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Due to rising environmental concerns, reducing energy consumption is a priority for many building and campus operators. As is always the case, you cannot properly control a system without accurately measuring it first. In steam applications, it is well established that the mass flow measurement of steam is more accurate than volumetric when steam pressure fluctuates. However, what is less understood is the degree of error that is caused by seemingly small variations in pressure.

It is common for pressure to fluctuate in process steam lines due to varying loads, pressure deadbands in the boiler control system and process changes. Since the density of low pressure steam (up to about 100 psig) varies greatly with just a small change in pressure, using a non-pressure compensated flowmeter can result in a significant cumulative error.

The precise way to calculate the flow error rate in a steam system with fluctuating pressure is with the following simple equation: E = [(calibrated p / actual p) – 1] x 100%; where E equals the error in % of actual flow, calibrated p equals the steam density at calibrated pressure and actual p equals the steam density at actual pressure (steam densities can be found in the steam tables for the corresponding pressures). However, the quick “rule of thumb” for velocity based flowmeters is that for every 1% change from the calibrated pressure results in an additional introduced error of 1%.

This means that a volumetric meter that is calibrated to a system pressure of 100 psi would experience an error of approximately 10% if the actual pressure is 90 psi. The result in this case would be for every 1000 lbs. of actual steam flow, the meter would measure 1100 lbs. – a whopping 100 lbs. of additional steam. Over a short period of time this error will compound, resulting in a huge difference between the measured and actual flows and skewing the system’s reported energy consumption. Fortunately this introduced error can be corrected by using a flow meter with pressure and/or temperature compensation technology.

Sierra Instrument’s Multiparameter Mass Vortex Flowmeter provides a perfect solution for measuring accurate steam flow under fluctuating pressure conditions. With its built in pressure and temperature sensors, only one pipe penetration is required for pressure and temperature compensated flow measurement, lowering installation costs. The electronics will calculate mass flow and display mass and volumetric flow rate, density, pressure, temperature and total flow. Also it can provide (3) 4-20 mA analog outputs that can be assigned to the variables. It also has Modbus and HART capabilities. The 240 model has an inline flanged body from 1/2” to 8” and the 241 is an insertion style for pipes 2” and greater.

For assistance on your next steam flow measurement application, please contact Jobe and Company at 855-805-2599 or sales@jobeandcompany.com for a more accurate and cost saving solution.