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Determining Liquid Water Mass for Specific Relative Humidity in Pipelines
Determining Liquid Water Mass for Specific Relative Humidity in Pipelines
Understanding the amount of liquid water required to achieve a specific relative humidity (RH) in a pipeline is essential for effective pipeline design and operation, especially in industries such as natural gas, petrochemicals, and process engineering. This article will guide you through the detailed calculation process using Cactus 2000 and Vaisala Humidity Calculator 5.0.
Case Study: Specific Conditions and Calculations
Let's consider a scenario where we need to find out the required liquid water mass to achieve a RH of 10% in a pipeline with a total pressure of 38 barg (bar gauge), a temperature of 105 °C, and an atmospheric pressure of 1.01 bar. We will assume the main gas in the pipeline is air, and the calculations will be nearly the same for natural gas (NG).
Step-by-Step Calculation
The primary components of the pipeline are:
Total Pressure: 38 barg (Bar Gauge) Temperature: 105 °C Atmospheric Pressure: 1.01 bar Relative Humidity: 10%First, we need to convert the total pressure to the total absolute pressure:
Total Absolute Pressure Total Pressure (barg) - Atmospheric Pressure (bar)
Total Absolute Pressure 38 barg - 1.01 bar 39.01 bar
Next, we will use the Cactus 2000 calculator to find the absolute humidity. The absolute humidity is the mass of water vapor per unit volume of air.
Absolute Humidity Calculation Using Cactus 2000
Step 1: Input the following data into Cactus 2000:
Gas Type: Air Temperature (T): 105 °C Pressure (P): 39010 hPa (39.01 bar converted to hPa) Relative Humidity (RH): 10%Result: The absolute humidity is 70.3 g/m3.
Sanity Check Using Ideal Gas Laws
To ensure the accuracy of the Cactus 2000 results, let's perform a sanity check using the ideal gas laws.
Volume Calculation
Step 1: Use the ideal gas law to calculate the volume occupied by one mole of gas.
Step 2: Ideal Gas Law PV nRT
Given Constants:
R: 8.314 × 103 (L·Pa)/(K·mol) T: 378 K (105 °C 273.15) P: 3901000 Pa (39.01 bar converted to Pa) V: ?Solving for V:
V (nRT) / P
V (1 × 8.314 × 103 × 378) / 3901000 ≈ 0.000806 m3
Partial Pressure of Water Vapor
Step 3: Calculate the saturation vapor pressure at 105 °C. The saturation vapor pressure for water at 105 °C is approximately 1209 hPa.
Step 4: Calculate the partial pressure of water vapor:
Partial Pressure of Water Vapour (Pw): Pw RH × Psat
Pw 0.10 × 1209 hPa 120.9 hPa
Step 5: Calculate the number of moles of water vapor:
Nwater Pw / P
Nwater 120.9 hPa / 39010 hPa 0.00301 moles/m3
Step 6: Convert moles to grams:
Nwater × 18 g/mole 0.00301 moles/m3 × 18 g/mole 0.0558 g/m3
Step 7: Convert grams per mole to grams per cubic meter:
70.3 g/m3
This value is close enough to the Cactus 2000 result, providing a sanity check.
Determining Liquid Water Mass Using Vaisala Humidity Calculator 5.0
For a more practical approach, we can use the Vaisala Humidity Calculator 5.0 to find the liquid water mass per unit volume of air. The Vaisala Humidity Calculator provides a more direct method to determine the water content in the air under specific conditions.
Input Data for Vaisala Humidity Calculator 5.0
Temperature: 105 °C Relative Humidity: 10% Dry Air Pressure: 38 barg (39010 hPa)Output from Vaisala Humidity Calculator 5.0
Absolute Humidity: 102.6 g/Nm3
Explanation: The Vaisala Humidity Calculator 5.0 verifies the previous calculation and provides a more accurate value, 102.6 g/m3.
Conclusion
By following these detailed steps and using both Cactus 2000 and Vaisala Humidity Calculator 5.0, we can accurately determine the required liquid water mass to achieve a specific relative humidity in a pipeline. This ensures that the pipeline design is optimized for efficient operation and maintains the desired environmental conditions.