Medidor de turbina

MEDIDOR DE TURBINA

Measurement of fluids applies to steady-state mass flow (free of pressure pulsations, a stable mass flow) conditions for fluids that, for all practical purposes, are considered to be clean (free of particulates and rouge), single phase (free of liquids), homogeneous, and Newtonian under the operating conditions of the facility.

If necessary, slug catchers, separators, filters, or gas scrubbers should be installed to minimize the presence of solid particles and liquid condensate. Gas that contains significant quantities of liquids

or solids cannot conform to the measurement standards. Even small amounts of liquids or solids increase the uncertainty in flow measurement.

Solid particles also can have an abrasive effect on vulnerable areas such as turbine meter bearings, stator and rotor edges, and so forth. A turbine flowmeter (flowmeter body, stator, rotor, bearings, and electronic rotor speed sensor) is a primary device (Figure 7-1). A turbine flowmeter without a high-performance flow conditioner (+1 .OO%) is classified as an energy extractive, inferential flowmeter. A turbine flowmeter with a high-performance flow conditioner has a U,, of *0.25%. At this time, the turbine flowmeter in combination with a high-performance flow conditioner is widely adopted for gas and dense phase fluid easurement downstream of the gas processing plant.

Turbine flowmeters that exceed their flow or differential pressure limits experience excessive bearing wear, which causes the flowmeter to understate flow quantities.

Sources of Error

A turbine flowmeter has a high sensitivity to the following uncontrolled parameters in the field environment: swirl, velocity profile, and turbulence similar between calibrations; buildup or decay

(rouge, oil, liquid) on the flowmeter’s internal surface (transducers and body); and bearing wear.

Swirl, Velocity Profile, and Turbulence

An assumption for a turbine flowmeter is that the conditions at the time of calibration (swirl, velocity profile, and turbulence) do not differ over the custody transfer period. In reality, there are differences, and the amount of error depends on the turbine flowmeter to compensate for these changes without introducing a significant error (2.0% or more). A high-performance flow conditioner (HPFC) in combination with a turbine flowmeter can compensate for changes without introducing a significant error. Savant Measurement Corporation’s proprietary research indicates the error to be within the laboratory’s uncertainty of *0.25% or less.

Film Buildup and Decay

An assumption for a turbine flowmeter is that the internal film buildup or decay at the time of calibration does not differ over the custody transfer period. The sensitivity to a given internal film buildup or decay is a function of the turbine flowmeter’s internal diameter (ID), the area between the turbine blades, and the boundary layer development of the turbine blades.

Bearing Wear

An assumption for a turbine flowmeter is that the bearing condition (and lubrication system) at the time of calibration does not differ over the custody transfer period. Excessive lubrication of the bearing and bearing wear produces excessive drag on the rotor (increases the gas slippage across the flowmeter). As a result, the turbine flowmeter undermeasures the amount of fluid.

Differential Pressure (dP)

Accurate differential pressure (dP) measurement is essential for orifice flowmeter applications. Smart dP transmitters (Figure 10-1) are preferred, in light of their impact on the total mass flow measurement uncertainty to the fiscal measurement and their “low drift” under field conditions. “Dumb” or “semi-smart” dP transmitters have a lower accuracy (lugher uncertainty) and are unlikely to meet the uncertainty requirements. For orifice flowmeters, the minimum dP should be limited to 20 in. H,O at 60°F in light of turbulence in the flow. Although dP transmitters with lower limits exist, they should be avoided due to the inability to distinguish between the differential across the orifice plate versus the turbulence in the flow field. For dual- and single chamber orifice flowmeters, the maximum reading of dP should be limited to 250 in. H,O at 60°F in light of the seal ring limitations. To monitor the performance of turbine and rotary displacement flowmeters, dP sensors may be installed to indicate excessive drag on the flowmeter due to bearings or particulates (dP versus qav. Smart dP transmitters are equipped with a programmable analog or digital output signal that is transmitted to a tertiary device over an analog loop.

Analog Signal

For a smart dP transmitter programmed with an analog output, the accuracy is normally specified as a percentage of the analog span. Therefore, care is needed in the selection and ranging of the transmitter, since the uncertainty, as a proportion of output signal, increases as the output decreases. This uncertainty at low readings is of particular significance in the case of dP sensing flowmeters (orifice, venturi, and subsonic flow nozzle), owing to the squareroot relation between mass flow and differential pressure (dP). For orifice flowmeters using an analog signal to communicate with the tertiary device, the range of an individual dP should be limited to about 10: 1 on differential pressure. This is equivalent to a 3: 1 turndown on flow rate, due to the square-root function for differential pressure flowmeters. When necessary, it is common practice to use two smart dPs covering low and high differential pressure ranges to minimize the total mass flow measurement uncertainty (approximately

a 6: 1 turndown in mass flow rate). The flow computer usually carries out switching between dPs. Using two dPs in this way, each range limited to 4:l differential pressure, it is possible to achieve a lower uncertainty over a flow range of 8: 1.

Digital Signal

For a smart dP transmitter programmed with a digital output, the accuracy is normally specified as a percentage of reading. The digital output is a designated protocol (modbus, hart, foundation fieldbus,

and others) that allows the dP to digitally communicate to the tertiary device.

For orifice flowmeters, when using a digital signal to communicate with the tertiary device, the range of an individual dP should be limited to the maximum and minimum range of the dP sensor,

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