The correct placement of gasket and proper tightening of bolts in a flanged joint will ensure a leak free joint. The following procedure describes how to achieve the same.
- PROCEDURE: –
Before actual tightening starts, the flange, gasket, bolts/nuts need to be observed as below:
- Before inserting the gasket in between the pipe flanges, check the flanges are parallel and co-axial (When none of the bolts is installed in the flange).
- As per ANSI B 31.3 clause no. 335-C, permitted tolerances are as follows: Flange faces shall be aligned to the design plane within 1mm in 200mm(1/16 in. /ft) measured across any diameter; flange bolts holes shall be aligned within 3mm (1/8 inch) maximum offset.
- If pipe flanges are not meeting the ANSI B31.3 requirement, piping shall be rectified.
- All gaskets seating area shall be cleaned properly before gasket insertion. If serration is damaged, reserration must be done.
2.2. GASKETS: –
- Ensure gasket of correct dimension and of specified material is used.
- Gasket shall be located correctly to ensure full sealing as indicated in drawing. For example – gasket on tubesheet of exchanger needs to be located so that outer periphery of gasket matches with OD of tubesheet at all points.
2.3 BOLTS / NUTS: –
- Ensure that material of all bolts and nuts is as specified.
- Bolts & nuts shall be cleaned with suitable solvents like Diesel / CTC using wire brush, especially in threaded portion.
- The bolt shall be lubricated with molybdenum di sulfide and it is essential especially in all bolts of size 7/8” and above to check the application as the bolt load developed by torque is dependent on the lubrication of threads.
- It is recommended to avoid use of short bolts on the flange joints.
2.4 TIGHTENING: –
- It is important that all bolted joints are tightened uniformly and in a diametrically staggered pattern (A typical sketch attached in Fig. 1).
- In case of pipe flanges 8″NB & upto 12″NB and having a pipe with spacer piece and minimum of two gaskets (which are getting compressed simultaneously during tightening) the distance between two flanges should be measured at four locations like 12’o clock, 3’o clock’ 6’o clock and 9’on clock positions and maintained equal for even tightening. The tightening shall be gradual and uniform. In case of flanges above 12” NB distance between the flanges shall be measured in 8 locations for uniform tightening.
- All stud bolts of 7/8″ and above shall be tightened using torque wrench. While tightening the bolt with torque wrenches (for pipe / Manway flanges) ensure that tightening load to each bolt is applied as uniformly as possible. The tightening shall be carried out in three or four stages in steps of 30, 50 & 70 percent of the final torque value. At fourth stage again use 70 percentage of the final torque value.
- For heat exchangers flanges torque values to be 30,70,100 and 100 percent of the final torque value.
- Where recommended to use hydraulic tensioner for bolt tightening follow the manufacturer recommendations.
A table is provided below to show an example of torques applied for bolt tightening.
TORQUE VALUES FOR STANDARD ANSI / MSS FLANGES 150lb
|NOMINAL SIZE(INCH)||BOLTING NO. / SIZE(INCH)||MIN. BOLT STRESS TO SEAL||RES. BOLT STRESS(PSI)||SUGGESTED APPLIED TORQUE VALUES (NM) BASED ON RESIDUAL BOLT STRESS & PUMP PRESSURE|
|*0.1||PUMP Psi||*0.13||PUMP Psi||*0.15||PUMP Psi||*0.2||PUMP Psi|
|12||12 X 7/8||25,338||48000||277||2039||365||2688||380||2801||515||3789|
|14||12 X 1||21,012||45000||388||2858||509||3748||532||3921||721||5317|
|16||16 X 1||19575||45000||388||2858||509||3748||532||3921||721||5317|
|18||16 X 1 1/8||18777||41160||404||2978||682||5027||747||5505||1013||7465|
|20||20 X 1 ¼||16321||42195||568||4187||700||5157||766||5645||1013||7465|
|24||20 X 1 ¼||13234||43250||785||3855||993||4874||1091||5357||1479||7264|
* CO-EFFICIENT OF FICTION DEPENDS ON LUBRICATION DONE ON BOLTS.
Please note that less toque (hardly 60%) is required when the bolts are properly lubricated
ASSUMED BOLT MATERIAL ASTM 193-B7; B16