1. What is pressure loss and why it matters for pumps
When a fluid flows through a pipe, it loses energy through friction with the pipe inner walls and through disturbances at direction changes, cross-section changes or obstructions. This lost energy is called pressure loss (head loss) and is measured in meters of liquid column (mlc), meters of water column (for fluids with density different from water, conversion is needed), bar or psi.
Pressure loss is inevitable — there is no zero-friction piping — but it can be minimized with correct sizing.
For industrial pump operators, pressure loss has direct impact on two points: (1) At suction, it reduces available NPSH — if the loss is too large, the pump cavitates. (2) At discharge, it increases the total head the pump must overcome — if not accounted for, the pump delivers less flow than designed.
FB Bombas, with over 80 years of experience, observes that poorly calculated suction pressure loss is the #1 cause of cavitation in industrial gear and centrifugal pump installations.
2. Distributed pressure loss (along the pipe)
Distributed pressure loss occurs from fluid friction with the pipe inner walls along the entire straight length.
It is calculated by the Darcy-Weisbach equation: Hf = f × (L / D) × (v² / 2g), where Hf is the head loss in meters, f is the friction factor (dimensionless, obtained from Moody diagram as function of Reynolds number and relative roughness), L is pipe length in meters, D is internal diameter in meters, v is fluid velocity in m/s, and g is gravitational acceleration (9.81 m/s²).
The friction factor f depends on the flow regime. For laminar flow (Reynolds < 2,100, typical in viscous fluid pumping with FBE pumps), f = 64/Re — and head loss is proportional to viscosity and velocity. For turbulent flow (Reynolds > 4,000, typical in water pumping with FBCN pumps), f depends on pipe relative roughness and is obtained from the Moody diagram or Colebrook-White equation.
In the transition zone (2,100 < Re < 4,000), behavior is unstable and should be avoided in design.
3. Localized pressure loss (fittings)
Localized loss occurs at each piping fitting: elbows, tees, valves, reducers, expansions, strainers, tank inlet and outlet. It is calculated by: Hf = K × (v² / 2g), where K is the specific loss coefficient for each fitting.
Typical K values for industrial design: 90° long radius elbow = 0.3; 90° short radius elbow = 0.9; 45° elbow = 0.2; tee straight-through = 0.3; tee branch = 1.5; gate valve open = 0.2; globe valve open = 6.0 to 10.0; check valve = 2.5; concentric reducer = 0.5; Y-strainer (clean) = 2.0 to 5.0; sharp-edge entry = 0.5; chamfered entry = 0.04.
Equivalent length method: alternatively, each fitting can be converted to an equivalent length of straight pipe (in diameters). Examples: 90° long radius elbow ≈ 20D, 90° short radius elbow ≈ 30D, gate valve open ≈ 8D, globe valve open ≈ 300D. This method is practical for quick field estimates. For critical NPSH designs (especially in FBE gear pump suction with viscous fluids), FB Bombas recommends calculating by the K coefficient method, which is more accurate.
4. Direct impact on pump NPSH
The available NPSH (NPSHa) formula includes suction head loss as a direct subtraction: NPSHa = Pa ± Hz - Hf - Pv. The Hf term (total suction head loss) is the sum of distributed loss + all localized losses in the suction line. Each meter of suction head loss is one meter less of NPSHa — and when NPSHa drops below the pump NPSHr, cavitation begins.
Practical example with FBCN 50-200 centrifugal pump: manufacturer NPSHr = 3.2 m. Installation with flooded suction (Hz = +2.0 m), atmospheric pressure (Pa = 10.33 mlc), water at 60°C (Pv = 2.03 mlc), DN65 piping with 5 m straight pipe + 2 × 90° elbows + 1 gate valve. Calculation: velocity v = Q/A = 2.5 m/s (turbulent, Re ≈ 330,000), f = 0.019 (Moody). Distributed loss = 0.019 × (5/0.065) × (2.5²/19.62) = 0.47 m.
Localized loss = (0.3+0.3+0.2) × (2.5²/19.62) = 0.26 m. Total Hf = 0.73 m. NPSHa = 10.33 + 2.0 - 0.73 - 2.03 = 9.57 m. Margin over NPSHr = 9.57 - 3.2 = 6.37 m — safe situation. But if the same installation had 15 m pipe + 5 elbows + dirty strainer (K=8.0) + negative suction (-3 m): Hf would rise to 3.8 m and NPSHa would drop to 1.5 m — below NPSHr. Cavitation.
5. Pressure loss with viscous fluids (gear pumps)
In FBE gear pump applications, the fluid is typically viscous (oils, asphalt, resins, chocolate). In laminar flow (Re < 2,100), pressure loss is directly proportional to viscosity: doubling viscosity doubles pressure loss. This has severe practical consequence: an oil at 20°C with 5,000 cSt has ~50× greater pressure loss than water in the same piping and velocity.
Therefore, FB Bombas recommends that suction piping for FBE pumps with viscous fluids have maximum velocity of 0.5 m/s (versus 1.5 m/s for water in FBCN centrifugal pumps).
Another particularity: many viscous fluids have temperature-dependent viscosity (e.g., asphalt CAP goes from solid at 25°C to pumpable liquid at 180°C). Pressure loss must be calculated for the WORST condition — usually at startup, when the fluid is colder and more viscous. FB Bombas offers heating jackets (CA option) on FBE pumps precisely to keep the fluid heated in the suction zone during startup, reducing local viscosity and consequently pressure loss.
6. Practical suction sizing rules
Based on FB Bombas field experience, these are the rules that minimize suction pressure loss: (1) Suction pipe diameter always ≥ pump connection diameter — never reduce before the pump; (2) Total suction line length: as short as possible, ideally < 10 diameters; (3) Avoid globe-type valves in suction (K = 6–10) — prefer gate (K = 0.2) or butterfly (K = 0.3); (4) Eccentric reducer (not concentric) in horizontal suction — avoids air pockets; (5) Suction strainer with passage area ≥ 3× pipe area, with differential pressure gauge to monitor clogging; (6) Flooded suction whenever possible — each meter of positive head is one extra meter of NPSHa; (7) For FBE gear pumps with viscous fluids: consider heating suction piping if viscosity at startup exceeds 50,000 SSU.
7. Need help with the calculation?
The FB Bombas application engineer calculates the pressure loss for your installation and verifies NPSH as part of the pump selection process — at no cost. Send the piping layout (length, diameter, fittings), fluid data (type, viscosity at operating temperature, density) and operating conditions (flow, pressure, temperature) to comercial@fbbombas.com.br or WhatsApp +55 11 97287-4837. Within 24-48 business hours, you receive the model recommendation with NPSH analysis included.