The Good, the Bad and the Ugly of Magmeters

The following tip is from a new book by Greg McMillan and Hunter Vegas titled 101 Tips for a Successful Automation Career, inspired by the ISA Mentor Program. Today’s Tip #12 is by Hunter Vegas.

101 Tips for a Successful Automation CareerEarly in my career, I was asked to specify a meter to measure the sideboiler bottom flow of an ammonia vaporizer. The sideboiler boiled off the ammonia for the process, and over time water would build up in the bottom of the sideboiler and had to be drained off through the meter. I had Operations pull a sample, and the conductivity of the ammonia/water mixture was very high. Given the high conductivity and the need to measure low flows, I considered a magmeter (magnetic flowmeter) to be a good choice for the application. Two weeks after the meter was installed, Operations called in a panic saying that the meter was reading zero even though liquid was obviously pouring through it. A quick investigation showed I had failed to consider the fact that if too much water was drained off, pure ammonia would go through the meter. Ammonia has extremely low conductivity, and at that point, the meter could not function and read zero. Since then, I always ask for the conductivity of the fluid during normal AND upset conditions!

Concept: Similar to the vortex meter, a magmeter can be an excellent choice for a large variety of applications, but it too has limitations. Understanding these limitations can help avoid a misapplication of this technology.

Details: When a conductor moves through a magnetic field, it generates a voltage. The higher the velocity of the conductor (with the magnetic field strength held constant), the higher the voltage generated. A magmeter uses this phenomenon to measure flow. In this case, the fluid is the conductor, and it flows through a non-conductive line sized tube that has a magnetic field passing from top to bottom. The meter has a pair of small electrodes (one on either side of the tube), which detect the resulting voltage and calculate a fluid velocity. The velocity times the cross sectional area of the meter provides a volumetric flow rate. Some meters include an additional electrode at the top and bottom of the tube to detect whether or not the pipe is full. (A half full pipe will read high because the calculation assumes that the pipe is full.)

Here is a quick list of the pros and cons of this type of meter:

PROS:

1) The meter is a full bore device and has practically no pressure drop.

2) The only metal contacting the fluid is the electrodes, which are very small. Therefore, the meter can be used to measure strong acids and caustics. Even if the electrodes must be made of an exotic metal (platinum, tantalum, etc.) the additional cost is not great.

3) The meter has no problems measuring acids, caustics, or conductive liquids with entrained solids. It can also work well for viscous fluids.

4) The meter effectively averages the flow profile of the pipe so it does not require the long, straight upstream and downstream meter runs that orifice or vortex meters require. Two to three diameters upstream and downstream are usually all that is required.

5) Recent improvements in electronics have made 2-wire magmeters available. (Older models required a separate source of 120VAC power.) The 2-wire devices are cheaper to install.

6) Magmeters do not have a low flow cutoff problem and will generally read as low as 1 foot/second.

7) If a remote transmitter is used, magmeters can handle very high temperatures and pressures.

CONS:

1) The meter only works on conductive fluids and will read zero if the fluid has no or very low conductivity. Most require at least 5 micromho, though some units can measure below that.

2) Cheaper magmeters use a PTFE or PFA liner with no additional reinforcement. When these meters are “steamed out,” the tube can soften and if the pipe is then blocked in, the resulting vacuum in the line can collapse the liner and ruin the meter. A better meter uses a PTFE or PFA liner that has been reinforced by a wire mesh or frame to prevent this problem.

3) Ceramic magnet magmeters can easily handle higher temperatures and vacuum conditions; however, they can be prone to damage from thermal shock. If the fluid temperature is high, be sure to specify a remote mounted transmitter.

4) A magmeter will not work on a lined pipe unless ground rings are added between the flanges of the meter and the lined pipe. (These rings complete the circuit that allows the voltage to be generated.) Recognize that these rings will also touch the fluid and should utilize the proper material of construction.

5) Beware of tantalum electrodes (which are often used for strong acids). If these electrodes are exposed to air they will generate a non-conductive oxide coating which will keep the meter from operating immediately. Once they are again exposed to the acid, it will burn the coating off but this can take some time and the meter may not function at all during this time.

6) A magmeter measures volumetric flow—not mass flow. It can calculate a mass flow based on an assumed density but if the fluid density changes, the reading will be in error.

Watch-Outs: Beware of gravity flow measurements when using a magmeter. Unless the meter is properly located, partially empty pipe conditions will occur, and the meter may be inaccurate.

Exceptions: Always ask about the upset conditions that the meter might see. As mentioned previously, steam-outs can irreparably damage the meter and very low conductivity conditions can prevent the meter from reading at all.

Insight: Many magmeters employ a combination of AC and DC excitation on the coils to provide a means of detecting and compensating for coating of the sensing electrodes. While this may not make them impervious to coating conditions, it will allow the meter to continue to operate longer before a cleanout is required. 

Rule of Thumb: A magmeter is an excellent choice for measuring the flow rate of a conductive liquid with reasonable accuracy. This meter is also well suited for measuring the flow rate of viscous liquids, acids, caustics, and slurries. 

Look for another tip next Friday.

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