Measuring Air Flow in Power Plants
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Measuring air flow is easy. Measuring air flow accurately can be very difficult. The trend to improve efficiency in today’s power plant environment drives the need for effective boiler trim controls which, in turn, requires accurate air flow measurement. An air flow measurement that is not repeatable, accurate, or representative of mass flow can destabilize the airflow control loop. Here are the primary causes of inaccurate air flow measurements: • • • • • • Improper selection of measurement technology Blockage of the flow element (due to plugged pitots or duct collapse) Inadequate straight runs of duct upstream and downstream of the measurement Collapse, or blockage of flow straightening devices (honeycomb, etc.) Change in duct size due to heat or pressure Measurement uncompensated by duct tem‐ perature and static pressure Why choose pitot measurement technology? Pitot tubes are a time‐tested method of measuring airflow in ducts. Pitot technology is cost‐effective, cally balanced against the mass (and BTU) of fuel. easily maintained, and applicable to a variety of duct configurations. A significant advantage of In rare cases where duct pressure and air pitot tubes over venturi, orifice plates, and airfoils temperature change minimally, uncompensated is the minimal unrecovered pressure drop across air velocity measurement can be adequately used the flow element. Use of pitot tubes can represent to infer mass air flow and in trimming the boiler. cost‐savings associated with reduction in fan Conventional pitot tubes are single‐port measure‐ ment devices. Duct‐mounted pitot tubes often horsepower requirements. have multiple ports, which average the velocity over a number of ports, providing a more representative sample of duct velocity in a single plane. Several multi‐port pitots can be arranged in an array to increase the number of sample locations and improve accuracy. In most cases, pitot tubes have one port facing into the air stream (high), and one port at 90° to the air stream (low). The differential pressure VAP3® Pitots between this “high” and “low” port represents the Pitot tubes, like other differential pressure‐based velocity, as noted above. The port facing the air technologies, measure the velocity of air in a duct. stream (impact port) is vulnerable to plugging The velocity, or speed, of the air is often confused from airborne particulate. Some manufacturers with the mass, or amount, of air moving through a offer purge systems that periodically blow out the duct. The combustion process is a “mass‐based” ports to prevent interference with pitot measure‐ relationship. The mass of air must be stochiometri‐ ment. 416 Landmark Drive Wilmington, NC 28412 www.easterninstruments.com Phone:(910) 392‐2490 Fax: (910) 392‐2123

Purge systems are costly, add complexity to the measurement, and are prone to plugging over time as hydration of airborne particulate occurs inside the flow element. A notable exception to the plugging problem is pitot technology from Eastern Instruments. The VAP3®/PA pitot places its “high” port inline with airflow, which means it has no impact ports and is not subject to plugging. Typically no purge system is required. often used to reduce the straight‐run require‐ ments for the measurement. These devices can be ...

A notable exception ‐ the High Beta® Flow Conditioner from Eastern Instruments. The High Beta® Flow Conditioner, which integrates the flow‐straightening vanes with the VAP3®/PA pitots into a duct section, is easily installed, adds structural integrity to the duct section, and is not susceptible to breaking or plugging. Eastern Instruments CF Dynamics Model demonstrating cyclonic flow in round duct, and straightened (recovered) flow through High Beta Instruments Ducts always expand and contract. As duct High Beta® Flow Conditioner ...

Comparative Attributes of Various Flow Straighteners a® et B gh Hi er en ht ig ra St b s om ne yc Va ne g in Ho en ht ig ra St Flow Straightening Selection: Comparible Attributes: Ability to Eliminate Swirl Adequately Straightens Flow Ease of Installation Integral Flow Measurement Resistant to Breakage Resistant to Plugging Fair Fair Poor No Fair Good Good Good Fair No Poor Poor Good Good Good Yes Great Good Comparative Attributes of Various Air Flow Conditioning and Measurement Devices VA P 3 an St ®/ Hi gh B et a® Low Yes 9 4 No No Yes Yes No 95B No No e High No 7 4 Yes No No...

Calculating Mass Air Flow Using Differential Pressure The DPS calculates mass flow for absolute flow measurement As with all differential pressure‐based measure‐ ments, velocity signals must be compensated for air temperature and duct static pressure. Temperature compensation is accomplished through direct measurement of air temperature (Thermocouple or RTD), wired to a transmitter and then to the DCS or to a compensating trans‐ mitter. Pressure compensation is accomplished through T‐ing the “low” side measurement to an absolute ...

High Beta® Flow Conditioner: Shown Installed in a Duct W FLO ION AIR ECT DIR EXISTING DUCT SECTION SHOWN TRANSPARENT AND CUT AWAY FOR CLARITY DAMPER INLET SECTION (304 STAINLESS STEEL) HIGH AND LOW IMPULSE TUBING (304 STAINLESS STEEL MOUNT PLATE (CARBON STEEL) TAG MANIFOLD BLOCK 1/2” NPT CUSTOMER CONNECTIONS (6061 ALUMINUM) THROAT SECTION (304 STAINLESS STEEL) 416 Landmark Drive Wilmington, NC 28412 VAP3®/PA WITH STIFFENERS PITOT NODE ASSEMBLY www.easterninstruments.com Phone:(910) 392‐2490 Fax: (910) 392‐2123