Typical gas quality specification in transactions of natural gas
Natural gas, by definition is a mixture of hydrocarbons and non-combustible components which are in a gaseous state. The amount of each of the components in the gas stream can affect measurement, operation, pipeline efficiency, and customer use. Therefore an acceptable quality specification is assigned in transactions of natural gas. Gas companies use many different quality specifications; a typical gas quality specificatio is given below.
| Component | Specification |
|---|---|
| Oxygen | <0.1% by volume. |
| Hydrogen sulfide | <0.25 grains per 100 cubic feet of gas. |
| Mercaptans | <0.25 grains per 100 cubic feet of gas. |
| Total sulfur | The total sulfur content, including mercaptans and hydrogen sulfide shall not exceed 2 grains per 100 cubic feet of gas. |
| Carbon dioxide | < 2% by volume. |
| Liquids | The gas shall be free of water and other objectionable liquids at the temperature and pressure at which the gas is delivered, and the gas shall not contain any hydrocarbons which might condense to free liquids in the pipeline under normal pipeline conditions and shall, in no event, contain water vapor in excess of 7 pounds per 1 million ft3. |
| Dust/gums/solid matter | The gas shall be commercially free of dust, gums, gum-forming constituents, and other solid matter. |
| Heating value | The gas delivered shall contain a daily, monthly or yearly average heating content of not less than 975 nor more than 1175 Btu per cubic foot on a dry basis. |
| Temperature | The gas shall not be delivered at a temperature of less than 40ºF and not more than 120ºF. |
| Nitrogen | <3% by volume. |
| Hydrogen | The gas shall contain no carbon monoxide, halogens, or unsaturated hydrocarbons, and no more than 400ppm of hydrogen. |
| Isopentane | The gas shall not contain more than 0.20 gallons of isopentane or heavier hydrocarbons per thousand cubic feet. |
The objective in the gas industry has been to maximize the heating value, but it is necessary to minimize the detrimental effects such as corrosion and damage to pipeline equipment caused by various components of natural gas which are abnormal, corrosive, inert, dangerious, or useless.
Isopentane and heavier components though belongs to the desirable hydrocarbon components, they can drop out as free liquids at some point in the range of normal operating conditions. Liquids of any source are undesirable because they cause equipment problems.
Other naturally occuring or frequently found components are hydrogen sulfide, sulfur, carbon dioxide, water and nitrogen occupy space and reduce pipeline efficiency. They either do not support combustion or they absorb energy when the natural gas is burned. All of these components except nitrogen can form acidic compounds contributing to corrosion.
Carbon dioxide is a strong acid former and can cause significant pipeline corrosion where water dropout occurs. Allowable levels for this component may be as high as 3% by volume.
Water also causes corrosion and equipment damage, and in fact corrosion cannot occur in the absence of liquid water.
Oxygen is not naturally occuring in natural gas, but it can intrude through leaks in low pressure system. It cause significant corrosion in very small concentrations and combustion in larger portions. Some companies reportedly allow as much as 1% oxygen. Even when oxygen is not in the free, gaseous state, it can contribute to reactions if it is present in sulfates, nitrates, water, or other oxygen containing molecules.
