Chapter 5 – Surge Protection
Surge Analysis and the Wave Plan Method
Supplementary Material: Example Problems and Solutions
Chapter 5 – Problem 5.12
5.12 Suppose you are an engineer working for a manufacturer supplying bladder surge tanks to a pipeline project. The surge analysis for this project was done by some other consulting firm and they proposed a compressor surge tank (see Section 5.6.2) to protect the pipeline system. You are asked to compute the equivalent bladder surge tank size and pre-charge pressure (see section 5.6.3) based on the following information gathered from the surge analysis report.
- Air pressure inside the tank at steady state conditions = 85.5 m (838.75 kPa gage)
- Initial air volume (at time 0) in the compressor surge tank = 5 m3
- Maximum expanded air volume during the unsteady state = 18 m3
- Proposed compressor surge tank volume = 22 m3
- A short (approximately 5m long) connecting pipe of 300 mm internal diameter with a 300 mm gate valve is added to the pipe as an isolation valve.
Solution:
Use the ideal gas law of isothermal processes (Eq. 5.1) to compute the pre-charge pressure.
If the bladder is sized to occupy the entire volume of the tank, the theoretical bladder tank volume is the same as the maximum expanded air volume.
If the atmospheric pressure is 10.34m (101.425 kPa abs) then P1 (of Eq. 5.1) = 838.75 + 101.425 = 940.175 kPa (abs).
V1 = 5 m3 and V2 = 18 m3.
Therefore P2 = 261.16 kPa (abs) and the pre-charge pressure is 261.16 – 101.425 = 159.73 kPa (or 16.28m).
However, the actual bladder tank volume should be slightly larger (about 5% to 10%) than the theoretical bladder volume of 18 m3 in case the fully expanded bladder shape is not same as the tank shape (which would result in the bladder not expanding into all corners of the tank). For this case, we will increase the bladder tank volume to 19 m3.
While normally the resistance to flow in and out of a compressor surge tank is only due to the resistance within its connecting pipe and isolation valve, the perforated grid placed at the bottom of the bladder surge tank adds additional resistance to flow. Adjust the connecting pipe and the isolation valve size, accounting for the additional resistance due to the perforated grid. The diameter of the bottom flange of the bladder surge tank may need to be increased (compared to the bottom flange of the compressor surge tank) to accommodate a larger grid to match the total resistance values.
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