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Dear Milisters,
Does anyone know how flow compensation for gas DP flow ends up like below in DCS?
-quote from DCS-
Equation 1
for mass-flow compensation
Q correction = Qmeasured x SQRT(Pmeas/Pref x Tref/Tmeas)
Equation 2
for volume-flow compensation
Q correction = Qmeasured x SQRT(Pmeas/Pref x Tref/Tmeas x MWref/MW)
-unquote-
I've tried to derived the equation from basic equation in AGA , and I can only proved equation (1). I don't have any idea where equation 2 derived from. So, this is my confusion :
1. I always assume MW is constant, independent from P and T change. Our Process engineer also has confirmed this. So, how does Equation 2 ask for MWref?
2. Even when they request MW ref and MW to be input for volume-flow compensation, most what DCS engineer do is to input MWref/MW = 1. So the equation (2) is exactly the same with equation (1). Which is not make sense.
3. I've checked this thread [http://control.com/thread/1026237316] and I read this statement
-quote-
Effect of temperature & pressure is negligible in
mass flow as it is movement of mass/time, but affects volume as density changes with pressure & temperature
-unquote-
Orifice in nature is mass-flow meter, but its calculation on mass-flow depends on square root of density (which is depends on P,T). So, how does it is negligible ?
4. To add another confusion, in other project I found different calculation has been used for mass-flow compensation and volume-flow compensation. For example, Equation (1) is used for volume-flow equation instead of mass-flow compensation.
If anyone knows good reference for flowcomp equation in DCS, it would be really helpful. Thanks for your answer.
Does anyone know how flow compensation for gas DP flow ends up like below in DCS?
-quote from DCS-
Equation 1
for mass-flow compensation
Q correction = Qmeasured x SQRT(Pmeas/Pref x Tref/Tmeas)
Equation 2
for volume-flow compensation
Q correction = Qmeasured x SQRT(Pmeas/Pref x Tref/Tmeas x MWref/MW)
-unquote-
I've tried to derived the equation from basic equation in AGA , and I can only proved equation (1). I don't have any idea where equation 2 derived from. So, this is my confusion :
1. I always assume MW is constant, independent from P and T change. Our Process engineer also has confirmed this. So, how does Equation 2 ask for MWref?
2. Even when they request MW ref and MW to be input for volume-flow compensation, most what DCS engineer do is to input MWref/MW = 1. So the equation (2) is exactly the same with equation (1). Which is not make sense.
3. I've checked this thread [http://control.com/thread/1026237316] and I read this statement
-quote-
Effect of temperature & pressure is negligible in
mass flow as it is movement of mass/time, but affects volume as density changes with pressure & temperature
-unquote-
Orifice in nature is mass-flow meter, but its calculation on mass-flow depends on square root of density (which is depends on P,T). So, how does it is negligible ?
4. To add another confusion, in other project I found different calculation has been used for mass-flow compensation and volume-flow compensation. For example, Equation (1) is used for volume-flow equation instead of mass-flow compensation.
If anyone knows good reference for flowcomp equation in DCS, it would be really helpful. Thanks for your answer.