C135 PCS 7 (PCS7)
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Configuring Valve_Lean (VlvL) Templates with Single Feedback Limit Switches and Block-and-Bleed Pairs in SIMATIC PCS 7

Configuring Valve_Lean (VlvL) Templates with Single Feedback Limit Switches and Block-and-Bleed Pairs in SIMATIC PCS 7

In SIMATIC PCS 7 Advanced Process Library (APL), configuring the VlvL (FB1899 - Valve Lean) block with only a single physical limit switch (e.g., Closed limit switch ZSC only) requires deleting the unused open feedback channel driver (Pcs7DiIn) and setting block parameter NoFbkOpen = 1 (No Open Feedback). This informs the valve state machine not to expect or monitor an open feedback signal, preventing spurious 'Travel time exceeded' or 'End position error' alarms when commanded open. For Block-and-Bleed pairs sharing a single common pilot solenoid valve where one valve opens while the other closes, engineers must never connect both valve block outputs to the same physical driver pin; instead, use a single coordinated VlvL block or evaluate the complementary open/closed limit switches through an intermediate logic gate before driving a single digital output.

Table of Contents

Architecture of the VlvL (FB1899) APL Lean Valve Block

The VlvL (FB1899) block from the PCS 7 Advanced Process Library is designed for controlling two-position switching valves (such as open/close pneumatic shutoff valves or motor-operated ball valves). Unlike the full-featured VlvS or modulating VlvAnL blocks, VlvL minimizes memory consumption and execution cycles while retaining essential DCS functionalities: manual/auto switching, interlock protection (Intlock, Permit, Protect), travel time monitoring, and standardized APL faceplate visualization.

In standard industrial piping designs, economic or space constraints often lead to installing only one limit switch—typically a Closed limit switch (ZSC) to confirm tight shutoff for safety, leaving the Open position unmonitored.

Configuring Single-Feedback Operating Modes (NoFbkOpen / NoFbkClose)

When deploying the standard Valve_Lean template from the PCS 7 APL template library with only a single closed feedback switch:

  1. Delete the Unused Channel Driver: Remove the Pcs7DiIn driver block pre-assigned to FbkOpen to save controller memory and prevent broken symbol warnings during compile.
  2. Deactivate Open Feedback Monitoring: Set block input parameter NoFbkOpen = 1 on VlvL. When NoFbkOpen = 1, the block considers the valve successfully opened as soon as the control output (Ctrl = 1) has been energized and the configured monitoring time (MonTiDynamic) has elapsed, without checking physical input pins.
  3. Deactivate Closed Feedback Monitoring (Alternative): If the installation only possesses an Open limit switch (ZSO), leave FbkOpen connected and configure NoFbkClose = 1.

Engineering Block-and-Bleed Valve Pairs with Common Solenoids

In gas train and chemical feed systems, a 'Block-and-Bleed' arrangement uses two valves driven by a single common pilot solenoid: when energized, the block valve opens to feed the process while the vent/bleed valve closes; when de-energized, the block valve snaps shut and the bleed valve opens to vent trapped pressure. When configuring this in CFC:

  1. Never Wire Dual Outputs to One Driver: If two separate VlvL blocks write to the same physical digital output driver (Pcs7DiOu), the block executing latest in the OB runtime group will overwrite the earlier block, causing unpredictable solenoid chatter.
  2. Master Valve Approach (Recommended): Model the pair as a single logical valve unit using one VlvL block. Connect the single solenoid driver to Ctrl.
  3. Feedback Combination: The pair is fully verified when Valve 1 is Open AND Valve 2 is Closed. Wire the Block ZSO and Bleed ZSC through an AND block into FbkOpen. Wire the Block ZSC and Bleed ZSO through an AND block into FbkClose. This ensures both valves have traveled to their correct opposite positions before clearing monitoring timers.

Signal Inversion Strategies in Pcs7DiIn and CFC

If an engineer attempts to create an artificial 'Open' feedback by simply inverting the 'Closed' limit switch (Not01 gate):

  1. The Transit State Hazard: When a valve leaves its closed seat, it travels for several seconds before reaching the open position. An inverted closed switch instantly reports 'Open' the microsecond the valve begins moving, masking mechanical binding or broken stems.
  2. Proper Inversion on Pcs7DiIn: If physical field wiring is Normally Closed (NC) instead of Normally Open (NO) for fail-safe wire-break monitoring, do not use external NOT gates in CFC. Instead, configure parameter InvIn = 1 on the Pcs7DiIn channel block. This inverts the logical evaluation while preserving proper diagnostic quality codes (ST).

Comparison of Valve Control Blocks (VlvL vs VlvL with Dual Feedback)

The following table compares the operational characteristics and diagnostic coverage of single-feedback versus dual-feedback valve configurations:

| Configuration Parameter | VlvL with Dual Limit Switches | VlvL with Single Closed Switch (NoFbkOpen=1) |
|---|---|---|---|
| Hardware Limit Switches | 2 (ZSC - Closed, ZSO - Open) | 1 (ZSC - Closed only) |
| Open Position Verification | Verified by physical field contact | Assumed true after MonTiDynamic timer |
| Mechanical Jam Detection | Immediate travel time fault in both directions | Detected during closing only; undetected on opening |
| Channel Driver Requirement | 2x Pcs7DiIn + 1x Pcs7DiOu | 1x Pcs7DiIn + 1x Pcs7DiOu |
| Process Object (PO) Impact | 1 Process Object | 1 Process Object |
| Recommended Deployment | High-pressure isolation, toxic service, burner fuel | Low-pressure cooling water, utility drains |

Verification

Verify single-feedback and block-and-bleed valve configurations in CFC test mode:
1. In CFC, command the valve to open (OpenAut = 1 or via faceplate); verify that output Ctrl asserts true (1).
2. Observe the runtime monitoring timer; verify that after MonTiDynamic expires, the valve state transitions to 'Open' without triggering a travel error alarm.
3. Command the valve to close (CloseAut = 1); verify that Ctrl drops to 0.
4. Toggle the closed limit switch input (FbkClose = 1); confirm that the valve immediately transitions to 'Closed' and the green closed status icon illuminates.
5. Leave FbkClose = 0 during closing; verify that upon expiration of MonTiDynamic, the block asserts an End Position Error (MonDynErr = 1) and turns red on the OS.

Limitations and Operational Precautions

Never set NoFbkOpen = 1 on safety-instrumented valves (SIS) or burner shutoff valves governed by NFPA or EN standards; safety standards strictly mandate positive mechanical confirmation of valve travel. When using inverted limit switch inputs (InvIn = 1), clearly label the wiring diagrams and Step 7 symbol comments to prevent field instrumentation technicians from misinterpreting open/close test meter readings.

Engineer FAQ

Why does VlvL show an error when I command it open if I don't have an open limit switch?

By default, VlvL expects both open and closed feedback contacts. If no contact is wired to FbkOpen and NoFbkOpen is left at 0, the block waits for MonTiDynamic to expire and then generates an end position error alarm. Setting NoFbkOpen = 1 resolves this.

Can I use an inverted closed switch to simulate open feedback?

While technically possible via a NOT gate, it is poor engineering practice. An inverted closed switch reports 'Open' the moment the valve cracks open, completely failing to detect if the valve stem broke mid-travel.

What does the parameter NoFbkClose do on VlvL?

Setting NoFbkClose = 1 disables monitoring for the closed limit switch, allowing valves with only an open limit switch to operate without generating close-travel alarms.

Technical References

Related Articles

Related Tags

  • VlvL
  • APL
  • Valve Control
  • Feedback Monitoring
  • Block and Bleed
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