°, 90°, 135°, 180°

Outer diamete(D): D≤1800mm
Wall thickness(T): T≤120mm
Straight Length (L): The length between two ends general from 300mm-1500mm

Example: Find the length of pipe required to make a 90 bend with a radius of 30"

Delivery

Inspection

Visual Inspection is conducted on fittings to check any surface imperfections. Both fittings body and weld are checked for any visible surface imperfections such as dents, die marks, porosity, undercuts, etc. Acceptance as per applicable standard.

Packing

For packing of carbon steel flanges with painting,we would use the bubble wrap to protect the painting.For flanges without painting or oiled with long-term shipment,we would suggest client to use the anti-tarnish paper and plastic bag to prevent the rust.