Pressure/Temperature Compensated Flow (PPTC)

This information applies to the CompactLogix 5380P and ControlLogix 5580P controllers.

The Pressure/Temperature Compensated Flow (PPTC) instruction calculates a flow at standard temperature and pressure, essentially a mass flow rate, given a volumetric flow rate or differential pressure measurement. This instruction requires measurements of the actual temperature and pressure of the flowing gas.

Available Languages

Ladder Diagram

Function Block Diagram

Structured Text

PPTC(PPTC_01);

Operands

IMPORTANT:

Unexpected operation may occur if:

Output tag operands are overwritten.

Members of a structure operand are overwritten.

Except when specified, structure operands are shared by multiple instructions.

There are data conversion rules for mixing numeric data types within an instruction. See Data conversions.

Configuration Operands

Operand	Type	Format	Description
PlantPAx Control	P_PRESS_TEMP_COMPENSATED	tag	Data structure required for proper operation of the instruction.

P_PRESS_TEMP_COMPENSATED Structure

Public members are standard, visible tag members that are programmatically accessible. Private, hidden members are used in HMI faceplates and are not programmatically accessible. Private members are listed in separate tables after public members.

Public Input Members	Data Type	Description
EnableIn	BOOL	Enable input. Ladder Diagram: Corresponds to the rung-condition-in. Default is true.
Inp_InitializeReq	BOOL	1 = Request to initialize the instruction. The instruction is normally initialized in instruction first run. Use this request when reinitialization is needed. The instruction clears this operand automatically. Default is true.
Inp_TAct	REAL	Actual (measured) temperature, can be absolute or common units. Valid = any float. Default is 0.0.
Inp_PAct	REAL	Actual (measured) pressure, can be absolute or common units. Valid = any float. Default is 0.0.
Inp_DPAct	REAL	Actual (measured) differential pressure. Valid = any float. Default is 0.0.
Inp_FAct	REAL	Actual (measured) uncompensated flow in volumetric units. Valid = any float. Default is 0.0.
Cfg_LoFlowCutoff	REAL	If Out_Flow is less than this cutoff value, it is shown as 0.0. Valid = 0.0 to maximum positive float. Default is 0.0.
Cfg_TStd	REAL	Standard temperature in Inp_TAct units. Valid = any float. Default is 0.0.
Cfg_PStd	REAL	Standard pressure in Inp_PAct units. Valid = any float. Default is 0.0.
Cfg_TOffset	REAL	Zero input-units temperature in absolute units. Typically 273.15 Kelvins or 459.67 Rankine. Valid = 0.0 to maximum positive float. Default is 273.15.
Cfg_POffset	REAL	Zero input-units pressure in absolute units. Typically 14.696 PSIA. Valid = 0.0 to maximum positive float. Default is 14.696.
Cfg_DPRef	REAL	Reference (full-scale) differential pressure. Common value 100.0 inches WC. Valid = 0.0 to maximum positive float. Default is 100.0.
Cfg_FRef	REAL	Reference flow in volumetric units at reference dp. Valid = 0.0 to maximum positive float. Default is 1.0.
Cfg_UseDP	BOOL	1 = Use Inp_DPAct (square root curve) to calculate flow. 0 = use Inp_FAct (linear). Default is false.

Public Output Members	Data Type	Description
EnableOut	BOOL	Enable Output. This output state always reflects EnableIn input state.
Out_Flow	REAL	Compensated flow (at standard temperature and pressure: mass flow).
Sts_Initialized	BOOL	1 = Instruction is initialized. Use Inp_InitializeReq to reinitialize.
Sts_Err	BOOL	1 = Error in configuration: See detail bits (Sts_Errxxx) for reason.
Sts_ErrTStd	BOOL	1 = Invalid standard temperature (Cfg_TStd + Cfg_TOffset must be > 0.0).
Sts_ErrPStd	BOOL	1 = Invalid standard pressure (Cfg_PStd + Cfg_PStd must be > 0.0).
Sts_ErrDPRef	BOOL	1 = Invalid reference differential pressure (must be > 0.0 if DP used).
Sts_ErrFRef	BOOL	1 = Invalid reference flow (at reference DP) (must be > 0.0).

Operation

The PPTC instruction is intended as a calculation function only, between other blocks. If a faceplate or alarms are needed, the calculated output from the instruction can be sent to a PAI (analog input) instruction for alarming and display.

The PPTC instruction:

Takes as its primary input either a volumetric flow rate or a differential pressure across a flow element, such as an orifice plate or pitot tube. When a differential pressure is used, the PPTC instruction allows configuration of the volumetric flow rate for a given differential pressure.

Accepts a temperature in common units (Fahrenheit or Celsius degrees) or in absolute units (Rankine degrees or Kelvins).

Accepts a pressure in common units (PSIG, kPa Gauge, or MPa Gauge) or in absolute units (PSIA, kPa Absolute, MPa Absolute).

Has user-configurable standard conditions, such as 14.696 PSIA and 60 °F, or 101.325 kPa and 0 °C.

Determines flow at the specified standard conditions by using the Ideal Gas Law (PV = nRT) to adjust from the given temperature and pressure to the standard temperature and pressure.

Virtualization

Virtualization is not applicable to the PPTC instruction.

Initialization

The instruction is normally initialized in the instruction first run. Re-initialization can be requested any time by setting Inp_InitializeReq = 1. For proper initialization, when adding the instruction while performing an online edit of the code, make sure that Inp_InitializeReq = 1, the default value.

Configuration of Strings for HMI

Configure strings for HMI faceplates (FT View) and for the Logix Designer configuration dialog box.

Description – Description of PPTC tag

Label for graphic symbol – Label metadata of PPTC tag

Display Library for HMI Faceplate call-up – Library metadata of PPTC tag

Instruction name – Instruction metadata of PPTC tag

Area name – Area metadata of PPTC tag

URL link – URL metadata of PPTC tag

Actual differential pressure units – Engineering Unit metadata of .Inp_DPAct

Actual uncompensated flow in volumetric units – Engineering Unit metadata of .Inp_FAct

Actual pressure, can be abs or common units – Engineering Unit metadata of .Inp_PAct

Actual temperature, can be abs. or common units – Engineering Unit metadata of .Inp_TAct

Compensated flow (at standard temperature and pressure: mass Flow) units – Engineering Unit metadata of .Out_Flow

Monitor the PPTC Instruction

Use the operator faceplate from the PlantPAx library of Process objects for monitoring.

Affects Math Status Flags

No.

Major/Minor Faults

None specific to this instruction. See Index Through Arrays for array-indexing faults.

Execution

Ladder Diagram

Condition/State	Action Taken
Prescan	Rung-condition-out is cleared to false.
Instruction first run	The instruction executes normally.
Rung-condition-in is false	Set rung-condition-out to rung-condition-in.
Rung-condition-in is true	Set rung-condition-out to rung-condition-in. The instruction executes.
Postscan	Rung-condition-out is cleared to false.

Function Block Diagram

Condition/State	Action Taken
Prescan	EnableOut is cleared to false.
Instruction first run	See Instruction first run in the Ladder Diagram table.
Instruction first scan	See Instruction first run in the Function Block Diagram table.
EnableIn is false	EnableOut is cleared to false.
EnableIn is true	EnableOut is set to true. The instruction executes.
Postscan	EnableIn and EnableOut bits are cleared to false.

Structured Text

In Structured Text, EnableIn is always true during normal scan. The instruction executes when it is in the control path activated by the logic.

Condition/State	Action Taken
Prescan	See Prescan in the Function Block Diagram table.
Instruction first run	See Instruction first run in the Function Block Diagram table.
EnableIn is true	See EnableIn is true in the Function Block Diagram table.
Postscan	See Postscan in the Function Block Diagram table.

Example

In this example, the PPTC instruction to determine the flow rate of compressed nitrogen at a standard pressure and flow. This can provide a more accurate measurement for custody transfer or control calculations where there is variability in environmental conditions and the flow transmitter is not capable of performing the compensation.

In this case, the PPTC instruction measures flow from a dp-transmitter. The transmitter provides the controller with a value that has been scaled to volumetric flow but not compensated for environmental temperature and pressure. We also have temperature and pressure measurements from where the flow is measured. In this example, the desired standard pressure and flow is 0 psig and 15 °C.

Ladder Diagram