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.
For example, orifice meters create a pressure drop in the measured fluid proportional to the square of the velocity.
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.
The examples are here for gas flow, but the same meter types can be used on liquids as well, with similar turndown ratios.
Conversely, in applications where the operational conditions are not expected to vary significantly (for example, a large power plant), a low turndown ratio will be sufficient.
If the heating plant is only working at a small fraction of its maximum and the turndown ratio is too low, at some point the burner will still need to be shut off when the desired pressure/temperature is achieved.
[1] This is undesirable, as flue gases are purged during both the shut-down and start-up phases, leading to energy losses and therefore inefficiency.
Additionally, typical startup times for boilers are in the order of one to two minutes, leading to an inability to respond to sudden load demands.
[3] Small domestic "vaporising" (i.e. burning kerosene or 28 second oil) burners do not modulate at all and are relatively inefficient.
However, due to problem of mixing the oil and air, turndown ratios of greater than four are uncommon.