Pump Impeller Selection for High Concentration Caustic ·

Table of Contents

Selecting the right pump impeller for handling high concentration caustic solutions is crucial for ensuring efficient and safe operation. Caustic solutions, such as sodium hydroxide (NaOH), are highly corrosive and can cause significant damage to improperly selected pump components. This post will guide you through the key considerations and mathematical principles involved in selecting the appropriate impeller for high concentration caustic applications.

Key Considerations #

Material Selection: The impeller material must be resistant to corrosion from caustic solutions. Common materials include:
- Stainless Steel (e.g., 316L): Suitable for moderate concentrations.
- Hastelloy: Ideal for high concentrations and extreme conditions.
- Titanium: Offers excellent corrosion resistance but is more expensive.
Impeller Design: The design of the impeller should be optimized for the specific viscosity and flow rate of the caustic solution. Common designs include:
- Open Impellers: Suitable for low viscosity fluids.
- Closed Impellers: Better for handling higher viscosity fluids and providing better efficiency.
Flow Rate and Head: The pump’s flow rate ($Q$) and head ($H$) are critical parameters. The relationship between flow rate, head, and impeller speed ($N$) can be described by the affinity laws: $$\frac{Q_1}{Q_2} = \frac{N_1}{N_2}$$ $$\frac{H_1}{H_2} = \left(\frac{N_1}{N_2}\right)^2$$ where $Q_1$ and $Q_2$ are the flow rates, $H_1$ and $H_2$ are the heads, and $N_1$ and $N_2$ are the impeller speeds.
Specific Speed ($N_s$): Specific speed is a dimensionless number that helps in selecting the appropriate impeller design. It is defined as: $$N_s = \frac{N \sqrt{Q}}{H^{0.75}}$$ where $N$ is the impeller speed, $Q$ is the flow rate, and $H$ is the head.

Example Calculation #

Let’s consider an example where we need to select an impeller for a caustic solution with the following parameters:

Flow rate ($Q$): 100 m³/h
Head ($H$): 20 m
Impeller speed ($N$): 1450 RPM

First, calculate the specific speed: $$N_s = \frac{1450 \sqrt{100}}{20^{0.75}} \approx 2500$$

Based on the specific speed, we can select an appropriate impeller design. For $N_s \approx 2500$, a mixed-flow impeller would be suitable.

Conclusion #

Its always a good idea to invite pump’s vendor onsite

Further refer to API 610 & Aramco SAES-G-005