Accuracy of venturi flow meters can be impacted by corrosion and internal build-up.Venturi flow meters, which have no moving parts, can function reliably and accurately for decades, with some examples in service for more than 100 years. Furthermore, Venturis are independently proven to perform at ± 0.25% accuracy if laboratory flow calibrated, and ±0.5% to ± 1.0% in the field. This is based on bench calibration and dependent on the type of Venturi meter, with modified short form devices at ± 0.5% and classical type devices in the range of ± 1.0%.
Their elegant mechanical design is so fundamentally sound that individual meters can outlast even the most durable material components. However, the design and materials used in Venturi meters have not remained static over the last century, and although the fundamental principles have remained the same, some important updates have made today's technology even more accurate and reliable than past versions.
Materials and design elements which have become obsolete can be responsible for performance degradation of some meters, adding to the age related issues of general wear, for example:
- Annular pressure-averaging rings or chambers, found in classical long and short form models, are prone to sediment accumulation or galvanic corrosion, unlike modern, static-tap designs typical of today's modified short form Venturis;
- Bronze components can oxidize and wear away over time, and also react with dissimilar metals like iron or steel to cause galvanic corrosion;
- Old internal and external protective coatings can erode, especially underground.
The lack of observation ports on older flow primaries also can make it difficult for hydraulic engineers to determine if there is any significant change to the metering cross-sections of the Venturi. This can impinge upon system function, particularly within the inlet high pressure and throat low pressure portions of the meter.
Venturi Rehabilitation
One of the virtues of Venturi flow primaries is their durability and a simple design that lends itself to rehabilitation. When carried out by experts, rehabilitation can return a flow primary to its original functioning parameters and accuracy at a fraction of the cost of replacement, often within only a few days.
Venturi meters can be rehabilitated in the field or in a factory, depending upon the situation and size of the meter. Flanged insert-style meters and meters made of uncommon materials can be restored in most cases. As with any quality meter installation, Venturi suppliers conducting rehabilitation services develop technical specifications to protect their customers and guarantee both quality and scope of service.
