How to avoid danger

Plant managers and engineers need to be more careful when specifying flow control meters, writes Martyn Rowlands of Durapipe UK. Choosing the wrong unit, he says, could be a very costly mistake.

The accuracy of flow measurement systems is crucial in the water and waste treatment industry.

Inaccurate flow measurement units can cause drinking water to be polluted, add to the breakdown of a pipework system, or generate high levels of leakage. This is caused by meters being installed, which cannot measure to a precise enough accuracy. And it is caused by the wrong equipment being specified.

Plant managers and engineers need to give greater consideration to the type of flow control meters they are specifying, and ensure they are selecting the right unit for the application.

Chemical dosing is an application that relies on accurate flow measurement data. If chemicals are being used to treat water, it only takes the meter to dispense too much of the chemical for it to pollute the water.

This is why it is extremely important that flow control solutions are specified that can control flow down to one tenth of a millilitre.

This absence of reliable flow data can have significant commercial, environmental and safety consequences. Inaccurate flow measurement can pollute the material that is passing through the system. And unreliable flow data can result in inefficient plant operation and unnecessary investment in plant upgrades. Despite this, it can not always be given the attention that is required.

The attention put on flow control has increased slightly in recent years. The demand for more accurate flow control measurement has been greater over the last decade due to stricter regulation of the water and waste treatment industry by Ofwat.

The drought in 1995 drastically changed standards and guidelines within the water industry. Flow control was put on the agenda, and there has been a realisation that there is a need for flow control solutions. But not everyone in the industry is aware of the different options available. And it is imperative the correct system is chosen for the application to ensure the efficient running of the plant.

Specifying the wrong product could be a costly operation, and could potentially result in legal action and fines been issued against water authorities.

The main considerations that need to be taken on board are the type of media that is being put through the pipe and how accurate the measurement needs to be, as different systems work to different measurements.

The following is a guide detailing the main types of flow control meters and sensors, and which applications they are best suited to.

Mechanical flow meters
The most popular mechanical meter is operated by a paddlewheel flow sensor, which is designed for use with solid-free liquids.

This system incorporates a paddle, which is caused to rotate by the velocity of the fluid passing through it.

This type of meter can only support clear fluids to work efficiently, as slurries or abrasive liquids will cause the paddle to clog up.

Electromagnetic flow meters
These are the most frequently specified meters as they are suitable for use in a wide range of applications, this unit has no mechanical parts so it can measure liquids where suspended solids are present as long as they are conductive and homogeneous.

This type of meter uses Faraday's law of magnetic induction as the basis of its operation. So, it needs to be used in conjunction with plastic pipework because it is important that the magnetic field is not disturbed.

For each type of flow meter, there are also three varieties
of flow sensors available, dependent on the requirements of the operator. these are detailed below.

Battery-powered flow meter
These sensors can calculate current flow rate and total flow volume that has passed through the system without the need for an external power source.

This sensor is typically used within installations where remote access is required, without direct power supply.

Flow monitor and transmitter
In addition to measuring current and total flow rates, this sensor can also convert the signal from the flow sensor into a 4-20mA signal for long-distance transmission.

It also has the capability of setting relay points so that if a flow drops below or rises above a specific level, an alarm is activated, providing piece of mind that someone will always be immediately alerted if this occurs so
that it can be dealt with quickly.

Batch controller
These accurately batch or blend liquids, which is particularly important in chemical dosing applications.

This sensor allows only the set volume of liquid to be dispatched, reducing the risk of poor water quality.

The importance of accurate flow control measurement cannot be underestimated.
To ensure the safe and efficient running of plants involved in water and waste treatment, accurate flow control measurement is key to the process.

And plant managers and engineers need to start considering the systems they put in place if they are to meet targets and avoid incurring heavy costs.

Martyn Rowlands is head of marketing at Durapipe UK.
T: 01543 279909.

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