Pipelines in every process industry are outfitted with flow meters to detect the actual amount of fluid in the system. All flow meters are utilized for automatic control of the flow to process equipment in this age of automation. Variable area (Rota meter), differential pressure (orifice, venturi), electromagnetic, turbine, vortex and other types of flow meters are available on the market.
In the first part of this blog series, we have discussed how flow meters function and how process characteristics, circumstances and practices affect flow detection. Delta pressure (DP)-type flow meters are mostly used in the process industries because of their economical simplicity. In this blog series, we will discuss DP-type flow meters only, and in this part, we will discuss the different process parameter variation effect on DP-type flow meter readings. This article will also provide some suggestions for making them excellent, precise and valuable. It will not consider the instrumentation factors such as accuracy, drift and resistance and transmission and errors.
There are three main parameters— pressure, temperature and molecular weight or densities — that mostly vary based on process situation.
As per Bernoulli's theorem, pressure energy and kinetic energy are interchangeable and conserved. A well-known formula has been developed for flow measurement based on delta pressure across a restricting object.
When any flow measurement device such as venturi or orifice is added in the system, flow rate is solely dependent on the density (which varies with temperature, pressure, molecular weight) and pressure difference, therefore the equation can be adjusted.
For liquid density, (ρ) does not vary with pressure and temperature significantly, but for gas fluid it does vary. ρ=(P*MW)/(R*T)
These above basic fundamental equations show how gas flow meters deviate the measurement readings from actual when pressure, temperature and molecular weight change.
1. Let’s use an example to understand. A fuel gas pipe is connected to furnace with a flow meter, using ethane (MW 30) fuel for combustion at 4 bar pressure and 40°C temperature.
The flow meter (venturi/orifice) reads one ton per hour (mass flow rate as a fixed density is considered and that is multiplied in transmitter). Due to an extra intake of ethane, the pressure increased to 5 bar, and the furnace consumed the same one ton of ethane. Due to the additional one bar of pressure, gas is compressed, volumetric flow is reduced (density is raised) and velocity through the throat of the flow meter is lowered, resulting in a lower delta P over the throat. Delta P across the throat is a function of velocity. ∇P=k*ρ*V2
Now, the flow meter will read less than one ton of fuel gas, while the real flow rate is one ton of ethane. Temperature decreasing of ethane and higher molecular weight (mixing of propane MW 44) generates the same effect as described before. The opposite is also true.
2. In many refineries and petrochemical plants, flow meters read the volumetric flow rate of the process. To better understand how temperature, pressure and molecular weight affect the compressor's volumetric flow rate, let's look at another example. The compressor compresses hydrogen and methane gas mixture (50-50%) to higher pressure, and a volumetric flow meter is installed in the compressor's suction.
When hydrogen gas composition increases, the molecular weight of the mixture would reduce and density of the mixture of gas would reduce. For the same volumetric flow of gas, the velocity of the gas through the throat is the same. But due to density reduction, delta P also drops significantly, and the volumetric flow meter will read a lower volumetric flow even though the actual volumetric flow rate to compressor is the same.
The vice versa of the above is also true. As the mixture of gas molecular weight increases or temperature reduces or pressure increments, the volumetric flow meter reads the higher volumetric flow rate, even though there is the same volumetric flow through the orifice or venturi.
Where act is actual of operating condition and design is design condition for flow meter.
Every flow meter should be corrected for pressure, temperature and molecular weight (if varying) where process factors vary or deviate from design parameters during normal operation.
Here are some instances:
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