When engineers select a centrifugal pump for a chemical application, the most common mistake is treating specific gravity as a flow curve correction and stopping there. The flow-head curve of a centrifugal pump does not change with fluid density — it is the same curve whether you are pumping water at SG 1.0 or sulphuric acid at SG 1.84. But the shaft power required to operate at any point on that curve scales directly with specific gravity, and if the motor was sized for water, it will be undersized for a dense chemical — sometimes significantly.
What Changes with Specific Gravity and What Does Not
A centrifugal pump’s head-flow curve is generated by the impeller geometry and the rotational speed. Head is expressed in metres of liquid column, which is a measure of energy per unit weight of fluid. Because head is expressed this way, the curve is the same for any fluid density — the impeller imparts the same energy per kilogram of fluid regardless of how much that kilogram weighs.
What changes is pressure. Pressure is head multiplied by fluid density (P = ρgh). At the same head and flow rate, a pump moving sulphuric acid at SG 1.84 is generating 1.84 times the pressure it would generate moving water at the same head. The system receives 1.84 times the hydraulic power per unit volume — and the motor must supply 1.84 times the shaft power to deliver it.
Motor power scales with specific gravity at every point on the pump curve. A pump that draws 4 kW at its best efficiency point when pumping water will draw approximately 7.4 kW at the same operating point when pumping a fluid at SG 1.84. If the motor is rated for 5.5 kW, it will run within its nameplate rating on water and trip its overload protection on the dense chemical before reaching the design flow rate.
Common Dense Chemicals and Their SG Values
The specific gravity values for common industrial chemicals vary widely and are not always intuitive. Sodium hydroxide 50% solution has an SG of approximately 1.52. Ferric chloride solution used in PCB etching and water treatment is typically SG 1.40–1.50 depending on concentration. Phosphoric acid 85% is SG 1.69. Sulphuric acid 98% is SG 1.84. Hydrochloric acid 33% is SG 1.16 — relatively modest but still 16% more power demand than water. Calcium chloride brine at 30% concentration is SG 1.28.
For any chemical with SG above 1.2, motor sizing deserves explicit recalculation rather than assumption that a motor sized for water will be adequate.
How to Calculate the Correct Motor Size
The approach is straightforward. Determine the pump operating point — the flow rate and head required — and read the shaft power from the pump manufacturer’s power curve at that operating point, which will be stated for water (SG 1.0). Multiply that power figure by the SG of the actual fluid. Apply the motor efficiency and any drive losses to get the motor input power. Add a service factor of 10–15% for conservative sizing, and select the next standard motor size above the calculated figure.
For example: a pump requiring 3.2 kW shaft power at the duty point for water, pumping a fluid at SG 1.45, requires 3.2 × 1.45 = 4.64 kW of shaft power. With a motor efficiency of 90%, the input power is 4.64 / 0.90 = 5.16 kW. With a 10% service factor: 5.7 kW. The appropriate motor is a 7.5 kW unit (the next standard size above 5.7 kW), not a 5.5 kW motor that might appear adequate when looking at the water-based power figure.
Consequences of Getting This Wrong
An undersized motor on a dense chemical application typically fails in one of two ways. The first is thermal overload: the motor draws more current than its rated capacity, heats up, and trips the thermal overload protection. The pump stops. Depending on the process, this may be a nuisance or a production incident. If the overload protection is incorrectly set too high (a common field adjustment when operators want to prevent nuisance trips), the motor runs hot continuously and fails prematurely due to winding insulation breakdown.
The second is torque limit: some drive systems have current limiting that prevents the motor from drawing above a set threshold. The pump simply cannot develop full speed or reach the design operating point. Flow is lower than expected, the system appears to be undersized, and the actual cause — motor under-specification for fluid SG — is not immediately obvious.
Applying This to the Fluimac Compass and Dragon
When sizing the Fluimac Compass (mag drive) or Fluimac Dragon (mechanical seal) centrifugal pumps for chemical transfer applications at Autoflo, specific gravity is always part of the motor sizing conversation. The pump curves are published for water, as is standard industry practice. The motor is selected after applying the SG correction to the water-basis power figure. For dense chemicals — any fluid above SG 1.2 — this calculation reliably produces a motor specification at least one frame size larger than a water-basis selection would suggest.
Related reading: Why Engineers Choose AODD Pumps Over Centrifugal for Chemical Transfer, Why a Semi-Open Impeller Handles Dirty Fluids Better, Mechanical Seal vs. Magnetic Drive for Chemical Transfer.
If you are selecting a centrifugal pump for a chemical with SG above 1.0, contact Autoflo at info@autoflotechnology.com and we will confirm the correct motor size for your actual fluid.