Magnetic Flow meter installation in Chlor-Alkali process applications | Automation.com

Magnetic Flow meter installation in Chlor-Alkali process applications

Magnetic Flow meter installation in Chlor-Alkali process applications

By Neelesh Shah, Westlake Chemicals

The operating principle of the magnetic flowmeter is based on Faraday's law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field. 

Image taken from processautomatic.com

Faraday's Law: u = kBvd

The magnitude of the induced voltage u is directly proportional to the velocity of the conductor v, conductor width d, and the strength of the magnetic field B. The transmitter supplies the controlled current to the coils to generate the magnetic field, and amplifies, filters, and converts the resulting signal to user outputs such as 4 - 20 mA, frequency, or digital communication information (HART, Fieldbus).

In typical magmeter application, the grounding of the meter is very important and critical because the generated induced voltage signal is actually quite small. Steps must be taken to minimize noise and to maximize noise rejection. One of these steps is proper grounding of the system. However in  electrolytic application    the flowmeter must never be grounded. Not only flowmeter but also electrolyzers, accessing platform, structure around electrolyzers, piping supports and accessories in feed and outlet pipes remain ungrounded (commonly known as floating ground).                                                              

Electrolytic application

In an electrolytic application, a large direct current (DC) is fed to the electrolyzers typically in multiple of 1000 amperes with high voltage. The magmeters are installed in the liquid circuits feeding to the electrolyzers i.e. feed caustic and feed brine. The sample picture shown here includes one set of electrolyzer and flowmeters in Chlor-Alkali application. Practically there could be multiple electrolyzers and flowmeters in the configured arrangement and different electrolyte application other than Chlor-Alkali.

Whatever the arrangement and application would be, the large voltages and currents present may cause currents to flow in unexpected ways. The current flows of interest here are generally of two types:

• Current flow in the fluid through the magmeter

• Current flow through grounding components

Both types of current may be present in a typical application of these types. In the first case, the flow of current in the fluid passing through the flowtube generates noise that may interfere with the low-level flow signal. Usually this noise varies with current level and has components that can easily interfere with the flow signal. The result is usually not an inaccurate flow measurement, but rather an unstable flow measurement that can make control difficult or impossible. In this situation, ground rings provide a path to shunt the current around the fluid in the magmeter.

Current flow through grounding components may occur if:

1. Multiple magmeters are used in a system

2. They are at different potentials and

3. The grounding components for multiple magmeters are tied to a common point. The most frequent common point may be through the grounded green wire. Situations like this have resulted in high corrosion of grounding components, up to and including the loss of sealing around ground electrodes. In addition, current through grounding components generates noise that can result in an unstable magmeter output.

                                                TYPICAL ELCTROLYZER APPLICATION

 
   

TYPICAL ELECTROLYTIC FLOWMETER APPLICATION

Solution   

The following recommendations must be taken into consideration      

 AC power version( 120V AC): Use the AC power version of the flowmeter. Use a power isolator to eliminate any power interference.

Unground Transmitter (Floating ground): The magmeter should be connected electrically only to the process, because the magmeter is not earth grounded.

Use remote mount transmitter and flowtube version

Grounding rings and connection

Use ground rings. Do not use ground electrodes. Ground rings expose more surface area to the process. If currents flows, less noise will be produced, providing a more stable reference for the amplifier.

Disable the Empty Pipe Detector. An empty pipe is detected by making a low frequency impedance measurement.  These applications can generate high levels of low frequency noise that can cause sporadic false Empty Pipe indications.

Reference

E+H flowmeter Promag 55S installation manual Rosemount technical manual 00830-2400-4727, Rev AA, Allen Bradley power isolator 4983-DC

About the author:

Neelesh( Neel) Shah is presently with Westlake chemicals as a senior instrumentation engineer. He has strong background and skill set for design engineering in process industry. He has designed and commissioned several Chlor alkali plants all around the world.

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