Time of flight is a principle that aids these flow meters.
TOM MICHALOWSKI | Panametrics
When someone thinks of steam, heating and cleaning are the most common applications. Heating could take the form of warming a house or building to being a part of a large district heating system in which steam consumption is a significant concern due to billing and energy rebates. For cleaning, steam is consumed for sterilization or deep cleaning in the pharmaceutical, food and beverage and other industries.
In the chemical and petrochemical industry, steam is commonly used for heat exchanging and as a reactant for steam cracking. Heat exchangers use steam to transfer energy to increase or maintain the optimal temperature of another fluid. In steam cracking, steam is introduced with a feedstock and sent to a cracking furnace to break the feedstock molecules into more valuable components. With these process steam flow measurements, it is critical to optimize performance and not waste energy or money from the generation of steam.
Unfortunately, measuring steam flow is not easy. Steam has the challenge of operating at high temperatures and across a wide pressure range, from close to ambient pressures to significant high pressures, based on final use cases and requiring little pressure drop to not waste energy. Steam users also desire a relatively high turndown or flow measurement range to understand energy usage during peak and low demand needs.
When it comes to measuring steam, ultrasonic technology may provide advantages over traditional technologies. For example, ultrasonic flow meters are known to have a high turndown ratio, which means users can measure steam during seasons when consumption is typically low. A single ultrasonic flow meter can cover a wide range of flows, providing users with additional savings. Using a single ultrasonic flow meter to cover the full range results in lower capital and installation costs.
In addition to a high turndown, ultrasonic meters provide no pressure drop. When measuring steam flow rate, pressure drop caused by orifice or vortex shedding meters robs energy from the steam, reducing the amount of power and heat delivered to the user. However, using a flow meter with two ultrasonic transducers that do not protrude into the flow stream is more effective as transducer installation causes no pressure drop and reduces steam generation costs.
Ultrasonic meters minimize the long-term cost of ownership. They have no moving parts to wear out or collect debris and require no regular maintenance or calibration. Titanium transducers are not affected by erosion from condensate droplets and will not fail due to thermal expansion cycles. Considerable costs are involved in maintaining, retesting and recalibrating other traditional flow technologies. As end users are increasingly looking to technology to optimize operations and reduce cost, these are the ultrasonic solution has risen in popularity.
Compact ultrasonic transducers are either installed in a flow cell (spool piece) or directly in the steam pipe, one upstream of the other with mounting nozzles. Transit-time ultrasonic flow meters take advantage of a simple principle called “time of flight,” as illustrated in Image 1.
Specifically, the time it takes for an ultrasonic signal to travel against the flow (i.e., upstream), tup, is longer than that it takes following the flow (i.e., downstream), tdn. The difference between upstream and downstream traveling times, Δt, is directly proportional to the flow velocity as seen in Equation 1.
Where V is the flow velocity to be measured, P is the ultrasonic path length, and Θ is the acute angle between the ultrasonic path and the axis of the flow cell or pipe section. In Equation 2, volumetric flow, Q, is then calculated by multiplying the velocity of the fluid, V, by the cross-sectional area of the conduit, A, and a meter factor, K, which depends on the interrogation path and the flow profile.
Q =K x V x A
In Equation 3, mass flow, M, is further derived through the density of the fluid, ρ.
M =ρ x K x V x A
In the case of steam, steam density, ρ, can be readily computed using a steam table if temperature, pressure and steam quality are known. Equation 1 shows that the operation of an ultrasonic flow meter strongly depends on the timing of tup, tdn, and the dimensional measurement of path length P and angle 0. In addition, it is shown that flow velocity measurement is independent of the medium flowing inside the pipe.
The transducers send and receive ultrasonic pulses through the steam. The meter measures the difference between the upstream and downstream transit times and uses digital signal processing to calculate velocity and volumetric flow rate. The mass flow is then calculated from temperature and pressure inputs and built-in steam tables.
Steam flow measurements will continue to be critical in chemical and petrochemical industries as users continue to minimize energy costs and optimize process performance. Ultrasonic flow meters will remain a measurement of choice due to their accuracy, reliability, wide range, no pressure drop and no regular maintenance.