After the sequence is corrected, the furnace retains a proven discharge path throughout the transfer, while pressure feedback detects a restricted or failed path before the WKM valve can isolate the Pyrofrac outlet. Equal valve positions are not the design target: the larger WKM valve and the smaller Decoke valve have different flow characteristics, so the DCS must sequence them from measured process response and confirmed valve status.
Isolation and Transfer Prerequisites
Before anything else, confirm whether opening the Decoke path while the Pyrofrac path remains open creates an unacceptable connection between the two destinations. That decision comes from the piping arrangement, process pressure envelope, and operating procedure—not from valve logic.
- Verify the approved decoking lineup, including drains, vents, downstream equipment availability, and the destination selected to receive flow.
- Confirm the required physical isolation. Swing elbows or full-strength blinds provide positive separation where air could otherwise enter the process. MOV position indication is not equivalent to physical isolation.
- Read the furnace outlet pressure, Pyrofrac-side pressure, and Decoke-side pressure from independent, serviceable instruments. A single pressure reading cannot show whether the receiving path is blocked.
- Confirm that both MOVs have valid open and closed limit indications, remote control availability, healthy motor protection, and no active trip.
- Obtain each valve's flow-versus-travel data, actuator travel time, allowable differential pressure, and motor torque or current limits from its documentation.
Do not move on until the downstream lineup and isolation method have been independently verified. The automated transfer mitigates a closed outlet; it does not replace positive isolation required for decoking.
Initial Pressure and Status Check
The first DCS branch tests whether a controlled transfer is permissible. Calculate differential pressure across each valve from simultaneous readings taken as close to the valves as practical. Trend the individual pressures as well as their differences; a small difference created by two uniformly rising pressures is not proof of a usable outlet.
| Reading | Meaning | Next action |
|---|---|---|
| WKM proven open; Decoke proven closed; pressures within the approved starting envelope | Normal starting lineup | Proceed to Decoke-path proving |
| Any valve indication contradictory or unavailable | Position is not proven | Block the automatic sequence and diagnose indication or actuator faults |
| Decoke-side pressure outside its approved receiving range | The destination may be unavailable, pressurized, isolated, or incorrectly lined up | Hold the WKM valve open and correct the downstream condition |
| Furnace outlet pressure already high or rising abnormally | Outlet capacity may already be restricted | Prevent transfer initiation and execute the approved operating response |
Use quality-qualified pressure values. A failed, stale, out-of-range, or maintenance-overridden transmitter must make the permissive false unless the approved design provides another independent measurement path.
Decoke-Path Proving Check
Commanding both valves at the same instant creates an unverified interval: the WKM valve may move faster, the Decoke valve may stall, or one position indication may disagree with actual flow. Open and prove the receiving route before reducing the established route, provided the process hazard review permits temporary overlap.
- Issue the Decoke MOV open command while holding the WKM MOV open.
- Confirm that the Decoke valve leaves its closed limit and progresses toward open. Monitor actuator trip status and motor current or torque feedback when those signals are available.
- Confirm the open limit within the documented actuator travel allowance. Set the DCS timeout from measured stroke tests and the actuator documentation, not from an arbitrary delay.
- Check for the expected pressure response: stable furnace outlet pressure and a Decoke-side response showing communication with the receiving system.
- If the valve stops, trips, times out, or produces the wrong pressure response, abort the transfer and keep the WKM path available.
An open limit switch proves that the actuator reached its adjusted endpoint; it does not prove downstream flow. Combine position proof with pressure response and downstream lineup status. Where consequence analysis requires higher integrity, use protection independent of the basic DCS sequence.
WKM Closure Decision
Different cross-sectional areas make synchronized percentage travel unsuitable. Flow depends on valve geometry, pressure drop, fluid properties, and downstream resistance. Gate valves also have strongly nonlinear flow behavior and can develop high local velocity while partly open. Repeatedly bumping the motors turns the MOVs into crude throttling elements and can produce thermal or torque trips.
| Observed condition during Decoke opening | Decision |
|---|---|
| Decoke open proven, destination available, outlet pressure stable | Permit WKM closure |
| Decoke moving but not open proven | Hold WKM open |
| Pressure rises toward the approved abort threshold | Stop WKM closure and command recovery to the safe established path |
| WKM actuator trips or position stops changing | Freeze automatic progression and invoke the defined abnormal-response procedure |
| Pressure signals disagree beyond the approved diagnostic tolerance | Abort automatic transfer and investigate instrumentation |
Once the Decoke path is proven, command the WKM MOV closed as a continuous stroke if the valve and actuator are rated for that duty. Monitor outlet pressure throughout travel. If pressure control during transition requires continuous modulation, use equipment selected for throttling service rather than pulse-driving gate-valve MOVs.
Abort and Fail-Safe Behavior
The sequence must define the response to every loss of proof before commissioning begins. A generic “sequence failed” alarm is insufficient because operators need to know which path remains available.
- On loss of Decoke open proof before WKM closure, cancel the WKM close command and retain the established Pyrofrac path.
- On high or rapidly rising furnace outlet pressure during WKM travel, stop further isolation and command the designated recovery action established by the process safety review.
- On DCS communications loss, invalid pressure, MOV motor trip, or contradictory limits, prevent automatic progression. Annunciate the initiating condition and the last confirmed valve states.
- Do not automatically reverse a valve if reversal could exceed actuator duty, trap pressure, or create an unsafe cross-connection. Define reversal behavior from the valve, actuator, piping, and process review.
- Retain an independent means of protecting against a closed outlet. The DCS transfer sequence is vulnerable to common power, logic, instrumentation, and actuator failures.
Commissioning and Final Verification
- Test each pressure input for correct range, engineering units, bad-quality handling, alarm action, and displayed trend direction.
- Stroke each MOV individually under an approved test condition. Record travel time, limit-switch operation, motor current or torque behavior, and response to a simulated trip.
- Test every permissive by withholding one condition at a time. Confirm that the sequence cannot close WKM unless the Decoke route has both required position and process proof.
- Simulate a Decoke timeout, invalid pressure input, contradictory valve limits, and abnormal outlet-pressure rise. Confirm the programmed abort action and alarm text for each case.
- Run the complete transfer under controlled operating conditions. Trend both valve positions, all relevant pressures, motor status, commands, permissives, and abort states on one time base.
Frequently Asked Questions
Why does equal MOV travel not balance the two outlet flows?
The WKM and Decoke valves have different areas and flow-versus-travel characteristics. Balance the transfer from measured pressure response and documented valve data, not matching position percentages.
Why does the Decoke valve need to open before the WKM valve closes?
Opening and proving the receiving path first prevents a closed-outlet interval if actuator speeds differ or one MOV stalls. Temporary overlap is permissible only when the approved piping and process review allow the two destinations to be connected.
Why does an open limit switch not prove Decoke flow?
The switch reports actuator endpoint position, while a downstream blind, closed valve, blockage, or incorrect lineup can still prevent flow. Require an acceptable pressure response and downstream-availability permissive as separate proof.
Why does bumping gate-valve MOVs cause trips?
Repeated starts increase motor heating, and partial travel can expose the gate to high differential-pressure forces. Use the actuator duty and torque or current limits from its documentation; select a throttling valve if continuous modulation is required.
How do I verify the WKM-to-Decoke transfer sequence?
Trend valve commands and positions, all relevant pressures, motor status, permissives, and abort states on one time base. Complete the test only when the Decoke path is proven before WKM closure, outlet pressure remains inside the approved envelope, and every injected fault produces its specified abort action.