Wrong fixes and their failure modes
High noise, hydraulic pressure rising to 150 bar, and no movement on the pump discharge gauge indicate that drive energy is reaching a stalled load without producing useful flow. In a hydraulic drive, oil flow determines motor speed while pressure produces torque. When the driven shaft cannot turn, pressure rises rapidly toward the hydraulic circuit's limiting device, and continued operation converts input power into thermal load. This is heat, not logic: stop the drive when pressure rises without shaft motion or product delivery.
Several plausible checks fail to address that mechanical condition:
| Attempt | Why it fails | What it can establish |
|---|---|---|
| Reverse the hydraulic motor | A seized pump remains seized in either direction. Reverse rotation may also loosen or displace an impeller when its retention method is direction-sensitive. | Nothing useful unless the pump is uncoupled and the required rotation direction is known. |
| Rely on the absence of shaft play | Low radial or axial play does not prove that the rotating assembly is free. A seal, bearing, impeller, or loose metal fragment can lock a shaft that has no detectable play. | A play check can reveal worn support components, but it is not a rotation test. |
| Assume transport-preservation fluid is blocking the pump | A normal liquid preservative does not explain a shaft that cannot be turned manually. An unidentified material should be identified from supplier documentation before flushing, but it is not a substitute diagnosis for mechanical lockup. | The supplier can identify any factory-applied preservative and its removal procedure. |
| Clear only the tank valve and suction strainer | These checks address inlet restriction but not a seized rotating assembly, an inlet protection plug, loss of prime, or trapped vapor inside the pump. | They confirm only the particular inlet components inspected. |
| Continue running to see whether flow starts | A stationary pump receives no useful cooling or lubrication from the pumped liquid. Heat, rubbing, and seal damage accumulate while the hydraulic circuit remains highly loaded. | No further diagnostic value once zero delivery and mechanical resistance are established. |
Pressure, torque, and thermal load
The number that matters is the combination of 150 bar hydraulic pressure and essentially zero product discharge pressure rise. High hydraulic pressure does not prove that the pump is developing LPG pressure. It shows that the hydraulic motor is encountering high resisting torque or that the hydraulic circuit itself has reached a restriction or pressure limit.
The installation separated those possibilities by disengaging the hydraulic motor. The motor operated independently, while the pump shaft could not be rotated manually. That isolates the fault to the pump or its immediately attached coupling components. Dismantling then exposed corrosion, a broken mechanical seal, and loose metal pieces. Those findings explain the stall far better than an inlet-flow theory: internal contact or debris restrained the shaft, the hydraulic motor applied torque against that restraint, and the system pressure climbed without moving LPG.
| Quantity or limit | Observed condition | Engineering meaning | Where to read or verify it |
|---|---|---|---|
| Hydraulic pressure | Up to 150 bar
|
High drive-side load; compare it with the permitted motor, pump-drive, hose, and relief settings before another test. | Hydraulic pressure gauge, hydraulic schematic, component nameplates, and manufacturer data |
| Pump discharge pressure | No rise of even 1 bar
|
The pump is not adding measurable product pressure. | Pump discharge gauge, checked for isolation and serviceability |
| Pump shaft rotation | Could not be turned manually after decoupling | Direct evidence of mechanical lockup inside the pump or at the pump-side coupling. | Approved shaft-turning point with the drive isolated |
| Hydraulic motor rotation | Motor operated when disengaged | The motor can rotate unloaded; loaded torque capability still comes from its data and hydraulic measurements. | Visual rotation check and hydraulic motor documentation |
| LPG inlet path | Tank internal valve and strainer checked; pump mounted below the tank | These facts support gravity supply but do not prove a liquid-filled, vapor-free pump casing. | Valve position, strainer inspection, approved venting or priming point, and liquid indication |
Symptom-to-cause decision matrix
| Symptom combination | Most useful cause branch | Deciding test |
|---|---|---|
| High hydraulic pressure, no discharge, pump shaft immovable when uncoupled | Seized bearing or seal, impeller contact, displaced rotating part, corrosion, or internal debris | Controlled manual rotation followed by internal inspection or supplier teardown |
| Hydraulic motor will not rotate even when uncoupled | Hydraulic motor, valve, supply, return, or mechanical motor fault | Hydraulic schematic checks plus pressure and flow measurements at the motor |
| Pump shaft rotates freely, but no LPG is delivered | Loss of prime, vapor lock, closed inlet path, retained protection plug, wrong rotation, or inadequate liquid supply | Verify liquid at the pump, provide the approved vapor escape path, inspect both ports, and compare rotation with the pump arrow |
| Noise begins immediately after a direction change | Wrong rotation or a direction-sensitive impeller attachment that has loosened or jammed | Compare actual rotation with the marked direction and inspect impeller retention |
| Free shaft before startup followed by leakage, scraping, or lockup | Dry-running damage, seal failure, loss of lubrication, or internal contact | Inspect seal faces, rub marks, bearings, impeller clearance, and recovered debris |
A successful supplier test establishes only that the unit operated at the time and under the test conditions used. Storage condition, transport, installation, rotation direction, liquid filling, and the first startup can change the mechanical state before commissioning.
Isolation and inlet preparation
LPG adds fire, pressure, and cold-burn hazards to an otherwise mechanical diagnosis. Do not separate piping, loosen a seal, or open the casing while LPG or trapped pressure remains in the assembly. Isolate the tank and hydraulic energy, depressurize and purge through the approved system procedure, control ignition sources, and confirm the work area and equipment are in the required safe state.
Before dismantling, establish whether liquid can actually occupy the pump casing. A side-channel pump requires liquid priming and can become vapor-locked. Mounting the pump below the tank creates static head, but liquid still cannot fill the casing if vapor has no escape path, an internal valve is not open, the strainer is blocked, or a shipping plug remains inside an inlet. A plastic protection plug can be pushed farther into a threaded port when piping is installed over it, leaving an apparently complete connection with a closed flow path.
Use an approved priming or venting connection to verify a liquid-filled casing without releasing LPG unsafely. Inspect the inlet and discharge ports physically when the system is isolated. Confirm that all temporary plugs, caps, packing pieces, and transport restraints have been removed. If the supplier specifies a preservation material, obtain its identity, safety data, compatibility information, and removal method before introducing a solvent or flushing liquid.
Decoupled rotation procedure
- Isolate, depressurize, purge, and lock out both the LPG and hydraulic sides using the applicable site and vehicle procedure.
- Mark the installed coupling arrangement and identify the pump's required rotation from its physical direction marking or manufacturer documentation.
- Disengage the hydraulic motor from the pump so neither shaft can transmit torque to the other. Inspect the coupling for displaced elements, interference, or incorrect assembly.
- Test the hydraulic motor independently under controlled conditions. Record its direction and whether it rotates smoothly. Stop if its pressure climbs abnormally while it is unloaded.
- Use the pump manufacturer's approved method to turn the pump shaft manually. Apply only the permitted turning torque; forcing the shaft can conceal the original contact point or create additional seal and impeller damage.
- If the shaft is immovable or has hard angular stops, classify the pump as mechanically locked and proceed to supplier evaluation or controlled teardown.
- If the shaft rotates freely through full revolutions, return to the inlet, priming, vapor-lock, rotation, and port-obstruction branches before condemning the pump.
This separation test is stronger than interpreting noise alone. It divides one coupled machine into two testable loads and identifies which shaft resists motion. Reversing the complete assembly does not make that distinction and can compound damage when the impeller attachment depends on the specified rotation.
Internal inspection and repair decision
The teardown found corrosion, a broken mechanical seal, and metal fragments. Corrosion can increase friction or interfere with running clearances; a failed seal can bind, leak, or release pieces; and loose fragments can wedge between stationary and rotating parts. A fragment may fall free during dismantling, so photograph its discovery location and preserve it for identification.
Inspection should cover the mechanical-seal faces and drive features, bearings or bushings, shaft condition, impeller attachment, impeller-to-casing rub marks, side clearances, and every loose metal piece. Match each fragment to a missing feature before reassembly. Fracture faces, directional rub marks, heat discoloration, corrosion distribution, and the impeller's retained position determine whether the seal failed first, whether another part jammed the shaft and then broke the seal, or whether dry operation created the initial damage.
Dry running is a credible damage path because the pumped liquid normally removes heat and may provide lubrication at internal interfaces. A pump that contains vapor rather than liquid can make noise without developing delivery pressure while contact surfaces heat quickly. The verified gravity inlet and cleaned strainer narrow the suction investigation, but the casing fill and vapor escape path still decide whether the pump was actually primed.
For a newly purchased unit, preserve photographs, coupling orientation, debris, seal pieces, pressure observations, and the supplier's test record. Supplier evaluation is preferable when teardown could affect warranty responsibility or when dimensional inspection and replacement clearances are unavailable. Replace or repair all damaged components according to the pump manufacturer's instructions; free rotation after debris removal alone does not make a fractured seal serviceable.
Controlled restart and verification
- Complete the documented repair or install the supplier-approved replacement. Confirm free manual rotation through complete revolutions before coupling the drive.
- Verify hydraulic motor direction independently, then compare it with the pump's marked rotation. Correct the hydraulic connections or control arrangement if the directions differ.
- Inspect both pump ports and the connected piping for retained plugs or obstructions. Confirm the tank valve and strainer condition again after reassembly.
- Fill and prime the pump through the approved arrangement, providing a safe path for displaced vapor. Verify liquid presence by the method specified for the LPG system.
- Couple and align the motor and pump without introducing axial load or coupling interference. Repeat the manual rotation check where the design permits it.
- Start under controlled delivery conditions while watching hydraulic pressure, pump discharge pressure, actual flow, noise, vibration, and seal leakage. Stop immediately if hydraulic pressure again rises toward
150 barwhile discharge pressure and flow remain at zero. - Record stable operating readings and compare them with the pump, hydraulic motor, and system documentation. Confirm that the discharge gauge responds and that the repaired seal remains dry during the observation period required by the manufacturer.
A valid restart produces the required shaft direction, measurable LPG delivery, a responsive discharge gauge, and hydraulic pressure appropriate to the actual load. Noise should be evaluated together with flow and pressure; quieter operation alone cannot prove that the pump is primed or internally sound.
Frequently asked questions
Why does a side channel pump show 150 bar hydraulic pressure but no LPG flow?
The 150 bar reading is on the hydraulic drive side, so it indicates high resisting torque rather than LPG discharge pressure. If the uncoupled pump shaft cannot be turned, inspect the pump for mechanical seizure instead of increasing drive pressure.
Why does reversing the hydraulic motor not restore pump delivery?
A mechanical jam resists torque in either direction. Reverse rotation can also loosen or displace an impeller when its attachment is direction-sensitive, so verify the marked direction with the pump uncoupled.
Why does a pump mounted below an LPG tank still lose prime?
Static head cannot fill the casing if trapped vapor has no approved escape path or if a valve, strainer, or retained shipping plug blocks the inlet. Verify liquid at the pump through the system's approved priming or venting arrangement.
Why can a pump shaft have no play but still be seized?
Shaft play measures support clearance, not freedom of rotation. A broken seal, corroded running surface, impeller contact, or loose metal fragment can lock a well-supported shaft.
When should I stop testing and contact official support?
Stop when the shaft cannot be turned by the approved method, internal parts or the mechanical seal are broken, LPG leakage appears, or drive pressure rises without discharge. For a new pump, preserve the parts, photographs, pressure readings, and supplier test information rather than performing an undocumented repair. Escalate to Pompetravaini or the authorized supplier for teardown limits, rotation confirmation, preservation-fluid identification, replacement clearances, and warranty direction.