'Turndown ratio' is a flow measurement term that indicates the range a specific
flow meter, or meter type, is able to measure with acceptable accuracy. It is also known as rangeability. It is important when choosing a flow meter technology for a specific application. If a gas flow to be measured is expected to vary between 100,000 m³ per day and 1,000,000 m³ per day, the specific application has a turndown ratio of at 10:1. Therefore the meter requires a turndown ratio of at least 10:1. For example: if the meter had an advertised maximum flow of 2,000,000 m³ per day then the required turndown ratio would be 20:1.
The turndown ratio of each type of meter is limited by theoretical considerations and by practical considerations. For example, orifice meters create a pressure drop in the measured fluid proportional to the square of the velocity. Therefore the range of differential pressure can become too large and compromise accuracy. It can also create process problems such as hydrate formation, and in the case of measuring the discharge of a compressor, there is a limit to how much pressure loss is acceptable.
Turndown ratio is one of a number of considerations in choosing a meter type for an application. Some other considerations are price, maintenance cost, accuracy, the fluid type and the velocity of the flowing fluid.
Typical turndown ratio of various meter types
The examples here are for gas flow, but the same meter types can be used on liquids as well, with similar turndown ratios. Note that meter manufacturers state their products' turndown ratios -- a specific product may have a turndown ratio that varies from the list below.
orifice plate meterhas a practical turndown ratio of 3:1. This means that if an orifice meter with a design flow rate of 200,000 m³ per day is installed, the flow range that the meter can measure accurately will be between 100,000 m³ per day and 300,000 m³ per day.
turbine meterhas a turndown ratio of 10:1. In the above application, the flow range that can be measured will widen to 60,000 m³ per day on the low side and 600,000 m³ per day on the high side. Rotary displacement metershave a rangeability of between 10:1 and 80:1, depending on the manufacturer and the application. Diaphragm metersare considered to have a turndown ratio of 80:1.
Multipath ultrasonic meters often have a stated turndown ratio of 50:1. In our example application, the flow range expands to 28,300 m³ per day on the low side and 1,414,000 m³ per day on the high side.
from sahli ruzaini & fyrd_ost..
ASME MFC-1M-1991 Glossary of Terms Used in the Measurement of Fluid Flow in Pipes (reaffirmed 1997) defines rangeability as "the ratio of the maximum flow to the minimum flow of a meter. Accuracy tolerance limits must be specified" (paragraph 3.8). Turndown is nowhere to be found in MFC-1M, but I vaguely remember reading (an ASME document?) where turndown and rangeablitiy were terms that could be used interchangeably.
The definition of rangeability in the on-line ISA Comprehensive Dictionary of Measurement and Control effectively agrees with the ASME definition above. The definition for turndown is the "ratio of maximum plant design for flow rate to the minimum plant design flow rate".
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