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FEATURE MONITORING & METERING Choosing the right flowmeter for steam


When it comes to metering, steam has many different properties to other fluids. So it’s important to procure flowmeters specifically made for steam from a company with specialist knowledge of the steam and condensate loop. Andy Cowan of Spirax Sarco explains the pitfalls of using general metering technologies in a steam environment and how using steam flowmeters helps businesses accrue the greatest financial, energy-saving and productivity benefits


W


hile most organisations wouldn’t think twice about the value of metering


electricity consumption or gas use, relatively few invest in flowmeters to find out how much steam they use. It’s a lost opportunity because a significant proportion of energy expenditure is used to generate steam. Therefore, unless its use is measured, it can be hard to justify energy-saving investments and, equally importantly, to prove they work after they have been implemented. Accurately measuring steam consumption requires meters made for use with steam. Selecting the wrong type of flowmeter can produce inaccurate measurements and lead to costly maintenance. Unfortunately the symptoms of poorly performing meters are hard to spot, often not being detected until the meter is factory calibrated. By then, potentially costly errors could have accumulated unnoticed, leading to excessive fuel consumption or under- charging for steam supplied as a utility. It’s vital that your steam metering is accurate and reliable because the more accurate the metering, the better the control over consumption. The boiler house is where significant energy savings can be made by eliminating wasteful steam consumption.


STEAM METERING CONSIDERATIONS So, what must be taken into consideration when metering steam? Firstly, pipework has a relatively large bore to limit the steam velocity to around 35ms-1 to reduce erosion and avoid water hammer. This can reduce the accuracy and turndown of many flowmeters that only achieve their optimum performance at higher velocities. For example, vortex meters are typically based on velocities of up to 75ms-1. It’s important to specify a meter suitable for slow flows and with a turndown to meet changing demand without needing to reduce the pipe bore, which can increase the velocity of the steam to potentially dangerous levels.


30 WINTER 2015 | ENERGY MANAGEMENT


Figure 1: Accurately measuring steam consumption requires meters made for use with steam. Selecting the wrong type of flowmeter can produce inaccurate


measurements and costly maintenance


Figure 2: Different flowmeter types need longer or shorter unobstructed, straight pipe lengths upstream and downstream for accurate readings


Some meters can also be affected by entrained water droplets in saturated steam. For example, turbine meters have multiple moving parts that are easily eroded. Similarly, the sharp edges in an orifice plate meter or the precise shape of the bluff body in a vortex meter can be eroded. This can lead to a slow drift in accuracy that is unlikely to be detected even by in-situ calibration. Meters for use with steam are best designed with smooth internal surfaces that reduce turbulence and erosion.


STEAM CONDITIONING MAY BE NEEDED Wet steam also affects the mass and energy of the steam, which can lead to significant errors of reading, regardless of the measurement principles used. It is important to remove as much of the entrained water in the steam as possible by installing conditioning equipment such as separators and traps. Many installed steam systems have been


modified over the years to suit changing needs. This can result in pipework that follows tortuous routes making it difficult to find the necessary straight runs of pipe required to allow a turbulent-free flow into the meter for accurate results. It is important therefore, to select a meter that can fit within the upstream and downstream straight pipe runs available. The length of unobstructed pipework before and after a meter is usually expressed in terms of the number of pipe diameters of pipeline needed and varies from meter to meter. Very often, greater energy efficiency,


achieved as a result of installing metering, can help recoup the cost of installation within just two years. However, with such a large choice of flowmeters and many variables to consider, it is easy to choose the wrong meter so it makes sense to speak with a company which has specialist knowledge of the steam and condensate loop. If in doubt contact a trusted steam specialist, such as Spirax Sarco, for more information. Spirax Sarco recently welcomed Dame Janet Trotter Lord-Lieutenant of Gloucester to their Runnings Road Technology Centre. Dame Janet Trotter presented a rose bowl on behalf of Her Majesty the Queen to recognise the company’s recent grant of a Queen’s Award for Enterprise in Innovation. The Award is the UK’s highest accolade for business success for design and manufacture.


Spirax Sarco www.spiraxsarco.com/uk 01242 521361


/ ENERGYMANAGEMENT


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