How to...
How to...
select and install pipe flanges
Pipe systems in the irrigation sector use a number of different jointing systems dependent on the pipe material, the operating pressure of the system and the need to be able to dismantle the system for maintenance or operational reasons. Flanged connections satisfy all of the above criteria and are an essential jointing system in the piping Industry.- by Mike de Villiers
INTRODUCTION
Standard pipe jointing systems vary from solvent cement and integral rubber ring joints in PVC, to compression and butt-welded joints in HDPE, insert joints in LDPE and quick couple joints in removable surface irrigation systems. However, when it comes to the interconnection of differing pipe and fitting materials, the jointing systems have to be compatible with each other. For example, HDPE pipes will utilise a compression male adapter to connect the pipe to say a female brass valve. The most commonly used jointing systems for interconnections in the smaller diameters are BSP threaded fittings, whereas flanged connections are used in the larger diameters. High pressure valves and pumps are usually manufactured from ductile iron, a specialised form of cast iron. The need to remove these items from the line for maintenance or replacement makes the use of flanged connections an essential feature. Flanged connections also enable the direct connection of different pipe materials to these items over a wide range of diameters.
FLANGE SPECIFICATIONS
A flange is a round piece of steel or plastic, with holes drilled around its circumference. Two flanges are used to complete a joint. They are bolted together with a rubber gasket between the faces, in order to achieve a water-tight joint. However, flanges are not that simple. Apart from the pipe diameter, one needs to consider the pressure rating as well as the flange specification, also known as the“ flange drilling”.
South Africa has its technology rooted in Imperial dimensioning( inches). With the formalisation of the use of metric systems in the RSA in 1962, flange options became far more complex and we live with this history to this day. BS 10 table D flanges( Imperial dimensions) are historically very popular in the Irrigation sector, however metric flanges to SANS 1123 is the industry standard for valves and pumps in the RSA. Note that Table D flanges are pressure rated at 600 kPa( 6 bar) and that SANS 1123 has tables that cover the entire pressure range up to 4000 kPa( 40 bar).
Needless to say, one cannot directly connect a Table D flange to a SANS 1123 flange because of the different dimensions. Flange suppliers, however, hold stocks of blank flanges and drill these according to demand, which mitigates some of the historical preferences for imperial flanges.
The pressure rating of a flange is dependent on the thickness of the disc as well as the size and number of holes drilled in the flange. The holes are drilled to accommodate the securing bolts that, when tightened, hold the two flange faces together. The size of the bolts is a very important aspect of the flanged joint, as the bolts have to withstand the massive pressure differentials that are at play, especially in valves during opening and closing functions.
The holes are drilled around the flange in a circle and each flange size, corresponding to the pipe size, has a different diameter measured between the bolt centres. This is known as the Pitch Circle Diameter, or PCD of the flange. Some of the smaller flanges, called universal or multi-drilled flanges, are compatible with both Table D( Imperial) and Table 1600 / 3( metric) flange drillings, however multi-drilled flanges have a maximum pressure rating of 10 bar. Other drilling tables are available, such as BS 4504( Imperial) and ASME 16.5( American) but these are not common in the irrigation sector, they may be used on some specialised equipment. The“ tables” mentioned above refer to the dimensional tables contained in the flange specifications, which are shown below. The tables below show the popular flange sizes. Refer to the full tables for the complete range of flange sizes.
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SABI | APRIL / MAY 2017