Flowcomp Equation in DCS (Confusion)


Thread Starter


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)


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

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

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.