Achieving Optimal Turbulence: The Role of the Reynolds Number in Oil Flushing

Reynolds number (Re) is a dimensionless value used to predict whether fluid flow in a pipe will be laminar, transitional, or turbulent. In oil flushing, Re matters because turbulent flow improves the ability to dislodge and transport contamination to filtration rather than letting debris settle in low-velocity regions. The goal is not simply “some turbulence,” but enough turbulence to keep contaminants moving. 

To use Reynolds number effectively in flushing procedures, it helps to clarify how it’s calculated and what inputs matter. Re is calculated by multiplying the average fluid velocity by the pipe inside diameter, then dividing by the fluid’s kinematic viscosity: Re = (V·D)/ν. If you only have flow rate (Q), convert it to velocity first (V = Q/A) or use a flow-rate form of the equation. 

For those who prefer a manual calculation, you can use the formulas shown in Figure 1; alternatively, many Re calculators are available in app form or via a search engine.

Flow regimes are categorized by their Reynolds number. Here’s a rule of thumb for internal pipe flow:

• Laminar: Re < 2000 (smooth, consistent flow)
• Transitional: Re 2000 – 4000
• Turbulent: Re > 4000

Achieving turbulent flow (sometimes called chaotic flow) is often essential for effectively dislodging and removing contaminants from piping systems. For this reason, many OEM and industry procedures use Re ≥ ~4,000 as a minimum turbulence target that must be reached during flushing activities.

However, targeting this specific minimum can be an imprecise goal for service providers and customers who attempt to "self-perform" these activities.

A high-velocity oil flush primarily serves to remove contaminants such as hydrocarbon buildup (sludge), foreign materials from installation or maintenance, or debris from equipment failure.

The key to a successful flush is flow velocity (and resulting turbulence), not chasing pressure. Pressure is mainly a consequence of restrictions and required flow. While pressure may naturally occur in small-bore tubing due to flow requirements, achieving adequate flow is paramount.

To ensure maximum flow and a greater Re, you must:

• Remove restrictions like orifice plates.
• Install temporary jumpers/bypasses (see Figure 2) or bypass material around in-line control valves, directional valves, or other restrictive critical components.
• Bypass components that should not see flushing debris, including bearings, gearboxes, and booster pumps.

Figure 2 – “Jumper” bypass material including fittings, valves, and hoses to run flush from bearing lubricant supply pipe to bearing lubricant return pipe. 

These measures protect vital equipment. Pushing contaminants into critical components creates a risk of trapped particulates, which can lead to immediate wear or failure upon start-up. 

The Problem with Sub-Optimal Flow

The display below (Figure 3) illustrates how turbulent flow affects contamination at a Re of 4000. Using a smooth-bore pipe with loose particulate and water shows that flow at Re 4000 is insufficient to effectively pick up and transport materials back to the filtration system or reservoir.

In a real central lubrication system, this problem is worse. Particulates would stick to oily surfaces, lodging in pipe imperfections, 90-degree turns, or reducers.

This inadequate turbulent flow results in:

• Significantly longer flush times.
• The risk of leaving behind material that can severely damage the system's critical components. 

Figure 3 – Fluid is running at a Re 4000 and loose contamination sits on the bottom of the pipe. 

Action-Oriented & Concise

For over 30 years in the industry, we have advised clients on the critical difference between the widely referenced Re 4000 minimum and the more effective Re 10000 minimum Figure 4 for flushing scopes.

While the verbiage around achieving a minimum Turbulent Flow or Re 4000 persists in our field, we prefer to aim much higher.

Example Application: Using a 2,000 Gallon Per Minute flushing skid with a 6-inch header pipe and ISO VG 32 lubricant at 150°F can achieve Re well above 20,000 (exact value depends on the pipe ID and the oil’s kinematic viscosity at temperature). as shown in Figure 5).

Procedures for High-Velocity Oil Flushing

Achieving the correct turbulent flow is crucial for a successful high-velocity oil flush. It is strongly recommended to engage a professional service provider who can accurately incorporate and calculate the necessary flow rates into their process. 

A reputable provider will furnish comprehensive, written procedures that cover key documentation and technical aspects:  

• Scope of Work: The complete project overview.
• Cleanliness Criteria:
  • Inspection media
  • ISO Cleanliness standards
  • On-site testing capabilities
• Process Steps:
  • Signature pages for procedure acceptance and process completion
  • Documentation for any deviations and approval authority
• An orange metal object with a round metal circleSafety Factors
• Equipment and Flow Rates:
  • Filter size and beta ratio
  • Bypass materials
• Flush Flow Path:
  • Marked drawings 

Techniques for Enhancing Turbulence 

To further assist in moving contamination out, consider these techniques for achieving turbulence within the pipe:

Mechanical Aids: Use pneumatic vibrators or rap the pipe with rubber mallets to loosen stubborn materials and induce turbulence.

Nitrogen Sparging: Introduce nitrogen bubbles (sparging) at the front end of the flushing activities. These fine, volatile bubbles create chaotic movement, knocking loose and carrying contaminants through and out of the system.

Professional Guidance

When in doubt, consult a professional in the flushing business. Ensure you check references, tour their facility, inspect their equipment, ask pertinent questions, and require a formal written procedure before job execution.