formula to compensate saturated steam with pressure and temperature

  • Thread starter jameswesleyjones1974
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jameswesleyjones1974

We have a yokogawa steam meter with a yfa flow converter that sends a 4-20 ma signal to the plc and we have a yokogawa pressure transmitter that also sends a 4-20 ma signal to the plc. I want to compensate my steam similar to what a mass flow computer does in the plc. Also the range of the steam meter is 4-20 ma and 0-12 kpph and the pressure is 4-20 ma for 0- 250 psi. How do i do this?
 
You have to make use of Steam Tables which tells the density of the steam with respect to each pressure/temperature. And to implement this in PLC I dont know which PLC you are using you can use Array to use the density at process pressure to be mutiplied with Volumatric flow to get Mass flow.
 
I suggest that you buy a flow computer for about $700 and output the compensated signal to the PLC. If the steam is saturated, do not use both a temp and a pressure input because you may get a superheat calc if the two are not exactly in agreement with the saturation curve. If you don't like that answer:

PPH = Velocity(ft/sec) x Area(sq ft) x Density(lb/cf) x 3600(sec/hr)

Have Yokogawa tell you what the meter scaling is in ft/sec and what the meter cross-sectional area is in sq ft. Solve for density (rho) with a pressure input (Pf) in PSIG as follows. I caution you that this is a highly workable approximation but is only valid at moderate pressures like yours (ie < 500 PSIG):

rho={2.121(14.69+Pf)-38.12[1/Ln(14.69+Pf)]+20.37}/1000

You may have to do a little math work to fit this algorithm into the instruction set for your PLC. For instance, natural logs are not a commonly available instruction. Or you could call Contrec and make the problem go away for a few hundred dollars.
 
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