Sizing & Selection of Surge Vessel
Table of Contents
In one of my design project I have to select a bladder tank to suppress hydraulics surges because of extremely high flows & a problematic check valve.
Bladder tanks, also known as anti-surge devices, are essential components in various industrial and commercial applications. They are designed to mitigate pressure surges and fluctuations in fluid systems, ensuring smooth and efficient operation.
Why pressure surge? #
Hydraulic surge, or “water hammer,” is a dangerous pressure wave \( \Delta P \) that occurs in a fluid-filled pipe system. It’s caused by a sudden change in the fluid’s velocity \( \Delta v \). Common culprits include:
- Rapid pump starts or stops
- Sudden valve closures
This event can be modeled by the Joukowsky Equation:
$$ \Delta P = \rho \cdot c \cdot \Delta v $$
Where:
- \( \rho \) (rho) is the fluid density.
- \( c \) is the acoustic wave velocity in the fluid.
- \( \Delta v \) is the change in fluid velocity.
The consequences are severe, from \( P_{max} > P_{design} \) (pipe rupture) to \( P_{min} < P_{atm} \) (pipe collapse from vacuum).
The Solution: The Bladder Surge Tank #
A bladder tank is a hydropneumatic device designed to absorb and control these pressure transients. It’s a pressure vessel divided by a flexible, elastomeric bladder.
- Gas Side: The bladder is pre-charged with a gas (usually Nitrogen, \( N_2 \)) to a specific pressure (\( P_{precharge} \)).
- Fluid Side: The other side is connected directly to the main pipeline.
How It Works #
The tank’s operation is governed by Boyle’s Law for the compressed gas:
$$ P_1 V_1 = P_2 V_2 $$
During a High-Pressure Surge (\( P_{surge} > P_{system} \)):
- The high-pressure wave forces fluid into the tank.
- This compresses the gas-filled bladder, decreasing its volume (\( V_2 < V_1 \)) and increasing its pressure (\( P_2 > P_1 \)).
- This action effectively absorbs the surge energy, dampening the pressure spike.
During a Low-Pressure Surge (\( P_{down-surge} < P_{system} \)):
- The system pressure drops, potentially creating a vacuum (\( P_{min} \approx 0 \)).
- The pre-charged bladder (\( P_{precharge} \)) is now at a higher relative pressure and expands.
- It pushes its stored fluid out of the tank and into the pipeline, raising the low pressure and preventing vacuum conditions.
By properly sizing the tank’s volume (\( V_{tank} )\) and pre-charge pressure (\( P_{precharge} )\), bladder tanks are essential for protecting pumps, valves, and pipes from the destructive forces of hydraulic surge.