Why the Steam Drum Is Crucial for Ensuring Proper Boiler Water Chemistry continued
blowdown sample is greater and therefore, less susceptible to error in collection and testing (3). The flowrate through the sample lines should be checked to ensure that velocity is approximately 6 feet per second (ft/s) with sample surplus flow going to drain. The amount of time required to adequately flush the sample line depends on the length of the line and the size of the sample tubing. Table A, adapted from ASME’s Consensus on Operating Practices for the Sampling and Monitoring of Feedwater and Boiler Water Chemistry in Modern Industrial Boilers, provides recommended flushing times for various sizes of commonly used sample tubing. Best practices dictate a minimum of three pipe volumes be run through the system before it can be ensured that a sample is representative (4, 5). Using the table—if the sample line is constructed of 3/8-inch tubing with a wall thickness of 0.049 inches, the required flush time would be 4.3 seconds per foot (s/ft) of sample line. So, if the sample line from the pipe tap to the sample panel is 500 ft, the required flushing time is 2,150 s, or a minimum of 36 minutes. The sample flowrate should not be readjusted after or during the flush period. Table A: Flush Times Required for Representative Sample Collection
Sample Tubing Size (inches)
Wall Thickness (inches)
Tube ID (inches)
Recommended Purge Time at Flowrate of 500 mL/min (s/ft)
¼-inch tubing
0.035 0.042 0.049 0.058 0.065
0.180 0.166 0.152 0.134 0.120
1.8 1.5 1.3 1.0 0.8
3/8-inch tubing
0.035 0.042 0.049 0.058 0.065
0.305 0.291 0.277 0.259 0.254
5.2 4.7 4.3 3.7 3.6
½-inch tubing
0.035 0.042 0.049 0.058 0.065
0.430 0.416 0.402 0.384 0.370
10.3 9.6 9.0 8.2 7.6
Note: ID = inside diameter Source: Reference 4
Most steam drums are provided with multiple saturated steam take-offs, with each take-off having its own sample collection nozzle. The sample lines from each take-off are commonly tied together to produce a composite saturated steam sample that is then routed down to the sample conditioning equipment for pressure reduction, cooling, and measurement. In other instances, the saturated steam sample is collected at a single location after the various saturated steam take-offs have been combined into a common steam header upstream of the superheater. If the plant is provided with the capability, it is most desirable to sample the saturated steam from each individual take-off to evaluate the carryover. This permits the steam purity to be evaluated along the entire length of the drum and helps to identify any location along the length of the drum where degradation or damage to the steam separation devices may have occurred. According to the Electric Power Research Institute (EPRI), there have been numerous instances reported where excessive carryover was discovered in only one area of the drum, and the damage to the steam separation equipment may have been missed or underappreciated if testing had only occurred on the combined saturated steam sample (2). To obtain samples that are truly representative of the bulk flow
for fluid steams that exhibit two-phase, isokinetic sampling procedures as detailed in ASTM D1066 Standard Practice for Sampling Steam should be used. Isokinetic sampling ensures that all phases of the sampled fluid enter the collection nozzle with the same velocity and direction of flow and that the flow velocity into the nozzle is the same as the sampled stream velocity (6). Fluids exhibit different density and inertia in different phases and are, therefore, not proportionally represented in a sample that is not collected with an isokinetic device (6). To ensure that the data collected during a drum carryover test accurately measures the purity of the steam phase as well as the water droplets present in the steam, the saturated steam samples should be collected using isokinetic sample nozzles in accordance with ASTM D1066. During the test planning phase, it is important to verify that isokinetic sample nozzles are installed in the saturated steam lines where sample collection will be performed.
“The amount of time required to adequately flush the sample line depends on the length of the line and the size of the sample tubing.” 20
the Analyst Technology Supplement 2020
