Mechanical seal flush plans are essential for chemical pump reliability, providing lubrication, cooling, and contamination control at the seal faces. This guide explains common API 682 flush plans and their application to chemical process pumps.
Seal Flush Fundamentals
Purpose of Flush
- Cool seal faces (friction heat)
- Lubricate seal faces (prevent dry running)
- Prevent solids/contaminants from reaching seal faces
- Maintain pressure (prevent vaporization at seal)
- Control environment (temperature, pressure, chemistry)
API 682 Standard
- Defines standard piping plans for mechanical seals
- Categories: Plan 01-99 (various configurations)
- Selection: based on fluid, temp, pressure, solids, hazard
- Components: orifice, valve, cooler, separator, reservoir
- Documentation: P&ID, datasheet, installation manual
Common Flush Plans
Plan 11 (Internal Recirculation)
- Fluid from pump discharge recirculates to seal chamber
- Components: orifice (control flow), internal passage
- Advantages: simple, no external supply, reliable
- Limitations: only for clean fluid, limited cooling
- Use: clean, non-hazardous, moderate temp (<80°C)
Plan 21 (Recirculation with Cooler)
- Plan 11 + external cooler (heat exchanger)
- Components: orifice, cooler (water/air cooled), valves
- Advantages: cools flush, suitable for hot fluids
- Limitations: external cooler, maintenance, water supply
- Use: hot fluids (>80°C), clean, non-hazardous
Plan 23 (Recirculation with Cooler and Circulator)
- Closed loop: seal chamber fluid circulated through cooler
- Components: pumping ring (circulator), cooler, orifice
- Advantages: efficient cooling (small volume), isolates process
- Limitations: more complex, pumping ring required
- Use: hot, hazardous, valuable fluids (minimize process exposure)
Plan 32 (External Flush)
- Clean external fluid injected into seal chamber
- Components: external supply, regulator, orifice, filter
- Advantages: clean flush, cools, keeps solids away
- Limitations: external supply required, flush enters process
- Use: dirty/abrasive fluids, slurry, polymerization
Plan 53 (Pressurized Barrier Fluid)
- For double seals: pressurized barrier fluid between seals
- Sub-plans: 53A (reservoir), 53B (accumulator), 53C (piston)
- Components: reservoir, pressurization, cooler, level switch
- Advantages: zero process leakage, containment, failure warning
- Limitations: complex, expensive, barrier fluid maintenance
- Use: hazardous, toxic, flammable, expensive fluid
Plan Selection Guide
| Fluid Condition | Recommended Plan | Reason |
|---|---|---|
| Clean, cold (<80°C) | Plan 11 | Simple, sufficient |
| Clean, hot (>80°C) | Plan 21 or 23 | Cooling needed |
| Dirty/abrasive | Plan 32 | Clean external flush |
| Hazardous/toxic | Plan 53 (double seal) | Zero leakage |
| Volatile (high VP) | Plan 23 + cooler | Prevent vaporization |
| Polymerizing | Plan 32 | Prevent buildup at seal |
| Valuable/expensive | Plan 23 | Minimize process exposure |
| Slurry/solids | Plan 32 + filter | Keep solids from seal |
Flush Plan Components
Orifice
- Controls flush flow rate (fixed orifice)
- Sizing: calculated for required flow at differential pressure
- Material: compatible with fluid (316SS, PTFE, Hastelloy)
- Location: in flush line (before seal chamber)
- Maintenance: check for clogging (especially dirty fluid)
Cooler
- Water-cooled or air-cooled heat exchanger
- Types: shell & tube, plate, air-cooled
- Sizing: based on heat load (friction + process heat)
- Material: compatible (tube side = process, shell = coolant)
- Maintenance: clean tubes, check for leaks, monitor temp
Reservoir (Plan 53)
- Barrier fluid reservoir with level, pressure, temp indication
- Components: level switch (alarm), pressure gauge, relief valve
- Barrier fluid: clean, compatible (water, glycol, oil)
- Pressurization: gas (nitrogen) or spring/piston
- Monitoring: level drop = inner seal failure (alarm)
Chemical Pump-Specific Considerations
Corrosive Fluids
- Flush components: compatible material (PTFE, Hastelloy, 316SS)
- Plan 11: process fluid circulates (must be compatible)
- Plan 32: external flush (clean, compatible fluid)
- Avoid: copper, aluminum (attacked by many chemicals)
- O-rings: EPDM (acid), Viton (oil), FFKM (universal)
Hazardous Fluids
- Double seal with Plan 53: zero leakage
- Barrier fluid: compatible, non-hazardous (water, glycol)
- Monitoring: level switch (inner seal failure alarm)
- Consider: magnetic drive (no seal at all) for extreme hazard
- Regulatory: EPA, OSHA, local emission limits
Hot Fluids
- Plan 21 or 23: cooling required
- Seal faces: SiC/SiC (high temp, chemical resistant)
- Barrier fluid: high-temp compatible (oil, glycol)
- Cooler: sized for heat load, water supply
- Thermal expansion: piping design, expansion loops
Dirty/Abrasive Fluids
- Plan 32: clean external flush (keep solids from seal)
- Flush pressure: > seal chamber pressure (positive flow into pump)
- Filter: remove solids from flush (5-50 micron)
- Seal faces: SiC/SiC (abrasion resistant)
- Monitor: flush flow, pressure, filter differential
Installation and Maintenance
Installation Best Practices
- Flush piping: sloped, drained, no dead legs
- Valves: isolation (for maintenance), check (prevent reverse)
- Orifice: accessible (cleaning, inspection)
- Cooler: accessible (cleaning, tube replacement)
- Instrumentation: pressure gauge, temp indicator, flow switch
Maintenance Checklist
- Check flush flow: visual (sight glass) or flow switch
- Check cooler: inlet/outlet temp, water flow, cleanliness
- Check reservoir (Plan 53): level, pressure, barrier fluid condition
- Check orifice: clean (clogging reduces flow)
- Check seal leakage: none (single), barrier fluid level (double)
- Schedule: monthly inspection, annual overhaul
Common Issues
- No flush (Plan 00): Seal overheats, fails. Use Plan 11 minimum.
- Plan 11 for hot fluid: Seal overheats. Use Plan 21/23 with cooler.
- Plan 11 for dirty fluid: Solids damage seal. Use Plan 32 (clean flush).
- Orifice clogged: No flush, seal fails. Clean orifice, install filter.
- Cooler not working: Hot flush, seal failure. Check water supply, clean tubes.
- Plan 53 level drop: Inner seal failure. Investigate, replace seal, refill barrier.
- Wrong barrier fluid: Incompatible, seal damage. Verify compatibility with process.
- No monitoring: Flush failure undetected. Install flow/temp/pressure alarms.
Procurement Recommendations
- Specify API 682 flush plan per fluid conditions
- For hot fluid: specify Plan 21 or 23 with cooler
- For dirty/abrasive: specify Plan 32 with filter
- For hazardous: specify Plan 53 (double seal) with reservoir
- Specify flush components material (compatible with fluid)
- Specify seal faces: SiC/SiC for chemical/abrasive
- Include instrumentation: pressure gauge, temp, flow switch
- For Plan 53: include level switch (alarm), pressure gauge
- Require P&ID and installation manual
- Require flush plan test (verify flow, cooling)
- Verify supplier has API 682 seal application experience
Related Industries
Seal flush plans apply to chemical processing (acid/caustic/solvent), oil & gas (refinery, pipeline), pharmaceutical (API, solvents), power generation (boiler feed, chemical feed), pulp & paper (stock, bleach), and general industrial (process pumps). The core principle — proper API 682 flush plan selection based on fluid properties for reliable mechanical seal operation — is consistent across all chemical pump applications.