The pressure dip appears when the motor-driven pump stops, but the quantity removed from the system is flow capacity, not half of the pressure. With two matched pumps, available flow changes from approximately 16 gpm to 8 gpm; bearing demand remains approximately 4 gpm. A full-flow, modulating relief valve should reduce bypass flow while holding the oil header near its regulated pressure. Any dip below the 5 psig auxiliary-pump threshold points to transient response, inadequate shaft-pump delivery, excess bearing flow, valve behavior, or control interaction—not an automatic 50% pressure loss.
Wrong fixes and misleading assumptions
Several common responses obscure the actual mechanism:
| Attempt or assumption | Why it fails | Engineering action |
|---|---|---|
| Assume pressure must halve when one of two pumps stops | Parallel positive-displacement pumps add flow. Pressure comes from downstream resistance and relief-valve regulation. | Compare pump flow, bearing demand, bypass flow, and the relief-valve pressure-versus-flow characteristic. |
| Raise the relief setting immediately | A higher setting can conceal insufficient flow or a slow valve while increasing bearing-header pressure and heat generation. | Record dynamic pressure and verify delivered flow before changing an adjustment. |
| Change the 5 psig start threshold to stop cycling | This treats the protective response rather than the pressure loss. A lower threshold also reduces the margin protecting the bearings. | Find why the running shaft-driven pump cannot keep the header above the existing threshold. |
| Install an accumulator without measuring the transient | An accumulator can supply short-duration flow, but it cannot correct sustained pump under-capacity, excess bearing consumption, or a malfunctioning relief valve. | Measure transient depth and duration, then size the accumulator from the missing oil volume. |
| Treat the relief valve as an instantaneous pop valve | This service requires modulating full-flow bypass. Crack pressure, operating pressure, droop, travel, and return-line resistance affect the transition. | Confirm the valve type and obtain its pressure-versus-bypass-flow data. |
A check valve also does not regulate header pressure. Its job is to isolate the inactive pump and prevent reverse flow. A sticking check valve or excessive cracking pressure can contribute to a transient, but replacing it without differential-pressure data is guesswork.
Flow balance behind the pressure
Both listed IMO pumps are three-screw, positive-displacement pumps. Within their operating limits, a screw pump delivers approximately fixed displacement flow at a given speed. It does not contribute a fixed fraction of system pressure. The discharge pressure rises only as required to move the delivered oil through bearings, piping, check valves, and the bypass path.
For the stated operating case, the shaft-driven IMO C3E3B-143 delivers approximately 8 gpm at 12 psig. Approximately 4 gpm goes to the pump bearings and the remaining flow returns to the reservoir. If the motor-driven IMO A3EB-87 is matched at the actual oil viscosity, speed, and operating pressure, the idealized balances are:
| Operating state | Total pump flow | Bearing flow | Calculated excess flow |
|---|---|---|---|
| Both matched pumps running | Approximately 16 gpm | Approximately 4 gpm | Approximately 12 gpm to bypass |
| Shaft-driven pump alone | Approximately 8 gpm | Approximately 4 gpm | Approximately 4 gpm to bypass |
The total available flow halves when the motor-driven pump stops. The calculated bypass flow changes from approximately 12 gpm to 4 gpm; it does not halve. The bearing branch can still receive its approximately 4 gpm demand, so a correctly selected regulating valve closes toward its seat and preserves header pressure.
The number that matters is the shaft-driven pump's actual flow margin above bearing demand at the oil's operating viscosity. The stated 8 gpm is approximate, and the motor-driven pump's matching capacity is an assumption for this calculation. Confirm each pump at its actual speed and pressure rather than treating model numbers as proof of identical performance.
Relief-valve and check-valve behavior
A full-flow relief valve in this circuit functions as a bypass pressure regulator. Its nominal setting is not necessarily a perfectly flat 12 psig. The valve begins to open at its crack pressure; increasing bypass flow requires additional force and usually produces a higher upstream pressure. Friction, spring rate, moving-element mass, damping, oil viscosity, and return pressure shape the dynamic response.
This means the two-pump operating pressure may differ from the one-pump pressure even when both states are normal. With approximately 12 gpm passing through the bypass, the regulating pressure can be higher than it is with approximately 4 gpm passing through the same valve. The single-pump pressure should follow the valve's operating curve, not an assumed mathematical half of the two-pump pressure.
| Quantity | Known value or condition | Where to read or measure it |
|---|---|---|
| Bearing demand | Approximately 4 gpm | Flow measurement in the bearing supply branch |
| Shaft-pump stated duty | Approximately 8 gpm at 12 psig
|
Pump data and an operating flow test |
| Normal pressure target | 12 psig | Header gauge or pressure transmitter |
| Auxiliary-pump action threshold | Below 5 psig | Pressure-switch setting and control logic |
| Relief crack and full-flow pressure | Not stated | Valve data, bench test, or measured pressure at several bypass flows |
| Check-valve loss | Not stated | Differential pressure across each check valve |
Each pump's check valve must open fully in forward flow and seal against reverse flow. When the motor-driven pump stops, its check valve should close while the shaft-driven pump continues feeding the common header. Reverse leakage into the stopped pump consumes capacity and can pull pressure down even though the shaft pump itself remains healthy.
Sources of a real pressure transient
A brief pressure dip can occur because fluid and mechanical elements do not change state instantaneously. The header and piping release stored elastic volume, the relief element travels toward closed, the stopped pump decelerates, and its check valve changes position. Oil compressibility, entrained air, pipe expansion, valve inertia, and sensor location determine what the pressure trace shows.
Relief-valve chatter is useful diagnostic evidence. Rapid motion or noise during transfer can indicate unstable interaction among bypass flow, valve dynamics, line volume, and the check valve. Chatter alone does not identify the 5 psig threshold because relief-valve motion and pressure-switch reset are separate events. Capture both pressure-switch state and analog header pressure on the same time base.
| Observed symptom | Probable mechanism | Deciding test |
|---|---|---|
| Short dip followed by stable pressure near the target | Relief or check-valve transition exceeds the stored oil volume available to the header | Trend header pressure, bypass pressure, and pump command through shutdown |
| Pressure remains low with the shaft pump alone | Insufficient shaft-pump delivery, excessive bearing flow, reverse leakage, or excessive internal leakage | Measure shaft-pump flow and bearing-branch flow at operating temperature |
| Pressure changes substantially with bypass flow | Relief-valve droop or return-line backpressure | Plot upstream pressure against measured bypass flow |
| Auxiliary pump repeatedly starts and stops | Control deadband is too small, stop logic is not latched, or the two pumping states straddle the switch thresholds | Trend start and stop states with the actual switch trip and reset pressures |
| Stopped pump turns, warms, or receives reverse flow | Its check valve leaks or fails to seat | Measure reverse differential pressure and inspect the valve |
This is heat, not logic, when excess flow is continuously throttled back to the reservoir. With both pumps running, approximately 12 gpm may be bypassed under the matched-pump assumption. That pressure loss becomes heat in the oil, so continuous two-pump operation is not a neutral workaround for an unresolved transfer problem.
Controlled pump-transfer procedure
Use a recorded transfer test to separate a normal short transient from a sustained capacity problem.
- Bring the oil to its normal operating condition. Record oil temperature because viscosity changes screw-pump slip, bearing restriction, valve damping, and pressure loss.
- Confirm the motor-driven pump is running, both pump discharge check valves are in service, the reservoir level is acceptable, and header pressure is stable above 5 psig.
- Record pressure at the common bearing header. If practical, add pressure measurements upstream of the relief valve and immediately downstream of each pump check valve.
- Record motor-pump command, motor-running feedback, shaft speed, the low-pressure switch state, and any auxiliary-pump restart command on the same trend.
- Stop the motor-driven pump using the normal operating command. Capture the minimum pressure, transient duration, final shaft-pump-only pressure, and any relief or check-valve noise.
- Measure or infer the steady flows only after pressure stabilizes: shaft-pump delivery, bearing-branch flow, and bypass return flow. The balance should satisfy
pump flow = bearing flow + bypass flow + leakage. - Repeat only after the system has returned to the same temperature and initial condition. Reproducibility separates hydraulic behavior from intermittent valve sticking or instrumentation noise.
If the final pressure stays above 5 psig but the instantaneous trace crosses it, address transient volume and valve response. If the final pressure remains below 5 psig, an accumulator will only delay the low-pressure event; correct the steady-state flow deficit first.
Control strategy and cycling prevention
The operating sequence starts the motor-driven pump before the equipment and permits equipment startup only while oil pressure remains above 5 psig. After the shaft-driven pump is established, the motor-driven unit stops. A subsequent pressure drop below 5 psig calls for the motor-driven pump to cycle.
Automatic cycling becomes unstable when the auxiliary pump raises pressure above its stop or reset condition, stops, and immediately allows pressure to fall below the start threshold again. The motor then repeats the sequence until a protection device trips or the logic changes state. Determine the actual pressure-switch trip, reset, and control delays from the switch calibration and program; only the 5 psig low-pressure value is stated here.
Two established control arrangements address this interaction. A latched auxiliary-pump command starts on low pressure and remains active until an operator or a defined reset sequence clears it. Alternatively, systems with separate pump relief settings can place the automatic threshold between the main- and auxiliary-pump pressures. The cited example uses a main-pump relief setting of 20 psi, an auxiliary-pump setting of 10 psi, and an automatic switch setting of 15 psi. Those values describe one control concept, not settings for the IMO C3E3B-143 and IMO A3EB-87 installation.
Before selecting either strategy, identify whether there is one common regulating valve or an individual relief valve for each pump. Separate relief settings cannot be applied to a circuit that has only one common relief element. Preserve the bearing-protection function when modifying latching, reset, or manual Auto/Off behavior.
Accumulator use and final verification
A bladder accumulator can bridge a short mismatch between required bearing flow and delivered pump flow while the relief and check valves reposition. Size it from the measured pressure trace and the missing oil volume. The required delivered volume is the integral of the flow deficit over the transient; a pressure dip alone does not provide that volume.
Accumulator precharge, minimum acceptable header pressure, maximum system pressure, oil temperature, and allowable gas compression determine usable oil volume. Read those inputs from the accumulator data, the pressure trace, and the machine lubrication requirements. An oversized connection restriction can prevent a correctly sized accumulator from discharging quickly enough, so include connection and isolation-valve pressure losses in the review.
Accept the transfer only when all of the following are demonstrated:
- The measured minimum pressure remains above the approved bearing-protection limit and does not cause an unintended 5 psig auxiliary-pump action.
- The shaft-driven pump alone supplies bearing demand plus a positive bypass margin at operating temperature.
- The relief valve modulates without chatter and settles at the expected one-pump operating pressure.
- The stopped motor-driven pump has no material reverse flow through its check valve.
- The auxiliary-pump logic starts, latches or resets as designed, and cannot enter repetitive short cycling.
Frequently asked questions
What happens if one of two matched IMO pumps stops?
Total theoretical flow changes from approximately 16 gpm to 8 gpm in the stated case, while bearing demand remains approximately 4 gpm. The relief bypass should reduce from approximately 12 gpm to 4 gpm; system pressure should follow the relief-valve curve rather than fall automatically by half.
What happens if the pressure falls below 5 psig?
The stated control sequence calls for the motor-driven pump to cycle on low pressure. Trend the analog pressure, switch state, and pump command together to determine whether the event is a brief transfer transient or a sustained shaft-pump flow deficit.
What happens if the relief valve chatters when the auxiliary pump stops?
Chatter indicates unstable valve motion during the change in bypass flow and can amplify the header transient. Check the valve's full-flow regulating behavior, oil condition, return pressure, and check-valve transition before changing its setting.
What happens if the auxiliary oil pump repeatedly starts and stops?
Verify the pressure-switch trip and reset points, then use a properly reviewed latching strategy or differentiated pump-pressure strategy to prevent repeated cycling. Stop the test if bearing pressure remains below the approved limit, valve chatter persists, or the motor approaches a protective trip. Escalate to IMO through an official support channel when pump performance, allowable pressure, relief selection, or accumulator compatibility cannot be verified from the applicable product data.