We can't believe our vortex shedding flowmeter

This month's edition of Problem Solving Ideas

We can't believe our vortex shedding flowmeter used in NH3 service when compared to a mass flowmeter. The shedder has been compensated with square-root P/T compensation. The flow valve is in manual, the line pressure is constant, with dropping temperature only. But the reading from the flowmeter decreased and the mass flow increased. Looking at trend data, the NH3 vaporizer pressure dipped. Because the meter uses the calculation: (volume)=M(mass)/D(density), where D is inversely affected by temperature, we have come to the conclusion that it is not compensating for increased mass that the vaporizer is pushing into the line.
How would I compensate for this in a formula?


Clarify the Scenario
Before addressing compensation, we need to clarify the scenario. Because vortex flowmeters are linear devices, there should be no square root involved. The vortex meter will give you the actual volumetric flow rate. You need to determine the density directly from the equation of state of the NH3 without any square root. By multiplying the actual volumetric flow rate by the density you will get the mass flow rate. This should agree with your mass meter, provided you have only gas present (e.g., single phase flow), and the equation of state in your flow computer is correct.
If there is still a discrepancy (after removing the square root function), check:
*Is the flow single phase?
*Is the equation of state for the NH3 correct?
*Is the pressure truly constant? ( if not you need to add pressure compensation)
Based on the scenario presented, I believe that two-phase flow is present. If so, using the equation of state to calculate the true mass falls apart. The bottom line is the vortex meter should be something you can trust for this measurement, provided the flow is single phase. If the discrepancy persists, further analysis of the situation with a flow expert may be required.

Wade Mattar
Foxboro Measurements and Instruments Div. of Invensys

The Root of the Problem
A vortex shedding flowmeter measures actual volume flow. It does not need pressure and temperature compensation unless you want to know mass flow or standard volume flow.  Since you are comparing it to a mass flowmeter, I assume that you are compensating the vortex meter to convert actual volume flow to mass flow. A vortex shedder is a linear flowmeter, so the use of square-root P/T compensation may be the source of your problem.  To calculate mass flow:
W = Da * Qa where: W is mass flow (lbm/min)
Da is actual density (lbm/ft^3)
Qa is actual volume flow (ACFM)

Pv = nRTz where: P is absolute pressure (psia)
v is volume (ft^3)
n is number of lbm-moles
R is universal gas constant: 1545 (ft-lbf)/(lbm-mole deg R)
T is absolute temperature (deg R)
z is supercompressibility factor

Note: A conversion factor of 144in^2/ft^2 is necessary to make the units work out

n = m/MW where: m is mass (lbm)
MW is molecular weight (lbm/lbm-mole)
Da = m/v = (144/1545)*(MW/z)*(P/T) by substitution and rearranging the equations above

For ammonia (MW = 17.03) and an ideal gas (z = 1, but an actual value can be obtained from compressibility charts for actual P and T):

W = (144/1545)*(17.03/1)*(P/T)*Qa = 1.5873 * (P/T) * Qa

Unfortunately we have not been given any actual operating conditions. For example’s sake, assume:

P is 100 psia
T is 80 deg F (540 deg R)
z is 0.925
Qa is 1000 ACFM

Da = (144/1545)*(17.03/0.925)*(100/540) = 0.3178 lbm/ft^3

W = 0.3178*1000 = 317.8 lbm/min

Dick Morris
Allied Control Services, Inc.

March’s Problem

Can You DO That?

We have a level measurement problem with a differential pressure level transmitter that doesn’t seem to track level properly. The vessel is a glass-lined reactor, with an internal agitator and baffles. The only available opening is a 1-in. port on the top of the vessel, and the vessel is 60-in. in diameter and 12-ft. high. There are cooling coils filled with Dowtherm, and a glass-wool-and-aluminum-lag insulating jacket that is about 6-in. thick. We are having real problems on high level, and the process upsets if the vessel is too full.
Please send us your comments, suggestions, or solutions to this problem. Solutions will be published in the March issue, but we need your responses by February 15 to meet our editorial deadlines. If you have a solution to this problem, or a problem of your own that you would like to pose to your colleagues, email it to CONTROL Ask the Experts at wboyes@putman.net.

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