APPENDIX D—DEFINITION OF A SOUR ENVIRONMENT (REPRINTED FROM NACE STANDARD MR0175-94: STANDARD MATERIAL REQUIREMENTS SULFIDE STRESS CRACKING RESISTANT METALLIC MATERIALS FOR OILFIELD EQUIPMENT2")

Sour Environments

D.1.1 Sour Environments are defined as fluids containing water as a liquid and hydrogen sulfide exceeding the limits defined in Pars. D.I. 1.1 and D.I. 1.2; these environments may cause sulfide stress cracking (SSC) of susceptible materials.

CAUTION: It should be noted that highly susceptible mate­rials may fail in less severe environments. The SSC phe­nomenon is affected by complex interactions of parameters including:

a. chemical composition, strength, heat treatment, and mi-
crostructure of the material;

b. hydrogen ion concentration (pH) of the environment;
с hydrogen sulfide concentration and total pressure;

d. total tensile stress (applied plus residual);

e. temperature; and

f. time.

The user shall determine whether the environmental con­ditions fall within the scope of this standard. (Editorial Com­ment: The critical hydrogen sulfide levels in D.I.1.1 and D.I.I .2 and Figures D-l and D-2 were developed from data derived from low alloy steel.)

Sour Gas

Materials shall be selected to be resistant to SSC or the en­vironment should be controlled if the gas being handled is at a total pressure of 0.4 MPa (65 psia) or greater and if the par­tial pressure of hydrogen sulfide in the gas is greater than 0.0003 MPa (0.05 psia). Systems operating below 0.4 MPa (65 psia) total pressure or below 0.0003 MPa (0.05 psia) hy­drogen sulfide partial pressure are outside the scope of this standard. Partial pressure is determined by multiplying the mole fraction (mol % + 100) of hydrogen sulfide in the gas by the total system pressure. Figure D-l provides a conve­nient method for determining whether the partial pressure of

hydrogen sulfide in a sour environment exceeds 0.0003 MPa (0.05 psia). A few examples are provided:

a. partial pressure of hydrogen sulfide in a system contain­
ing 0.01 mol % hydrogen sulfide (100 ppm or 6.7 grains per
100 standard cubic feet [SCF]) at a total pressure of 7 MPa
(1,000 psia) exceeds 0.0003 MPa (0.05 psia) (Point A on
Figure D-l).

b. partial pressure of hydrogen sulfide in a system contain­
ing 0.005 mol % hydrogen sulfide (50 ppm or 3.3 grains per
100 SCF) at a total pressure of 1.4 Mpa (200 psia) does not
exceed 0.0003 Mpa (0.05 psia) (Point В on Figure D-l).

Sour Oil and Multiphases

Sour crude oil systems that have operated satisfactorily us­ing standard equipment are outside the scope of this standard when the fluids being handled are either crude oil, or two- or three-phase crude, water, and gas when:

a. the maximum gas:oil ratio is 5000 SCF:bbl (barrel of oil);

b. the gas phase contains a maximum of 15% hydrogen sul­
fide;

c. the partial pressure of hydrogen sulfide in the gas phases
is a maximum of 0.07 MPa (10 psia);

d. the surface operating pressure is a maximum of 1.8 MPa
(265 psia) (see Figure D-2); and

e. when pressure exceeds 1.8 MPa (265 psia), refer back to

The satisfactory service of the standard equipment in these low-pressure systems is believed to be a result of the in-hibitive effect of the oil and the low stresses encountered un­der the low-pressure conditions.