Wednesday, 16 November 2011

Product Specifications

Specifications for natural gas liquid products:
Fig. 1: GPA specifications for commercial propane, commercial butane, commercial butane-propane mixtures, and Propane HD-5.1

Fig.2: GPA specifications for natural gasoline.
These are "official" industry standards, representing a broad industry consensus for minimum quality products. Producers, purchasers, or pipeline companies may adopt variations of these specifications.

The gas plant designer and operator, as well as purchasers, will also be concerned with specifications for other plant products, including residue gas, raw mix streams, ethane, propane, ethane-propane mixes, normal butane, iso-butane, and plant condensate. Although there are no "official" industry specifications for normal butane, common commercial transactions for normal butane stipulate that the product shall meet all specifications for commercial butane and, in addition, be composed of a minimum of 95 volume percent normal butane.

Common commercial specifications for iso-butane stipulate that the product contain a minimum of 95 volume percent isobutane, and also meet all specifications for commercial butane. Likewise, there are no industry standard specifications for ethane or ethane-propane (EP) mixes.
of typical quality criteria in industry use as shown in Fig.3.
Quality specifications for natural gas have historically been individually negotiated and prescribed in contracts between purchasers or pipeline companies and the producer or processor. Specification parameters for pipeline quality natural gas may include heating value, composition, contaminants, water content, and hydrocarbon dew point. Specification limits for these parameters may vary widely depending on the pipeline system, climatological conditions, end use, and other factors. Example pipeline quality gas specification parameters are shown in Fig.4.
LP-gas specifications of GPA Standard 2140, shown in Fig.1, are the industry standards in the United States. International specifications, adopted in ISO 9162, are shown in Fig.5

In many cases, specification parameters for LP-gas are based on simple "pass-fail" test methods that can be performed quickly and easily by field personnel. These specifications and test methods are intended to assure products that can be safely handled in transport systems, and that will perform adequately and safely in their end-use markets. Unfortunately, many of these tests tell the design engineer or plant operator little about product composition or quantitative limits. The following discussion is intended to provide an indication of product composition and quantitative limits imposed by these industry specifications.

Vapor Pressure
Vapor pressure is a critical specification that must be observed for safe and efficient utilization of propane, butane, and butane/propane mixtures in domestic and commercial installations, and to comply with various regulations governing transport vessels and cylinders. 
The GPA vapor pressure specification limit for propane meets the requirements of U.S. Department of Transportation regulations by effectively limiting the ethane content of commercial propane and propane HD-5 to a maximum of approximately 7 volume percent. Any appreciable quantity of propylene, permitted in commercial propane only, would necessarily reduce the amount of permissible ethane due to the  higher vapor pressure of propylene relative to that of propane.
Likewise, variations in the butane content of propane, limited to 2.5 volume percent, will impact the amount of ethane permitted by the vapor pressure specification.
Moisture Content
Moisture in propane must be controlled to very low concentrations to avoid hydrate formation in pipelines and freezing in tanks, regulating equipment, and other equipment in the distribution system.

Although a properly designed and operated dehydration system produces very dry propane, moisture can and does enter the transportation and distribution system at many points, such as storage tanks, loading racks, and transport vessels. 
There are two recognized methods for determining acceptable levels of moisture in propane products: the GPA Cobalt Bromide Test, and the Valve Freeze method (ASTM D-2713). Both are "pass-fail" tests that provide qualitative determinations of commercially "dry" propane, but neither method yields quantitative measures of moisture in the product. The Cobalt Bromide test is based on the work of Hachmuth4, which determined acceptable levels of moisture in commercial equipment, and correlated these levels with results of the test procedure. The test is based on observation of color changes of cobalt bromide salt caused by the humidity of the gas or vapor surrounding it. In practice, the cobalt bromide is supported on white cotton wadding and exposed to a stream of propane vapors chilled to 32°F. The color of the cobalt bromide changes from green to lavender at about 30% relative humidity, indicating "wet" propane. Propane-water system data7,8,11 indicate that the water content of saturated propane vapors at 32°F is approximately 530 ppmw. The water content of saturated propane liquid is approximately 35 ppmw at 32°F. At 30% saturation at 32°F, commercially "dry" propane as measured by the Cobalt Bromide test will be about 159 ppmw in the vapors and about 10  ppmw in the liquid. Based on these specification limits at 32°F, Fig.6 gives maximum allowable water content of liquid propane at other system temperatures.
The valve freeze method was developed to detect excessive moisture in liquid propane, and is preferred by some over the Cobalt Bromide test. The test device is a specially constructed and calibrated orifice valve designed to simulate expansion of  propane through a pressure regulator. A liquid sample of the product to be tested is passed through the valve at a preset flow rate. The time required for the valve to freeze and interrupt flow due to moisture in the product determines whether or not the product is commercially "dry." Test data reveals that a freeze-off time of more than 60 seconds indicates less than  30 ppmw moisture in the liquid product. The method is not applicable to propane products containing anti-freeze agents such as methanol. It is also affected by the temperature of the liquid sample.
A third method, the Bureau of Mines dew point tester, is a simple field test still used by some, but is not recommended because its accuracy is dependent on many poorly controlled variables, such as temperature and pressure of the system. This method was originally developed by the U.S. Bureau of Mines and is still used as a field method to determine moisture content in natural gas systems. Butane specifications stipulate "no free water." Since butane cannot be used in vapor withdrawal systems at temperatures below its boiling point, water content is not detrimental for most butane uses.

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