Plan 11 at a glance
API Plan 11 is one of the most common mechanical seal piping plans for single seals in centrifugal pumps. It sends process liquid from the pump discharge through a restriction orifice and back into the seal chamber. This simple flush arrangement helps remove heat from the seal faces, vent the seal chamber on horizontal pumps, and improve vapor pressure margin around the seal.
Although Plan 11 looks simple, its reliability depends on the actual liquid, pump pressure, seal chamber design, orifice size, and the cleanliness of the flush line. If the orifice becomes blocked or the liquid is easy to freeze, polymerize, crystallize, or vaporize, the seal may run hot and fail early.
Contents
- What is API Plan 11?
- How Plan 11 works
- Main functions of Plan 11
- Orifice and piping notes
- When Plan 11 should not be used
- Pressure check: two pumps, one orifice
- Maintenance checks
- FAQ
What is API Plan 11?
API Plan 11 is a recirculation flush plan for mechanical seals. The flush liquid is taken from the pump discharge, passes through an orifice, and enters the seal chamber. After cooling and flushing the seal faces, the liquid returns naturally through the pump internal circulation path.
In normal service, Plan 11 uses the pumped product itself as the seal flush liquid. No external clean liquid supply is required. This makes the plan simple, compact, and economical for many clean, non-polymerizing services.
Plan 11 is often used on single mechanical seals in horizontal centrifugal pumps. It can also be applied to other pump arrangements when the seal chamber pressure, fluid condition, and heat load are suitable.
How Plan 11 works
The pump discharge side normally has a higher pressure than the seal chamber. Plan 11 uses this pressure difference to drive a small controlled flow through the flush line. The restriction orifice limits the flow so that the seal receives enough cooling and flushing without wasting too much pump capacity.
A typical Plan 11 route is:
- Pump discharge connection
- Flush piping
- Restriction orifice
- Seal gland or seal chamber connection
- Seal chamber and seal faces
The flush should normally enter near the top of the seal chamber or gland, often around the 12 o’clock position on horizontal pumps. This helps remove vapor and air from the chamber instead of trapping gas near the seal faces.
Main functions of Plan 11
Heat removal from the seal faces
Mechanical seal faces generate heat during operation. This heat comes from sliding contact, fluid shear, pressure load, and friction during start-up or unstable conditions. If heat is not removed, the liquid film between the rotary face and stationary face can become too thin or begin to vaporize.
Plan 11 provides a steady flow of process liquid across the seal area. This helps carry heat away from the seal faces and keeps the seal chamber closer to a stable operating condition.
Seal chamber venting
Horizontal pumps can trap air or vapor in the upper part of the seal chamber. If the seal faces run in a gas pocket instead of a liquid film, the faces may overheat quickly. This can lead to leakage, face cracking, carbon blistering, or elastomer damage.
Because Plan 11 is usually piped into the upper area of the seal chamber, it can help sweep vapor away and keep the seal faces wetted by liquid.
Improving vapor pressure margin
Some fluids have a narrow margin between operating pressure and vapor pressure. Light hydrocarbons and hot liquids are common examples. If the pressure around the seal faces drops too close to vapor pressure, the liquid film can flash into vapor. Once the seal faces run dry or semi-dry, seal life drops sharply.
Plan 11 can increase seal chamber pressure and improve vapor margin by bringing higher-pressure liquid from the pump discharge into the seal area. This is one reason Plan 11 is frequently considered for services where vaporization is a concern.
Orifice and piping notes
The restriction orifice is a critical part of Plan 11. Without an orifice, the flush flow may be too high. An undersized or blocked orifice can restrict the supply; erosion or an enlarged bore can increase recirculation.
Common practical checks include:
- Use an orifice size suitable for the pump and seal service.
- Avoid very small openings that clog easily.
- Keep the flush line short and simple when possible.
- Install the piping so it can be inspected and cleaned.
- Confirm that the flush enters the correct seal chamber or gland port.
- Check whether the pump vendor or seal supplier has a recommended flush rate.
For many services, a minimum orifice diameter around 3 mm, or 1/8 inch, is used as a practical lower limit to reduce clogging risk. Final sizing should still be confirmed according to pump pressure, liquid properties, temperature, seal size, and required flush flow.
When Plan 11 should not be used
Plan 11 is not suitable for every liquid. Because it uses the pumped product as the flush liquid, any problem in the pumped liquid can also affect the seal flush.
Avoid or review Plan 11 carefully when the liquid:
- Contains solids that may block the orifice
- Crystallizes when pressure or temperature changes
- Polymerizes in small hot passages
- Freezes in exposed piping
- Is highly viscous and difficult to circulate
- Is dirty enough to damage seal faces
- Has poor lubricating properties at seal chamber conditions
- Requires an external clean flush for seal reliability
In these cases, another piping plan may be safer, such as an external flush plan, a cooled recirculation plan, or a dual seal support system. The correct choice depends on the liquid, emissions requirement, pump construction, and seal arrangement.
Pressure check: two pumps, one orifice
A discharge pressure reading alone cannot establish whether a Plan 11 circuit has enough driving pressure. Compare pressure at the discharge take-off with pressure in the seal chamber at the same operating point. Suction pressure is not a substitute for seal chamber pressure.
Illustrative comparison, not a sizing recommendation: Pump A has 8 barg at the take-off and 6 barg in the seal chamber; Pump B has 8 barg and 7.5 barg respectively. Their available circuit pressure differences are 2 bar and 0.5 bar. With the same liquid, orifice geometry and negligible other losses, the idealized relationship Q ∝ √ΔP gives Pump B about half the flow of Pump A. The same discharge pressure and orifice diameter therefore do not imply the same flush performance.
In a real installation, piping losses, liquid viscosity, throat-bushing clearance and the interaction between flush flow and chamber pressure must be included. The circuit pressure difference is not automatically the pressure drop across the orifice. Use the pump and seal suppliers’ calculations before changing the bore.
Keep two checks separate: driving pressure moves the flush; vapor margin keeps the liquid from flashing. For the latter, compare seal chamber absolute pressure with the fluid vapor pressure at the local temperature, using the same pressure basis. A flowing line alone does not establish adequate vapor margin. Plan 11 has no cooler, so more recirculation cannot be assumed to solve every hot-service problem.
Maintenance checks
Before restart: follow the equipment isolation, cleaning, filling and venting procedure. Confirm the approved orifice identification and correct flush port against the drawing. Do not open a pressurized flush line to check for flow.
When the seal runs hotter: record pump speed, process temperature, discharge pressure and seal chamber pressure at the same operating condition. Compare against a known stable condition. A hot pipe can be heated by conduction even when circulation is poor; pipe temperature alone is not proof of flow.
After repeated plugging: retain a description or photograph of deposits and note whether the problem follows a product change, shutdown or cold-weather restart. Repeatedly cleaning the restriction may restore operation temporarily without correcting crystallization, polymerization or solids ingress. Review the fluid and piping plan before enlarging the opening.
For a useful technical review, provide the pump model and speed range, seal drawing, fluid composition, operating temperatures, pressure measurement locations, installed orifice bore, piping arrangement and the timing of leakage. Mark unknown values as unknown. See our Plan 11 operating checks or send the service details for review.
FAQ
Does a 3 mm orifice suit every Plan 11 installation?
No. A practical minimum intended to limit plugging is not a universal flow specification. Required flush flow and available pressure difference still need to be checked for the particular pump, seal and liquid.
Is Plan 11 the same as an external water flush?
No. Plan 11 recirculates the pumped product from the discharge. An external flush, commonly described by Plan 32, introduces a separate compatible liquid. It needs its own supply controls and a review of product dilution.
Can Plan 11 remove air before the pump starts?
Do not rely on it to replace filling and venting before startup. Its circulation depends on an available pressure difference during operation. Follow the pump and seal instructions, especially for arrangements with high points that can trap gas.
Technical reference: John Crane cartridge-seal installation instructions illustrate the discharge-to-seal Plan 11 route. The pressure comparison above is an explanatory example, not field-test data or an approved installation design.
