Why we all need flow measurement

Without flow, the water industry quite literally would not exist. Gigalitres of water and waste water are abstracted, treated, delivered, consumed, collected, treated and discharged every day of the week. How we manage this water has a very large impact on the way we live and on the environment that we live in. To misquote the old adage, in order to manage the water that flows through the water industry, we have to measure it

Measurement

Why we all need flow measurement
A typical flume and level based measurement device

The majority of this article will talk about measuring wastewater but, at its very essence, the principles are the same, although the wastewater side of things comes with its own complications. In the main we as an industry measure wastewater flows using two techniques. The first is in open channels using a primary device such as a flume or a weir. This regulates the velocity flowing through the device which causes the level to rise. This level change is used to measure the flow using the Universal Flow Equation where flow is equal to velocity multiplied by the area. The second method is using electromagnetic flow meters in closed pipes, where the area is fixed and the part of the equation that you measure is the velocity. These two technologies typically comprise about 90-95 per cent of all in-line measurement techniques.

However there are alternatives and, although relatively rare outside flow surveys in the wastewater network, these methods are still in use. The first of these is Time of Flight flow measurement which, like the electro-magnetic flow meter, measures the velocity of water in a closed pipe. The second is the area velocity device which measures both elements of the Universal Flow Equation by measuring both the velocity and the area. Typically, this device is used in an open channel, but more recently technological developments have seen this typically submerged method come out of the water and use non-contact methods.

Problems

The technology exists in the wastewater industry to measure flow in virtually any process application but there are problems and the skill in measuring flow is picking the right flowmeter for the right job. It’s rare that a flow meter (or any instrument) is installed in an installation that is absolutely perfect for the job and compromises normally have to be made and normally this is where measurement usually starts to go wrong. When installing any instrument the following should be considered:

Why – What is being measured and why, is there a purpose for the measurement. If the answer is no then stop and don’t install an instrument where it isn’t needed.

What – A flow meter, like any instrument, should be selected for the application. Anyone who says that their meter can be installed anywhere is wrong. Looking at the application and seeing what type of flow meter fits is key bearing in mind any interferences in channels or pipes. Mostly the errors that these interferences produce can be mitigated if thought through and incorporated into design in advance.

Where – This is going to depend upon what the use of the flow data is going to be. Is it for compliance, is it for operation of the main flows of the treatment works or is it for controlling an individual process. This will of course affect the application and type of flow meter.

How – The answer here is listen to your supplier. They will advise the best method to install a flow meter. Also, take into account how a meter is going to be installed, how it is going to be operationally maintained and also how it is going to be replaced.

Thinking of these three points in the installation of any flow meter or instrument means that it will actually work rather than being squeezed into an application which will not function, either  in terms of the application or sometimes of the operation. This eventually results in the meter failing to work correctly and trust in the reliability of the measurement failing.

Top five reasons for incorrect flow measurement

  •  Fouling – Be it an open channel or a closed pipe, all installations are subject to fouling from plants, trees and other vegetation or to overdosing of chemicals that can narrow the bore on pipes.
  • Physical damage to the measurement structure – More applicable to open channels, but with age, flumes start to peel away from the concrete or weir plates start to bend. The only option is replacement.
  • Poor installation – Open channels need to be relatively calm, flat and free discharging. In civil engineering, a tolerance of 10-20mm is normally considered as flat. In flow measurement, this tolerance reduces to 1-2mm.
  • Turbulence – Closed pipe meters usually need to be flooded and situated away from turbulence or disturbance. If not, this will affect measurement accuracy.
  • Telemetry errors – If a meter is being routed back to telemetry with a 4-20mA loop make sure the scales agree at both ends. Normally a no-brainer but often the biggest source of error

Example

Why we all need flow measurement
A V-Notch Weir in poor condition

If it is not accessible for cleaning, a simple V-notch weir system does not get cleaned by operational staff and plant life will grown on it. The plant life causes an obstruction in the measurement device, causing the level to rise and the actual measurement to read high. The operator does not believe the reading because it is “too high”; the root cause is, of course, the plant life in the weir. When it is removed, the flows return to “normal”, as long as damage has not been caused. This is a downward spiral that the wastewater industry had fallen into for quite some time before the advent of the MCERTS scheme, which brought standards to flow measurement within the wastewater industry. The standards and associated management systems that need to be in place have brought a standard to wastewater flow that has resulted in an increased reliability in flow measurement.

Compliance

When flow measurement is correct, it has many uses; when it is wrong, it can be counter-productive. The first use of flow measurement under the MCERTS Scheme is to measure the compliance of the various treatment works with their permit conditions in terms of dry weather flow compliance. However, this is extending in scope to cover flow in full treatment conditions.

There are some technical difficulties with this approach, primarily with the fact that most flow installations are on the effluent from treatment works, but this is not insurmountable. However, the uses of flow measurement are significantly more than just to measure compliance.

Example

 Compliance relies on accurate flow measurement. If your flow measurement is not accurate, it can appear that a site is non-compliant when it really is, or it could indicate growth or infiltration where they do not exist.

  • In operation and control – Flow measurement is a fundamental part of treatment works, both on the main treatment flow and on many sub-processes. On large works, for example, it is often used to control storm water management, return and recirculation flows, and settlement tank desludging.
  • Designing treatment works relies on correct flow measurement being recorded for many years to obtain a picture of site performance and to ascertain whether an existing works needs to be upgraded.

There are many more uses for flow measurement and this article cannot go into them all, but suffice it to say, the measurement of flow is one of the fundamental bases of both the engineering design and the operation of the water and wastewater industries  as long as it is installed and operated in a way that allows the data that is produced to be relied upon. The consequences of getting it wrong are to put the industry in the situation where errors in measurement, or the fact that they are perceived as untrustworthy, cost more than installing the technology correctly in the first place.

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