Reversing a Gear Pump: Complete Guide to Bi-Directional Operation, Safety, and Configuration

One of the most common questions plant engineers and maintenance teams ask about gear pumps is whether they can be reversed — either to change flow direction, empty a line back into a tank, or switch between two process paths. The answer depends entirely on the pump type, the gear geometry, the seal arrangement, and whether the relief valve is correctly configured for reverse operation. This guide covers everything: which gear pumps support reversal, what physically changes inside the pump when you reverse rotation, what must be modified in the pipework and valve arrangement, and the specific checks required before reversing a pump that was not originally specified for bi-directional operation.

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Quick Answer: Can a Gear Pump Be Reversed?

Pump Type	Rotation Reversible?	Flow Reversible?	What Changes
Internal gear pump (crescent or gerotor)	Yes — always bi-directional	Yes — inlet becomes outlet	Suction and discharge ports swap; relief valve must be re-piped or a dual relief valve used
External gear pump — spur gears	Yes — with correct seal and port configuration	Yes — inlet becomes outlet	Same as internal; check if gland plate is symmetric; re-pipe relief valve
External gear pump — single helical gears (UA Series)	No — rotation direction is specified	No	Helical gear axial thrust direction is fixed; running in reverse damages thrust bearings
External gear pump — herringbone gears	No — inherently directional	No	Double helical cancels axial thrust only in design direction; reverse causes bearing overload
Gear pump with fixed-port gland plate	No without modification	Only with pipework change	Non-symmetric gland plate must be replaced or rotated to swap port function

⚠ The single most important rule: Never reverse a helical or herringbone external gear pump. The gear geometry creates axial thrust in only one direction — reversing creates thrust in the opposite direction, which the bearing arrangement cannot handle. Bearing failure typically follows within hours. Always confirm gear tooth type before reversing any external gear pump.

Why Gear Pump Flow Reverses When Rotation Reverses

An external gear pump works by creating a low-pressure zone where the gear teeth unmesh (suction) and a high-pressure zone where the gear teeth mesh (discharge). When rotation direction reverses, the unmeshing zone and the meshing zone exchange positions — the port that was previously at low pressure (suction) is now at high pressure (discharge), and vice versa. The fluid path through the pump body stays the same, but the direction of fluid movement through it reverses.

An internal gear pump works identically in principle. The rotor and idler rotate in the same direction as each other regardless of CW or CCW drive direction — reversing the drive shaft direction simply reverses both rotors together, swapping the expanding and contracting cavities and therefore swapping suction and discharge.

What Must Be Modified When Reversing a Gear Pump

1. Relief Valve — The Most Critical Change

⚠ Reversing a gear pump without addressing the relief valve leaves the pump completely unprotected against over-pressurisation in the new flow direction. If the discharge is blocked in reverse mode, pressure builds without limit until a pipe fitting, seal, or pump casting fails. This is a serious safety hazard.

Three solutions, in order of preference:

Dual relief valve (one on each port): The best solution for a pump permanently operated in both directions.
Cross-port relief valve block: A purpose-designed valve block with a single relief valve internally connected between both ports.
Manual re-piping at each reversal: Move the relief valve from its current port connection to the new discharge port connection each time the flow direction changes. Only practical if reversal is infrequent.

2. Suction and Discharge Port Labels and Pipework

When a symmetric pump is reversed, the physical ports remain in the same location on the pump body — but their function changes. Any labels, direction arrows, or pipework annotations must be updated.

3. Mechanical Seal — Check Rotation Direction

Single coil spring mechanical seals (Type 41, DIN EN 12756) are rotation-direction-dependent. Check the seal hand marking before reversing. Multi-spring seals and bellows seals are rotation-independent — these require no change on reversal.

4. Gear Pump with Bypass Port — Check Bypass Direction

Some gear pump designs incorporate an integral bypass port. When the pump is reversed, verify that the bypass port remains connected to the correct system line.

Step-by-Step Procedure for Reversing a Gear Pump Safely

Confirm the pump is a reversible design: Check the manufacturer's datasheet — confirm spur gear (not helical or herringbone) for external pumps; any internal gear pump is reversible.
Isolate the pump: Close suction and discharge isolation valves. De-energise and lock out the motor (LOTO procedure). Relieve any trapped pressure.
Address the relief valve: Install a dual relief valve block or move the single relief valve to the new discharge port connection.
Check and change the mechanical seal if required: If the pump uses a single coil spring seal (Type 41), replace with the opposite-hand seal before reversing.
Re-label ports and update pipework: Update any port labels, flow arrows, and P&ID annotations.
Reverse the motor rotation: Swap two of the three motor phase connections (for 3-phase motors) to reverse rotation. Confirm direction arrow matches new rotation.
Check the coupling and shaft alignment: Any change involving motor work is an opportunity to verify coupling alignment.
Start with no-load check: Open isolation valves, start the motor, and observe for correct flow direction. Monitor discharge pressure.

📖 Read More: Gear Pump Maintenance Guide

Reversing an Internal Gear Pump — Specific Considerations

The internal gear pump is inherently bi-directional — both the rotor and idler reverse together when shaft rotation reverses, and the eccentric geometry creates identical pumping action in both directions. This makes it the preferred choice for applications requiring regular flow reversal without mechanical modification each time.

Common Reversal Mistakes and Their Consequences

Mistake	Consequence	How to Prevent
Reversing a helical or herringbone external gear pump	Bearing overload — failure within hours to days	Check gear type before reversing; helical/herringbone pumps must not be reversed
Not moving the relief valve to new discharge side	No overpressure protection in reverse — pipe or seal failure if discharge blocks	Install dual relief valve or move single valve before reversing
Not changing single coil spring seal hand	Spring unwinds — reduced face closing force — seal leak within hours to days	Confirm seal type; change to correct hand or replace with rotation-independent design
Assuming all gear pumps can be reversed	Damage to directional pump design	Always confirm with manufacturer's documentation or engineering team
Swapping motor phases without checking rotation direction	Pump runs in wrong direction — no flow or cavitation	Verify rotation direction arrow after phase swap; test with isolation valves open

FAQs — Reversing a Gear Pump

Q: Can all gear pumps be reversed?

No. Internal gear pumps and spur-tooth external gear pumps can be reversed with appropriate modifications (relief valve, seal hand). Single helical and herringbone external gear pumps must never be reversed — the gear geometry creates axial thrust in one direction only, and reverse operation causes bearing failure.

Q: Does reversing a gear pump reduce its performance?

For correctly reversed bi-directional pumps, performance is identical in both directions — the displacement, pressure capability, and efficiency are the same. The pump produces the same flow at the same RPM regardless of rotation direction.

Q: Can I reverse a gear pump without stopping it?

Never reverse a gear pump while it is running. The mechanical impact of reversing rotation while the gears are moving under load causes immediate gear and bearing damage. Always stop the pump completely, make all necessary modifications (relief valve, seal), and then restart in the new direction.

Q: Which gear pump is best for regular bi-directional operation?

The internal gear pump is the best choice for applications requiring frequent or regular flow reversal. Its symmetric geometry requires no mechanical changes between directions (other than the one-time installation of a rotation-independent seal and a dual relief valve arrangement). Contact Unique Pump Systems for bi-directional internal gear pump specifications.

Unique Pump Systems manufactures rotary gear pumps across the full UA Series range. For bi-directional application advice, relief valve specification, and rotation direction confirmation for your specific pump model, contact our application engineering team.

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