Selecting API Plan 14 for Vertical In-Line Pump Seals

At startup, the control panel may show the pump running normally while the upper seal cavity remains vapor-bound. Without seal-chamber flow, pressure, or temperature indication, the panel cannot distinguish liquid-wetted faces from faces running in trapped gas. Start with the venting path, not the motor controls.

Reject the fixes that leave gas at the seal faces

A single-stage pump and a single seal do not make Plan 11 self-venting. Those details affect the seal arrangement and pump hydraulics, but they do not remove the gas pocket created when the seal faces sit at the top of a closed vertical cavity.

• Do not accept the single-stage exception without a hydraulic explanation. Vertical in-line pumps are generally single-stage apart from uncommon specialty designs. A claim that the API 682 recommendation applies mainly to multistage vertical in-line pumps does not address the normal single-stage geometry.
• Do not treat one manual pre-start vent as the complete fix. It depends on an operator performing the task correctly before every start. It also cannot continuously remove vapor or entrained gas that reaches the chamber after startup.
• Do not increase Plan 11 flow before correcting the flow path. More discharge-to-seal flow may improve cooling, but a one-way supply does not provide a dedicated return from the high seal cavity to suction.
• Do not change the orifice by guesswork. An oversized restriction can reduce chamber pressure or waste recirculation flow. An undersized passage can starve the faces of cooling flow. Base the change on the required seal flow and calculated pressure losses.
• Do not use the single seal as proof that the fluid is nonhazardous. Confirm the process classification and permitted vent destination independently.

The useful default for this arrangement is Plan 14: discharge to the seal chamber, then seal chamber back to suction. Retain a high-point vent for initial filling and purging.

Trace the real fault to the upper seal cavity

A vertical seal places the faces near the top of the chamber. Air introduced during maintenance, gas released from solution, or process vapor can collect there. If the circulation path does not sweep that high point, the faces can lose their liquid film even though the pump casing is full.

Plan 14 addresses the geometry by establishing both an inlet and an outlet. The discharge leg brings higher-pressure process liquid to the chamber. The return leg carries liquid, entrained gas, and vapor from the chamber toward suction.

Determine the actual seal-chamber pressure

Do not select between Plan 13 and Plan 14 from pump orientation alone. Impeller construction, balance holes, throat-bushing clearance, internal leakage paths, and operating point determine chamber pressure.

If the pump has a closed impeller with wear rings and balance holes connecting the back side of the impeller toward suction, the holes reduce pressure imbalance and bearing thrust. The seal chamber may then operate near suction pressure. Confirm that construction on the sectional drawing; an API-related specification does not prove that the installed impeller has those features.

Other vertical pump arrangements can place discharge pressure in the seal chamber. Where chamber pressure remains high enough to drive flow back to suction, Plan 13 can provide a chamber-to-suction circulation path. It generally needs more flow than Plan 11 to remove the same seal heat because it does not introduce a fresh discharge-side stream at the chamber.

Use this decision path:

• If chamber pressure is close to suction pressure, do not expect a chamber-to-suction-only Plan 13 circuit to develop useful flow. Evaluate Plan 14.
• If chamber pressure is substantially above suction pressure, evaluate Plan 13 flow and heat removal as well as Plan 14.
• If the fluid can flash at the faces, favor the arrangement that maintains the required chamber pressure and continuous circulation. Check vapor-pressure margin at the maximum local seal temperature.
• If measured pressures disagree with the vendor calculation, resolve the pump’s internal leakage model before approving an orifice or throat-bushing change.

Check the process assumptions before approving Plan 14

The recommendation for Plan 14 rests on more than pump orientation. Record the fluid hazard, normal and maximum temperature, suction pressure, discharge pressure, vapor pressure, solids behavior, seal heat load, and required face-cooling flow.

The simplified recommendation assumed a nonhazardous fluid because a single seal was selected. Treat that only as a screening assumption. If the fluid is hazardous, confirm whether a single seal is acceptable and route every vent to the approved containment or recovery system.

The discussion also used an elevated-temperature screen written as greater than 60 °C/150 °F. Those values are not equivalent: 60 °C equals 140 °F, while 150 °F is about 65.6 °C. Use the specified design temperature and the seal supplier’s cooling limits rather than treating that pair as an exact conversion.

For light hydrocarbons or another flashing service, cooling flow alone may not solve the problem. The chamber pressure must remain above the fluid’s vapor pressure at the local temperature by the required design margin. Obtain that margin from the approved seal design basis rather than assigning an arbitrary value.

Configure the Plan 14 flow path

The pump needs usable connections at discharge, the seal gland or chamber, and suction. Locate the chamber outlet so gas rises into it rather than collecting above it. Route piping continuously where practical and remove unintended high points.

1. Take the supply from the pump discharge connection and route it to the seal-chamber inlet.
2. Connect a separate seal-chamber outlet back to pump suction. Confirm that this outlet sweeps the upper face area.
3. Install the calculated restriction in the appropriate circuit location. The return restriction controls how chamber pressure and flow divide between the suction return and internal leakage paths.
4. Review throat-bushing clearance. Change it only after calculating how the change affects chamber pressure, recirculation flow, and leakage into the pump.
5. Add an accessible high-point vent valve for initial purging. Route the vent according to the fluid’s hazard and emissions requirements.
6. Check the complete line for pockets, blocked fittings, closed valves, reversed connections, and restrictions smaller than those used in the hydraulic calculation.

The return to suction makes the circuit self-venting during operation, but it does not remove the need to fill and vent the chamber before the first start or after the seal cavity has been opened.

Set flow without sacrificing pressure margin

The circulation rate must remove seal-generated heat while maintaining liquid at the faces. Use the seal supplier’s required flow or heat-load calculation, then calculate the available differential pressure through both external legs and their restrictions.

Measure or calculate four interacting paths: discharge-to-chamber piping, chamber-to-suction piping, leakage through the throat bushing, and internal pump connections such as balance holes. Changing one path changes chamber pressure and the flow through the others.

• Open the return restriction or enlarge its orifice only when the resulting chamber pressure still provides the required vapor-pressure margin.
• Reduce throat-bushing clearance only when the pump and seal design permit it; the change can raise chamber pressure and redirect more discharge flush through the external return.
• Increase circulation when the measured temperature rise or supplier heat balance calls for it, not simply because Plan 13 or Plan 14 is installed.
• Recalculate at the limiting operating point. Discharge and suction pressures change across the pump curve, so a circuit that works at one condition can lose flow or pressure margin elsewhere.

Verify the seal before design acceptance

1. Review the pump sectional drawing. Identify the impeller type, wear rings, balance holes, throat bushing, seal-chamber connections, and the chamber’s physical high point.
2. Mark the proposed path from discharge to chamber and from chamber to suction. Reject any layout that leaves the face area above the effective outlet.
3. Obtain the vendor’s chamber-pressure and flush-flow calculation for the specified operating range. The calculation must include restriction sizes and throat-bushing leakage assumptions.
4. Before startup, fill the chamber and open the high-point vent until gas is displaced by process liquid. Close or isolate the vent as required by the approved piping arrangement.
5. Start the pump and record discharge, seal-chamber, and suction pressures at the same operating point. Confirm flow direction through both legs.
6. Check seal temperature and leakage during stabilization. Investigate rising temperature, intermittent leakage, noise, or repeated gas discharge rather than treating a normal motor indication as proof of seal lubrication.
7. Repeat the pressure and temperature checks at the relevant operating extremes. Verify that the lowest chamber pressure still meets the approved vapor-pressure margin and that circulation remains adequate for heat removal.

Document the final orifice, throat-bushing clearance, valve lineup, venting method, measured pressures, and acceptance readings. That record makes later seal failures diagnosable and prevents an undocumented restriction change from defeating the circuit.

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