API 682 defines standard piping plans for mechanical seals in centrifugal and rotary pumps. The correct flush plan controls seal chamber pressure, temperature, and fluid condition โ directly affecting seal life. A poorly selected flush plan can reduce seal life from 5+ years to months. This guide covers API 682 flush plans (Plan 01 through Plan 76), selection criteria, and common mistakes.
API 682 Seal Types
| Type | Description | Typical applications |
|---|---|---|
| Type A | Single seal, O-ring pusher | General purpose, clean fluids, <200ยฐC |
| Type B | Single seal, metal bellows (non-pusher) | High temp (>200ยฐC), polymerizing fluids, coking |
| Type C | Single seal, O-ring pusher, high pressure | High pressure (>40 bar), abrasive fluids |
| Type D | Double seal (dual, pressurized) | Hazardous, toxic, carcinogenic fluids (zero leakage) |
| Type E | Tandem seal (dual, unpressurized) | Hazardous fluids with backup containment |
Common API 682 Flush Plans
| Plan | Description | Best for | Key components |
|---|---|---|---|
| Plan 01 | Internal recirculation from pump discharge to seal chamber | Clean, non-coking fluids, <150ยฐC | Internal recirculation orifice |
| Plan 02 | No flush โ dead-ended seal chamber | Clean fluids, low temp, vertical pumps | None (convection cooling only) |
| Plan 11 | Single flush from discharge to seal chamber via orifice | Most common for clean fluids, general service | Orifice, flush line |
| Plan 13 | Recirculation from seal chamber back to suction | Vertical pumps, low NPSH, light hydrocarbons | Orifice, recirculation line |
| Plan 14 | Combination Plan 11 + Plan 13 (discharge to seal, seal to suction) | High vapor pressure fluids, vertical pumps | Two orifices, two lines |
| Plan 21 | Flush from discharge through cooler to seal chamber | Hot fluids (>150ยฐC), clean service | Cooler, orifice, flush line |
| Plan 23 | Closed-loop recirculation: seal chamber โ cooler โ seal chamber | Hot, dirty, or polymerizing fluids (isolates seal from process) | Cooler, circulating device (pumping ring) |
| Plan 31 | Flush from discharge through cyclone separator (solids removed) to seal | Slurries, abrasive fluids, <2% solids | Cyclone separator, orifice |
| Plan 32 | External flush from clean source to seal chamber | Slurries, dirty fluids, when clean external fluid available | External clean fluid, regulator, orifice |
| Plan 41 | Plan 21 + cyclone separator (discharge โ cyclone โ cooler โ seal) | Hot, abrasive fluids | Cyclone, cooler, orifice |
| Plan 52 | Unpressurized dual seal with external reservoir (barrier fluid) | Hazardous fluids, backup containment, emissions control | Reservoir, cooler, level switch |
| Plan 53A | Pressurized dual seal with external reservoir (piston/pneumatic pressurization) | Toxic/carcinogenic fluids, zero leakage required | Pressurized reservoir, cooler, pressure switch |
| Plan 53B | Pressurized dual seal with bladder accumulator (barrier fluid) | Toxic fluids, high pressure, zero leakage | Bladder accumulator, cooler, pressure switch |
| Plan 53C | Pressurized dual seal with piston accumulator (process pressure pressurizes barrier) | Toxic fluids, variable process pressure | Piston accumulator, cooler |
| Plan 54 | Pressurized dual seal with external barrier fluid circulation system | Multiple pumps, central barrier fluid system | External tank, pump, cooler, controls |
| Plan 55 | Unpressurized dual seal with external reservoir (no circulation โ gravity) | Light duty, backup containment | Reservoir, level switch |
| Plan 62 | External quench to atmospheric side of single seal (steam/water) | Hot fluids, coking, polymerizing, crystallizing | Quench connection, drain |
| Plan 65A/B | Leakage detection (orifice + pressure switch or flow switch) | Emissions monitoring, single seals | Orifice, pressure/flow switch |
| Plan 71 | No external connections โ containment seal only | Dry containment, non-hazardous backup | None |
| Plan 72 | External buffer gas to containment seal (unpressurized dual gas seal) | Gas seals, dry gas containment | Gas supply, regulator, flow switch |
| Plan 74 | Pressurized barrier gas to dual gas seal | Dry gas seals, toxic gas service | Gas supply, regulator, pressure switch |
| Plan 75 | Leakage drainage from containment seal to reservoir | Condensing leakage, liquid collection | Reservoir, level switch |
| Plan 76 | Leakage drainage to atmosphere (non-condensing gas) | Gas leakage monitoring | Flow switch, vent |
Critical: Flush Plan Selection by Fluid Type
| Fluid type | Recommended plan | Why |
|---|---|---|
| Clean, non-coking, <150ยฐC | Plan 11 | Simple, reliable, adequate cooling |
| Clean, hot (>150ยฐC) | Plan 21 or Plan 23 | Cooler reduces seal temp; Plan 23 isolates seal from dirty process |
| Abrasive/slurry (<2% solids) | Plan 31 or Plan 32 | Cyclone removes solids (31) or external clean flush (32) |
| Hot + abrasive | Plan 41 | Cyclone + cooler combined |
| Polymerizing/coking (e.g., bitumen, sulfur) | Plan 23 + Plan 62 | Closed-loop cooling + steam quench prevents coking at atmospheric side |
| High vapor pressure (LPG, propane) | Plan 13 or Plan 14 | Recirculation to suction prevents flashing in seal chamber |
| Toxic/carcinogenic (VCM, Brโ, phosgene) | Plan 53A/B/C (dual pressurized) | Zero leakage โ barrier fluid pressure > process pressure |
| Hazardous but less critical | Plan 52 (dual unpressurized) | Backup containment, emissions monitoring |
| Vertical pumps | Plan 13 or Plan 14 | Seal chamber above impeller; recirculation to suction |
| Dry gas (compressors) | Plan 72/74 | Gas barrier/buffer for dry gas seals |
Critical: Plan 23 โ The Most Misapplied Plan
Plan 23 (closed-loop recirculation with cooler) is often specified but frequently misapplied:
- How it works: A pumping ring (in seal chamber) circulates seal chamber fluid through an external cooler and back. The seal chamber is ISOLATED from the pump process fluid (only a small initial fill).
- Best for: Hot, dirty, or polymerizing fluids where you don't want process fluid reaching the seal. Common in refineries for hot hydrocarbon, bitumen, and sulfur service on API 610 pumps.
- Common mistake: Using Plan 23 for clean, cool fluids where Plan 11 is simpler and cheaper. Plan 23 adds a cooler, circulating device, and piping โ unnecessary complexity for clean service.
- Another mistake: Inadequate cooler sizing. The cooler must remove heat from both seal face friction AND process heat conduction. Undersized cooler โ seal overheats โ premature failure.
Critical: Plan 53A/B/C โ Zero Leakage Seals
For toxic, carcinogenic, or environmentally hazardous fluids, dual pressurized seals (Plan 53 series) provide zero leakage:
- Principle: Barrier fluid (clean oil or water) is pressurized ABOVE process pressure (typically 1.5โ2 bar above). If the inboard seal fails, barrier fluid leaks INTO the process (not process to atmosphere). The outboard seal contains the barrier fluid.
- Plan 53A: Piston or pneumatic pressurization of reservoir. Simple, but barrier fluid pressure varies with process pressure.
- Plan 53B: Bladder accumulator maintains constant barrier pressure. Better for variable process pressure. Most common for refinery hazardous service.
- Plan 53C: Piston accumulator uses process pressure itself to pressurize barrier (1:1 ratio). Always maintains barrier > process. Best for high-pressure or variable-pressure service.
- Barrier fluid selection: Must be compatible with process fluid (if inboard seal fails, barrier mixes with process). Common: clean lubricating oil, water/glycol, or process-compatible fluid. Must be clean (filtered to <10 ฮผm).
Critical: Plan 62 Quench โ Preventing Coking and Crystallization
Plan 62 (external quench) is essential for fluids that solidify, coke, or crystallize at the atmospheric side of the seal:
- Steam quench: For hot fluids (bitumen, sulfur, molten salts) that coke/cool-solidify. Steam keeps atmospheric side warm and flushes away deposits.
- Water quench: For crystallizing fluids (urea, caustic, salts). Water dissolves crystals and cools the seal.
- Common mistake: No quench on coking service โ seal face cokes โ seal hangs up โ catastrophic failure. Always specify Plan 62 for fluids with melting point >40ยฐC or coking tendency.
Common Problems and Solutions
Problem 1: Seal fails in 3โ6 months on clean water service. Plan 11 with undersized orifice โ insufficient flush flow, seal overheats. Solution: verify orifice size per API 682 (typically 3โ6 mm). Check flush flow rate (should be 5โ15 L/min). Ensure flush line is not blocked. Consider Plan 23 if fluid has any contamination.
Problem 2: Seal leaks on hot bitumen pump (coking). No Plan 62 quench โ bitumen cools and solidifies at atmospheric side, damaging seal face. Solution: add Plan 62 steam quench (1โ2 bar steam, continuous). Use Type B metal bellows seal (no O-rings to coke). Plan 23 closed-loop cooling. Our API 610 BB2 pumps for hot service include Plan 23 + 62 as standard.
Problem 3: Dual seal barrier fluid pressure drops (Plan 53B). Bladder accumulator loses pressure or inboard seal fails. Solution: install pressure switch with low-pressure alarm. Check barrier fluid level (reservoir sight glass). If inboard seal fails, barrier fluid level rises in reservoir (process fluid enters barrier). Schedule seal replacement during planned outage.
Problem 4: Plan 31 cyclone plugs on slurry service. High solids (>2%) overwhelm cyclone separator โ solids reach seal. Solution: use Plan 32 (external clean flush) instead for >2% solids. Ensure cyclone underflow drain is open (not blocked). Size cyclone for actual solids loading. Consider slurry pump with mechanical seal designed for solids.
Problem 5: Seal overheats on Plan 23 application. Cooler undersized or pumping ring not generating enough flow. Solution: verify cooler heat duty (must remove seal face heat + process conduction). Check pumping ring direction (must match pump rotation). Ensure cooler is not fouled. Increase cooler size by 50% margin.
Selection Summary
- Clean <150ยฐC: Plan 11 (most common, simple)
- Clean >150ยฐC: Plan 21 (cooler) or Plan 23 (closed-loop, isolate seal)
- Abrasive <2%: Plan 31 (cyclone) or Plan 32 (external clean flush)
- Hot + abrasive: Plan 41 (cyclone + cooler)
- Coking/polymerizing: Plan 23 + Plan 62 (closed-loop + quench)
- High vapor pressure: Plan 13 or 14 (recirculate to suction)
- Toxic/carcinogenic: Plan 53A/B/C (dual pressurized, zero leakage)
- Hazardous backup: Plan 52 (dual unpressurized, containment)
- Vertical pumps: Plan 13 or 14
- Gas seals: Plan 72/74
- Critical: match plan to fluid properties (temp, solids, coking, toxicity), don't over-specify Plan 23 for clean service, Plan 53 barrier fluid must be process-compatible, Plan 62 essential for coking/crystallizing fluids, verify orifice sizes and cooler sizing
- Our API 610 pumps support all standard flush plans; magnetic drive pumps eliminate seals entirely for hazardous fluids. For seal flush plan selection, talk to our engineers with your fluid properties, pump type, and operating conditions.