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In the flow

Even with the right flow measurement equipment, there may be no way to avoid ‘installation effects’, writes Louisa Hearn

Whether it is for tracking upstream oil flow or monitoring rogue environmental emissions, accuracy is the watchword when it comes to selecting flow-metering equipment.

Because flow measurement is used for both gases and liquids, there is an abundance of different devices on the market – some highly specialised and others created for more general use.

Their common objective is to allow companies to monitor and record the most important flow data in their processes, with minimal day-to-day intervention.

Even so, there is no guarantee any device will escape so-called ‘installation effects,’ says Craig Marshall, flow measurement consultant at NEL (formerly known as the National Engineering Laboratory).

“These are to do with the nature of fluid flowing through a pipe. Because the middle is a hollow cylinder, where the fluid meets the wall you have frictional forces,” says Marshall.

“A flow meter only stands a chance of living up to the manufacturer’s accuracy and uncertainty claims if it is installed correctly.”

The primary objectives for deploying a flow meter are to track fiscal transactions (for custody transfer in the energy or water industries, or for government taxation), environmental monitoring for regulatory purposes, or process optimisation and safety.

To satisfy each of these, there are four main categories of flow-metre devices, says Marshall.

At the top end are Class 1 devices offering the absolute accuracy required for custody transfer applications. Down at the lower end of the spectrum are class 4 devices for servicing less critical operations such as flare measurement for waste gas disposal.

But regardless of its ranking, any number of physical limitations can undermine device accuracy, says Marshall. This is especially so in areas where space is at a premium, such as an offshore oil or gas site. (See box, page xx)

“For a successful measurement, flow profile must be symmetrical at the

NEL’s flow measurement facilities

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centre line. The issue we see is that to achieve a fully developed flow profile, you need a lot of straight pipe - but in reality we don’t have that,” he says.

“So when fluid flows through a bend, the mechanics in its flow profile change too because it tends to flow faster on the outside of a bend.”

Some devices are designed for a single application and others for more general use - and each of these will have their own individual response to installation effects, says Marshall.

McCrometer is a US company that supplies of flow instrumentation equipment to the water industry globally. Its flagship product is a differential pressure flow meter called the V-cone.

“Our core business is the measurement of water, so you’ll find our products anywhere where water is being used,” says McCrometer V-cone product manager, Nick Voss.

The V-cone is used for custody transfer in the water industry, which Voss describes as a relatively mature sector which is facing tighter environmental regulations amid falling government funding.

As a result, he says water treatment plants in both the US and elsewhere are being forced tighten their focus on exactly where their water is going.

“Many water companies are having to add more metering points throughout their networks of water distribution systems to measure usage,” he says.

He says the V-cone has unique advantages over orifice plates, because it is almost completely insensitive to upstream or downstream piping disturbances.

“This is useful in the process industry where steam is used quite a lot,” says Voss.

“So instead of needing long strait runs of pipe you can collapse all that down and put the V-cone down to straighten out the velocity profile.”

He adds that this is especially useful in newly constructed sites where every extra couple of meters of pipe counts.

Davidson Water in the US had been using Venturi flow meters to measure water leaving its plant, but needed to boost accuracy to assess the loss of non-revenue water – which had left the plant but remained unaccounted for in terms of delivery and billing.

“We had to start with an accurate reading of the finished water

that was leaving the plant, in gallons-per-minute and total gallons per month,” says Dale Draughn, Davidson’s cross-connection control coordinator.

The McCrometer team recommended implementation of its electromagnetic Full Profile Insertion (FPI) Mag flow meter, a solution that Davidson has now rolled out.

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control & instrumentation

A flow meter only stands a chance of living up to the manufacturer’s accuracy and uncertainty claims if it is installed correctly

Davidson Water wanted to improve the accuracy of its flow measurements to assess the loss of non-revenue water

CALIBRATION

Installation effects

Installation effects such as an insufficient length of straight pipe, can cause flow conditions to vary from laboratory conditions. However these effects can be dramatically reduced if you just give them a little consideration, says Craig Marshall, flow measurement consultant at NEL.

The first line of attack, says Marshall, is to alter flow profile at the point of measurement using either of the following methods.

A flow conditioner – This is a plate with holes in it. When it is placed into the pipe, it destroys the existing flow profile and creates a new flow profile. This serves to shorten the length of flow that would normally be required for an accurate flow meter reading.

A flow straightener – An example is a tube bundle, which is a series of smaller pipes joined together to try to straighten out flow.

The alternative to changing flow profile is to calibrate the meter in situ within process conditions, using a pipe prover or a master meter to verify its accuracy.

Because of the logistical issues around removing pipe work and transferring it to a calibration facility, Marshall says most high-end users would be more likely to settle on a mix of calibration and flow conditioning.

“Those are the traditional methods to try to eliminate installation effects but some meters [such as ultrasonic meters] now offer their own secondary diagnostics for flow,” says Marshall.

Building on this, NEL is researching new ways to develop the relationship between diagnostics and the mechanism or performance shift of the meter. The goal is to produce powerful tools to make truly smart meters, says Marshall.

“This is a 10 to 15 year-project that will involve a lot of research, development and interaction with industry,” he says.

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PROCESS ENGINEERING

“Davidson Water has already been able to decrease non-revenue water loss from 16.7% to 13%, and there is a similar story across other process sectors as companies seek to do a better job of accounting for everything,” says Voss.

McCrometer has also recently released a product called FPIX.

“It is one step further in the evolution of magnetic flow meter technology,” adds Voss.

“It measures multiple points in the water flow itself in two-dimensional, cross-sectional space. This gives a complete measurement within actual flow itself, which is brand new.”

Environmental considerations are also emerging as a key driver in the flow measurement industry - a trend that has been noted by Fluid Components International (FCI), a manufacturer of thermal-dispersion based gas-flow meters.

The company’s key markets include oil and gas, power and energy, and chemical plants and waste water treatment.

A new raft of environmental considerations demand continuous emissions monitoring systems to meet what FCI marketing director Randy Brown describes as an “alphabet soup” of regulations.

“[Gas] flares are always measured for leak detection, and environmental legislation also wants to know what is being burned.”

Brown says the other key trend influencing these sectors is the drive to improve operational productivity and efficiency.

“They want to measure the gas flow in their process because it will ensure the process will run more efficiently, or for less cost,” he says.

An enabling technology that is helping them achieve this, says Brown, is the increased deployment of digital communication bus (DCB) technology.

“Instead of traditional analogue transmission, emerging DCB technologies such as Foundation Fieldbus, Profibus or Hart offer benefits you can’t get with a simple analogue signal,” he says.

“If I can measure every instrument, I know which device has to be recalibrated and can send an engineer to the right place.”

Trevor Foster, managing director of Titan Enterprises, says when choosing a flow meter, it is important to weigh up strengths and weaknesses of each technology prior to implementation.

Titan Enterprises has a broad customer base ranging from food and drink additives to adhesive dosing, medical and agricultural industries.

“Our customers are increasingly applying our ultrasonic meters which satisfy low-flow applications for pilot plant and general small bore flow situations where no moving parts are required or desired,” says Foster.

He says the aim is to develop its Atrato ultrasonic technology into new product areas.

“Currently low flow meters are either quite inaccurate or very expensive,” he says.

“Our patented ultrasound technology should cross these barriers and create relatively low cost and accurate meters.”

control & instrumentation

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Water companies are adding more metering points throughout their networks of water distribution systems to measure usage

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JUNE 2014