Engineering Valve_Lean (VlvL) Templates: Best Practices for Shared Solenoid Outputs and Complementary Feedback

In SIMATIC PCS 7 Advanced Process Library (APL), when engineering two physical valves operating as a paired subsystem (such as a Block-and-Bleed manifold) sharing a single common pilot solenoid valve where one valve opens while the other closes, automation engineers must NEVER connect the control output pins (Ctrl) of two distinct VlvL blocks to the same digital output driver (Pcs7DiOu). Doing so causes output contention, where the block executed latest in the cyclic OB runtime sequence overwrites earlier commands. The robust, standard practice is to utilize a single logical VlvL block representing the entire block-and-bleed assembly driving a single Pcs7DiOu, while evaluating the complementary limit switch feedbacks (Block ZSO + Bleed ZSC for Open; Block ZSC + Bleed ZSO for Closed) through intermediate Boolean AND gates into FbkOpen and FbkClose.

The PLC Scan Cycle Rule: Why Multiple Blocks Must Not Write to One Output

A fundamental principle of deterministic programmable logic controllers (PLCs) is that the physical process image output (PIQ) reflects the final value assigned to an address during the cyclic program scan. In Step 7 and PCS 7 CFC:

Execution Order Determines Final State: If Valve Block A writes Ctrl = 1 to output %Q4.0 in Run Sequence 1, and Valve Block B writes Ctrl = 0 to %Q4.0 in Run Sequence 2, the physical solenoid valve will de-energize (0), completely ignoring the command of Block A.
APL Driver Overlap: When multiple Pcs7DiOu channel blocks reference the exact same hardware symbol, the driver generator flags compilation warnings or generates erratic edge-detection signals on the physical card.

Architectural Modeling of Block-and-Bleed Valve Pairs

In piping systems, Block-and-Bleed pairs consist of an in-line process isolation valve (Block Valve) and an atmospheric vent valve (Bleed Valve). When energized by a single 3-way/4-way solenoid:

Energized State: Block Valve is forced OPEN; Bleed Valve is forced CLOSED. Material flows through the process pipe.
De-energized State (Safe State): Block Valve snaps CLOSED; Bleed Valve springs OPEN. Any trapped fluid vents safely, preventing cross-contamination.

Because the two physical valves are mechanically and pneumatically slaved to a single solenoid, treating them as two independent asynchronous DCS valves creates false travel alarms and operator confusion. They should be modeled as a single functional automation object.

Coordinating Solenoid Driving and Complementary Feedback Logic

To configure this in the CFC editor using standard APL components:

Single Valve Block: Place one instance of VlvL (FB1899) named VLV_BB_101.
Drive the Shared Solenoid: Connect VlvL.Ctrl to the PV_In pin of a single digital output driver Pcs7DiOu, which writes to the physical solenoid output address.
Evaluate Open Feedback (FbkOpen):
- Wire the Block Valve Open limit switch (ZSO_Block) and Bleed Valve Closed limit switch (ZSC_Bleed) to separate Pcs7DiIn blocks.
- Connect both outputs through a standard Boolean AND block (And01). Connect the Out of And01 to VlvL.FbkOpen.
Evaluate Closed Feedback (FbkClose):
- Wire the Block Valve Closed limit switch (ZSC_Block) and Bleed Valve Open limit switch (ZSO_Bleed) through a second AND block (And02). Connect Out to VlvL.FbkClose.
Dynamic Travel Supervision: Set MonTiDynamic (e.g., 5.0 seconds). If either valve fails to stroke to its complementary seat within 5 seconds, the block triggers an end position error.

Handling Single-Feedback Fail-Safe Fallbacks in VlvL

If the bleed valve lacks limit switches entirely and only the block valve has contacts:

Wire the Block Valve ZSO directly to FbkOpen and Block Valve ZSC directly to FbkClose.
If only one limit switch exists across the entire pair (e.g., Block ZSC only), configure NoFbkOpen = 1 on VlvL as established in standard single-feedback practices, ensuring the block does not monitor the missing open feedback.

Comparison of Valve Pairing Implementation Methods in CFC

The following table compares different approaches to configuring paired valves sharing a common solenoid:

Engineering ApproachSolenoid Contention RiskCFC ComplexityDiagnostic ClarityProcess Object (PO) CostSingle VlvL with Dual-AND Feedback (Recommended)Zero (Single driver writes output)Low (1 VlvL + 2 AND gates)100% Comprehensive (Validates both valves)Consumes 1 POMaster/Slave Dual VlvL Blocks with OR LogicZero (OR gate arbitrates single driver)Moderate (2 VlvL + custom logic)Can create conflicting faceplate alarmsConsumes 2 POsDirect Dual VlvL Driving Same OutputSevere (Last executed block wins)Low (Naive parallel wiring)Broken (Valves chatter or fail to stroke)Consumes 2 POsModulating Valve ApproximationZeroHigh (Misapplication of VlvAnL)Poor (Discreet valves treated as analog)Consumes 1 PO

Verification

Verify the Block-and-Bleed valve coordination in CFC:
1. In CFC test mode, command VlvL to open (OpenAut = 1); confirm that Ctrl asserts true (1) and drives the single solenoid output.
2. Simulate Block ZSO = 1, but keep Bleed ZSC = 0 (simulating a jammed bleed valve); verify that FbkOpen remains 0.
3. Observe that after MonTiDynamic expires, VlvL generates an End Position Error (MonDynErr = 1) and turns red, proving the safety interlock detected the stuck bleed valve.
4. Set Bleed ZSC = 1; confirm that FbkOpen transitions to 1, clearing the travel timer and establishing solid green Open status.

Limitations and Operational Precautions

In hazardous petrochemical and natural gas burner feeds, ensure that the solenoid de-energized state matches the fail-safe piping layout (typically de-energize to close fuel block, de-energize to open vent). Never rely on software timers alone to verify bleed valve closure if venting toxic gas; physical limit switches on both block and bleed valves are strongly advised to satisfy environmental and safety auditing requirements.

Engineer FAQ

Can I use an OR gate to combine the outputs of two VlvL blocks?

Yes, an intermediate OR gate can merge control outputs from two blocks into a single digital driver. However, this is inferior to using a single coordinated VlvL block because operator commands on the two faceplates can conflict, leaving one faceplate showing 'Opening' while the physical output is already overridden.

Why did my valve solenoid chatter when I configured two VlvL blocks in CFC?

Because both blocks were writing contradictory values to the same output driver. Depending on execution ordering and dynamic feedback changes, the output was alternately set to 1 and 0 every PLC cycle.