Variable area (VA) flow meters will respond like the canary in the mine when the air quality changes. If process conditions change there usually is an impact on flow rate. For example if back pressure changes on gas flows the float/flow will change just as changes in liquid viscosity will have a similar impact to the float/flow.
VA meters reacting to changes in process conditions can be a good or bad thing based on a user's viewpoint, which brings us back to the real question. So why do we need so much information to specify the proper VA meter? I will go through the questions and explain why it is necessary.
The information needed is:
- Normal operating temperature & pressure
- Maximum operating temperature & pressure
- Flow rate – minimum, maximum, normal
- Reference conditions and volumetric flow type, if a gas
- Fluid: Density & viscosity
1. Normal operating temperature & pressure
What are the normal operating temperature and the maximum operating temperature? When the fluid is a gas or some liquids it is absolutely necessary to know the operating temperature because of the impact to fluid conditions (density and viscosity). The maximum temperature is necessary because many times the operating temperature is much lower than the maximum temperature. A great example is selecting a plastic meter based on the operating temperature but then realizing the maximum temperature far exceeds the capability of the meter.
2. Maximum operating temperature & pressure
What are the normal operating pressure and the maximum operating pressure? Again when the fluid is a gas it is absolutely necessary to know the operating pressure because of the impact to fluid conditions (density and viscosity). I have seen applications where the maximum operating pressure was so high that we didn't have a suitable meter even though the normal operating pressure was quite low.
The next post
will dig into how important flow rates and reference conditions are in specifying a variable area flow meter.
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