【国外标准】 Standard Practice for Flow Control and Temperature Control for On-Line Water Sampling and Analysis

5.1 Sample conditioning systems must be designed to accommodate a wide range of sample source temperatures and pressures. Additionally, efforts must be made to ensure that the resultant sample has not been altered during transport and conditioning and has not suffered excessive transport delay. Studies have shown that sample streams will exhibit minimal deposition of ionic and particulate matter on wetted surfaces at specific flow rates (1-5). 35.1.1 To ensure that the physical and chemical properties of the sample are preserved, this flow rate must be controlled throughout the sampling process, regardless of expected changes of source temperature and pressure, for example, during startup, or changing process operating conditions.5.2 The need to use analyzer temperature compensation methods is dependent on the required accuracy of the measurement. Facilities dealing with ultra-pure water will require both closely controlled sample temperature and temperature compensation to ensure accurate measurements. The temperature can be controlled by adding a second or trim cooling stage. The temperature compensation must be based on the specific contaminants in the sample being analyzed. In other facilities in which some variation in water chemistry can be tolerated, the use of either trim cooling or accurate temperature compensation may provide sufficient accuracy of process measurements. This does not negate the highly recommended practice of constant temperature sampling, especially at 25°C, as the most proven method of ensuring repeatable and comparable analytical results.5.3 A separate class of analysis exists that does not require or, in fact, cannot use the fully conditioned sample for accurate results. For example, the collection of corrosion product samples requires that the sample remain at near full system pressure, but cooled below the flash temperature, in order to ensure a representative collection of particulates. Only some of the primary conditioning criteria apply in this case, as in others. Temperature compensation is not applicable since the material being analyzed is not in a liquid state.1.1 This practice covers the conditioning of a flowing water sample for the precise measurement of various chemical and physical parameters of the water, whether continuous or grab. This practice addresses the conditioning of both high- and low-temperature and pressure sample streams, whether from steam or water.1.2 This practice provides procedures for the precise control of sample flow rate to minimize changes of the measured variable(s) due to flow changes.1.3 This practice provides procedures for the precise control of sample temperature to minimize changes of the measured variable(s) due to temperature changes.1.4 The values stated in SI units are to be regarded as standard. The values given in parentheses after SI units are provided for information only and are not considered standard.1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.