A flanged joint is composed of three components; flanges, gaskets, and bolting; which are assembled by pipe fitter. Special controls are required in the selection and application of all these elements to attain a joint, which has acceptable leak tightness.
However, it is not advisable to use a flange connection in underground piping when it supposed to be buried. The flange is also the most common source of leak and fire in a process plant. There are a variety of flanges available to suit the requirements. A flange can be classified in several alternate ways as follows:
- Based on Types of Connection
- Based on Flange facing Types
- Based on Pressure Temperature Ratings
- Based on Material Types
Threaded Flanges are also known as screwed flange, and it is having a thread inside the flange bore which fits on the pipe with matching male thread on the pipe. This type of joint connection is Speedy and simple but not suitable for high presser and temperature applications. Threaded Flanges are mostly used in utility services such as air and water.
Socket-Weld Flanges has a female socket in which the pipe is fitted. Fillet welding is done from outside on the pipe. Generally, it is used in small bore piping and only suitable for low pressure and temperature application.
Slip-On flange has a hole with matching outside diameter of pipe from which pipe can pass. The flange is placed on pipe and fillet welded from both inside and outside. Slip-On Flange is suitable for low pressure and temperature application. This type of flange is available in large sizes also to connect big-bore piping with storage tank nozzles. Normally, these flanges are of forged construction and are provided with the hub. Sometimes, these flanges are fabricated from plates and are not provided with the hub.
Lap Joint Flanges
Lap flange is having two components, a stub end, and a loose backing flange. Stub end is butt-welded to the pipe and Backing flange freely moves over the pipe. The backing flange can be of a different material than stub material and normally of the carbon steel to save the cost. Lap flange is used where frequent dismantling is required, and space is constrained.
Weld neck flange is the most widely used type in process piping. It gives the highest level of joint integrity due to Butt-welded with a pipe. These types of flanges are used in high pressure and temperature application. Weld neck flanges are Bulky & costly with respect to other types of the flange.
The blind flange is a blank disc with a bolt hole. These types of flanges are used with another type of flange to isolate the piping system or to terminate the piping as an end. Blind flanges are also used as a manhole cover in the vessel.
Flange Facing Types
Based on Flange facing Types, it can be further classified as
- Flat Face (FF)
- Raised Face (RF)
- Ring Joint (RTJ)
- Tongue and groove (T&G)
- And Male and Female type
As the name suggests, the flat face flange has a flat face. Flat face flanges are used when the counter-flanges are flat face. This condition occurs mainly in connection to Cast Iron equipment, valves, and specialties. Full face gasket is used when a flat face flange is used.
Raised face flange has a small portion around the bore is raised from the face. The gasket seat on this raised face. The height of the raised face depends on the flange pressure-temperature rating that is known as a class of the flange. For 150# & 300# height of the raised face is 1/6” and above 300# it is 1/4”. The inside bore circle type of gasket is used with a raised face flange.
Ring joint type face flange has a specially designed grove in which metal gasket seat. This type of flange is used in high pressure and temperature services.
Serration on the Flange Face
The flange face has small grooves as you can see in the image. This machining is known as a serration. Flange faces can be smooth or serrated type. Which type of face to use it depends on the type of gasket and service of the fluid.
Smooth finish is used with a metallic gasket whereas a serrated finish is used with a non-metallic gasket. The soft material of gaskets is set in this serration and prevents liquid or gas from passing from the flange joint.
Serration can be spiral or concentric rings as you can see in the slide. Concentric rings type finishing is used when fluid is of very low density. If you use spiral type finish with very low-density fluid, it may find a leakage path through the spiral cavity.
Serration of flange face is specified in RMS (Root Mean Square) or AARH (Arithmetic Average Rough Height), the most common value of serrated face is 120-250 AARH. Comparator gauge is used to check the serration of the flange. In the image, you can see how gauge is used to verify the value of serration.
Flange Pressure-Temperature Class (Service Rating)
The flange is classified as per their pressure-temperature ratings which are designated as 150#, 300#, 400#, 600#, 900#, 1500# and 2500#. Large diameter flanges that are 24” to 60” are available up to 900# class. Pressure-temperature ratings are maximum allowable working gauge pressures in the bar & the temperatures in degrees Celsius.
Higher the rating, heavier the flange and can withstand higher pressure and temperature. When the temperature goes up, the pressure goes down, and vice versa. Please note that different material has different pressure ratings.
Flanges are manufactured from
- Carbon steel
- Low alloy steel
- Stainless steel
- Combination of Exotic materials (Stub) and other backing materials
List of materials used to manufacture flanges is covered in ASME B16.5 & B16.47.
- ASME B16.5 -Pipe Flanges and Flanged Fittings NPS ½” to 24”
- ASME B16.47 -Large Diameter Steel Flanges NPS 26” to 60”
Commonly used Forged material grads are
- Carbon Steel: – ASTM A105, ASTM A350 LF1/2, ASTM A181
- Alloy Steel: – ASTM A182F1 /F2 /F5 /F7 /F9 /F11 /F12 /F22
- Stainless Steel: – ASTM A182F6 /F304 /F304L /F316 /F316L/ F321/F347/F348
Following to be confirmed during an inspection of a flange
- Outer & Inner Diameter of body
- Bolt Circle & Bolt hole Diameter
- Hub Diameter & thickness of weld end
- Length of the Hub
- Straightness and alignment of the bolt hole
Permissible tolerances are given in B16.5 and B16.47 standards.
The following shall be marked on flange body
- Manufacturer logo
- ASTM material code
- Material Grade
- Service rating (Pressure-temperature Class))
- Thickness (Schedule)
- Heat No
- Special marking if any QT (Quenched and tempered) or W (Repair by welding)