Fluid Services

A general term concerning the application of a piping system, considering the combination of fluid properties, operating conditions, and other factors which establish the basis for design of the piping system.

B31.1 Fluid Services Applies to all fluids where steam vapour is generated at 15 psig or higher. Steam condensate (liquids) where water is 160 psig or temperatures exceed 250F.

B31.3 Fluid Services Category D Fluid Service

Fluid is not flammable, not toxic or otherwise harmful to human tissues. Harmful means exposure that can harm skin, eyes or mucous membranes that irreversible damage may result unless immediate action is taken.

Design Gauge pressure is below 150 psi (1,035 kPa) Design Temperature is between -20F to 366F (-29C to 186C)

An example of Category D fluid might be utility water in a plant or low pressure steam condensate.

Category M Fluid Service

Category M service is a fluid where the potential for personnel exposure is considered to be significant and where a single exposure to a very small amount can cause serious irreversible harm to people. This could be death, respiratory failure, blindness, etc. Design for Category M Fluid service should also meet the requirements of Chapter VIII of B31.3.

An example of Category M fluid could be Hydrogen Sulphide gas, high concentrations of Hydrochloric Acid or high pressure steam.

High Pressure Fluid Service

High Pressure Fluid Service is one the pressures are in excess of ANSI Class 2500. It is important to note that B31.3 notes that there are no specified pressure limitations for the application of the High Pressure Piping Rules. Design for High Pressure Fluid service should also meet the requirements of Chapter IX of B31.3.

Normal Fluid Service

Fluid service that is covered by 31.3 but not subject to the rules for Category D, Category M or High Pressure Fluid definitions.

Sour gas

Sour gas is natural gas or any other gas containing significant amounts of hydrogen sulphide (H2S). Natural gas is usually considered sour if there are more than 5.7 milligrams of H2S per cubic meter of natural gas, which is equivalent to approximately 4 ppm by volume under standard temperature and pressure. However, this threshold varies by country, state, or even agency or application. For instance, the Texas Railroad Commission considers a sour gas pipeline one that carries gas over 100 ppm by volume of H2S. However, the Texas Commission on Environmental Quality has historically defined sour gas for upstream operations which requires permitting, reporting, and possibly additional emission controls as gas that contains more than 24 ppm by volume. Natural gas that does not contain significant amounts of hydrogen sulphide is called "sweet gas."

Although the terms acid gas and sour gas are sometimes used interchangeably, strictly speaking, a sour gas is any gas that specifically contains hydrogen sulphide in significant amounts, whereas an acid gas is any gas that contains significant amounts of acidic gases such as carbon dioxide (CO2) or hydrogen sulphide. Thus, carbon dioxide by itself is an acid gas, not a sour gas. In addition to being toxic, hydrogen sulphide in the presence of water also damages piping and other equipment handling sour gas by sulphide stress cracking. Natural gas typically contains several ppm of volatile sulphur compounds, but gas from one well in Canada is known to contain 90% hydrogen sulphide and others may have H2S contents in the tens of percent range.

Stress Critical lines Flexibility as defined in the design codes is a measure of the ability of a piping system to absorb its own expansion and that of connected equipment to ensure that the loadings do not cause:

Fatigue failure of the piping system due to repeated overstress. Failure of the piping system due to excessive plastic strains. Leakage at any flanged joints. Overloading of valves and other components in the piping system which would impair their operation. Detrimental distortion of connected equipment from excessive thrusts and movements and machinery

misalignment. A flexibility analysis is required if the Stress Engineer is in any doubt as to the ability of the system to satisfy the above requirements. Typically a comprehensive analysis is not required if:

They are duplication of successfully operating installations. They may readily be judged adequate by comparison with previously analyzed systems. They are of uniform size, have not more than two points of fixation, no intermediate restraints and fall

within the limitations of the empirical equation from ASME B31.3.

The following guidelines may be used for defining stress critical lines: Process lines to and from strain sensitive equipments such as pumps, compressors and blowers. Lines to and from steam turbines and reactors. Lines with operating temperature differences from base temperatures >= 180C for pipe sizes 6" and

larger and >= 330C for pipe sizes 4" and smaller. Process lines in toxic services for verification of flange leak tightness. Two phase flow piping such as blow down, and flare piping. Non-metallic piping with design temperature above 50C. All category "M" piping as defined in ASME B31.3. Lines connected to wellheads (applies to onshore and offshore oil and gas installations) Lines subject to excessive deflection resulting from differential settlement (e.g. lines connected to

storage tanks), structural and/or equipment displacement. Typically, engineering analysis, by visual inspection and quick manual calculations, are performed on the following systems:

Lines >= 4" to air coolers. Lines >= 16". Lines with operating temperature differences from base temperatures >=80C for piping sizes 6" and

larger. Lines subjected to vacuum or external pressure such as jacketed lines. Relief systems - closed or relieving to atmosphere