The number that matters on a delayed coker heater charge pump is not the 500-525 kW on the motor nameplate. It is the 3.5 gpm of clean gas oil each mechanical seal injects into the suction stream, and what the heater does to that gas oil once it is inside the pump.
Where the Flush Ends Up
An API Plan 32 injects clean, cool product from an external source directly into the seal chamber. Part of it leaks outboard across the faces; the balance flows inboard, past the throat bushing, and joins the pumped stream. That is the design intent - the flush displaces hot, solids-laden vacuum residue from the seal faces and replaces it with a clean, lower-viscosity, lower-temperature fluid. Face life goes up, and the seal chamber stops carrying coke fines.
In coker charge service the inboard leg is a one-way trip. Everything injected goes forward into the heater passes at coking-severity outlet temperature. Coker operating experts put the cracked fraction of that gas oil at 10 to 20 percent - converted to coke on the drum and to light products - with the remainder recovered in the fractionator. The coke fraction is gone. It leaves the unit as solid carbon, not as sellable distillate. Every other Plan 32 gas oil service in the refinery returns its flush to inventory; this one burns a slice of it on every pass.
This is a yield loss, not a reliability problem. The seals may run for years on Plan 32 and still be the most expensive seals in the plant.
The Annual Loss Number
Run the arithmetic before the flush plan is frozen in the datasheet. For two double-ended (between-bearings, two seals per pump) charge pumps at 3.5 gpm per seal:
Seals = 2 pumps x 2 seals = 4
Flush rate = 4 x 3.5 gpm = 14 gpm
Daily volume = 14 x 1440 / 42 = 480 bbl/day
Annual volume = 480 x 360 days = 172,800 bbl/yr
Cracked to coke = 172,800 x 10% = 17,280 bbl/yr
Value at $30/bbl = US$518,000/yrThat US$500,000 per unit per year uses the low end of the cracking range and a conservative gas oil valuation. At 20 percent cracking, or with higher flush rates, larger pump counts, or a deep-cut crude unit that has already spent capital and energy to recover that gas oil in the first place, the number climbs. One refinery running this configuration puts its own loss at US$1,500,000 per year on a single unit. Substitute your own flush rate from the seal datasheet, your own gas oil transfer price, and your licensor's cracking yield, and the conclusion moves very little.
Gas Oil Plan 32 by Service
The coker heater charge pump is the outlier. The same flush plan, the same fluid, and the same injection rate carry completely different economics elsewhere in the refinery:
| Service | Fate of injected gas oil | Real cost of the flush |
|---|---|---|
| Gas oil hydrotreater / desulfurizer charge | Fully recovered downstream | Energy to reprocess the same gas oil repeatedly |
| Same, but unit is charge-limited by pump capacity | Fully recovered, but displaces fresh feed | Lost throughput on fresh feed - can be severe |
| Coker gas oil product / fractionator reflux | Returns to the same stream either way; recovered in the fractionator | Energy to cool and re-inject |
| Vacuum bottoms | Normally no injected flush - seal chamber under vacuum | Plan 54 or a variation without injection is standard practice |
| Coker heater charge | 10-20% cracked to coke and light ends; coke fraction unrecoverable | Direct yield loss, US$0.5-1.5M/unit/yr |
A Plan 32 gas oil flush in vacuum bottoms service would produce the same coke-pile loss, which is one reason it is not used there.
Plan 32/52 Against Plan 54
Plan 52 adds an unpressurized buffer fluid loop with a reservoir between a tandem pair of seals. It contains and vents primary leakage; it does nothing about the gas oil being pushed inboard by the Plan 32. Plan 54 replaces the injection entirely: an external system circulates pressurized barrier fluid between dual seals at a pressure above seal chamber pressure, so the leak direction reverses - barrier fluid leaks inboard in small, controlled quantities and no process-derived flush is consumed.
| Criterion | Plan 32 + 52 | Plan 54 |
|---|---|---|
| Gas oil to coke pile | 3.5 gpm/seal, continuous | None |
| Seal face environment | Clean, cool, injected product | Clean barrier fluid, temperature set by the circulation system |
| Leak direction | Inboard into process, outboard to buffer | Barrier inboard only, at low rate |
| Leak detection | Buffer reservoir level/pressure rise | Barrier reservoir level fall and make-up rate |
| External hardware | Flush source, filter, flow control per seal | Barrier pump, cooler, reservoir, pressure control |
| Dominant failure mode | Loss of flush supply exposes faces to residue instantly | Loss of barrier pressure - seal reverts to buffer duty until tripped |
| Recurring cost | US$0.5-1.5M/yr yield loss per unit | Barrier system power, cooling, make-up oil |
Plan 54 wins on the only criterion large enough to matter. The capital and complexity of a barrier circulation system are recovered in months against a seven-figure annual yield loss. Where seal chamber pressure allows, a pressurized reservoir variant can substitute, but a charge pump seal chamber sits at suction pressure plus stage rise and needs continuous barrier cooling; a circulated Plan 54 system handles that heat load where an accumulator-based plan struggles.
Recommended Arrangement: Plan 54 with Face-to-Face Stationary Bellows
The proven configuration for this service is a face-to-face stationary bellows pair, with the process fluid on the inside diameter of the primary seal and the barrier fluid on the outside diameter of both bellows. A refinery in the same corporate group runs exactly this arrangement in coker charge service with excellent results.
Two physical reasons drive the geometry. Bellows are hydraulically loaded by whichever fluid contacts the convolutions; putting clean barrier oil on the OD of both units keeps coke fines and vacuum residue out of the convolutions, where they would otherwise pack, stiffen the bellows, and destroy its ability to track face runout. Stationary bellows also remove the centrifugal pumping of solids into the flexing element that a rotating bellows suffers in a dirty fluid at high surface speed.
- Fix the barrier fluid, its viscosity at seal chamber temperature, and its compatibility with gas oil, before the seal vendor sizes the faces.
- Set barrier pressure at a defined margin above the maximum seal chamber pressure the pump datasheet shows at rated flow and at settle-out, not above suction pressure alone.
- Size the barrier circulation system for the heat soak from the process side plus face-generated heat at maximum speed, and confirm the cooler duty against the seal vendor's heat load calculation.
- Specify low-level, low-pressure and high-level alarms on the barrier reservoir, and a make-up rate trend as the primary seal-wear indicator.
- Confirm with the licensor in writing that the flush plan change does not alter the guaranteed feed composition or their seal-related warranty position.
Fallback: Tandem Seal on the Licensor's Plan 32/52
If the licensor's document holds and the seal stays Plan 32/52, the arrangement changes. A face-to-face pressurized set has no role without a barrier system; use a standard tandem arrangement with the primary seal built as a rotating bellows. Rotating bellows have long been preferred in dirty services of this type, and two of three cokers in one operator's fleet run them, with stationary bellows in the third.
The known weakness of the rotating bellows here is corrosion and erosion of the bellows shell by the residue stream. Watch shell wall thickness at every seal overhaul, and treat progressive shell thinning - not face wear - as the life-limiting item when specifying spares. Bellows alloy selection deserves the same scrutiny as the pump case material.
Vent and Drain Piping Metallurgy
Material class C-6 (12% chrome) for the pressure casing is a sound choice for this service, with one trap. Many pump manufacturers supply 300-series stainless steel vent and drain piping as standard with a C-6 pump. With high chlorides in the coker feed, that austenitic piping is exposed to chloride stress corrosion cracking under the wet, chloride-concentrating conditions that occur during start-up and shut-down, not during steady running.
Specify Incoloy 825 for vent and drain piping in coker charge service. Do not accept 300-series stainless on a case drain line here - a cracked case drain on a hot vacuum residue pump discharges above the autoignition temperature of the fluid and produces a major fire. Write the piping alloy into the datasheet as a separate line item; it is routinely missed when the material class alone is quoted.
Hydraulics, Bearings and Verification
The BB2 two-stage, radially split, between-bearings configuration is the correct frame for this duty, and both offered casing types are acceptable: a double volute cancels net radial load away from best efficiency point, and a diffuser achieves the same balance by a different route. Neither is a reason to reject an offer. Antifriction bearings with accelerometer provisions for off-line route monitoring are conventional; on a unit whose charge pump outage stops the coker, weigh continuous vibration and bearing-temperature monitoring against the route interval, because rolling-element bearing failures in hot residue service progress faster than a monthly route detects.
Numbers to pin down before order placement:
| Quantity | Value / limit | Where to read it |
|---|---|---|
| Flush rate per seal | 3.5 gpm (basis of the loss calculation) | Seal vendor datasheet, Plan 32 flow control orifice |
| Gas oil cracked to coke | 10-20% of injected volume | Licensor yield model for the unit |
| Seal chamber pressure, max | Sets barrier pressure setpoint | Pump datasheet, rated and settle-out cases |
| Barrier heat load | Sets cooler and circulation duty | Seal vendor heat balance calculation |
| Bellows shell thickness | Erosion/corrosion life limit | Overhaul inspection record, seal GA drawing |
| Vent/drain piping alloy | Incoloy 825 | Pump datasheet auxiliary piping schedule |
| Driver rating | 500-525 kW as offered | Vendor proposal against rated and end-of-curve power |
After conversion, verify by three measurements: gas oil injection flow reads zero at the former Plan 32 tie-in and the line is blinded; barrier reservoir pressure holds its setpoint margin above seal chamber pressure through a full start-stop cycle; and barrier make-up volume over the first 90 days trends flat. A rising make-up rate is primary seal wear. A falling reservoir level with no make-up is a leak to atmosphere across the outboard seal.
Stop and escalate when the licensor's specification and the seal economics genuinely conflict - the flush plan sits inside the licensed process guarantee, and changing it is a licensor decision, not a procurement one. Take the calculated annual loss, the reference installation running Plan 54 in the same service, and the seal vendor's heat load figures to the licensor and the pump OEM together, and get the deviation approved in writing before the purchase order is released.
Frequently Asked Questions
What happens if we keep the Plan 32 gas oil flush on the coker heater charge pumps?
The pumps will run, but 10-20% of every gallon injected cracks in the heater and leaves as coke. At 3.5 gpm per seal across four seals, that is roughly 17,000 barrels per year converted to coke, or about US$500,000 per unit per year at $30/bbl, and up to US$1,500,000 on higher-rate units.
What happens if the barrier pressure on a Plan 54 drops below seal chamber pressure?
The leak direction reverses and the dual seal degrades to buffer-fluid duty, admitting hot residue and coke fines to the inboard faces. Alarm on low barrier pressure and low reservoir level, and treat both as an immediate action to restore pressure or take the pump out of service.
What happens if the vendor supplies 300-series stainless vent and drain piping with a C-6 pump?
With high chlorides in the feed, the austenitic piping is exposed to chloride stress corrosion cracking during start-up and shut-down when moisture concentrates chlorides on the wall. A cracked case drain on this service produces a major fire; specify Incoloy 825 for vent and drain piping.
What happens to a rotating bellows seal in coker charge service?
The bellows shell suffers corrosion and erosion from the residue stream, and shell thinning - not face wear - becomes the life limit. If Plan 32/52 is retained, a tandem arrangement with a rotating bellows primary is the preferred build, with shell thickness checked at every overhaul.
What happens if the unit is charge-limited by pump capacity when Plan 32 flush is used?
The injected flush occupies pump capacity that would otherwise pass fresh feed, so the flush backs out feed rather than merely costing reprocessing energy. That converts a modest energy penalty into a throughput loss that can dwarf the flush volume itself.