Torque Seated vs. Limit Seated: Getting the PLC Logic Right
There are two fundamental ways a valve reaches its end position: torque seated and limit seated. They require completely different PLC logic to drive correctly, and mixing them up is one of the more common — and harder to catch — programming mistakes in valve control.
The Difference
A limit seated valve has a defined mechanical stop. The actuator drives the valve until a limit switch confirms it has reached the open or closed position. Once that limit is reached, the PLC de-energizes the output. The logic is simple: drive until feedback says you’re there, then stop.
A torque seated valve has no limit switch at the seat. Instead, the actuator drives the valve into its seat until the mechanical resistance (torque) exceeds a threshold, at which point the actuator’s internal torque switch trips and the actuator stops itself. The PLC keeps the output energized the entire time — it doesn’t decide when to stop. The actuator does.
This is the critical distinction: with a torque seated valve, the PLC drives the output until the actuator tells it that movement has stopped. With a limit seated valve, the PLC stops driving the output when the limit is reached.
Where It Goes Wrong in the PLC
The most common mistake is programming a torque seated valve like a limit seated valve — de-energizing the output as soon as any position feedback indicates “closed.” Here’s what happens:
- The valve travels toward the closed position.
- A position indication (sometimes a zone switch or a percentage threshold) indicates the valve is “at” the closed position.
- The PLC drops the close output.
- The actuator stops — but the valve isn’t torqued into its seat.
The valve shows closed on the HMI. No alarms fire. But the valve isn’t sealed. In a custody transfer or isolation application, product is leaking past the seat.
The opposite mistake also happens: programming a limit seated valve like a torque seated valve — keeping the output energized after the limit is reached. This can stall the actuator against the mechanical stop, overheat the motor, and burn out the actuator over time. You won’t see it fail immediately; it’ll show up as premature actuator failures months later.
What the Logic Should Look Like
For torque seated valves:
- Energize the close (or open) output and keep it energized.
- Monitor the actuator’s torque switch or “motor running” feedback. When torque is achieved (motor stops, torque switch trips), the valve is seated.
- The output can remain energized — the actuator handles de-energizing its own motor. Or, if your design requires it, de-energize the output only after confirming the torque switch has tripped.
- Your travel timeout alarm should account for the additional time needed to torque into the seat beyond the travel zone.
For limit seated valves:
- Energize the output to drive the valve.
- When the limit switch confirms the end position, de-energize the output immediately.
- If the limit isn’t reached within the expected travel time, alarm it — don’t keep driving.
Diagnostics That Get Missed
On the PLC side, a few things that often aren’t monitored but should be:
- Travel time trending — A torque seated valve that takes progressively longer to seat is telling you the seat is wearing or the actuator is losing torque. Log it. A 10-second increase in seating time over six months is early warning of a mechanical problem.
- Torque switch state during travel — If the torque switch trips mid-stroke (not at the seat), you have a mechanical obstruction or a packing problem. This should be a distinct alarm, not lumped in with “failed to close.”
- Output vs. feedback mismatch timers — The output is energized but the valve isn’t moving. How long do you wait before alarming? Too short and you get nuisance alarms on slow actuators. Too long and you’re running a stalled motor. Match your timer to the actuator’s rated stroke time plus a reasonable margin — not a generic default.
HMI Considerations
The HMI should make it obvious which type of valve the operator is looking at. A torque seated valve that shows “CLOSED” based on position percentage alone is misleading — the operator needs to see that the torque switch has confirmed the seat, not just that the valve reached a position threshold.
Consider adding a “SEATED” indication separate from “CLOSED” for torque seated valves. “CLOSED” means the valve is in the closed zone. “SEATED” means the actuator confirmed torque. They’re not the same thing, and operators who understand the difference will catch problems that position-only indication hides.
The Bottom Line
Know which type of valve you’re programming before you write the first rung. Check the actuator datasheet, confirm with the mechanical scope, and verify during commissioning that your logic matches the actual valve behavior. A five-minute conversation with the mechanical team about whether a valve is torque seated or limit seated can save you a full day of troubleshooting a “closed” valve that’s leaking.