Dual ranges 2

[khan101]

Hi Forum,
I have a scenario. I have an equipment which can use dry feed and wet feed or both. When we use dry feed we use lots of water (400 l/s), when we use mixed feed or wet feed we use very less water (75 to 100 l/s). The flow is measured by an orifice plate (16" line size) and transmitter is calibrated for 0 to 500 l/s. The problem is, when we are on total dry feed transmitter is in 75% of the range which is desireable for good accuracy. But when we are on wet feed or mixed feed we are running it at around 75 to 100 l/s which is below 25% of the range and the accuracy goes to ...you know where. My question is, shall I install two flow meters? one for hi range and one for lower range or I can do something else? I do not want to change my orifice plate or modify my piping.
Thanks in advance for the responses.

 

[ScottyUK]

Khan,

Two transmitters is a common way to deal with the problem of very large ranges. If the signals from the transmitters go in to a DCS or PLC, it would be a trivial matter to confgure a transfer block to select the high or low range transmitter depending upon the measured value. Include some hysteresis in the switching threshold.





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If we learn from our mistakes,
I'm getting a great education!

 

[hydrae]

Khan101
Most types of flow meters (pitot, orfice, ultrasonic, propellor, turbine, disk) are accurate in a 10 to 1 turndown or less, the one exception is a Mag meter they are accurate at a 30 to 1 turndown. The accuracy drops when the flow is less than 1 ft/sec (0.3m/sec) on most models. the drawback is the cost (rough number $1000 per inch). Replacement to a mag meter may be an option in your installation.
Caution: mag meter do not work well with ultrapure waters (DI,RO)

Hydrae

 

[jsummerfield]

A square-root relationship between flow and pressure differential applies to all head type meters. Thus 70.7%flow is 50% dp, 50% flow is 25% dp, etc. Head type meter turndown improvement is because the transmitter error is less significant now at 0.05% accuracy than 30 years ago when most transmitters had 0.5% accuracy. The elements still may still have error from 0.5% to 2% of the full scale flow rate. At 10% design flow the reading is quite low on the scale and error is high. Do not plan better than 6:1 turndown from head type meters.

John