The material path is a reciprocating supply pump feeding a common header. The header branches at the front gear pump and continues another 20 ft to the rear gear pump. During the 30 s cycle, the rear gear pump completes its demand 5 s before the front pump. Follow the material and pressure wave through that handoff before changing seals or adding restrictions.
Where does the flow stop?
The reciprocating pump continues at the same frequency while the rear branch stops accepting material. Because the supply pump displaces material on every stroke, reducing downstream demand does not automatically reduce supply. Header pressure then rises until flow finds another path, the source stops, stored volume expands, or a component leaks or fails.
| Path location | Physical condition | Diagnostic value |
|---|---|---|
| Reciprocating-pump discharge | Source continues through the cycle | Shows whether pressure rises when total demand falls |
| Front gear-pump inlet | Branch is closest to the tee | Shows the pressure applied to the leaking pump |
| Rear gear-pump inlet | Branch includes an additional 20 ft of line |
Separates header pressure from line-loss effects |
| Rear branch after completion | Stops accepting normal process flow 5 s early |
Identifies the event that changes total demand |
The statement that “all flow” transfers to the front branch needs pressure and flow measurements. Closing the rear path can increase front-branch flow if the front pump accepts it, but it can also primarily increase header pressure. With highly viscous sealer, hose diameter, fittings, temperature, and restrictions can make the pressure at each branch substantially different.
Does the five-second mismatch explain the shaft leakage?
The timing mismatch can produce a pressure transient, but leakage present since installation points to a baseline application or mechanical problem as well. Many gear-pump shaft seals are intended to operate near inlet or case pressure rather than full process pressure. Read the pump documentation for maximum inlet pressure, maximum case pressure, permitted seal arrangement, rotation direction, and any required case-drain connection.
| Observation | Likely mechanism | Check |
|---|---|---|
| Leakage throughout the cycle | Normal feed pressure exceeds the shaft-seal limit | Compare measured front-inlet and case pressure with the pump rating |
| Leakage increases when the rear pump stops | Demand reduction raises common-header pressure | Trend all three pressure locations across the handoff |
| Leakage occurs at low pressure | Seal damage, wrong seal material, shaft runout, poor alignment, incorrect rotation, or assembly damage | Inspect the seal, shaft surface, coupling, bearings, and rotation |
| Front and rear completion times differ | Different displacement, speed, outlet resistance, commanded quantity, slip, or calibration | Compare delivered mass or volume per cycle and drive commands |
Layer one first: inspect collapsed hoses, undersized passages, plugged strainers, hardened sealer, closed valves, and temperature differences before interpreting control behavior. A restriction near the front pump can elevate upstream pressure while starving its inlet, and a plugged case drain can load the shaft seal even when header pressure appears acceptable.
Will regulators, flow controls, or stronger seals solve it?
| Approach | What it controls | Effect when rear demand stops | Decision |
|---|---|---|---|
| Pressure regulator at each branch | Limits downstream branch pressure while flow remains within its operating range | The rear regulator can close, but displaced supply volume still needs a destination | Useful for branch pressure control only after protecting the source header |
| Flow-control valve at each branch | Adds resistance to limit branch flow | Raises the pressure required from the reciprocating pump and can approach deadhead when demand disappears | Do not use as the sole protection method |
| Higher-pressure shaft seal | Raises seal capability if the pump supports that seal arrangement | May stop external leakage while transferring load to the case, bearings, housing, piping, or coupling | Use only after verifying every affected component rating |
| Source bypass, unloading path, or compatible relief device | Limits common-header pressure by diverting excess delivery | Provides a destination when either branch closes | Preferred mechanical pressure-protection principle |
| Demand-controlled source | Reduces or stops displacement as branch demand falls | Prevents sustained excess delivery | Preferred operating control, backed by independent pressure protection |
A downstream pressure regulator is not an excess-flow disposal device. A flow control is more restrictive as flow rises, so it can aggravate the source-pressure problem. An accumulator can absorb a short transition, but it has finite capacity and cannot protect a source that continues pumping after the accumulator fills.
What configuration should be selected?
Protect the common header first, then balance the branches. Fit a pressure-limiting arrangement between the reciprocating-pump discharge and any downstream isolation point. Select wetted materials, passage geometry, temperature control, and return destination for the sealer. Size the device for the source pump’s maximum deliverable flow at the worst material viscosity, using the pump and valve manufacturers’ data.
If recirculation changes or cures the sealer, control the reciprocating pump from measured demand or header pressure so it pauses, unloads, or reduces delivery when one branch completes. The control function does not replace independent protection against a blocked outlet, failed sensor, or stuck command. The approved maximum pressure is the lowest rating among the shaft seal, pump case, hoses, fittings, valves, instruments, and connected process equipment.
After header protection is established, branch regulators may hold each gear-pump inlet below its permitted value. Correct the completion mismatch separately by comparing requested quantity, actual delivered quantity, gear-pump displacement, drive speed, outlet resistance, and internal slip. Restricting both inlets to force equal timing wastes pressure and makes performance sensitive to sealer viscosity.
How should the correction be implemented?
- Install suitable pressure instruments at the reciprocating-pump discharge, front gear-pump inlet, and rear gear-pump inlet. Use sensor interfaces that do not create material-trapping dead legs.
- Record pressure and delivered quantity through a complete
30 scycle. Mark the instant when the rear branch completes and compare the readings before and after that event. - Obtain the permitted inlet, case, and shaft-seal pressures for both gear pumps. Check hose, fitting, valve, and instrument pressure ratings as part of the same pressure boundary.
- Inspect the leaking front pump for seal compatibility with the sealer, shaft damage, coupling misalignment, bearing play, correct rotation, and a clear case-drain path where the pump design requires one.
- Provide a sealer-compatible pressure-limiting or unloading path at the source header. Route diverted material to an approved destination that does not create a blocked or cured return line.
- Add source demand control so reduced branch demand reduces reciprocating-pump delivery. Configure the shutdown threshold below the lowest permitted working pressure, allowing for switching overshoot and instrument accuracy.
- Add branch pressure regulators only if the two gear pumps require lower or different inlet pressures. Do not use inlet flow restrictions to substitute for source protection.
- Correct the rear/front completion mismatch through quantity, speed, displacement, or sequence settings after pressure protection is functional.
How is the repair verified?
Repeat the production cycle while logging all pressure points. The rear-pump completion event must not drive the front inlet or case above its documented limit. Check both completion orders because a future process change could make the front branch stop first.
Confirm stable delivered quantity at both outlets, no source deadhead, no chatter at the limiting device, and no accumulating pressure between closed valves. Inspect the front shaft seal during operation and after pressure decays. A dry exterior alone is insufficient if the modification causes case heating, shaft load, or unstable flow.
For a controlled blocked-demand test, use the approved commissioning procedure with the pressure-limiting path already active. Verify that the source unloads or stops and that the independent protective device limits pressure without plugging or trapping curing material.
FAQ
Can I install pressure regulators before both gear pumps?
Yes, if branch inlet pressure needs regulation, but the common header still needs an unloading, bypass, or relief path. Regulators close against excess supply; they do not consume the reciprocating pump’s displaced volume.
Does a flow-control valve stop gear-pump shaft leakage?
Not by itself. Added restriction can increase upstream pressure and worsen leakage unless the source reduces delivery or diverts the excess flow.
Can I replace the shaft seal with a higher-pressure seal?
Only use a seal arrangement approved for the pump, sealer, speed, and measured inlet or case pressure. Raising the seal rating does not raise the ratings of the housing, bearings, hoses, or other components.
Does the extra 20 ft of line make the rear pump finish early?
The added line changes pressure loss, but completion also depends on displacement, speed, commanded quantity, outlet resistance, and slip. Log source, front-inlet, and rear-inlet pressures with delivered quantity through the full 30 s cycle and verify the 5 s handoff no longer creates a pressure excursion.