GB/T 45182-2025Test procedures of gasket parameters based on leakage rate for the pipe flange joints (English PDF)

1 Scope

This document describes the test methods and test procedures for gasket parameters, including the maximum permissible gasket stress Qsmax, the unloading modulus of elasticity EG, the creep relaxation factor PQR, the change in gasket thickness due to creep, the minimum gasket stress on assembly Qmin(L), the minimum gasket stress under test conditions Qsmin(L), the axial coefficient of thermal expansion and the static friction factor.

This document applies to the parameter testing of non-metallic flat gaskets (rubber gaskets excepted), semi-metallic gaskets and metallic gaskets; gaskets of other shapes and sizes may also be tested by reference to this document.

2 Normative references

The contents of the following documents constitute indispensable provisions of this document through normative reference in the text. For dated references, only the edition corresponding to that date applies to this document; for undated references, the latest edition (including all amendments) applies to this document.

GB/T 9124.1 Steel pipe flanges - Part 1: PN series

GB/T 9124.2 Steel pipe flanges - Part 2: Class series

3 Terms and definitions

The following terms and definitions apply to this document.

3.1 maximum gasket stress, Qsmax. The maximum stress that may be applied to the gasket at a specified temperature without the gasket being crushed, fractured, failing in compression or suffering damage to its bearing surface.

3.2 minimum gasket stress required for leakage rate class L on assembly, Qmin(L). The minimum stress that has to be applied to the gasket when it is installed at room temperature to ensure that tightness class L is reached between the flange faces at the test internal pressure.

3.3 minimum gasket stress required for leakage rate class L under test conditions, Qsmin(L). The minimum gasket stress needed to maintain tightness class L under the test conditions of temperature and internal pressure.

3.4 tightness class, L. A range of leakage rates, or the maximum value of a defined leakage rate.

3.5 creep relaxation factor, PQR. A characterisation of the stress relaxation behaviour of a gasket exposed to the test temperature for a long period, expressed as the ratio of the residual gasket stress to the initial gasket stress.

3.6 unloading modulus of elasticity, EG. The ratio of stress to strain when the gasket is unloaded; its value is determined from the recovered thickness of the gasket when the initial compressive load is unloaded to one third of its value.

3.7 axial coefficient of thermal expansion. The expansion per unit length in the axial direction of the gasket produced by a unit change of temperature, at the service temperature and gasket stress.

3.8 change in gasket thickness due to creep. The change in thickness caused by creep between the completion of loading and the end of the test.

3.9 static friction factor. The ratio of the static friction force between the gasket and the flange sealing face to the normal force, under the test conditions.

4 General test requirements

4.1 Schematic diagrams of the general test rig, the compression and creep test rig, the room-temperature leakage test rig, and the leakage test rig permitting the use of replaceable platens are given in Annex A to Annex D respectively.

4.2 The test platens and the heating plates shall be stiff enough to ensure that they can carry the applied load without the deformation of the platen affecting the surface stress on the gasket. The modulus of elasticity of the platen material shall be 195 GPa to 210 GPa, the hardness of the sealing face shall be 40 HRC to 50 HRC, and the surface roughness Ra shall lie within the range 3.2 micrometres to 6.3 micrometres.

4.3 For gasket test pieces for raised-face flanges of nominal size DN 40 and nominal pressure PN 40, the dimensions of the test platens shall conform to GB/T 9124.1; for test pieces of nominal size DN 100 (NPS 4) and nominal pressure Class 300, the dimensions of the test platens shall conform to GB/T 9124.2. For gasket test pieces for tongue-and-groove flanges, the dimensions of the test platens shall conform to GB/T 9124.1 or GB/T 9124.2.

4.4 When the creep relaxation factor PQR and the maximum permissible gasket stress Qsmax are being measured, stainless steel foil or aluminium foil not thicker than 0.05 mm may be placed between the gasket and the platen to prevent sticking, and its use shall be recorded in the test report. The foil shall not be reused.

4.5 The compression of the gasket needs to be monitored during the test. To measure the compression of the gasket, a displacement transducer may be placed every 120 degrees around the circumference of the test platen, or a single transducer placed at the centre. Placing a displacement transducer at the centre to measure the compression of the gasket is not recommended for leakage tests.

4.6 The loading and unloading rate for polytetrafluoroethylene (PTFE) type gaskets shall be 0.1 MPa/s; the loading and unloading rate for all other gaskets shall be 0.5 MPa/s.

4.7 The heating rate for high-temperature tests is 2 degrees Celsius per minute.

4.8 For all tests, the initial thickness of the gasket is the thickness measured after a gasket stress of 1 MPa has been applied at room temperature and held for 1 min.

5 Test pieces

5.1.1 For gaskets of the PN series, a test piece of nominal size DN 40 and nominal pressure PN 40 should be selected; for gaskets of the Class series, a test piece of nominal size DN 100 (NPS 4) and nominal pressure Class 300 should be selected.

5.1.2 For sheet material, a test piece of inside diameter 49.0 mm and outside diameter 92.0 mm should be selected for the PN series, and a test piece of inside diameter 115.0 mm and outside diameter 181.0 mm for the Class series.

5.1.3 For gaskets for tongue-and-groove flanges, the dimensions of the test piece shall be diameters of 61 mm and 75 mm (PN series) or diameters of 54 mm and 73 mm (Class series). Some of the test parameters need to be adjusted or corrected in accordance with Annex E. The tolerances of the tongue-and-groove platens shall meet the following requirements: the tongue width shall carry a minus tolerance; the groove width shall carry a plus tolerance.

5.1.4 Where a tongue-and-groove sealing face is used for the test, this shall be stated in the test report.

5.1.5 Gaskets of other shapes and sizes may be selected as test pieces where required or by agreement between the parties.

5.2 Number of test pieces. Not fewer than three test pieces shall be selected at random from the same sample for testing.

5.3 Preconditioning. Before the test, the test pieces shall be kept for not less than 48 h in surroundings at a temperature of 23 degrees Celsius plus or minus 5 degrees Celsius and a relative humidity of 50 percent plus or minus 6 percent. Not more than 30 min shall elapse between removal of the test pieces from those surroundings and the start of the test.

5.4.1 The thickness of the gasket is measured with a gauge of accuracy not lower than 0.001 mm; the arithmetic mean of measurements at three points on equal arcs is taken as the result.

5.4.2 The diameters of the gasket, the inside diameter and the outside diameter, are measured with a gauge of accuracy not lower than 0.01 mm; the arithmetic mean of measurements at three points on equal arcs is taken as the result.

5.4.3 Where the gasket has an inner ring or a centring ring related to the sealing element, the thickness of that inner ring or centring ring shall also be measured and recorded.

6 Test conditions

6.1 The ambient temperature during the test shall be kept at 23 degrees Celsius plus or minus 5 degrees Celsius.

6.2 For room-temperature tests, the test temperature shall be kept at 23 degrees Celsius plus or minus 2 degrees Celsius.

6.3 For high-temperature tests, a suitable test temperature shall be determined from the material of the gasket under test; the recommended test temperatures are given in Table 1. Table 1 lists the following recommended temperatures, in degrees Celsius: 50, 100, 125, 150, 175, 200, 225, 250, 275, 300, 350, 400, 450, 500, 550 and 600.

7 Determination of the maximum permissible gasket stress Qsmax and of the unloading modulus of elasticity EG

7.1 Test temperature. The test may be carried out at room temperature or at one of the temperatures recommended in Table 1, but is not limited to those temperatures.

7.2.1 Load the gasket stress, calculated on the initial area of the gasket, at the appropriate rate (see 4.6) up to 20 MPa, hold for 5 min and record the gasket thickness.

7.2.2 For a high-temperature test, then raise the temperature at the appropriate heating rate (see 4.7) to the specified value, keeping the gasket stress unchanged during heating. Hold the temperature for 15 min and record the gasket thickness.

7.2.3 Unload the gasket stress at the appropriate rate (see 4.6) to 6.7 MPa, hold for 5 min and record the gasket thickness.

7.2.4 Load (see 7.2.1) and unload (see 7.2.3) the gasket cyclically at the gasket stresses given in Table 2, holding for 5 min and recording the gasket thickness on each occasion, until the gasket is crushed, until the maximum load of the testing machine is reached, or until the maximum gasket stress specified by the manufacturer is reached. Table 2 gives the gasket stresses in megapascals, pairing each loading step with the unloading step that follows it: loading 20 with unloading 6.7; 30 with 10; 40 with 13.3; 50 with 16.7; 60 with 20; 80 with 26.7; 100 with 33.3; 120 with 40; 140 with 46.7; 160 with 53.3; thereafter the loading stress increases by 20 MPa and the unloading stress by 6.7 MPa on each cycle.

7.2.5 Plot the temperature against time curve, the gasket stress against time curve and the gasket thickness against time curve shown in Figure 1. The symbols and item numbers in Figure 1 are: t, time in minutes; T, temperature in degrees Celsius; QA, gasket stress in megapascals; e, gasket thickness in micrometres; 1, temperature curve; 2, gasket stress curve; 3, gasket thickness curve; 4, crushing of the gasket; 5, Qsmax equal to 180 MPa.

7.3.1 The gasket stress corresponding to the loading cycle before the gasket is crushed is the maximum permissible gasket stress Qsmax at that temperature, as shown in Figure 1. Alternatively, plot the reduction in gasket thickness per unit of gasket stress against the gasket stress, as shown in Figure 2; the stress value corresponding to the cycle before the reduction increases sharply is the maximum permissible gasket stress Qsmax.

7.3.2 If no crushing of the gasket test piece is observed and there is no sign of damage, Qsmax may be taken to be the maximum stress applied.

7.3.3 If no crushing of the gasket test piece is observed but there are signs of damage, further tests need to be carried out following the procedure of Figure 3 to determine the value of Qsmax.

7.3.4 If, in that procedure, the inside bore of a DN 40 / PN 40 gasket test piece is reduced to less than 43 mm after the PQR test, or the inside bore of a DN 100 (NPS 4) / Class 300 test piece is reduced to less than 90 mm, the gasket stress is taken to have exceeded the value of Qsmax.

7.3.5 For spiral wound gaskets, the gasket stress at which the centring ring buckles or the wound layers burst is the maximum permissible gasket stress Qsmax.

7.4.1 As shown in Figure 4, plot the gasket stress against time curve for loading and unloading and the gasket stress against change in gasket thickness curve, then calculate the unloading modulus of elasticity for the various load cycles. The symbols and item numbers in Figure 4 are: 1, creep; t, time in minutes; QA, gasket stress in megapascals; the thickness term, change in gasket thickness in millimetres.

7.4.2 The value of the unloading modulus of elasticity EG (see Figure 5) is calculated by formula (1). The symbols in the formula are as follows: QA is the gasket stress in megapascals; the thickness term at QA is the gasket thickness after the pressure has been held at stress QA, in millimetres; the change term is the change in gasket thickness on unloading from QA to one third of QA, in millimetres.

8 Determination of the creep relaxation factor PQR and of the change in gasket thickness due to creep

8.1.1 Load the gasket under test at the appropriate loading rate (see 4.6) up to the specified gasket stress Q1.

8.1.2 Hold the load for 5 min and record the gasket thickness at that moment as the initial thickness of the creep stage; then raise the test temperature at the appropriate heating rate (see 4.7) to the specified value. During this process the displacement is held constant.

8.1.3 Hold the temperature unchanged for 4 h, and record the residual gasket stress Q2 and the final gasket thickness at that moment.

8.1.4 Calculate the creep relaxation factor and the change in gasket thickness due to creep.

8.1.5 Record the condition of the gasket after the test.

8.2 The creep relaxation factor PQR and the change in gasket thickness due to creep are calculated by formulae (2) and (3), the loaded area of the gasket AG being obtained from formula (4). The symbols are as follows. PQR is the creep relaxation factor. Q1 is the initial gasket stress, in megapascals. Q2 is the residual gasket stress, in megapascals. The creep term is the change in gasket thickness due to creep under the conditions K, Q1 and T, in millimetres, and the corresponding relaxation factor is the creep relaxation factor under the conditions K, Q1 and T. K is the stiffness of the test platen, in newtons per millimetre, the recommended stiffness being 500 kN/mm for PN series flanges and 1500 kN/mm for Class series flanges. T is the test temperature, in degrees Celsius. AG is the loaded area of the gasket, in square millimetres. Ds is the smallest of the outside diameter Do of the gasket, the outside diameter of contact between the test platen and the gasket, and the outside diameter of the sealing face of a raised-face flange, in millimetres. The corresponding inner term ds is the largest of the inside diameter Di of the gasket, the inside diameter of contact between the test platen and the gasket, and the inside diameter of the sealing face of a raised-face flange, in millimetres.

9 Determination of the minimum gasket stress on assembly Qmin(L) and of the minimum gasket stress under test conditions Qsmin(L)

9.1 Tightness class. The tightness class may be determined from Table 3, or determined by the party commissioning the test according to the needs of the test. The tightness classes of Table 3 may be extended further to L0.001, L0.0001 and so on. Table 3 pairs each tightness class with the corresponding range of leakage rate, in milligrams per metre per second: class L1.0, leakage rate not more than 1.0; class L0.1, not more than 0.1; class L0.01, not more than 0.01. Note: the leakage rate here is the quantity leaked per unit time per unit length, the unit length being the geometric mean of the circumference of the inner edge and that of the outer edge of the loaded face of the gasket.

9.2.1 The test gas shall be helium of purity not lower than 99 percent.

9.2.2 Different measuring methods may be used for the leakage test according to the tightness class. Where the leakage rate is below 0.001 milligrams per metre per second, measurement of the leakage rate with a helium mass spectrometer is recommended.

9.3.1 Load the effective gasket stress at the appropriate rate (see 4.6) up to 5 MPa and hold for 5 min; raise the pressure of the medium to 4 MPa, hold for 20 min or until the leakage rate is steady (the leakage rate changing by not more than 2 percent within 20 min), then measure the leakage rate.

9.3.2 Continue to load the effective gasket stress at the same rate up to 10 MPa, hold for 5 min or until the leakage rate is steady, then measure the leakage rate.

9.3.3 Unload the effective gasket stress at the appropriate rate (see 4.6) to 5 MPa, hold for 5 min or until the leakage rate is steady, then measure the leakage rate.

9.3.4 Load and unload the gasket cyclically at the effective gasket stresses given in Table 4, until the loading and unloading cycle at 160 MPa has been completed or until the next loading value would exceed Qsmax. Table 4 gives, for a medium pressure of 4 MPa, the effective gasket stress in megapascals at each loading step and the unloading steps that go with it: load 5, no unloading; load 10, unload to 5; load 20, unload to 10 and 5; load 40, unload to 20, 10 and 5; load 60, unload to 20, 10 and 5; load 80, unload to 40, 20, 10 and 5; load 100, unload to 40, 20, 10 and 5; load 120, no unloading; load 140, no unloading; load 160, unload to 80, 40, 20, 10 and 5.

9.4 Effective gasket stress. The effective gasket stress Qe is calculated by formula (5). The symbols are as follows: QA is the gasket stress on installation, in megapascals; P is the internal pressure of the medium, in megapascals, here equal to 4; ds is the largest of the inside diameter Di of the gasket, the inside diameter of contact between the test platen and the gasket, and the inside diameter of the sealing face of a raised-face flange, in millimetres; AG is the loaded area of the gasket, see formula (4), in square millimetres.

9.5.1 Plot the curve of effective gasket stress against leakage rate, as shown in Figure 6. The subscript of L in the figure indicates the maximum leakage rate permitted for that tightness class, and that value may change with the definition of the tightness class. The symbols and item numbers in Figure 6 are: Qe, effective gasket stress in megapascals; L, tightness class, in milligrams per metre per second; 1, measuring point; 2, loading curve; 3, unloading curve; 4, Qsmin(L) at L0.001; 5, Qsmin(L) at L0.0001.

9.5.2 The point at which the horizontal line for the specified tightness class L intersects the solid line in the figure, that is the loading curve, is the minimum gasket stress on assembly Qmin(L) needed to maintain tightness class L; the point at which it intersects the broken line, that is the unloading curve, is the minimum gasket stress under test conditions Qsmin(L) needed to maintain tightness class L.

9.5.3 The values of Qmin(L) and Qsmin(L) corresponding to other leakage rate classes may be obtained by interpolation.

9.6 Determination of Qsmin(L) at other internal pressures. Where another internal pressure is chosen for the test, the test is carried out according to the sequence of Table 4; depending on the internal pressure, several loading and unloading test points may be skipped as appropriate, so as to obtain the corresponding Qsmin(L), and the curve of effective gasket stress against internal pressure for the various tightness classes is plotted, as shown in Figure 7. The symbols in Figure 7 are: P, internal pressure of the medium, in megapascals; Qe, effective gasket stress, in megapascals.

9.7.1 The leakage curve obtained from each leakage test, as shown in Figure 6, shall be recorded in the test report.

9.7.2 The leakage curves may be used only to estimate the leakage rate under the test conditions; they shall not be used to calculate the leakage rate under actual operating conditions.

9.8.1 The rig that takes long-term heat ageing into account is shown in Figure F.1 of Annex F.

9.8.2 For materials for which heat ageing need not be considered, the high-temperature test may use the metallic materials and the elastomeric O-ring of the room-temperature leakage rig shown in Figure C.1; that O-ring may be used at temperatures up to and including 300 degrees Celsius. Testing at higher temperatures requires suitable materials to be chosen for the seals.

9.8.3 When the leakage rate of the gasket is measured at high temperature under different pressures, the pressure difference method is preferred, especially where an external vessel is used to raise the pressure.

10 Determination of the axial coefficient of thermal expansion

10.1 For metallic gaskets, the coefficient of thermal expansion of the material concerned is used directly.

10.2 For other types of gasket, the coefficient of thermal expansion of the flange metal may be used.

11 Determination of the static friction factor

The method for determining the static friction factor is given in Annex G.

12 Test report

The test report shall include at least the following: a) the number of this document; b) the type and specification of the gasket (for a semi-metallic gasket, its type and detailed construction); c) the dimensions of the gasket; d) the material of the gasket; e) the test curves; f) the gasket parameters or gasket performance and the corresponding test conditions (gasket stress, temperature, test duration, stiffness of the test platens simulating the flange and the like), including either the single value of the test result or the mean of several tests; g) the shape and dimensions of the gasket after the test and any change in the gasket, such as cracks, faults, delamination, blisters and deformation (photographs may be attached); h) where test platens other than those of this document are used, full details of them; i) the name of the tester; j) the test date; k) the name of the laboratory.