When dealing with pumps and process systems, one of the most important — but often overlooked — properties of a liquid is its specific gravity (SG). Understanding SG is critical for proper pump selection, sizing, and power calculations.
What is Specific Gravity?
Specific gravity is the ratio of a substance’s density to the density of water (measured at 4 °C, where water is densest).
Specific Gravity (SG):
\( SG = \frac{\text{Density of substance}}{\text{Density of water}} \)
Because it’s a ratio, specific gravity has no units. It’s a simple way of expressing whether a fluid is lighter or heavier than water.
- SG = 1 → same density as water.
- SG < 1 → lighter than water (floats).
- SG > 1 → heavier than water (sinks).
Everyday Examples of Specific Gravity
- Petrol: ~0.7 (lighter than water)
- Seawater: ~1.03 (slightly heavier than water)
- Sulphuric Acid (98%): ~1.8 (almost double the density of water)
- Mercury: ~13.6 (extremely dense)
Why Specific Gravity Matters in Pumping
While pump head (metres of liquid) is not affected by SG, pressure and power requirements are.
1. Pressure Conversion
Pump performance curves are usually expressed in head (m), which is independent of the liquid’s density. But when converting head into pressure, SG comes into play:
Pressure Conversion:
\( \text{Pressure (kPa)} = \text{Head (m)} \times SG \times 9.81 \)
That means pumping 10 m of water (SG = 1.0) results in 98.1 kPa, but the same head of sulphuric acid (SG = 1.8) results in ~177 kPa.
2. Power Requirement
The power a pump needs is proportional to the liquid’s SG. The higher the SG, the more energy required.
Pump Power Requirement:
\( \text{Power (kW)} = \frac{\text{Flow} \times \text{Head} \times SG \times g} {3.6 \times 10^{6} \times \eta} \)
This is why a pump that handles water efficiently may overload or trip its motor when switched to a heavier liquid without adjustment.
3. Material & Mechanical Considerations
High-SG fluids put more stress on pump bearings, shafts, and casings. Ignoring this can lead to premature wear, seal failures, or complete breakdowns.
Key Takeaways
- Specific gravity is a simple but vital property for any pumping system.
- It directly influences pressure and power, but not pump head.
- Always check the SG of the process fluid — never assume it’s “like water.”
Final Word
When selecting or operating pumps, knowing the specific gravity of your fluid can save you from costly mistakes. It ensures the pump is not only capable of delivering the required flow and head, but also that it won’t be overloaded or wear out prematurely.
At The Pump Expert, we always remind engineers and operators: know your fluid before you size your pump.
