API Plan 31 is a single-seal flush that pipes process fluid from the pump discharge through a cyclone separator, which sends the cleaned stream to the seal chamber and returns the solids-laden stream to pump suction. Among the API seal flush plans, it is Plan 11 with a cyclone added to the line. Plan 41 adds a cooler to the same arrangement, and Plan 12 cleans the Plan 11 flush with a strainer instead of a cyclone.
The cyclone works when the fluid carries dilute solids that are roughly twice as dense as the liquid and the pump supplies enough pressure differential to spin them out.
How the Cyclone Separator Works
The cyclone spins the flush so that solids heavier than the liquid are thrown to its wall and drained away, while clean liquid leaves through the centre. Fluid at discharge pressure enters the separator’s cylindrical top tangentially, and the narrowing cone below it speeds the spin up as the flow moves down.
Solids collect at the wall, spiral down to the bottom outlet and return to pump suction, the lowest-pressure point in the loop, taking only a small share of the liquid with them. Clean liquid turns up the centre of the vortex, leaves through the top outlet and is piped to the flush connection in the gland plate and onto the seal faces.
A particle is separated only if it reaches the wall before its liquid leaves the cone. Its outward speed grows with the square of its size and with the difference between solid and liquid density, and it falls as the liquid’s viscosity rises.
With no screen to load up, a working cyclone removes solids with little maintenance, where a strainer or filter in the same line needs regular cleaning.
When the Cyclone Separator Works
The cyclone keeps solids out of the seal chamber when four conditions hold at once: solid density, particle size and load, pressure differential across the separator, and a throat bushing in the seal chamber. It removes only suspended solids. Anything dissolved in the liquid passes straight through.
A settling test checks the solids before the plan is specified. Shake a sample of the pumped fluid in a clear container and let it stand for 10 to 15 minutes: if the solids settle out, a cyclone separator will work in most services.
Where these conditions are not met, a Plan 32 flush of clean liquid from an outside source is the usual alternative. In service, the separator wears from the solids it handles, and a change in the pressures around it, or grit found in the seal chamber at teardown, is the sign that it has stopped separating.
Solids Heavier Than the Liquid
The solids should have a specific gravity of roughly twice the liquid’s.
Check the ratio against the liquid actually pumped, not water. Sand and mineral grit in water meet the rule easily; organic and plastic particles usually do not.
Particle Size and Solids Load
Particles in a Plan 31 flush should be larger than about 1 micron, and the solids should make up no more than about a tenth of the fluid.
Particles near the 1-micron floor are only partly caught, so a fluid whose solids are mostly fines is a poor match. The upper size limit is what passes the separator’s inlet and bottom outlet without bridging; take it from the data sheet of the specific separator.
Fibres and flakes separate less efficiently than rounded grains, so fibre-laden stock such as paper pulp separates poorly in a cyclone.
Whether a cyclone suits a slurry pump depends on the slurry: one whose solids exceed about a tenth of the fluid is beyond the cyclone’s solids-load limit.
Pressure Differential Across the Separator
Published minimum differentials for a Plan 31 cyclone run from 1 bar (15 psi) to 1.7 bar (25 psi), but they are not one quantity: each maker measures between a different pair of points. Check which pair the separator maker means before comparing its figure with the pump.
| Where the differential is measured | Minimum quoted |
|---|---|
| Pump discharge to pump suction | 1 bar (15 psi) or 1.4 bar (20 psi) |
| Separator inlet to separator outlet | 1 bar (15 psi) or 1.7 bar (25 psi) |
| Pump discharge to seal chamber | 1.7 bar (25 psi) |
The separator maker’s stated maximum differential is the ceiling: fed harder, the separator sends abrasive-laden fluid through the flush into the seal chamber unseparated.
Some guides say the separator’s inlet and outlet pressures should be nearly equal, but the drop from inlet to outlet is the differential that has to stay between the minimum and the maximum. The pressures that should be close are those where the two outlets discharge, the seal chamber and the pump suction, so that flow leaves through both.
Throat Bushing Requirement
Plan 31 should have a close-clearance throat bushing at the bottom of the seal chamber. Seal makers differ on whether it is a requirement or a recommendation; on an open-bore seal chamber we would fit one.