Application of Turbine Meters in Liquid Measurement Class # 2020.1

David Smith

Chevron Pipe Line Company 4800 Fournace Place

Bellaire,TX 77401 USA

Introduction Turbine Meters are used successfully and applied worldwide in the petroleum industry. Its compact size, rangeability, low cost of ownership, superior accuracy, wide temperature and pressure range makes it attractive for liquid hydrocarbon measurement. While there are many advantages there are also weaknesses of a turbine meter such as flow conditioning requirement, back pressure control, high viscosity liquids, and susceptibility to fouling and deposits. Turbine meters are often found measuring light crude oils, refined products (gasoline, diesel, jet fuel) and light hydrocarbons (LPG and NGL). This paper will discuss pipeline metering utilizing conventional Turbine flowmeters for liquid measurement.

History: The development of the modern turbine flowmeter can be traced back to David Potter, founder of Potter Aeronautical. Potter pioneered the development of the turbine flowmeter for the Navy in the 1940s. The initial application for the Potter turbine flowmeter was in fuel flow measurement for Navy aircraft. With the publication of API 2534 in 1970, the liquid turbine meter became a recognized meter for use in custody transfer of petroleum liquids such as light hydrocarbons, distillates, and light crude oil.

Construction The body, internal assembly, and pickoff assembly are three basic parts of a turbine meter. The turbine meter incorporates a bladed rotor installed in a flowtube or meter body. The rotor is suspended axially to the direction of flow through the meter body. The rotor will rotate on its axis in proportion to the rate of the flowing liquid while employing a means to convert the rotation output.

Figure 1. Rim Type Turbine Meter

Body

The body of a turbine meter is a simple flow tube or meter housing. The body can be constructed in different material depending on the application. For the petroleum industry, the most common is carbon steel. Typical body sizes can range from 1 to 24 in diameter with a flange rating between 150# to 2500# ANSI class. Depending on the material of the body or housing, temperatures can range from -20 F to 400 F and are classified as standard and high temperature.

Internal Assembly

The internal assembly consists of the rotor assembly, bearings, diffuser, and support fins. There are different options for the rotor assembly depending on the application. Most meters either have a bladed or rimmed rotor. The rotors are generally constructed from 304 or 316 stainless steel. A bladed rotor is normally offered up to 6 in size. For larger sizes, the rotor is fit with a rim. The rim provides additional rigidity for the blades and increases the pulse resolution by adding additional magnetic buttons or slots to the rotor. Another important design of the rimmed rotor was to assist with viscosity effect at the tip of the blade. Most turbine bearings and journals are tungsten carbide. The bearings are very hard and abrasion resistant creating a long life span in turbine meters. The diffuser support assembly or stator secures the internal in the body.

Pickoff Assembly

Translating the turbine rotation into a usable signal is generally accomplished using a preamp and pickoff. The angular velocity of the turbine rotor is taken through the turbine meter wall by means of the magnetic pickoff. The pickoff coil is constructed on many coils of fine wire wrapped around a permanent magnet. Turbine blades made of a paramagnetic material rotate past the pickup coil, generating irregular shaped voltage pulses. Each voltage pulse represents a discrete volume. The total number of pulses collected over a period of time represents the total volume metered.

The sinusoidal signal from the pickup coil has a low amplitude and normally reliable within 20 feet. This is optimal for totalizers or rate indicators that are mounted locally to the turbine meter. The turbine meter may be fitted with more than one pickup offering additional outputs as required.

The signal preamplifier conditions the output signal and boosts them to a level suitable for transmission to the flow computer. Because the signal degrades with distance due to impedance, transmission is normally up to 3000 feet. Some preamps provide an option on the selection of output whether power pulse output, variable voltage output, or open collector output.

Figure 2. Signal Output

The blade count and volume relationship results in what is known as the KFactor for the turbine meter. The frequency output from the preamplifier is pulses per second and can be calculated using the formula:

Frequency (pulses/second) = Kfactor (pulses/barrel) * Flowrate (barrels/hour) 3600 seconds/ hr

Principle of Operation Turbine meters are volumetric flow measuring and transmitting devices that produce output signals directly proportional to the rate-of-flow of the liquid being measured. The primary output is a high resolution signal that is amplified and shaped by an integral amplifier mounted directly on the meter. This wave pulse can be fed directly to flow computers, digital readout devices, or totalizing counters.

Turbine Rotation As the liquid product strikes the front edge of the blades, a low-pressure area is produced on the opposite side. The blades of the turbine rotor will tend to travel toward this low-pressure area as a result of the pressure differential across the blades. The pressure drop constitutes the energy expended to produce movement of the rotor. The initial tendency of the rotor is to travel downstream in the form of axial thrust, but since it is physically restrained from excessive downstream movement by the thrust washers, the only resulting movement is rotation. Fluid flowing through the meter imparts an angular velocity to the turbine blades, which is directly proportional to the linear velocity of the liquid. The degree of angular velocity or rpm of the rotor is determined by the angle of the blades to the flowing stream and the approached velocity.

Rotor Balance With axial thrust forcing the turbine rotor downstream, the friction resulting from contact between the rotor and the downstream thrust washer would result cause excessive wear if there were not some means of balancing the rotor on its axis between the upstream and downstream restraints. Making the hub of the rotor smaller in diameter than the diffuser changes the cross section area of the rotor. Since the rotor hub is smaller than the inlet diffuser, the velocity of the fluid decreases as it passes through the rotor. Bernoullis theorem states that when the flow velocity decreases, the static pressure increases. Therefore, a high-pressure area exists at the downstream side of the rotor exerting an upstream force on the rotor. As a result, the rotor is hydro-dynamically balanced on its axis and friction contact is reduced. Tungsten carbide thrust washers and contact surfaces effectively minimize wear when line shock or surges may temporarily overcome this hydrodynamic balance.

Figure 3. Turbine Meter Assembly

Performance Characteristics The accuracy of a turbine meter is derived from its output and is a measure of the deviation of an indicated measurement from the referenced standard. Accuracy is an indication of a meters ability to correctly measure product and is usually expressed as either percent registration or Meter Factor. Accuracy can be expressed at any given flow rate as: Meter Factor = Actual Qty Metered Qty Percent registration is simply the reciprocal of meter factor expressed as a percentage. Typical accuracy for turbine meters is +/-0.15% from 10% to 100% of maximum rate (10:1 flow range). For smaller meters generally less than 3, the standard linearity is +/-0.25%. Some manufactures offer a premium linearity option. Repeatability is the ability of the turbine meter to reproduce the same reading each time under constant operating flow conditions at any point over its specified range. Typical repeatability for turbine meters is +/-0.02%.

Figure 4. Typical Linearity and Repeatability

Application Considerations

Viscosity Turbine meters are viscosity sensitive. Viscosity is a measure of the liquids resistance to flow generally expressed in centistokes (cSt). Viscosity has an effect on rangeability of a turbine meter. As the viscosity increases, the turbine meters linearity begins to suffer. This effect on linearity is primarily due to a change in the fluids velocity profile and skin friction between the fluid and the rotor blades. In order to maintain the turbines linearity on viscous fluids, the Reynolds Number must be maintained. The equation for Reynolds number below indicates that an increase in kinematic viscosity (v), decreases the Reynolds number. Increasing the fluid velocity (V) or pipe inside diameter (d) will result with an increase in Reynolds Number. Re = Vd v Turbine meters perform best in turbulent flow conditions (Re= 4,000 and above) as opposed to laminar flow (less than 2,000). A high Reynolds number is one of the factors needed to maintain turbulent conditions. Specific Gravity Turbine meters are also affected by specific gravity. As the specific gravity decreases (generally below .66), the lift forces on the blades decrease. As the velocity decrease, the lift forces decrease as well. These reduced lift forces are overtaken by bearing friction as the low flow rates are approached explaining the linearity deterioration. As the specific gravity decreases, the corresponding decrease in differential pressure allows an increase in the maximum flow rate. This can be an advantage for light hydrocarbon liquids such as LPG. Most turbine meters will accommodate overspeeding on an intermittent duty cycle basis as per the manufacturers recommendation. Flow Conditioning The need for flow conditioning is driven from the sensitivity of the meter to deviations from as calibrated conditions of swirl and velocity profile. Installation effects such as insufficient straight pipe, exceptional pipe roughness, elbows, valves, tees, and reducers causes the flow conditions within the pipe to vary from the reference condition and affect the meters performance. As outlined in API 5.3.5, research has shown that a straight pipe of 20 meter-bore diameters upstream of the meter and 5 meter-bore diameters downstream of the meter may provide effective flow conditioning.

Figure 5. Straight Pipe Requirement

If space does not allow the recommended 20 pipe diameters upstream, properly designed straightening element or flow conditioner may be installed to assist flow conditioning by eliminating swirl.

Figure 6. Straightening Element Another option is to use a flow conditioning plate where space may be limited. This is a non-traditional approach but has been used successful in load rack applications. The plate is securely positioned in the inlet flange of the meter and assures proper entry of product in the turbine meter without the use of an upstream flow straightening section. It is manufactured in different material for different model meters and limited to smaller size meters such as 3 and 4.

Figure 7. Integral Flow Conditioning plate Back Pressure To prevent cavitation, it is recommended to have a minimum back pressure at the outlet of the meter. Cavitation is defined as a formation of vapor bubbles within a liquid at low pressure regions that occur in places where the liquid has been accelerated to high velocities. The formation of bubbles and their collapse as it passes over the blade surface can cause erratic behavior in the turbine meter and excessive wear due to overspeeding. The result is over-registration. The formula to calculate the minimum back pressure as defined in API 5.3.6 is sufficient to prevent cavitation as shown in the below formula.

For higher vapor pressure liquids such as LPG, it may be possible to reduce the coefficient of 1.25 to some other practical and operable margin.

Figure 7. Effects of Cavitation on Rotor Speed

Strainers Strainers are a critical piece of equipment used to protect the turbine meter from damage and debris. Strainers should be installed upstream of the turbine meter. Strainers use a perforated and/or wire mesh for straining purposes. Liquid Turbine Meters typically use a 1/8 diameter perforated support with 40 x 40 mesh liner. If installed, a differential pressure gauge can continuous indicate the pressure drop across the strainer. If critical pressure differentials are reached, appropriate actions can be taken before damage occurs to the meter. Troubleshooting Tips A systematic approach is the best approach when troubleshooting a problem. The best and simplest starting point in troubleshooting a turbine meter is with the pickoff and preamp since this does not require the line to be drained or meter removed from the line.

Pickoff: The pickoff produces a low level sinusoidal signal. The best method of troubleshooting is measuring the resistance across the pickoff coil. A handheld digital volt meter is required to read the ohms. If the pickoff is connected to a preamp, remove the dc power source then remove the pickup wires from the preamp. Pickoff coil resistance may range depending on the manufacturer so it is best to check with the turbine manufacturer for normal resistance range. If the pickoff reads open, it has failed and will need to be replaced. Pickoff exposed to water or flowing liquid that enters the electronic housing can cause the resistance of the pickoff to read in the high mega ohms and eventually fail open. Another option is using a portable oscilloscope to measure the signal output directly from the pickoff. The pickoff coil will produce a small mV peak-to-peak output in the form of a low level a-c sine wave. Caution should be used as disconnecting the pickoff from the preamp while measuring flow would result in lost counts to the receiving device. Consult the manufactures literature for the actual specification of resistance and mV output for the pickoff. Preamplifier: The preamp is powered by a dc input from an external source generally by the flow computer or preset. The pickoff coil(s) is connected directly to the preamp. The preamp produces 5V TTL output, supply voltage output, or open collector output depending on the required input level of the receiving device. The type of output may be configurable on the preamp. Preamp configuration may be different depending on the manufacturer. A digital volt meter or portable oscilloscope is the basic tool used in troubleshooting the preamp. First, verify the wiring connections to the preamp are solid then check the power at the preamp. If no voltage is present, the external source may have a blown fuse, defective power supply board or the source may be turned off. With the digital volt meter, the frequency output can be measured from the preamp. If the flow rate and k-factor from the meter are known, the frequency output from the preamp can be compared with the calculated value. Frequency (pulses/second) = Kfactor (pulses/barrel) * Flowrate (barrels/hour) 3600 seconds/ hr

Figure 8. Preamplifier Using the portable oscilloscope, the signal output can be measured from the preamp. The square wave output can be viewed for a clean signal. Noise or grounding issues can generally be spotted using an oscilloscope. If the turbine meter is equipped with dual pickup coils for pulse fidelity checking, the timing of the 90 degree phase shift can be measured. The oscilloscope may capture added or missing pulses that occur due to electrical transients or equipment failure.

Internal Assembly: The most time consuming part of troubleshooting the turbine meter is removing the meter body for inspection of the internal assembly. Most internals can be removed from the meter body and disassembled by basic hand tools such as crescent wrench, screw driver, needle nose pliers, and plastic mallet. Deposits and abrasives in the liquid can affect the internals resulting in poor performance. Dirty bearings can be cleaned using a suitable solvent and polished using crocus cloth type material. The face of the bearing should have a mirror finish after clean and polished. If the turbine meter is equipped with the ball or turcite bearing design, the bearings must be replaced if worn. Abrasive foreign material can scratch the bearing. Thrust washers and journal bearings are made of tungsten carbide which is extremely hard making it less resistant to wear. However, if grooves can be felt on the face of the bearing then it should be replaced. Missing, loose or bent blades will also affect the performance of the internal. Generally this type of damage is the result of foreign material, line shock, and cavitation. If the rotor blades have been slightly bent, the manufacturer may be able to straighten and salvage the rotor assembly. In the case of loose or missing blades, the rotor must be replaced. After any repair, the internal should be re-calibrated to verify its accuracy. Conclusion For over 40 years, turbine meters have been a popular choice in the Petroleum industry. Turbine meters have a proven history of superior measurement performance. While there may be a few drawbacks, if properly applied in the flow system, the turbine meter will perform to its full capability and continue to be an important tool in petroleum measurement. References Mechanical Engineering, Vol 105, No. 2, February 1983 API 5.3, fifth addition, September 2005 DAN-LIQ-Turbine-Meter-TG-0411