Calculate Steam Temperature to Pressure

[DaDaDadeo]

The IAPWS-IF97 steam table is used as a measuring standard for the correlation between steam temperature and pressure. The calculation formula(s) for PLCs can be elusive. There are formulas such as the Antoine and Clausius-Clapeyron, but these deviate too much from the IF97 curve. EngineeringToolbox.com has one that may be accurate enough for boilers and pressures less than 15 Bar.


Below are different formats of the same formula (Remove the "*14.554" for Bar units):


RSLOGIX 500 (F8:0 = Float Tag Temperature in C)

(((2.71828**((77.345+(0.0057*(F8:0+273.15)))-(7235.0|(F8:0+273.15))))|((F8:0+273.15)**8.2))|100000.0)*14.554


RSLOGIX 5000 (SBR_Input_REAL = Float Tag Temperature in C)

2.71828**(77.345+(0.0057*(SBR_Input_REAL+273.15))-(7235.0/(SBR_Input_REAL+273.15)))/((SBR_Input_REAL+273.15)**8.2)/100000.0*14.554


Float Script (IDEC)

// "@" is for temporary storage

@1 = [IN_TEMPERATURE] +273.18 ;
@2 = ( ( 77.345 + ( 0.0057 * @1 ) ) - ( 7235.0 / @1 ) );
@2 = EXP(@2);
@3 = POW( @1 , 8.2 ) ;
[OUT_PRESSURE] = ((@2 / @3 ) / 100000) * 14.554;


EXCEL (A2 = Cell with Temperature in C)

=(2.71828^(77.345 + 0.0057*(A2+273.15) - 7235 / (A2+273.15) ) / (A2+273.15) ^8.2/100000)*14.554





Reference: https://www.engineeringtoolbox.com/water-vapor-saturation-pressure-air-d_689.html

 

[drbitboy]

Cool; this is for saturated steam.


What about poor old MicroLogix, with no exponentiation?


My dad converted the 1967 ASME Steam Table formulations (H=f(P,T); S=f(H,V), etc.) from FORTRAN-IV to Timex 2068 Basic, and then to PC Basic. Timex Basic was better because it had a five-byte real. There was a problem in convergence in one region using successive substitution, but we fixed it by switching to Newton-Raphson.

 

[DaDaDadeo]

Float Script (IDEC)

// "@" is for temporary storage

@1 = [IN_TEMPERATURE] +273.18 ;
@2 = ( ( 77.345 + ( 0.0057 * @1 ) ) - ( 7235.0 / @1 ) );
@2 = EXP(@2);
@3 = POW( @1 , 8.2 ) ;
[OUT_PRESSURE] = ((@2 / @3 ) / 100000) * 14.554;

Float Script (IDEC)

// "@" is for temporary storage

@1 = [IN_TEMPERATURE] +273.15 ;
@2 = ( ( 77.345 + ( 0.0057 * @1 ) ) - ( 7235.0 / @1 ) );
@2 = EXP(@2);
@3 = POW( @1 , 8.2 ) ;
[OUT_PRESSURE] = ((@2 / @3 ) / 100000) * 14.554;

Cheers!