Experience in South African and international industrial environments has shown that significant savings (in excess of 20%) can be obtained in the energy conscious area of utilities consumption, this covers water, air, gas, electricity and steam, collectively referred to as WAGES.
Regardless of the energy management concept used, the key ingredient is the proper measurement of these media at the points of supply (compressors, boilers and water & gas incoming lines) and also at the major points of consumption (factory feeds and other points of usage). The adage ‘if you do not measure you cannot control’ is absolute in the context of energy reduction schemes. So it is important that accurate flow metering is implemented at all points to measure correctly over the full plant operating range and take into account the practical realities of the installations (eg, pipe work) and flow media conditions (eg, cleanliness, quality, composition).
This article focuses on some of the practical issues of choosing the correct flowmeter and why so often users are disappointed in the final metering solution.
Choosing the correct flowmeter for the measuring point
Table 1 gives an example of some of the common choices open to the user when selecting a flowmeter for any particular measuring requirement.
Table 1 also illustrates the initial problem of meter type selection, namely “which meter is the correct one for my application?” The point here is that despite there being a wide range of possible flowmeters for any one metering requirement, the achievable performance of each varies considerably depending on the operating conditions they are required to work under. Each meter type has considerably different measuring ranges, sensitivity to different process conditions, installation and media quality, not to mention cost implications and ease of installation and usage.
Before the meter type can be considered, the first step of selection must be to spell out the application requirement in terms of performance and the conditions under which the meter must operate. This must be done for every measurement point in the system, and in some cases, you may end up with different meter types on the same flow media at different measuring points. An energy management system normally involves reconciling supply with consumption over the factory layout, so accuracy is a requirement in all but the smallest of installations.
The next step to be considered is the operating conditions of the metering locations:
1. The required flow range, the common mistake here is to neglect the minimum flow rate. All flowmeters have a minimum flow rate at which the meter accuracy falls away, or even worse, the meter stops measuring altogether. If you have selected and sized the flowmeter only considering the nominal and maximum flow rate values, there is a risk that if there are times when the flows drop below the minimum threshold for that particular chosen meter then you will have errors when reconciling your consumptions.
2. A very important step, often misunderstood, is the selection of the flow unit. In a consumption application such as steam, gas, water or compressed air, the flow unit of choice must be a mass flow unit rather than a volumetric unit. An extremely common choice is what is referred to as a ‘normalised’ ‘standardised’ or ‘corrected’ unit. This is expressed as a volume unit, but the media process conditions have been mathematically adjusted inside the flowmeter or externally in a computer to a common set of ‘reference’ pressure and temperature conditions. This is acceptable so long as the reference conditions are the same for all the flowmeters in the measuring system. The point to understand is that 1 Nm3 (normalised m³) of air referenced to 0°C and 1013,25 mbara is always numerically the same as 1,293 kg, so it is actually a mass flow unit and as such meets the requirement. Confusing the m³ volumetric unit with Nm³ is one of the most common reasons for errors in a measuring system.
3. The quality of the measured medium (gas dryness, solids content, gas bubbles in liquid, cleanliness). The quality of the flow media can have a huge effect on the performance and reliability of the selected meter type. The most common problems in the utilities measurements are water drop out (steam and compressed air), oil breakout and dirt particles (compressors). For instance, a vortex meter will work under very wet and dirty conditions. The dirt collected over the bluff body has no effect but a relatively small amount of dirt on the sensing paddle will affect or potentially even knock out the measurement.
4. Process conditions: if you are using a volumetric flowmeter such as a vortex, positive displacement, turbine or a differential pressure orifice or Pitot tube, then a knowledge of the minimum to maximum pressure and temperature ranges needs to be known to be able to size the flowmeter in terms of mass flow rather than the range in volumetric units.
For a table of today’s common flowmeter choices, their strengths and suitability vs the application gases and liquids most commonly found in the utilities department, visit http://instrumentation.co.za/+C17125
Meter installation considerations
Having chosen the best flowmeter type and size based on an assessment of the application requirements, flow ranges and process conditions and quality, we should be in a position to have a reasonable expectation of the metering performance, but, regardless of this initial work, the next most common reason for non-performance is the quality of the physical pipework installation.
The starting point is to understand one of the realities of calibration, namely that all flowmeters are normally calibrated under ideal installation and process conditions. All meters calibrated on a rig enjoy the ‘benefits’ of a perfect pipework installation and controlled pressures and temperatures, not to mention the guarantee of being set up correctly. Different flowmeter types for both gases and liquids will be affected to different degrees when some of those perfect conditions are not presented in the application.
In the real world of the installation, there is unfortunately often a considerable gap between the calibration and the operating conditions. As soon as you add practicalities such as pipe diameter changes, bends, valves or filters, then you start to distort the flow conditions inside the pipe that are presented to the meter.
It is not adequate on a plant schematic just to specify a point on the schematic as a metering point due to the process requirements. Often meters are installed to suit the convenience of the user rather than considering what the meter itself requires. A working example could be a compressor house output, you will want to measure the flow output of the compressor or compressor bank but there could be a huge difference in performance if you mount the meter before or after the receiver, before or after the dryer, before or after the manifolds etc. The manual will have specific instructions and guidelines as to the need for straight pipe before and after the meter point relative to pipework bends, diameter changes and most critically upstream valve locations. Also, the meter inlet diameter that matches the installation pipework may be critical – for example for a thermal flow cell meter this is critical, but it can be easily compensated in a vortex meter. For steam look out for low points that could trap water and do not mount a meter there, no matter how convenient the location may be for the operator.
The use of flow straighteners and conditioners to avoid swirl
The danger of simply relying on these types of devices to remove the effects of poor installation is that whilst these devices can work, they are not a guarantee of success and they come at a cost in terms of extra introduced pressure drop. Also they have installation requirements themselves in terms of upstream and downstream straight pipe requirements, so they do not entirely remove the need for clear pipe work.
A far better approach is to try to minimise or preferably avoid swirl in the first place. In an example reproduced on a flow rig, it was shown that 16 diameters of upstream clear pipe work was as effective as a perforated plate conditioner, the point being that the flow conditioner installation required 10 diameters of pipe work upstream of the flowmeter anyway, and it introduced a significant pressure drop. The 16 pipe diameter alternative produced no discernible pressure drop and did not have any extra installation cost. In this case less was more.
I will close with a list of some installation bloopers I collected over the years: the valve installed upstream of the flow meter; inadequate upstream pipe work; mismatched flanges; slipped gasket; unsealed cable gland; the process temperature compensation sensor not fitted in the pipework and the meter pipework bypass configuration missing a rather necessary isolation valve.
By avoiding these pitfalls and sticking to the flow measurement guidelines discussed in this article, you will go a long way towards the effectiveness of any energy reduction campaign based on WAGES measurement.
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