Configuring DoseL (Dose Lean) Block and Troubleshooting Creep Flow Alarms in SIMATIC PCS 7 APL

In the SIMATIC PCS 7 Advanced Process Library (APL), the DoseL (FB1834 - Lean Dosing) block integrates automatic batch and continuous dosing with advanced flow monitoring. When configuring DoseL, spurious 'Creep flow too large' alarms and dosing quantity freeze issues typically occur due to misconfigured creep flow limit parameters (CR_AH_Lim, CR_Flow_Mag_AH) or misunderstandings of Feature.Bit13 (Creep flow detection). Creep flow represents unintended flow measured when dosing is in the 'Off', 'End', or 'Pause' state. To prevent false creep flow trips or to disable creep flow evaluation entirely without halting dosing totalization, engineers can set Feature.Bit13 = 0 (evaluates creep flow only when CR_AH_Lim is exceeded) or deactivate creep flow monitoring by configuring CR_AH_En = 0.

Functional Overview of the DoseL (FB1834) APL Block

The DoseL (FB1834) block from the PCS 7 Advanced Process Library is engineered for controlling and monitoring dosing procedures of liquids or granular solids using flowmeters, load cells, or pulse counters. It provides coarse and fine dosing valve control, dosing setpoint management, automatic overshoot compensation, and flow rate supervision.

Unlike a generic totalizer block (TotalL), DoseL manages a strict internal state machine comprising Idle, Dosing, Post-dosing, Pause, and End states. It continuously integrates the process flow value (PV) into an accumulated actual dosing quantity (ActQty). To ensure product quality and pipeline integrity, DoseL incorporates specialized diagnostic alarms, including flow monitoring during dosing and creep flow monitoring when the valves are commanded shut.

Mechanics of Creep Flow Detection and Dosing States

Creep flow is defined as any measured flow rate or weight change that occurs when the dosing system is in an inactive state—specifically Off (0), End (4), or Pause (2). Such flow typically indicates a leaking shutoff valve, line drain-down, or sensor zero-point drift:

Creep Flow Detection Mode: DoseL calculates the flow magnitude during inactive states. If this unintended flow persists, the block flags the condition, generates a high-priority alarm, and sets the block icon into an alarm state (red frame).
Creep Flow Magnitude Parameter (CR_Flow_Mag_AH): Represents the absolute measured flow velocity or rate while the dosing valves are closed.
Creep Flow Limit Parameter (CR_AH_Lim): The upper engineering limit threshold above which a creep flow alarm is triggered.
Totalization Impact: If creep flow alarms are active and parameters are contradictory (such as setting limit to 0 while message generation is active), the internal state transition logic can lock the totalizer, preventing ActQty from updating during subsequent dosing runs.

Configuring Feature.Bit13 and Creep Flow Limit Parameters

To control how the DoseL block handles creep flow, automation engineers must configure Feature.Bit13 and the associated analog limit inputs:

Feature.Bit13 = 1 (Standard Factory Setting): Creep flow is continuously evaluated during all inactive states. Any flow reading above zero is accumulated into the dosing diagnostic buffer.
Feature.Bit13 = 0 (Selective Threshold Evaluation): Creep flow is only flagged when the measured rate exceeds CR_AH_Lim. Below this threshold, minor sensor noise or temperature fluctuations are ignored.
Disabling Creep Flow Monitoring (CR_AH_En = 0): Setting CR_AH_En = 0 disables the alarm evaluation circuit for creep flow entirely. The block ignores any residual flow when the dosing valve is shut.
Message Suppression (CR_AH_MsgEn = 0): Disables WinCC alarm generation while retaining internal diagnostic flags.

Troubleshooting Quantity Totalization Lockups and Alarm Inconsistencies

When commissioning DoseL in CFC, engineers occasionally encounter a scenario where changing the process value (PV) fails to increase ActQty, accompanied by a persistent red creep flow alarm. Follow these troubleshooting steps to restore proper totalization:

Check Dosing Command State: Ensure the block has received an explicit Start command (StartAut = 1 or Operator Start via faceplate) and is in the 'Dosing' state. If the block remains in 'End' or 'Off', incoming PV will be treated as creep flow rather than dosing quantity!
Reset the Quantity Buffer: Prior to starting a new batch, pulse the reset pin (RstActQty = 1 or RstOp = 1) to clear previous batch totals and clear active latching alarms.
Verify Limit Values: Never set CR_AH_Lim = 0 while CR_AH_En = 1. A limit of 0 means any infinitesimal floating-point noise from a 4-20 mA transmitter (e.g., 0.0001 L/s) instantly triggers an alarm. Set CR_AH_Lim to a realistic engineering threshold above transmitter noise (e.g., 2% of nominal flow).
Reconcile Feature Bits: If external reset or automatic acknowledgement is desired, verify Feature.Bit2 (Resetting the dosing quantity on start) and Feature.Bit30 (Resetting depending on operating mode).

Comparison of Dosing Block Configurations (DoseL vs Full Dose)

The following table compares the capabilities and behavior of DoseL against the full-featured Dose block:

Functional AttributeDoseL (FB1834 - Lean Dosing)Dose (Full APL Dosing Block)Primary Use CaseStandard liquid/solid single or dual valve dosingComplex dosing with dribble feed and scale tareCreep Flow EvaluationControlled via Feature.Bit13 and CR_AH_LimMulti-stage creep flow and leakage trackingOvershoot CorrectionAutomatic moving-average overshoot calculationMulti-algorithm dynamic predictive compensationProcess Object (PO) ConsumptionStandard 1 PO per instanceConsumes 1 PO per instanceExecution Time OverheadLow memory footprint and fast execution cycleHigher memory footprint and parameter countFaceplate ViewsStandard, Parameter, Trend, AlarmStandard, Parameter, Limit, Trend, Alarm, Diagnostics

Verification

Verify proper DoseL operation and creep flow suppression in CFC:
1. In CFC test mode, verify that with dosing idle (Off state), applying a small simulated flow (PV = 0.5) does not generate a creep alarm when CR_AH_Lim = 1.0 and CR_AH_En = 1.
2. Increase simulated flow to PV = 1.5; verify that the creep flow alarm bit (CR_AH_Act = 1) asserts and the faceplate displays 'Creep flow too large'.
3. Set CR_AH_En = 0; verify that CR_AH_Act immediately resets to 0 and the red alarm condition clears.
4. Command a dosing cycle (StartAut = 1); verify that the block enters Dosing state, the coarse valve output energizes (Ctrl = 1), and ActQty increments accurately according to PV * SampleTime.

Limitations and Operational Precautions

Disabling creep flow alarms (CR_AH_En = 0) must only be done if physical shutoff valves are verified leak-free or if the dosing sensor is prone to zero-point thermal drift. In hazardous chemical dosing or custody transfer systems, ignoring creep flow can lead to vessel overfills or inaccurate mass balances. Always ensure RstActQty is triggered before a new dosing run to prevent old residual volumes from being added into the current batch.

Engineer FAQ

What is the difference between `CR_AH_En` and `CR_AH_MsgEn` on DoseL?

CR_AH_En enables or disables the actual limit evaluation logic within the block algorithm. CR_AH_MsgEn only suppresses the ALARM_8P message transmission to WinCC while leaving the internal calculation and status word bits active.

Why did `ActQty` stop changing when I changed the process value `PV`?

Because the block was not in the active 'Dosing' state. If the block is in 'Off', 'End', or 'Pause', incoming flow is treated as leakage or creep flow and is intentionally NOT added to the dosing target quantity.

Can DoseL automatically correct for valve closing delays (overshoot)?

Yes. DoseL features automatic post-dosing overshoot compensation (AutoCorr = 1). After each dosing cycle completes, it measures the excess material delivered and adjusts the valve shutoff trip point for the subsequent cycle.

Technical References

Related Tags

DoseL
APL
CFC
Process Control
Dosing

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Configuring DoseL (Dose Lean) Block and Troubleshooting Creep Flow Alarms in SIMATIC PCS 7 APL