GB/T 12621-2008Standard test method for stress relaxation of gaskets for pipe flanges (English PDF)

2.1 Test apparatus

The test is carried out on a dedicated stress relaxation test apparatus consisting of upper and lower circular platens, a central bolt, a push rod, a washer, a nut, a dial gauge and its assembly, as shown in Figure 1. The upper and lower circular platens, central bolt, push rod, washer and nut shall be made of 42CrMo or a material of equivalent properties. The central bolt shall be calibrated in accordance with Annex A.

Figure 1 - Gasket stress relaxation test apparatus for test method A. Key: 1 - upper and lower circular platens; 2 - central bolt; 3 - dial gauge assembly (including connecting sleeve, push rod and set screw); 4 - dial gauge; 5 - nut; 6 - washer; 7 - specimen.

2.2 Specimens

2.2.1

Before testing, the specimens shall be dried in a hot-air oven at 100 °C ± 2 °C for 1 h and then cooled to 21 °C to 30 °C in a desiccator containing a suitable desiccant.

2.2.2

Unless otherwise specified, the specimen shall be a ring with an outside diameter of 44 mm, an inside diameter of 22 mm and a thickness of 1.5 mm.

2.3 Test conditions

Compressive force: 23.8 kN;

Test temperature: unless otherwise specified, 100 °C ± 2 °C;

Test duration: unless otherwise specified, 22 h.

2.4 Test procedure

2.4.1

Check all parts of the test apparatus, clean the surfaces of the upper and lower circular platens, and lubricate the bolt threads and the washer with a suitable lubricant.

2.4.2

Install the specimen centrally between the two circular platens.

2.4.3

Fit the central bolt, washer and nut, and tighten the nut by hand.

2.4.4

Fit the dial gauge assembly and set the dial gauge reading to zero.

2.4.5

Using a spanner, tighten the nut continuously and uniformly until the dial gauge reading corresponding to the specified compressive force is reached, and record this reading D0. The loading time shall not exceed 3 s. For a compressive force of 23.8 kN, the typical elongation of the bolt is 0.109 mm to 0.113 mm.

2.4.6

Remove the dial gauge assembly. Unless otherwise specified, place the test apparatus with the specimen in a hot-air oven at 100 °C ± 2 °C, keep it at that temperature for 22 h, then remove it and allow it to cool to room temperature.

2.4.7

Refit the dial gauge assembly and set the dial gauge reading to zero.

2.4.8

Without disturbing the dial gauge, loosen the nut and record the dial gauge reading Df.

2.5 Number of tests

Select a number of specimens from the same sample and test not fewer than three specimens chosen at random.

2.6 Calculation of stress relaxation and test results

2.6.1

The stress relaxation is calculated according to formula (1):

Stress relaxation (%) = (D0 - Df) / D0 × 100 ... (1)

where:

D0 - dial gauge reading before the test apparatus is placed in the oven (i.e. the initial elongation of the bolt), in millimetres (mm);

Df - dial gauge reading after the test apparatus has been removed from the oven and cooled to room temperature (i.e. the residual elongation of the bolt), in millimetres (mm).

2.6.2

The mean of all tests is taken as the final test result, expressed to two significant figures.

3.1 Test apparatus

3.1.1

The test is carried out on a dedicated gasket stress relaxation test apparatus consisting of a central bolt, upper and lower flanges, a spacer block, a loading nut, a heating device and other parts, as shown in Figure 2. The load measuring system consists of a dial gauge mounted at the top of the central bolt, an adjusting screw at the bottom and the central measuring rod connected to it. The heating device consists of an electric heater, a thermocouple and a temperature controller. The test apparatus shall be calibrated in accordance with Annex B.

Figure 2 - Gasket stress relaxation test apparatus for test method B. Key: 1 - dial gauge; 2 - clamping sleeve; 3 - dial gauge holder; 4 - loading nut; 5 - loading bar; 6 - thrust ball bearing; 7 - central measuring rod; 8 - central bolt; 9 - upper flange; 10 - specimen; 11 - lower flange; 12 - fastening nut; 13 - support; 14 - adjusting screw; 15 - thermocouple; 16 - insulating jacket; 17 - pin; 18 - screw plug; 19 - heating wire (1 kW); 20 - insulating cover plate; 21 - spacer block.

3.1.2

The central bolt shall be made of 35CrMoA, quenched and tempered.

3.1.3

The load-deformation characteristics of the central bolt at 20 °C and at the test temperature shall be strictly calibrated and converted into a stress relaxation calculation chart with gasket stress and dial gauge reading as coordinates.

3.1.4

The control accuracy of the temperature control system shall be within ± 1 °C.

3.2 Specimens

3.2.1

The specimens shall be preconditioned as specified in 2.2.1.

3.2.2

Unless otherwise specified, the specimens shall be of nominal size DN 32 and nominal pressure not greater than PN 50.

3.3 Test conditions

Unless otherwise specified, the test conditions shall be as given in Table 1.

Table 1 - Test conditions (specimen type: gasket stress, MPa; test temperature, °C; test duration, h):

Asbestos rubber gaskets: 40 MPa; 300 °C; 16 h;

Non-asbestos rubber gaskets, PTFE gaskets and modified PTFE gaskets: 35 MPa; 200 °C; 16 h;

Expanded PTFE gaskets: 25 MPa; 200 °C; 16 h;

Flexible graphite composite gaskets: 35 MPa; 300 °C; 16 h;

Serrated metal, corrugated metal and corrugated-serrated metal gaskets with non-metallic facings: 45 MPa; 300 °C; 16 h;

Metal jacketed gaskets: 60 MPa; 300 °C; 16 h;

PTFE envelope gaskets: 35 MPa; 150 °C; 16 h;

Spiral wound gaskets (asbestos, non-asbestos or flexible graphite filler): 70 MPa; 300 °C; 16 h;

Spiral wound gaskets (PTFE filler): 70 MPa; 200 °C; 16 h.

3.4 Test procedure

3.4.1

Place the specimen centrally between the upper and lower flange faces and set the dial gauge reading to zero by means of the adjusting screw at the bottom of the central bolt.

3.4.2

Tighten the loading nut with the loading bar to compress the specimen until the dial gauge reading corresponds to the specified gasket stress. If after 5 min the dial gauge reading is found to have fallen, tighten the nut again to the initial reading and record this reading A1 (µm).

3.4.3

Switch on the electric heater and heat uniformly from room temperature to the test temperature within 1 h.

3.4.4

Hold at the test temperature for 16 h, adjusting the temperature controller so that the temperature fluctuation is less than 1.5 %. Record the dial gauge reading A2 (µm).

3.4.5

At the test temperature, unload the bolt and record the dial gauge reading A3 (µm).

3.5 Number of tests

Select a number of specimens from the same sample and test not fewer than three specimens chosen at random.

3.6 Calculation of stress relaxation and test results

3.6.1

From the readings A1, A2 and A3, obtain from the calibration curve of gasket stress versus dial gauge reading (see Figure B.3 in Annex B) the gasket stress SK (MPa) corresponding to A1 (i.e. the initial compressive stress of the gasket at 20 °C) and the gasket stress SG (MPa) corresponding to (A2 - A3) (i.e. the residual stress of the gasket).

3.6.2

The stress relaxation of the gasket is calculated according to formula (2):

Stress relaxation (%) = (SK - SG) / SK × 100 ... (2)

where:

SK - initial compressive stress of the gasket at 20 °C, in megapascals (MPa);

SG - residual stress of the gasket, in megapascals (MPa).

3.6.3

The mean of all tests is taken as the final test result, expressed to two significant figures.