How to determine the tightening torque of a flange?

CONTENTS OF THE ARTICLE

TORQUE THEORY
ASSEMBLY
CALCULATION OF TORQUE
TABLES

TIGHTENING TORQUE THEORY

In this article we will teach you to determine the minimum torque applicable to the screws that join the flat gaskets and the flanges .

References:

Tightening torque methodology

Orifice gaskets require initial preload in high pressure systems. We must bear in mind that the internal pressure, the moment of thermal concentration and the moment of dead load are the loads that generally cause more losses in the joint.

To apply the torque we will use impact wrenches , torque wrenches or torque wrenches. The torque that we must apply is governed by the following variables:

Nut and bolt class
Burr of the nuts
Lubrication
Dust, chips and dirt on bolts and nuts
Notches
State of the flange surface on which to rotate

Adequate lubrication can increase efficiency compared to an unlubricated assembly of approximately 50% . Heavy graphite or oil mixture will suffice for normal applications.

Coefficients of friction of nuts and bolts:

Materials	Lubricants	Friction µ ± 20%
Steel	Graphite, petroleum or oil	0.07
Steel	Molybdenum disulfide grease	0.11
Steel	Machine oil	0.15
Cadmium Plated Steel	Does not require	0.12
Zinc plated steel	Does not require	0.17
Steel bronce	Does not require	0.15
Corrosion resistant alloy or nickel or silver based	Does not require	0.14
Titanium steel	Graphite in petroleum	0.08
Titanium	Molybdenum disulfide grease	0.10

Relevance of preload

The preload in the flanged end holes must be high enough to keep the assembly in contact and under pressure. Decreased pressure could result in leakage of pressurized liquid, loss of seal under cyclic loading conditions, and reduced seal life. In the following image we can see the relationship between the useful life of the seal and the preload .

An axially loaded flanged joint in which no preload was applied to the bolts is represented by the line OAB, in which the stress on the bolt is equal to that exerted on the joint. When the stress on the joint varies between Pa and Pb, the load variation on the bolt is represented by the section on the ordinate Pba and PBb. However, with the application of a preload, the load variation on the bolts will be lighter than that of the joint (as indicated by line PB1A, with less slope than OAB) because part of the load is absorbed as a reduction. compression of the gasket. In this way, the axial load applied to the bolts varies between PBa ‘and PBb’ when the modification of forces on the joint is between Pa and Pb. These conditions result in a considerable reduction of the variation of the cyclical load of the bolts which pays off in an extension of the useful life of the joint.

ASSEMBLY

For an effective seal, all components of the flange connection must be optimally mounted, the most common cause of a leak is improper mounting. Before starting to assemble, we must pay attention to the angularity and centering tolerances related to the pair of flanges that make up the set. The use of excessive force will lead to a deterioration of the joint with the proclivity to generate losses.

Assembly sequence

Place the gasket on the surface of the flange to be sealed.
Position the other flange in contact with the gasket.
Clean the studs and lubricate them properly, with a mixture of oil and graphite for example.
Place these studs in the holes.
Approach the nuts by hand.
Follow the adjustment sequence according to the figures.
Do not exceed 30% of the recommended torque values for the studs, as it may cause a bad seating of the joint.
Once the recommended torque according to the table has been reached, make a pass in hourly sequence for verification.
Due to stress relaxation and creep, it is essential to pre-tighten the studs to ensure the correct torque value during system operation.

CALCULATION OF TIGHTENING TORQUE IN FLAT JOINTS

To determine the torque necessary to apply, first of all we will determine the required surface pressure that must be applied to the joint to achieve an adequate seal, which will be:

Where:
Ecal: Calculated Surface Pressure in Bar.
Emin: Minimum Surface Pressure in Bar.
R: Radial Pressure where R = m. Pd.
y: Set Pressure in Bar.
H: Force that tends to separate the flanges H = Ai. Pd.
A: Gasket contact area.
Pd: Design Pressure in Bar.
Ai: Internal area of the duct.

El torque estará dado por la siguiente expresión:

Where:

Ejun: Surface pressure of the joint in Bar.-
A: Contact area of the joint.
d: Effective diameter of the Stud.
N: Number of Studs.

The torque to be applied to the studs of the flanges equipped with flat gaskets must be the one with the highest value, comparing the minimum with the calculation.

DETERMINATION OF TIGHTENING TORQUE FOR SCREWS

First of all, we will calculate the torques to apply to the flange bolts, which allow obtaining a surface pressure to be exerted on the joint, in such a way that a correct sealing of the joint is ensured, based on the pressure of design corresponding to the classes of pipes, their values being the following:

MINIMUM AND CALCULATED TORQUE FOR FLANGES ANSI S-150#

DN	Db	Dr	N	di	de	e	Pa	m	Pd	Ai	Ecal.	Tcal	Tmin
pulg.	mm.	mm.	c/u.	mm.	mm.	mm.	Bar	-	Bar	cm2	Bar	Nm.	Nm.
1/2	12.7	10.17	4	22.0	34.9	1.60	400	4.00	19.50	3.80	490.86	14	12
3/4	12.7	10.17	4	27.8	42.9	1.60	400	4.40	19.50	6.07	492.12	21	17
1	12.7	10.17	4	34.5	50.8	1.60	400	4.40	19.50	9.35	494.69	27	22
1 1/2	12.7	10.17	4	49.6	73.0	1.60	400	4.40	19.50	19.32	494.72	57	46
2	15.9	12.88	4	61.9	92.1	1.60	400	4.40	19.50	30.09	494.07	116	94
2 1/2	15.9	12.88	4	74.6	104.8	1.60	400	4.40	19.50	43.71	498.03	136	110
3	15.9	12.88	4	90.7	127.0	1.60	400	4.40	19.50	64.61	498.30	199	160
4	15.9	12.88	8	116.1	157.2	1.60	400	4.40	19.50	105.87	501.40	142	114
6	19.1	15.75	8	170.7	215.9	1.60	400	4.40	19.50	228.85	510.52	276	216
8	19.1	15.75	8	221.5	269.9	1.60	400	4.40	19.50	385.33	518.23	381	294
10	22.2	18.55	12	270.3	323.8	1.60	400	4.40	19.50	573.83	522.82	404	309
12	22.2	18.55	12	327.0	381.0	1.60	400	4.40	19.50	839.82	532.54	494	371
14	25.4	21.27	12	350.3	412.7	1.60	400	4.40	19.50	963.77	528.26	700	530

MINIMUM AND CALCULATED TORQUE FOR FLANGES ANSI S-300#

DN	Db	Dr	N	di	de	e	Pa	m	Pd	Ai	Ecal.	Tcal	Tmin
pulg.	mm.	mm.	c/u.	mm.	mm.	mm.	Bar	-	Bar	cm2	Bar	Nm.	Nm.
1/2	12.7	10.17	4	22.0	34.9	1.60	400	4.00	49.90	3.80	632.50	19	12
3/4	15.9	12.88	4	27.0	42.9	1.60	400	4.40	49.90	5.73	632.33	36	22
1	15.9	12.88	4	34.5	51.0	1.60	400	4.40	49.90	9.35	641.70	46	29
1 1/2	19.1	15.75	4	49.6	73.0	1.60	400	4.40	49.90	19.32	642.39	114	71
2	15.9	12.88	8	61.9	92.1	1.60	400	4.40	49.90	30.09	640.71	75	47
2 1/2	19.1	15.75	8	74.6	104.8	1.60	400	4.40	49.90	43.71	650.86	109	67
3	19.1	15.75	8	90.7	127.0	1.60	400	4.40	49.90	64.61	651.55	159	98
4	19.1	15.75	8	116.1	157.2	1.60	400	4.40	49.90	105.87	659.48	229	139
6	19.1	15.75	12	170.7	215.9	1.60	400	4.40	49.90	228.85	682.81	246	144
8	22.2	18.55	12	221.5	269.9	1.60	400	4.40	49.90	385.33	702.54	406	231
10	25.4	21.27	16	270.3	323.8	1.60	400	4.40	49.90	573.83	714.30	474	266
12	28.6	24.45	16	327.0	381.0	1.60	400	4.40	49.90	839.82	739.16	678	367
14	28.6	24.45	20	350.3	412.7	1.60	400	4.40	49.90	963.77	728.21	666	366
16	31.8	27.62	20	410.3	469.9	1.60	400	4.40	49.90	1322.19	759.73	865	455
18	31.8	27.62	24	461.8	533.4	1.60	400	4.40	49.90	1674.94	748.94	965	515

MINIMUM AND CALCULATED TORQUE FOR FLANGES S-600#

DN	Db	Dr	N	di	de	e	Pa	m	Pd	Ai	Ecal.	Tcal	Tmin
pulg.	mm.	mm.	c/u.	mm.	mm.	mm.	Bar	-	Bar	cm2	Bar	Nm.	Nm.
1/2	12.7	10.17	4	22.0	34.9	1.60	400	4.00	96.50	3.80	849.63	25	12
3/4	15.9	12.88	4	27.0	42.9	1.60	400	4.40	96.50	5.73	849.30	48	22
1	15.9	12.88	4	34.5	50.8	1.60	400	4.40	96.50	9.35	868.61	61	28
1 1/2	19.1	15.75	4	49.6	73.0	1.60	400	4.40	96.50	19.32	868.75	154	71
2	15.9	12.88	8	61.8	92.1	1.60	400	4.40	96.50	30.00	865.04	102	47
2 1/2	19.1	15.75	8	74.6	104.8	1.60	400	4.40	96.50	43.71	885.12	148	67
3	19.1	15.75	8	90.7	127.0	1.60	400	4.40	96.50	64.61	886.46	217	98
4	22.2	18.55	8	116.1	157.2	1.60	400	4.40	96.50	105.87	901.80	369	164
6	25.4	21.27	12	143.7	215.9	1.60	400	4.40	96.50	162.18	862.75	624	289
8	28.6	24.45	12	170.7	269.9	1.60	400	4.40	96.50	228.85	850.33	1190	560
10	31.8	27.62	16	221.5	323.8	1.60	400	4.40	96.50	385.33	870.87	1318	605
12	31.8	27.62	16	276.0	381.0	1.60	400	4.40	96.50	599.15	892.88	1334	598
14	34.9	30.80	20	327.0	412.7	1.60	400	4.40	96.50	839.82	948.77	1455	613
16	38.1	33.97	20	410.4	469.9	1.60	400	4.40	96.50	1322.83	1096.31	1532	559
18	41.3	37.15	20	461.8	533.4	1.60	400	4.40	96.50	1674.94	1074.81	2235	832
20	41.3	37.15	24	513.2	584.2	1.60	400	4.40	96.50	2068.54	1112.20	2107	758
24	47.6	43.49	24	615.9	692.1	1.60	400	4.40	96.50	2979.28	1153.27	3272	1135
30	50.8	46.67	28	730.3	857.0	1.60	400	4.40	96.50	4188.38	1041.81	5488	2107