Pump Differential Pressure vs. Head

The Illusion of Pressure in Centrifugal Pumps

One of the most common mistakes junior engineers make is selecting a centrifugal pump based on its pressure rating rather than its head.

As established in pump theory, a centrifugal pump is a constant-head machine, not a constant-pressure machine. A specific impeller spinning at a specific RPM will always impart the exact same velocity to the fluid, thereby generating the exact same Head (in meters or feet), completely regardless of the fluid's density.

Converting Head to Differential Pressure ($\Delta P$)

While the pump cares only about Head, the downstream piping and equipment (like reactors, heat exchangers, and safety relief valves) care entirely about Pressure.

To convert the Pump Head ($H$) into Differential Pressure ($\Delta P$), you must multiply the head by the fluid's density ($\rho$) and gravitational acceleration ($g$):

To calculate the pressure in bar (using standard metric units of $kg/m^3$ for density and meters for head):

The Specific Gravity (SG) Shortcut

In the field, engineers often use Specific Gravity (SG) as a shortcut. Since $1 \text{ bar} \approx 10.2 \text{ meters of water}$, the conversion becomes:

Why This Distinction is Catastrophically Important

Imagine you have a single centrifugal pump on a test bench. At its Best Efficiency Point (BEP), its performance curve guarantees it will generate exactly 100 meters of head.

Let's see what happens when we pump three different fluids through this exact same pump:

1. Pumping Water ($SG = 1.0$)

The pump discharges at 9.8 bar. The piping and flanges easily handle this.

2. Pumping Gasoline / Hexane ($SG = 0.65$)

Even though the pump is working exactly as hard (generating 100m of head), the low-density fluid exerts far less pressure on the pipe walls. The discharge pressure is only 6.4 bar.

3. Pumping Concentrated Sulfuric Acid ($SG = 1.84$)

This is the danger zone! The dense acid causes the discharge pressure to nearly double to 18.0 bar. If the downstream piping was only designed for standard ANSI Class 150 flanges (which max out around 19 bar at ambient temps), you are dangerously close to blowing a gasket or bursting the pipe, purely because you changed the fluid density!

Furthermore, because the pump is generating 18 bar of pressure instead of 9.8 bar, the electric motor must provide almost double the shaft horsepower to maintain the 100m head. The motor will immediately trip on high amperage or burn out.