Centrifuge Operating Water Guide | Dolphin

In a self-cleaning disc centrifuge, the bowl's sludge is discharged through a unique hydraulic mechanism. The bowl body consists of two main parts: the bowl shell (top) and the sliding bowl bottom (bottom), which can move axially (up and down).

During normal operation (bowl closed): Operating water fills the chamber below the sliding bowl bottom. The centrifugal force acting on this water creates a hydraulic upward pressure that holds the sliding bowl bottom firmly against the bowl body, sealing the sludge discharge slots closed. Process fluid cannot escape from the bowl periphery.

During ejection (bowl open): The operating water is released (drained) from the chamber below the sliding bowl bottom. Without the hydraulic upward pressure, the centrifugal force of the bowl contents pushes the sliding bowl bottom downward. The discharge slots open and all sludge (and in a total ejection, all bowl contents) discharges outward through the slots in a fraction of a second.

After ejection (bowl re-closing): Fresh operating water is re-admitted to the chamber. Hydraulic pressure builds, pushing the sliding bowl bottom back up and closing the bowl. The re-sealing sequence typically takes 2–5 seconds.

Operating water must meet specific quality standards. Non-compliant water causes scale buildup, corrosion, blocked orifices, and erratic bowl behavior. The operating water stays in the bowl operating chamber and over time some of this water evaporates. If the operating water has dissolved minerals, these impurities are left behind as crystallized deposits in the bowl operating space. Similarly, sediment from unfiltered water deposits in the operating water system and prevents the operating slide from moving freely.

Required water quality parameters:

• Clarity/turbidity: Clean, particle-free water. Suspended solids block the small-diameter orifices and nozzles in the operating water circuit. Use a 100-mesh (150 micron) or finer inline strainer on the operating water supply.
• Hardness: Low hardness preferred. Hard water (above 200 mg/L CaCO3) will deposit calcium carbonate scale in the water chambers and on the sliding bowl bottom sealing faces. Scale causes erratic bowl behavior and seal damage. Use softened water where hardness is an issue.
• Chloride content: Chloride causes stress corrosion cracking in stainless steel bowl components. For stainless steel bowls, operating water chloride content should be below 50 ppm. Never use seawater or brackish water as operating water unless the machine is specifically designed for it.
• Temperature: Operating water should be at or near ambient temperature. Hot water causes thermal expansion issues and reduces its density, affecting the hydraulic balance of the sliding bowl bottom mechanism.
• pH: Neutral to slightly alkaline (pH 6.5–8.5). Acidic water causes corrosion of the water circuit components.

Operating water must be supplied at adequate pressure for the machine's ejection mechanism to function correctly. Insufficient pressure causes failed ejections or incomplete re-sealing. Excessive pressure causes premature bowl opening or operating water overfill leaks.

Typical pressure requirements:

• Small disc centrifuges (bowl diameter under 250 mm): 2.5–3.5 bar (36–51 PSI) at the machine inlet
• Medium disc centrifuges (bowl diameter 250–400 mm): 3.5–5.0 bar (51–73 PSI) at the machine inlet
• Large disc centrifuges (bowl diameter above 400 mm): 4.5–6.0 bar (65–87 PSI) at the machine inlet

These are typical ranges only — consult the specific machine technical manual for the exact requirement. Pressure must be measured at the machine inlet connection, not at the supply header, to account for pressure drop in the supply piping.

Pressure measurement point: Install a pressure gauge at the machine operating water inlet connection. Check the pressure with the ejection solenoid valve open (active ejection) — this is the dynamic pressure under flow conditions, which is the relevant value. Static (no-flow) pressure is always higher and does not indicate whether the supply can maintain pressure during an ejection.

Effects of low pressure:

• Incomplete or failed bowl ejection — sludge accumulates and eventually overflows into the disc stack
• Slow or incomplete bowl re-sealing after ejection, causing product leaking from the discharge ports between ejections
• On machines with a closing water timer, the bowl may not fully close within the allotted time, triggering a fault alarm

Common causes of low pressure:

• Undersized supply pipe creating excessive pressure drop under flow
• Partially closed supply valve or a failing pressure regulator in the supply line
• Scale or debris buildup inside the supply piping reducing effective bore diameter
• Other equipment drawing from the same supply header simultaneously, reducing available pressure
• Clogged inline strainer (check and clean at every preventive maintenance visit)

Fix: Measure pressure at the machine inlet during an ejection cycle. If below specification, identify the pressure drop source. Clean strainers, verify valve positions, and check supply pipe sizing against the machine's flow rate requirement. A dedicated operating water supply with its own pressure regulator is the most reliable configuration.

How scale causes problems: Calcium carbonate and magnesium scale deposits from hard water accumulate on the internal surfaces of the water circuit: solenoid valve orifices, nozzles, the water chamber in the bowl bottom, and on the sealing faces of the sliding bowl bottom. Scale on sealing faces prevents the bowl from closing completely, causing continuous leaking. Scale in orifices and nozzles restricts water flow, causing slow re-sealing and erratic ejection behavior.

Detection: Inspect the water circuit components during every major overhaul. White or gray deposits on internal water circuit surfaces confirm hard water scale. Bowl leaking that cannot be corrected by O-ring replacement may be caused by scale on the sliding bowl bottom sealing face.

Removal: Scale can be dissolved with a dilute acid solution (5–10% citric acid or a commercial descaling chemical). Soak affected components for 30–60 minutes, then rinse thoroughly with clean water. Never use strong mineral acids (hydrochloric acid) on stainless steel bowl components — chloride attack can cause stress corrosion cracking.

Prevention: Use softened water or deionized water for operating water supply where water hardness is above 150 mg/L. Install a water softener or scale inhibitor dosing system on the operating water supply header.

The solenoid valves in the operating water circuit are the most active mechanical components on the centrifuge, opening and closing thousands of times per year. They are a common failure point that causes erratic ejection behavior, continuous bowl leaking, or complete loss of ejection capability.

Common solenoid valve failures:

• Valve stuck open: Water continuously supplied to the bowl causes the bowl to remain open (continuous ejection/leaking) or overfills the bowl.
• Valve stuck closed: No water reaches the bowl; ejection cannot occur. Sludge accumulates continuously until the machine fails.
• Valve leaking past the seat: Small continuous flow through a closed valve gradually fills the bowl water chamber when it should be empty, causing unexpected bowl opening between programmed ejection cycles.
• Sluggish valve: Coil wear, low voltage, or partial blockage by debris causes delayed opening or closing. Ejection timing becomes erratic and inconsistent.

Maintenance schedule: Inspect and clean operating water solenoid valves every 2,000 hours or annually. Replace solenoid valve coils at the first sign of sluggish operation. Replace complete valves every 3–5 years or whenever the valve body or seat shows wear. Dolphin Centrifuge stocks replacement solenoid valves for all major disc centrifuge models.

Bowl does not eject: Check operating water pressure (must meet minimum at machine inlet during ejection). Check solenoid valve coil with a multimeter — coil resistance should match specification. Manually override the solenoid valve to confirm hydraulic function independent of the electrical control. Check for scale or debris blocking the water inlet orifice.

Bowl leaks continuously (not at ejection): Check for a stuck-open solenoid valve on the closing water circuit. Inspect the sliding bowl bottom O-rings. Check for scale on the sealing faces. Verify operating water pressure is not too high (causing overfill leaking). See the full bowl leaking troubleshooting guide for a systematic approach.

Ejection alarms but bowl never fully opens: Low pressure is the most common cause. Check pressure during the actual ejection event. Also check for scale buildup on the sliding bowl bottom that prevents it from moving freely.

Bowl opens at unexpected times (spontaneous ejection): A leaking solenoid valve in the closing water circuit allows the closing water chamber to drain slowly. When pressure drops below the holding threshold, the bowl opens spontaneously. Replace the leaking valve immediately — uncontrolled ejections can cause product loss and liquid seal breakover.