Understanding the relationship between pressure and flow rate is fundamental to the design and operation of oil flowmeter systems. In any fluid system, the flow rate through a pipe is driven by a pressure difference between two points—the higher the pressure difference, the higher the flow rate for a given pipe resistance. Flowmeters themselves introduce a pressure drop (differential pressure) as the oil passes through their measuring elements, and this pressure drop must be within the capacity of the pumping system. Excessive pressure drop across a flowmeter can reduce system flow rate, cause cavitation, or exceed the pressure rating of downstream components.
The pressure drop across a flowmeter increases with flow rate—typically as the square of flow velocity for turbine and vortex types, and more linearly for positive displacement types. At maximum rated flow, the pressure drop can range from a fraction of a bar for large turbine meters to several bars for small, high-viscosity PD meters. The flowmeter pressure drop specification must be reviewed in the context of the system's available differential pressure. If the available pressure is marginal, selecting a meter with a lower pressure drop (e.g., an ultrasonic or Coriolis meter in a larger pipe size) may be necessary to ensure adequate flow performance.
The operating pressure at the meter also affects measurement accuracy. For volumetric meters, high pressure compresses the oil slightly, increasing its density and reducing its volume. At very high pressures, this compressibility effect may need to be accounted for in custody transfer calculations using a pressure correction factor (Fp). Temperature and pressure transmitters installed alongside the flowmeter provide the data needed for these corrections. Ensuring that the flowmeter is always operated within its rated pressure limits is also an important safety consideration—over-pressurizing a flowmeter can cause seal failure, housing rupture, or damage to measuring elements, potentially creating a dangerous oil release.