Understanding Nominal Flow Rate in Pumps

When reading a pump specification sheet, the first number most buyers look at is the flow rate. It tells you how much liquid the pump can move. However, you will often see this listed as the Nominal Flow Rate (sometimes called Rated Flow Rate). But what does "nominal" actually mean for your production line? As an experienced industrial pump manufacturer, I want to explain why nominal flow rate is not always the same as the actual liquid coming out of your pipe, and how it changes between internal gear pumps and sanitary twin-screw pumps.

1. What is Nominal Flow Rate?

The nominal flow rate is the theoretical or ideal amount of fluid a pump can move under perfect conditions. Pump manufacturers calculate this number based on a specific speed (RPM) using a standard, thin fluid—usually clean water—at zero or very low backpressure. We usually measure nominal flow rate in two common units: GPM (Gallons Per Minute) or m³/h (Cubic Meters Per Hour).

The Golden Rule: Nominal vs. Actual Flow Rate

In the real world, your pump rarely operates under perfect laboratory conditions. The actual flow rate—the real amount of liquid that enters your tank—is almost always lower than the nominal flow rate. This difference is caused by internal gaps in the pump and fluid thickness, a concept known as volumetric efficiency. If you are planning to run a pump at high pressure with thin liquids, you must account for this slip or risk production bottlenecks.

2. Nominal Flow Rate in Internal Gear Pumps

Internal gear pumps are positive displacement pumps. In theory, every time the gears turn, they push a fixed amount of fluid. If you look at the spec sheet for an internal gear pump, the nominal flow rate assumes zero "slip." Slip happens when fluid leaks backward through the tiny clearances between the gear teeth and the pump casing.

Why the Actual Flow Drops:

With Thin Fluids (Water/Solvents): Backpressure forces some liquid to slip backward. Your actual flow rate will be noticeably lower than the nominal flow rate. For example, a pump rated at 20 GPM nominal might only deliver 15 GPM in a high-pressure system with water.

With Thick Fluids (Glue/Resin/Oil): Heavy, viscous fluids cannot easily slip through the tiny gear gaps. Therefore, when pumping thick liquids, the actual flow rate of a gear pump gets very close to its ideal nominal flow rate. If you handle adhesives, this is your most reliable option.

3. Nominal Flow Rate in Sanitary Twin-Screw Pumps

Sanitary twin-screw pumps handle flow rate in a unique way because they can run at a huge range of speeds (from 200 RPM up to 3,600 RPM). Because these pumps do two jobs, they have two very different flow profiles:

Product Transfer Duty: When moving thick foods like chocolate or yogurt at low speeds, the pump delivers a steady, gentle flow. Similar to gear pumps, the thick fluid seals the screws, making the actual flow rate match the nominal calculations perfectly. This gives you precise batching for recipes.

CIP (Clean-in-Place) Duty: When the pump switches to maximum speed to flush the lines with thin cleaning water, the nominal flow rate skyrockets. Even though water has more internal slip, the ultra-high RPM creates a massive actual flow rate with high pressure to scrub the pipes clean. This dual-mode performance is why many food and dairy plants prefer twin-screw pumps over gear pumps.

4. How to Calculate Your Real Flow Needs (Buying Advice)

To avoid buying a pump that is too small for your factory, never design your system based on the maximum nominal flow rate alone. Instead, use these steps to choose correctly:

Check Your Viscosity: Know how thick your liquid is at your exact operating temperature. For thin liquids, anticipate 10-25% slip. For thick liquids, you can trust the nominal number more closely.

Account for System Pressure: Long pipes, bends, and filters create backpressure, which increases internal slip and lowers your flow. A pump that works at 0 psi may lose 30% flow at 100 psi with thin liquids.

Over-Size Slightly: As a general rule of thumb, choose a pump with a nominal flow rate that is 15% to 20% higher than your actual required production flow. This buffer ensures you can handle pressure spikes, viscosity changes, and future line expansions without buying a new pump next year. Remember, a pump running close to its nominal capacity is a pump that will last longer and save you maintenance headaches.