C135 PCS 7 (PCS7)
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How SafePos and OpenAut Work in the PCS 7 APL VlvL Block

How SafePos and OpenAut Work in the PCS 7 APL VlvL Block

In the APL VlvL block, SafePos defines the valve's idle/fail position; it does not mean that OpenAut must be inverted during normal automatic operation. OpenAut = 1 commands open and CloseAut = 1 commands close. Feature.Bit4 selects switch or pushbutton command behavior, while Feature.Bit3 can reset a command after the block evaluates it. Safe-position behavior becomes relevant when the configured monitoring, interlock, fault, or out-of-service conditions call for the idle position.

Table of Contents

Normal Automatic Commands

The APL valve block uses neutral command names: OpenAut requests opening and CloseAut requests closing. The block's Ctrl output is a control signal whose electrical polarity and resulting actuator motion depend on the valve wiring and configured signal path. Do not infer the valve's process position from Ctrl alone; compare the output with FbkOpen and FbkClose and the installed actuator behavior.

What SafePos Controls

The APL manual excerpt cited in the discussion defines SafePos = 0 as closed, SafePos = 1 as open, and SafePos = 2 as stopped for the applicable valve use. It is the configured idle position, not the meaning of the normal open/close commands. The block enters this position under documented conditions such as active interlock/protection, monitoring faults, startup behavior, out-of-service mode, or bad command quality when the relevant monitoring feature is enabled. MonSafePos controls whether the safe position is monitored in the applicable error response.

Select Command Mode

The APL command behavior also depends on feature settings:

SettingEffect described in APL documentationFeature.Bit4 = 0Pushbutton mode; automatic commands can be reset after the commanded control change is evaluated.Feature.Bit4 = 1Switch mode; the static OpenAut command selects the open/closed state, and CloseAut is not needed for this selection.Feature.Bit3 = 1Enables resetting the control command after evaluation; do not confuse it with selecting switch mode.

Confirm the precise feature descriptions in the manual for the installed APL release.

Trace the Output and Feedback

  1. Check the VlvL block revision and Feature word in the project.
  2. Confirm whether switch or pushbutton mode is intended and whether command-reset behavior is enabled.
  3. Apply one automatic command at a time in a controlled test and observe OpenAut, CloseAut, Ctrl, and both feedbacks.
  4. Separately test the documented fault/interlock condition with MonSafePos configured as intended and confirm the configured idle position.
  5. Verify that the field actuator's de-energized position matches the project's SafePos assumption.

Verification

For normal operation, confirm OpenAut = 1 produces the intended open action and CloseAut = 1 produces the intended close action. For fault handling, confirm the block and actuator reach the intended configured safe position only under the applicable documented conditions. Check the faceplate status and feedback signals against physical valve position.

Limitations and Operational Precautions

Ctrl polarity depends on the actuator and output wiring; this article does not specify a wiring diagram. Feature definitions can vary by block family/release, so verify VlvL FB1899 in the exact APL version. Test safety/interlock behavior only under an approved commissioning procedure.

Engineer FAQ

SafePos defines the idle/fail position; OpenAut and CloseAut remain open and close requests. Feature.Bit4 selects switch or pushbutton operation, while Feature.Bit3 controls command reset.

Technical References

  • Siemens, PCS 7 Advanced Process Library V9.1 SP2 Function Manual, document entry 109812806: https://support.industry.siemens.com/cs/document/109812806
  • Siemens, PCS 7 Advanced Process Library V9.1 SP2 Readme, confirming VlvL FB1899 version 5.6: https://support.industry.siemens.com/cs/attachments/109825794/AdvLib-Readme-Online_en-US.pdf
  • Siemens, PCS 7 Advanced Process Library V7.1 Function Manual, switch/pushbutton and feature-bit descriptions for the valve block (historical supporting detail; confirm against the installed release): https://cache.industry.siemens.com/dl/files/808/36209808/att_77975/v1/s7jal71b_en-US.pdf

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Related Tags

PCS 7 APL, VlvL, SafePos, valve control

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