Plan 14 is Plan 11 plus a suction-bypass vent above the throat bushing — designed for the failure mode where a vertical chamber dead-heads on accumulated vapor while the recirculation gauge still reads flow. Four numbers control whether it works: 50 psi vapor margin, two coupled orifices, and 1 mm throttle bushing clearance.
What Plan 14 Adds to Plan 11 — and Why That Resolves Vapor Lock
Plan 14 splits one recirculation circuit into two coordinated paths: a discharge-to-chamber inlet (the Plan 11 leg) and a chamber-to-suction vent leg above the throat bushing. API 682 4th Edition sets the test any flush plan on volatile service must clear — chamber pressure at least 50 psi above the pumped-fluid vapor pressure, or a 1:3 absolute-pressure ratio, whichever is greater. Plan 14 satisfies that margin through three mechanisms Plan 11 cannot deliver on a vertical chamber:
- Continuous venting at the chamber’s high point clears trapped vapor before it dead-heads the circuit
- Discharge-side pressurization rides the chamber above suction independent of throat-bushing tolerance — useful where cartridge bushing clearance is hard to verify in the field
- Flow refresh keeps chamber temperature close to process temperature because nothing stagnates
Sizing the Two Orifices: The Coupled-System Rule
Plan 14’s two orifices form a coupled hydraulic system. Oversize the vent and chamber pressure collapses below the vapor margin; undersize it and vapor accumulates. The sizing decision happens on three coordinated specs:
- Flow-rate anchor (John Crane chief engineers Gordon Buck and Ralph Gabriel): target 1 gpm per inch of seal diameter on low-pressure, low-speed services. Above 3600 rpm or 500 psig box pressure, calculate explicitly
- Minimum orifice floor: API 682 / ISO 21049 fix the floor at 3 mm (1/8 in) — not a sizing recommendation. Field practice on dirty services steps the vent orifice one drill size above the inlet
- Choke-tube substitute on dirty services: on slurry or polymer-bearing condensate where disk orifices clog, substitute ¼-inch heavy-wall tubing sized so pressure drop equals (discharge − chamber) on the inlet leg and (chamber − suction) on the vent leg
Throttle Bushing Clearance Under API 682 4th Edition
The throttle bushing limits leakage past the seal into the gland atmosphere, and the standard sets clearance, not historical practice. API 682 4th Edition specifies 1 mm diametral for shafts at or below 60 mm and 2 mm diametral above 60 mm — roughly 0.5 mm and 1.0 mm radial. Pre-API-682 installations ran 0.025 inch diametral, and Gordon Buck’s 19,000-record failure dataset shows the tighter historical clearance carried roughly 1.6% rub rate versus 0.12% at 0.040 inch. The standard’s larger gap is a deliberate reliability trade.
Where Plan 14 Fits — and Where It Doesn’t
Plan 14 belongs where the seal chamber traps vapor AND the discharge-to-suction differential can drive flow through both orifices. The chamber-traps-vapor side covers four named service types:
- Vertical inline pumps on hydrocarbon service
- OH3-OH5 overhung verticals — Flowserve application engineer Garrett Marsala notes Plan 14 also prevents the long dry-running window during startup that Plan 11 chambers can suffer
- NPSH-marginal cold water — local boiling at the high point fails the seal
- Light-end hydrocarbon condensate — vapor accumulates faster than Plan 11 can clear
When chamber-to-suction ΔP is very low, neither orifice develops enough pressure drop to drive the vent leg — Plan 13 is the correct call there. The decision is ΔP-driven, not pump-orientation-driven. The API 682 piping plan sub-pillar maps every plan-number to its driving mechanism.
When Plan 14 Runs Out of Headroom: Escalation to Plan 23
Plan 14 stops being enough when recirculation flow cannot keep chamber temperature within API 682’s ≤10°F rise above pumping temperature. Three triggers signal the jump to Plan 23:
- Measured chamber rise exceeds 10°F at design flush rate
- Light-hydrocarbon vapor pressure sits within the 50 psi margin even after venting — flashing is inevitable without cooling
- Pumped fluid is low-viscosity enough for a pumping ring to circulate through an external heat exchanger
Past Plan 23, the next step is a dual-seal arrangement — see the broader API plans escalation ladder.
Where Plan 14 Specifications Most Often Go Wrong
The mistake I see most often is treating the vent orifice as “open enough” and the inlet orifice as the only one controlling chamber pressure. The two are coupled — oversize the vent, chamber pressure collapses below the 50 psi vapor margin, and the seal flashes silently while the flow gauge reads fine. Measure pressure at the chamber tap and confirm the margin against the pumped fluid’s vapor pressure at operating temperature. If the math does not clear, the orifice ratio is wrong before the seal selection is.
