Fundamentals of Gas Turbine Meters

(A review of the design and operation of gas turbine meters)

Class # 1170.1 - 2014

Philip A Lawrence. Lead Engineer (Measurement) ENABLE™ Midstream Partners

Oklahoma City, Oklahoma, USA. Introduction

The Turbine Flow Meter is a velocity measuring device that has been used for many years in natural gas service. This paper will focus on the basic theory, operating principles, performance characteristics and installation requirements used in natural gas turbine meter applications. A discussion of fundamental turbine meter terminology is also included in the paper. The Turbine Meter is a volumetric integrated device which means it does not measure the flow rate directly but infers it from the speed of rotation of a centrally mounted multi-bladed turbine rotor. It comprises of a bladed rotor either flat or helical multi-bladed, sometimes helical twin bladed designs are used usually for liquid applications and only as per the requirement of the fluid viscosity being measured.

The turbine rotor assembly sits on a shaft with sleeve or ball bearings or on out-rigged shafts and sleeves, usually fabricated in stainless steel or sometimes tungsten carbide when used for liquid applications only, which then allow it to spin freely in a parallel axis to the to the direction of the fluid flow.

The design for a liquid turbine meter differs than that of a gas turbine meter the liquid type of meter usually has a smaller central hub because the extraction of torque is higher than a gas meter per diameter because the fluid is more dense. Natural Gas turbine meters have a large central hub or diffuser which is used to increase the velocity at the rotor extremities and thus improve the mass flow through the meter around the diffuser /rotor. This is to increase the Kinetic Energy through the meter rotor this is the driving force behind a turbine meters operation and relies on both velocity and density.(see equation 1.0). Some liquid application designs have floating type rotors, and some do not.

K Factor

The speed of rotation is generally directly proportional to the fluid velocity passing the blades and the device and is usually calibrated to pre-determine the relationship between a known volume per unit time passing the meter to a pulse count. Generally this is obtained by using an electro-magnetic pick up coil situated in the body of the meter. This speed of rotation for each blade that passes past the pick-up coil or datum point results in one cycle of output , which is usually defined in actual non-corrected units. ie. ACF/ unit time (actual cubic feet)

When a known and defined start point has passed through 360 degrees, the number of pulses can be integrated to represent a discreet volume of flowing fluid.

The counting of the blades passing the pick-up is a measure of the total cycles which are proportional to total volume flow. The counting of the each of the blades per a time period is a measure of frequency in cycles per second (Hz), which is proportional to flow rate.(Curve 1) Real World Effects (friction in the bearings geometric shifts due to manufacturing tolerances) can cause some skewing of the curve from the ideal.

At any point on this curve the meter K factor (K) as it is called can be derived by dividing the frequency (f), by the flow-rate (Q) sometimes this may also be called a Rotor Factor,(Figure 1.0 and Equation 2.0).

Figure 1.0 - Typical “K” Factor Curve.

From this information a linearity curve is derived which indicates the meters acceptability for use. The range-ability of liquid turbine meters is normally 10-1 but this nominal value can vary with meter size, viscosity/quality, pressure and density of the gas ( see Figure 2.0) K f Figure 2.0 - Linearity Curve Viscous and Mechanical Drag The dark line parts of the curve shown in Figures 1- K factor & 2-Linearity represents the idealized performance. This performance would be possible only if turbulent (fully developed) flow exists at the rotor, and all retarding forces (friction) were eliminated. The broken line portion of the curves (1 & 2) represents the actual performance, with the segments A) , B), and C, relating to the velocity profile conditions shown in figure 3 – (A) ,4 - (B), & 5 - (C). Fig 3 - A Fig 4 -B Fig 5 -C Figure 5 -C shows a typical Turbulent Flow Velocity profile 10,000ReD. or greater on Gasses, Fig 4 -B shows the Transitional Region between turbulent and laminar, this region can cause certain issues, one being the possibility of erratic measurement (Usually around 8-10,000ReD. on gas and 4000ReD. on liquids) Fig 3-A shows the Laminar Range, (4-8000ReD on Gas) which are normally associated with viscous or low velocity flows the retarding viscous drag forces bearing attributed to friction, drag and low k.e. which will cause degradation in linearity and K factor. Larger diameter meters and devices with a large blade area (helical meters-liquids) are less sensitive to liquid viscous effects than small bore devices. The pick-up usually operate by a variable reluctance principle however some meters use magnets directly in the blade structure and pick up coils. Some devices use a newer principle of measuring shaft rotation directly by hall effect transistors and shaft magnets which are more precise , but may have reliability issues The Turbine Meter also may have a built in temperature measurement, in certain cases attached directly into the meter body.

C B A

Tolerable Linearity usually; +/- 1/2 - 1% A – 80% flow rate +/- 1% - 1�/� % B -20% flow rate +/- Out of range - C

Ideal

Real World -----

Meter Designs

It can be seen that increasing either the flow rate (velocity) or pressure (mass) of the gas will also increase the meters turbine rotor kinetic energy. The meter design motive should be to allow the maximum amount of driving energy that can be extracted whilst minimizing the amount of wasted kinetic energy. This need is usually achieved by using light designed rotors and instrument precision ball bearings, smooth surfaces etc.

The photograph below Figure 6.0a shows a cross-sectional view of a top entry style turbine meter. This provides a diagrammatic explanation of the basic parts used in the meters operation.

The flowing gas impacts the angular blades of the rotor or impeller immediately after leaving the annulus channel created by the nose cone and the pipe wall, as shown in item #10 the gas is accelerated by the diffuser /cone (Figure 6.0 a &b) at the periphery of the blades offering maximum velocity and driving force.

Figure 6.0a - Exploded View of a Typical Top Entry Gas Turbine Meter.

The turbine rotor assembly shown figure 7.0 and Item #9 above, usually feature a blade angle of either 30°or 45°, depending upon the manufacturers model and the desired capacity/flow rate. A blade shaped with a lesser angle results in a larger capacity rating because of the angle at which the flowing gas strikes the rotor blade.

An example of this at identical flow rates means that a rotor with a blade angle at 30° will operate at a lower rpm than its 45° counterpart. Therefore the blade angle at 30° can be operated at a considerably higher flow rate with a longer bearing or service life in the photograph (Figure 7.0) the 30°rotor is (left) and the 45° rotor (right). The rotor blade with a steeper angle is generally used where a low flow-rate per diameter is needed.

Figure 7.0 - Turbine Rotors Both rotors can be mounted on one shaft on some designs to facilitate real time proving using comparison of the rotors output this will be mentioned later in the paper. The turbine meter rotor is normally mounted on a stainless steel shaft (Item #6) usually supported by two lubricated instrument grade ball bearings to reduce friction. Note that an external lubrication system for this turbine is shown as Item #4 this is fitted since non sealed bearings are used on this part. The horizontal drive shaft in turn is connected by a bevel or 90 degree gear system to a vertical counter drive shaft (item #8).

Figure 6.0b - Alternate Diffuser

This shaft is also supported by two bearings, however, these bearings are usually of a sealed design and generally do not require any lubrication. Then the vertical shaft is linked to a magnetic coupling (Item #3). This magnetic coupling is unique in that it allows the turbine meter to overcome a basic but difficult problem – the transference of a rotating motion from the pressurized region to the non-pressurized (exterior) area.

The gear train (Item #2) converts the shaft motion created by the turning rotor into a more useful format. Generally the desired mechanical output is displayed in standard incremented units such as 1, 10, 100 or 1000 cubic

feet per revolution (0.1, 1 or 10 ��per revolution in S.I. units), so allowing it to easily convert to a manually read index, or an inductive pulsing device or volume corrector. The link provided between the gear train and totalizing device is commonly referred to as an index plate or instrument mounting plate (Item #1).

One other function of the drive gear train is to provide a solid base to which the timing gears or change gears may be mounted. These gears allow the turbine’s performance to be calibrated shifted or changed in a linear fashion by changing them out with different gear wheel types. The use of

these calibration gears are especially critical during the initial manufacturers calibration procedure and should be re-established after any activity that may affect the accuracy of the meter.

A large variety of low frequency and high frequency electrical pulse outputs are also offered for the turbine meters found in industry today. These electrical outputs may be found in both the pressurized and non-pressurized areas of the turbine meter, from pick up coils to read metal-bladed devices to inductive or hall-effect magnetic sensors or sometimes reed switches. End-entry turbines, another popular option that have been used require the removal of the meter body from the pipeline to fully access the measuring module or cartridge

Meter Performance

AGA 7 lists the minimum performance guideline that a natural gas turbine meter should be able to achieve today. Turbine meters can generally supersede this requirement. The requirement states that the top 80 % of the meters range should meet a specific accuracy criteria, and the low 20% or transition flow rate. The chart below shows the AGA 7 accuracy requirement at atmospheric pressure.

Figure 8.0 AGA 7 and ISO 9951 Performance Statements

Proving of Gas Turbine Meters

All gas turbine meters for fiscal duty or custody transfer of natural gas must be proved and calibrated. A reference proving system or master meter can be used to determine an absolute or true volume. The minimum accuracy of a turbine meter is usually application specific and generally accepted to be +/- 1.0% in the North American oil and gas industry. Pressure and temperature correction is also needed to derive a standardized volumetric output. This performance can often be enhanced when the meter is calibrated under more defined conditions, such as elevated pressures or calibrated over specific flow rates. A turbines maximum rated capacity (Qmax) is normally defined as the maximum flow rate that a particular model can manage without sacrifice to the longevity of the meter. The ASME MFC-1M-1991 definition for accuracy:

Rangeability

Qmax (maximum rated flow)is usually stamped on the meter and also represents a certain rotor speed or rpm that should not be exceeded for extended periods of time. At elevated pressures, the maximum capacity in SCFH (Standard Cubic Feet per Hour) increases directly as the density changes due to this pressure thus impacting the kinetic energy.

The minimum rated capacity (Qmin) for a turbine is generally considered to be the lowest flow rate at which the Meter can still maintain an accuracy of +/- 2.0% according to AGA3 / ISO 9951. A decrease in kinetic energy attributed to low flow velocities and also at low pressure can cause the frictional forces to act upon the turbine causing it to under-register.

However, this loss of flow rate capacity is partially compensated for by an increase in-line pressure. Therefore, at elevated pressures the minimum capacity in SCFH increases directly as does the square root of Boyle’s Law pressure multiplier. For this reason, the term Qmin, when expressed in ACFH, can improve at elevated operating pressures.

The rangeability or turndown of a turbine meter is simply denoted as Qmax/ Q min for that model. This value is a representation of the entire operating range of flow rates at which the turbine will maintain a specified performance accuracy. The rangeability of all turbine meters improve at elevated pressure due to the increase in gas density as mentioned before there is also improvement in the rated Qmin.

A turbine’s rating is usually listed at atmospheric conditions for reasons of comparison and may have the possibility to exceed 200:1 at high pressure applications an example say at 15 psig the turndown may be 15-1 and at 1400 Psig the same meter could exhibit a range of 140-1 on the same gas product. Repeatability Repeatability refers to a meter's ability to duplicate measurement results when multiple tests are performed at similar conditions. Many turbine manufacturers supply repeatability information for both the mechanical and electronic outputs of that model. The ASME MFC -1M-1991 definition is:

Repeatability data should represent the meter only, and all errors or uncertainties associated with the calibration system are usually to be ignored. The physical condition of a turbine meter and it’s in field use/application plays a critical role when evaluating repeatability over an extended period of time.

The pressure loss in a pipeline caused by a turbine meter, also referred to as the pressure drop across the meter, is attributed to the energy expended to compensate for frictional and driving forces. Frictional forces include fluid skin friction, while driving force is necessary to operate the mechanics of the meter. Pressure drop is measured between a point directly upstream of the meter and one immediately downstream. Selection of a Meter Turbine manufacturers generally provide pressure loss information at both atmospheric and elevated pressure conditions. The rating is usually listed at Qmax for that model under those conditions. Important also is the module/cartridge interchangeability for a turbine meter and how it represents the change in performance accuracy

seen when installing specific modules or cartridges into an infinite number of bodies. This factor is an important consideration for field upgrades when determining the need for a complete meter versus simply changing the measuring cartridge. In addition, the interchangeability factor should be examined when deciding to remove the complete meter or only remove the cartridge from the field for repair, service, and re-calibration.

Module cartridge interchangeability fidelity for modern turbine meters generally range between +/- 0.1% to +/- 0.5% depending upon the model, vintage and technology.

Figure 8b - Cartridge Dual Rotor Turbine Meter

Approximately 35 years ago the dual rotor turbine meter was introduced into the industry. This model features a secondary rotor used to provide full or partial compensation for measurement inaccuracies attributed to the first or primary rotor. In general terms, the first rotor still maintains a mechanical output used to drive a direct reading index, volume corrector or pulse emitting device (pick up coils-Figure 9b). This mechanical volume is similar to that of any single rotor turbine in that it performs no correction for measurement errors, such as non-uniform flow profiles or typical meter problems (bearing wear, mechanical friction, component damage, contamination, etc.).

The two rotors are mounted upon separate shafts, which in turn are supported by instrument grade bearings. The turbine meter lubrication system supplies oil to both sets of shaft bearings during each lubrication procedure. Figure 9.0 shows the secondary rotor that is located immediately downstream of the primary rotor. Please note that the blade angle of the secondary rotor is much smaller that the primary main rotor.

Figure 9.0a-Dual Rotor Design Turbine Meter.

Thus the secondary rotor is rotating at approximately one tenth of the main rotor’s speed for the same gas flow rate. Also the secondary rotor is driven by the exit angle of the gas as it leaves the trailing edge of the primary rotor. The speed relationship between these two rotors will allow the secondary rotor to provide an adjustment to the volume measured by the first rotor. This compensation is performed through calculations done by preset algorithms unique to the manufacturer and meter model used.

9.0 b Reluctance and Inductance Pick up Coils

In addition to this self-correcting feature, the dual rotor turbine also provides a type of “in-line” calibration. This is done by examining any change in the rpm relationship between the two rotors, the technician can compare the adjusted output to that of the original factory calibration. Since the original factory calibration was performed under ideal flowing conditions thus creating a baseline analysis for all future checkpoints/proves. This concept has become a useful tool in both troubleshooting these meters as well as the surrounding piping configuration.

The adjusted output and calibration features produced by the turbine are a product of the electronic signals emitted by both rotors via either inductive or reluctance pick up coils. A pre-programmed flow computer is needed to manage the data from these devices.

Installation and Operation

The up and downstream piping configuration has an effect on most inferential measurement devices. As a velocity / inferential meter gas turbines are no exception. The American Gas Association has provided the optimum or recommended installation in Report No. 7. This arrangement consists of ten pipe diameters of straight pipe immediately upstream of the meter and five pipe diameters downstream Figure 10.0 next shows the correct orientation for a standard set-up.

Figure 10.0 – Piping Installation Diagram

A second piping set-up is listed in the AGA 7 standard, known as the “short or close coupled installation” methodology and is often used when there are space limits the use of a standard installation, (Figure 11.0)

11.0 Close Coupled Installation

The use of an integral flow conditioner is recommended for these types of installation described within the standard.

Field Maintenance The use of the spin test methodology to check a turbine meters fidelity and also by using a planned maintenance scheme incorporating the correct lubrication for these types of turbine meters a long and accurate life for the meter can be enabled. Over lubrication is to be avoided as it can cause the oil to be deposited on the blades and ruin the calibration and change the K factor. Generally the manufacturer will indicate what is best for the meter in question if the manufacturer is on his game, and should be providing the necessary information in the service manual for the device. Typical Spin Test Protocol

Figure 12.0 Typical Spin-Down Time Curve Conclusions Turbine Meters are a very cost effective instrument and provide good value for money when used in a clean gas condition. Modern turbine meter devices when used with a flow computer can also perform with high accuracy if properly calibrated. When using a twin rotor design it is possible to input 2 data streams into a SCADA system to provide real time diagnostics regarding the meters operating condition. The cost per measurement point is at a medium level compared with other gas measurement devices. References and Reference Materials

Natural Gas Measurement by Turbine Meter AGA 7.0 -Latest Revision

Fundamentals of Gas Turbine Meters, AGMS 2013 -Gorham,J.A.

Turbine Meters for Gas Measurement-Lawrence, P.A. - Cameron Measurement Systems 2006

Crude Oil Measurement by Turbine Meters, ISHM 1996-Lawrence, PA.