FUNDAMENTALS OF INSERTION TURBINE METERS Les Bottoms Thermo Electron Corporation, Flow Systems

FUNDAMENTALS OF INSERTION TURBINE METERS Les Bottoms Thermo Electron Corporation, Flow Systems 9303 W. Sam Houston Parkway, Houston, TX 77099 INTRODUCTION The insertion turbine meter is well suited for large pipeline measurement. It is presently used in many areas of the gas industry, such as compressor efficiency and surge control, pipeline leak detection, pacing odorizers and samplers, as well as checking pipeline throughput. The insertion turbine meter provides good accuracy, and offers the user cost saving advantages at the initial purchase phase, and during installation and pipeline maintenance. DESCRIPTION The insertion turbine meter (Figure 1) is a measurement device that can be inserted through an isolation valve and flanged riser attached to the side of a pipeline. It can measure both liquids and gases and can be installed into an active line without interruption of flow. The insertion turbine meter can be used in line sizes of 2 and larger. direction and measure bi-directional flow. An RTD can also be installed in the stem, along with the pickup to provide temperature measurement. In some cases, the stem tip, which includes the pickup and RTD, is removable in the field for ease of maintenance. 3)The velocity sensor attaches to the tip of the stem assembly by means of a locking nut. Units for liquid applications are normally fitted with journal bearings made from Tungsten Carbide and various alloys. Units for gas applications are fitted with stainless steel, ball bearings. Head assemblies can be fitted with different rotors to allow accurate measurement at virtually any pipeline flow rates and pressure conditions, up through ANSI 2500. These meters typically consist of three (3) basic parts; the housing, the stem assembly and the velocity sensor. 1)The housing assembly allows the insertion turbine meter to be connected to the pipeline. This connection is normally accomplished using a flange with a size and pressure rating appropriate for the specific application. The housing also provides a method to seal off the insertion mechanism (stem assembly), which extends the velocity sensor into the pipeline. A pressure tap is included in the housing to allow pipeline pressure measurement, if required. 2)The stem assembly holds the velocity sensor and allows its proper positioning in the pipeline. The stem assembly is available in several different configurations, depending on line pressure rating. Manual insertion assemblies are the most common in the industry, but some manufacturers offer hydraulic or pneumatically powered mechanisms which automatically insert and retract the stem assembly when pipeline pigging. In this case, a hydraulic reservoir is pressurized from either the pipeline or an external source, and can be manually or automatically controlled. The tip of the stem of an insertion meter houses the pickup, which is used to sense rotation to pulses proportional to the flow in the line. Some units can be fitted with dual pickups to detect changes in FIGURE 1. (See also Figure 6) Some models can be fitted with air purges and/or special bearings (such as Teflon lined), for use in applications where contaminants in the line would normally cause damage to the bearings. Meter mounted amplifiers are available for use where long distances or noisy environments are involved. INSTALLATION The insertion turbine meter is normally installed through an isolation valve and riser welded to the pipeline. Entry PAGE 42

point can be from any location around the pipeline circumference, as long as the meter axis is 90 degrees from the pipeline axis. Normal installation entries are from the top or side of the pipeline. (I.D.) of the pipe by 0.12, (or 1/8th of inside diameter). This assumes an ideal flow profile. The insertion turbine meter can be installed in an active pipeline without interruption of flow, using the hot tapping procedure. This procedure is well know in the pipeline industry and has been used for many years. A typical installation would be accomplished as follows: 1)A flanged riser of appropriate size (normally 2 to 4 inches) and pressure rating is welded to the pipeline at a pre-selected location. 2)An isolation valve of appropriate size and pressure rating is bolted to the flanged riser. 3)A hot tapping device is bolted to the isolation valve. 4)The isolation valve is then opened and the hot tapping device is extended to cut a hole in the pipeline. When the hole is through, the hot tapping device retains the cut metal (coupon), which is then retracted with the cutting tool. 5)The isolation valve is then closed. The hot tapping device is removed and the installation is ready to accept an Insertion meter. Good installation practices improve the flow meter performance. Successful installations have the following characteristics: 1)Meter location on pipeline with at least 10 to 20 unobstructed, straight pipe diameters upstream and 5 to 10 diameters downstream. Longer, straight, unobstructed runs are preferred. 2)Riser, flange and valve centered on the pipeline and perpendicular to it, to within +/- 1. 3)A pipeline tap performed by qualified personnel to assure that the hole is centered in the riser, free from burrs or slag and large enough to accommodate the insertion turbine head without interference. 4)A line that has operated at least 12 hours after any welding or construction work was performed to ensure purging of all loose debris. The turbine head assembly (velocity sensor) should be positioned in the pipeline so that it is aligned to within ±2 of the pipeline axis. The head assembly should also be located in the pipeline at the mean velocity point. In order to determine the flow rate, it is first essential to place the rotor assembly in such a position as to detect the average velocity. Due to mechanical constraints it has traditionally been the practice to install the rotor on the center line for pipes up to 10 diameter. More recent research suggests that the center line should be used for pipes up to and including 16 diameter. However, when using the center line, an offset factor must be calculated to compensate for the higher than average velocity that can be expected. In line sizes greater than 16, the mean velocity point is assumed to be at the location represented by multiplying the inside diameter FIGURE 2. Unusual or abnormal conditions can create disturbances in a pipeline that might affect the location of the mean velocity point. These conditions can be caused by; bends, reducers, tees, valves and regulators, which are located in close proximity to the insertion meter installation. The result is that the flow profile can be distorted. The condition can also be described and either laminar or turbulent. When turbulent flow exists, the flow velocity across the pipeline is fairly constant and flat. When laminar flow exists, the flow velocity at various points across the pipeline varies at different flow rates. The location of the precise mean velocity point under these conditions can be determined by profiling. Profiling can be performed with the insertion turbine meter after it is installed and prior to connection to its readout electronics. Profiling is easily accomplished, using a flow rate indicator or frequency counter. Because profiling is a comparison of the flow rate across the line, the readings may be taken in any engineering unit (GPM, ACFM, Ft/Sec etc.). FIGURE 3. Profiling is accomplished by first knowing the exact distance between the center of the turbine head assembly and a point on the outside of the insertion meter PAGE 43

housing, (normally the bottom of the stem top nut). The head assembly is then traversed across the pipeline, at even distances, to determine the profile for that line. These readings can be averaged together to locate the mean velocity point in the line. Therefore, the insertion turbine meter can detect swirl conditions and be installed to compensate for them. PERFORMANCE Performance, when referring to an insertion turbine meter, consists of three main elements: Accuracy, rangeability and reliability. ACCURACY In most turbine meters, accuracy is a product of two factors. Those factors are, linearity and repeatability. 1)Linearity is defined as the maximum percent deviation from the average calibration factor of the flow meter over a specific flow range. In real terms this is the difference between indicated flow rate and actual flow rate. 2)Repeatability is defined as the ability of a flow meter to duplicate a specific calibration factor under the same flowing conditions. In other words the meter will produce the same result under the same application conditions. This is often accompanied by a probability (or certainty) factor (say 95%). If 100 tests were conducted under the same conditions, the worst 5% would be discarded and the rest should be within the manufacturers stated deviation. that if a turbine is designed to perform at the maximum flow rate of a particular pipe size, then the lowest flow rate that can be accurately monitored is likely to be 10% of that flow rate. The linear range is in fact limited strictly by mechanical considerations. However, the repeatable flow range of a turbine meter (in general) is considerably wider, and may offer turndown of 20 to 1 or more. Below the 10% point, the K factor of the instrument starts to decline as the rotation is impaired by frictional and magnetic forces. However, as indicated in the previous paragraph, the K factor at a specific flow rate (below 10% flow), may remain remarkably constant. With the introduction of simple and inexpensive flow computers, this additional repeatable range can be utilized by linearizing the K factor electronically on the lower points in the range. For gas service, the (linear) operating range of the turbine meter will be limited by the density, which is directly related to pipeline pressure. Where widely fluctuating flows are likely, the working range of the insertion meter can be extended by using an inexpensive microprocessor signal conditioner or low cost flow computer with linearization capability. It is important to note at this point that when used on low flows, the insertion turbine is less repeatable on gases than on liquids, due to density related factors. The following chart gives an indication of how range ability will be affected by pressure changes in the pipeline. For most liquid applications it is generally accepted that the insertion meter can achieve accuracy of +/- 1%. For gas service attainable accuracy is more likely to be in the range of 2% to 3%. Repeatability of 0.1% can be readily achieved. Tests by independent laboratories, which have been conducted to internationally acceptable standards, have shown that under ideal conditions, the insertion turbine meter can achieve the accuracies required for use as a Custody Transfer meter. However, for gas service, in a field environment, the insertion turbine meter would not generally be considered fiscally acceptable for this purpose. However, there are many other applications outside the realm of custody transfer where the insertion meter offers outstanding performance. The best performance is achieved where the meter is installed at least 20 diameters from an obstruction where the profile has been fully developed. RANGEABILITY The linear flow range of most turbine flow meters when used for liquid service is generally accepted as being limited to approximately 10 to 1 turndown. This means RELIABILITY FIGURE 4. Reliability is an important part of a flow measurement application. Meters that are reliable require minimal maintenance, and when properly installed, will operate for many years, trouble free. The insertion turbine meter has proved to be a reliable performer over the last thirty years. End users have enjoyed many thousands of hours of service during this period. The secret of trouble free service is attributable to one thing. PREVENTIVE MAINTENANCE The insertion turbine meter requires very minimal maintenance. In general, bearing wear does not cause a PAGE 44

degradation of accuracy until abrupt failure occurs. Periodic checking of the meter output signal waveform can often detect bearing problems as they develop. Preventative maintenance minimizes loss of measurement. Since removal of the device is comparatively simple, maintenance is straightforward and cost effective. From a service point of view, the end user only needs to stock a limited selection of parts. In many cases, the pickup can be removed and replaced in the field. The turbine head assembly (velocity sensor or rotor) can be removed and replaced in the field. One head assembly often fits a variety of insertion meter mechanisms, regardless of pressure rating. Different head assemblies are available for different flow ranges and/or velocities. For gas measurement, the rotor design varies according to the application pressure. Most manufacturers offer separate designs for low and high-pressure applications. When the insertion turbine meter does require maintenance, it can easily be removed from an active line through an isolation valve without line shutdown. Parts can be replaced or repaired and the meter returned to service in less than an hour. OPERATING CHARACTERISTICS 1)Flow Range: Gas velocities from 1 to 300 ft/sec. Liquid velocities from 0.5 to 45 ft/sec (turbine heads must be selected for each specific application). 2)Accuracy: Linearity of better than ±0.5% over a 10:1 flow range. Repeatability of 0.1%. 3)Operating Pressure: Up to ANSI 2500, when fitted with proper size flange and insertion mechanism. 4)Operating Temperature: Up to 250 F which satisfies most applications. 5)Line Size: 2 inches and larger. 6)No power required, unless amplifier or other electronic readouts are mounted on meter. 7)Connection to Pipeline: Normally using a 2 to 4 flange with appropriate pressure rating. PRACTICAL CONSIDERATIONS The operator should not lose sight of the fact that the insertion turbine meter is measuring the velocity of the gas or liquid in the pipeline. All other parameters are inferred from the velocity measurement. This makes it extremely important to know (or find out) the actual I.D. of the pipe before setting up and using the device. Generally speaking a 1% error in the inside diameter will result in a 2% error in flow rate indication. ECONOMICS Insertion turbine meters are economical to use as well as accurate. By using an insertion device where appropriate, the user can acquire more measurement systems for less money. In fact, as many as 4 or 5 insertion meter systems can be purchased for the price of a single orifice meter system. A typical insertion turbine meter system would merely require a flanged riser, an isolation valve, an insertion turbine meter and readout electronics. No by-pass piping is required, since the unit can be removed at any time without shutting down the process. A full size inline measuring system would require upstream and downstream block valves, a by-pass line, a by-pass valve, a measurement device and readout electronics. This system might typically cost several times more than the insertion meter system, depending on line size. When wide flow ranges exist, conventional measurement devices require multiple metering systems. This configuration could typically cost as much as twenty (20) times more than one insertion meter system. A single insertion turbine meter can measure an entire pipeline flow, eliminating the need for headers and multiple meter runs in those locations where custody transfer accuracy is not required. OBSTRUCTION FACTOR Finally when attempting to obtain maximum accuracy from the insertion device, it is necessary to consider the obstruction factor created by the insertion stem and turbine head assembly. When an insertion device is installed in a pipeline. The restriction is minimal and hence pressure drop associated with the use of the device is negligible. However, for maximum accuracy the area occupied by the mechanical components must be considered since the gas flowing in the pipeline will accelerate as it passes the restricted area created by the turbine assembly. In essence, the area is reduced, because a portion is occupied by the assembly. The following chart has been assembled which indicates the obstruction as a fraction of the pipe area for different pipe sizes. The chart is based on the dimensions of the Thermo Flow Automation high-pressure insertion turbine but other manufacturers will generally produce something based on their own designs. PIPE TURBINE HEAD NORMAL SCH. 40 INSERTION OBSTRUCTION DIAMETER LD DEPTH ON FACTOR 4 4.026 C/L.844 6 6.065 C/L.899 8 7.981 C/L.922 10 10.020 C/L.937 12 11.938 C/L.947 16 15.000 C/L.958 20 18.812 2.257.991 24 22.624 2.715.993 30 28.500 3.420.994 36 34.500 4.140.995 42 40.500 4.860.996 FIGURE 5. PAGE 45

ADVANTAGES OF INSERTION TECHNOLOGY The advantages of using an insertion turbine meter as a flow measurement device are: 1)High Accuracy: Able to achieve a linearity of better than ±0.5% and a repeatability of 0.1%. 2)Linear Output: Proportional to flow velocity. 3)Rapid Response: Able to respond to changing flow conditions in the order of milliseconds. (Can track pulsating flow due to low mass flow). 4)Bi-directional: Can be made to measure and indicate flow in both directions. 5)Low Pressure Drop: When installed in large lines, pressure loss is negligible. 6)Low Equipment Cost: When compared to other measurement devices now being used, can cost as much as 1/10th to 1/20th for the equipment alone, depending on line size. Further savings are achieved in the cost of the installation. 7)One Measurement Device: For all line sizes, (w/same pressure rating). 8)Ease of Installation: Can be installed in an active pipeline by the Hot Tapping method through an isolation valve without line shutdown 9)One Pipeline Entry: Flow, pressure and temperature measurement devices can be installed through a single isolation valve and riser. 10)Low Installation Cost: As much as 1/5 to 1/20 of the cost of other meter installations, depending on line size. 11)Low Maintenance Cost: Maintenance is minimal Only a turbine head assembly is recommended as a spare part. Both turbine head assembly and pickup can be replaced in the field. Both items fit all meter mechanisms. If an isolation valve is installed, maintenance is possible without line shutdown. TYPICAL HARDWARE REQUIREMENTS THE INSERTION METER FOR GAS MEASUREMENT The insertion meter has been widely used in the past for various duties in the gas industry. Gas measurement varies considerable from liquid measurement and consideration should be given to the pressure, velocity and range requirements. TYPICAL APPLICATIONS 1)Because of the low mass feature of the rotor, the insertion turbine meter provides both an ideal means of measuring COMPRESSOR THROUGHPUT vs FUEL CONSUMPTION and COMPRESSOR SURGE. Much consideration is currently given to the gas industry for compressor automation and control. Attractive features include: Negligible pressure drop; no compressor head loss. Fast response time (usually 3 to 5 milliseconds). Tracts well in pulsating conditions. 2)Other applications may include: PACING ODORIZERS LEAK DETECTING PACING SAMPLERS ALLOCATION METERING SUMMARY The insertion turbine meter has provided both the oil and gas industries with field-proven, accurate measurement for over thirty years. There have been very few design changes in recent years though materials of construction have been improved. The device is still a useful and cost effective way of measuring gas and liquid flows, particularly where a power supply is not available. An insertion turbine metering point might consist of the following: 1)An insertion turbine meter with appropriate pressure rating and calibrated for a specific application. 2)A temperature measurement device installed as in integral part of the insertion meter stem. 3)A pressure transducer installed on the housing of the insertion meter. (If gas service) 4)A straight section of pipe of appropriate length with a flanged riser for the insertion meter. (A section of an existing pipeline may suffice). 5)An isolation valve. 6)A flow computer, indicating corrected volume. (Compensating for variations in pressure, temperature, specific gravity, and super compressibility). FIGURE 6. PAGE 46