INTRODUCTION TO ULTRASONIC METER STATION DESIGN

THOMAS KEGEL SENIOR STAFF ENGINEER COLORADO ENGINEERING EXPERIMENT STATION, INC (CEESI)

Introduction

Meter station design is a topic that is not an ultrasonic meter is usually expressed in terms addressed by industry standards but is very of the minimum and maximum velocities. important in day-to-day operations of many gas Similar velocity range limits are observed with companies. This paper discusses a number of all meter sizes. Different operators can have topics that pertain to pipe layout aspects of different rangeability policies; company policy meter station design. is the primary source for this information. In the present discussion the operating range of 10-70 Topics from a companion paper [1] that ft/s is selected. The rangeability of 7:1 is quite describes run switching are included in the conservative, larger values are quoted by some. discussion. Adding a second meter of the same size in parallel, flowing at 70 ft/s, doubles the Meter Sizing and Rangeability maximum flowrate. The rangeability of the two meters is now 14:1. The value increases to 21:1 The first design task is to determine the flowrate with a third meters and 28:1 with a fourth meter. range required in a meter station. The nominal Installing additional meters do not affect the flowrate will determine the meter size while the minimum flowrate. range will determine the number of meters. As an example a station is required to measure a For the purpose of meter station design range of 3.6 to 77 MMCFD at a nominal ultrasonic meters are independent of pressure pressure of 250 psia and 60°F. The uncorrected and temperature. The design process is more flowrate range is 8,474 - 180,791 acfh based on difficult with orifice meters because of the non- a volume correction 17.7. Typical flowrates at linearity and pressure dependence. Selecting the 10 ft/s and 70 ft/s for several meter sizes are best ultrasonic meter(s) is a much simpler shown in Table 1; maximum flowrates are process. tabulated for 2, 3 and 4 meters in parallel. A single six inch meter matches the minimum The rangeability, or turndown, of a meter is the flowrate; 7,223 acfh from the table is slightly range of flowrates that can be measured, less than the design value. Four six inch meters additional discussion in [1]. The rangeability of will measure 202,232 acfh which provides 12% additional capacity for future expansion. Headers Looking at Table 1 it is noted that the high flowrate conditions can be achieved with one A multiple meter run station needs to distribute six inch and two eight inch meters, or one six flow equally to the parallel meters; a header is inch and one ten inch meter. Reducing the the most common approach. The two most number of meters can reduce the initial meter common configurations, designated “T” and station cost (CAPEX) but will likely increase “F”, are shown in Figure 1. Headers can be operating cost (OPEX). Multiple parallel runs above ground or underground with the addition are generally designed with the same size meter. of elbows. Underground headers can reduce Advantages include balanced flow, simpler run ambient noise an important consideration when switching algorithms [1], and the ability to neighbors are nearby. Above ground headers maintain flow while on meter is shutdown. provide better access for inspection and Ultrasonic meters offer numerable diagnostic maintenance. Removing accumulated liquid, for parameters that can assist in troubleshooting example, is easier with above ground headers. measurement problems. These diagnostics are The drawings of Figure 1 are not to scale, more useful when all te meter are the same size. additional details are discussed below.

The discussion above is based on achieving The header diameter is a balance between cost rangeability using multiple meters. Another and operations: larger headers will be more application is metering flow that is very steady expensive but will improve measurement and there is no need for broad rangeability. integrity. Unfortunately the improvement in Multiple smaller meters are often selected measurement results is difficult to quantify and instead of fewer larger meters for the same economic analyses are weak. The terms reasons listed above. “hogging” and “starving” refer to flow imbalance between meter runs as a result of an undersized header.

Table 1: Typical Ultrasonic Meter Flowrate Ranges

Minimum Maximum Flowrate [acfh] Nominal Flowrate Single Two Three Four Size [acfh] Meter Meters Meters Meters 2 738 5168 10335 15503 20670 4 3183 22278 44556 66834 89112 6 7223 50558 101116 151674 202232 8 12507 87547 175095 262642 350189 10 19714 137995 275990 413985 551980 12 28274 197920 395841 593761 791681 16 45664 319645 639290 958934 1278579

Figure 2: T Header Design

A common header design equation is: Figure 3: Z Header Design

unavailable a reducer can be added. Another option is an extruded header; a more costly Where dh is the header diameter and d1 - dn are option that allows for more design freedom. the individual meter diameters. Typically K = 2, Most operators do not allow saddle fittings slightly smaller values are acceptable for larger based on poor structural integrity. Also typically headers while slightly larger values are not used are weld fittings (Weld-o-let and recommended for smaller headers. The header similar) larger than 2 inches. These fittings are diameter for the present example is calculated good for pressure, temperature and sampling as: ports, not for meters.

Velocity Profile

Gas does not travel through a pipe with a uniform velocity. A property called “viscosity”

will reduce the velocity near the pipe walls to An 18 inch header would be a good near zero. Another layer of gas will flow at a conservative choice for the four six inch meters. slightly faster velocity; moving further from the 2 2 2 2 wall the velocity becomes progressively faster. The header length is another design parameter; In general the velocity increases as the distance the dominant factor is the spacing between from the pipe wall increases; the center of the meter runs. In the present example, with six pipe will flow at maximum velocity. The inch meters, five foot spacing between runs velocity distribution is often called a “flow should be adequate for access. Larger dimeter profile” or “velocity profile”. Over the years runs should be further apart, smaller runs can be theory and experimental work have closer. mathematically defined the “fully developed”

profile that is observed at the end of a long pipe. Most headers are fabricated from forged 22 Manufacturers and calibration laboratories reducing tees. d If a size combination is K H 1 d 2 n d  .  . . d   d KH 4 6  .0  6  5 2 9 4 .3

d 2H 9 4 .3 1 7 .1 5 attempt to create fully developed flow when package such that profile distortion and other designing and testing meters. installation effects contribute no more than 0.3% additional uncertainty. Figure 4 shows a Obtaining fully developed flow in a meter conservative meter package design. The total station is difficult because the various pipe straight tube length upstream of the meter is fittings (elbows tees, headers, valves) will expressed in multiples of nominal meter distort the velocity profile. The traditional worst diameter. For example, 10ND = 60 inches for a case is out of plane double elbow (OPDE). With six inch meter. The location of the thermowells a distorted velocity profile and swirling flow is a tradeoff between being close enough to the this distortion was originally investigated in meter to register meter temperature, but not so conjunction with multiple run orifice meter close that the flow profile in the meter is station design. With a small diameter header the affected. The tees or elbows are not part of the flow into the orifice makes two or three out of package but are normally part of a meter station. plane turns. The swirling flow will produce These components are discussed further below. measurement errors; the same problem exists with turbine and ultrasonic meters. Flow conditioner

Most ultrasonic meter station designs Orifice gas flow standards are “design based”; compensate for profile distortion in three ways. they describe exactly how to build an orifice First, the multiple path USM design averages meter and accompanying tube bundle flow the velocity across the meter flow area. Second, conditioner. With the increased use of turbine the meter station design includes two straight and ultrasonic meters most gas custody transfer tubes installed upstream of the meter. Third a measurements are made with proprietary flow conditioner is installed between the tubes. technology. As a result standards are becoming “performance based”; the minimum The potential for distorted profiles and swirl is performance rather than the design of a meter is addressed in the AGA 9 [2] standard. The standardized. Flow conditioner designs are also standard defines a meter package that is more commonly proprietary, flow conditioner typically interpreted to include inlet and outlet are being standardized based on performance. tubes as well as a flow conditioner. The As a result, considerable test data have been manufacturer provides the design details of a published describing the effects of various

Figure 4: Conservative Meter Package Design combinations of distorting element, flow turbulent intensity profile present at the inlet to conditioner, and ultrasonic meter. the meter. When the conditioner is rotated, the profile is also rotated, the ultrasonic path Traditionally a flow conditioner has been trajectories cross through different turbulent thought to produce a fully developed “outlet” intensity distributions and the meter output profile independent of “inlet” profile. Recent changes. research and testing has focused on the turbulence profile instead of velocity profiles or Noise swirl. A more relevant definition of a flow conditioner is to produce a consistent “outlet” Valve noise has been a persistent challenge for profile independent of “inlet” profile. This ultrasonic meters. A gas control valve operates definition maintains the traditional utility of a based on a relationship between pressure drop, flow conditioner but places more emphasis on flowrate and valve position. The pressure drop the need to calibrate with the flow conditioner, a represents a “loss” of gas flow energy; the direction supported by the AGA 9 package energy is not actually lost but rather converted definition. into increasing downstream velocity. Some of the energy also appears as acoustic pressure and Care must be taken to maintain the conditioner heating. The traditional valve design produces in the same position in the field as it was in the audible noise (20Hz - 20kHz); recent design lab. The effect of rotating a flow conditioner has improvements have greatly reduced the been investigated [3]; results indicate that intensity of audible noise. The apparent noise measurement shifts of as much as 0.2% can be reduction represents acoustic energy that has produced. been shifted to ultrasonic frequencies (80 - 400kHz) used by flowmeters. The results have Turbulence ranged from minor signal distortion to complete meter failure. The velocity profile is the result of viscosity that produces shear forces within the flow. The shear Three potential solutions to the noise problem forces also result in rotating structures of are commonly applied: various sizes and velocities; in total these structures are called “turbulence”. An example 1. Physical design of meter station that many of us have experienced is turbulence 2. Transducer design, different natural while on board an airplane; the buffeting can be frequency quite severe. Much like a fully developed flow, 3. Signal filtering in software turbulence occurs naturally, with a well-known intensity distribution. In an ultrasonic meter Items 2 and 3 are meter design solutions. In turbulence will cause apparent random general the user needs to consult with the variations in ultrasonic meter transit time vendors, they have different approaches suited measurements. For this reason an ultrasonic for the particular product designs. CEESI meters operates with averaged transit time data. experience indicates the current USM products have greater noise immunity than older The shift resulting from a rotated conditioner is products. CEESI has conducted noise testing for thought to be the result of a non-symmetric several of the vendor. While the results are proprietary and cannot be shared users are direction arrow corresponds to an installation strongly encouraged to ask for test results if upstream of the meter package; a mirror image noise is a potential problem. is installed downstream. The upper tee is popular because the ends can be opened for There are similarities and differences between inspection of the meter package. An easily noise and turbulence. At a fundamental level removable inspection cap is shown; a blind turbulence is velocity fluctuations while noise is flange can also be installed. pressure fluctuations. From the perspective of meter station design both turbulence and noise Aside for the maintenance consideration, it can be generated from the same source. Some would appear that an elbow and a tee are features that can cause noise or turbulence: hydrodynamically identical. The two flow paths are quite different; an elbow provides a more • Protruding gaskets gradual turn. Figure 7 [4] shows the velocity • Tube/meter mis-alignment distribution in an in-plane double elbow (IPDE) • Rough weld beads and in-plane double tee (IPDT). The blue is the • Corrosion lowest velocity, yellow is highest, and green is • Flow conditioners in between. Figure 8 [4] shows the IPDE and IPDT turbulence intensity. The dark blue is Elbows and Tees lowest intensity, yellow and red are highest, green is in between. Another point of The first station design rule is to install the comparison is in regard to measured pressure noise source, usually a valve, downstream of the drop: A tee flowing from the branch connection ultrasonic meter. If the problem is still likely to produces three times the pressure drop of an be present, the second rule is to use piping elbow. Recalling that pressure drop represents a system components to isolate the meter from loss of energy; the energy is released as outside noise sources. turbulence and noise.

Over the years various combinations of elbows Problems are occasionally observed in the field and tees have been used for noise isolation; two when IPDT “noise filters” are used. One designs are shown in Figure 5 and 6. The flow particular meter run generated extreme pressure and flow fluctuations during calibration at high

Figure 5: Noise Reducing Fixture Figure 6: Noise Reducing Fixture Figure 7: Velocity Distributions in Tees and Elbows Figure 8: Turbulence Distribution in Tees and Elbows velocity. Removing the tees, essentially a Temperature Measurement straight run, eliminated the fluctuations. A new device, called an “elliptical deflector”, was Field observations have identified meters that installed with the IPDT as shown in Figure 9. have been shut in but still report flow. A The immediate problem was resolved, the common source of flow is the result of device appeared to work. One description differential solar heating that creates convection suggests that turbulence in the flow triggers currents within the closed meter package. fluctuations at a resonant frequency much like Ultrasonic meters are very sensitive to low an organ pipe. The pressure waves travel back velocities, more so than other technologies. The and forth through the meter tube reflected by the meter is able to sense the very low convection inspection cover or blind flange. The deflector velocities. One solution is to install a shade blocks the parallel surfaces and eliminates the above the meter. Another solution is to use a fluctuations. Based on the above described low flow cutoff in the software to force the data experience the elliptical deflector was included to zero. in a test program [3], the results were inconclusive because the fluctuating behavior Another temperature related problem is could not be reproduced. The elliptical deflector observed with thermowells at high velocity. A is currently being studied [5] using CFD cylindrical structure can develop vortex analysis. The results will be published upon shedding under certain flowing conditions. The completion. vortex shedding will impose a fluctuating load that can result in structural failure over time. Fluctuating flow and pressure, as noted above, Reference 6 contains information for has been observed in meter stations. Flowing thermowells manufacturers to assure that their gas adjacent to an open meter tube can produce designs are likely not to fail. The user should noise, like blowing across the mouth of a bottle. ask the vendor for evidence of compliance to The designs in Figures 2 and 3 both contain this standard. potential noise sources when meter runs are shut in. Very little information is available to guide a Diagnostics designer to avoid this problem; it is mention here as a potential troubleshooting route for Ultrasonic meters are equipped with a broad noise in existing meter stations. An acoustic range of diagnostic parameters that can be used noise case study is contained in Reference 7. to help troubleshoot specific measurement problems. Numerous independent publications as well as vendor literature are readily available. Diagnostics are useful in addressing some of the issues described in this paper; selected examples combination of diagnostic parameters that are are briefly described below. associated with a meter; much like a human fingerprint. Consulting with the vendor will A single measurement is made up of the average identify the recommended fingerprint format of multiple transit time measurements. The appropriate to their product. standard deviation of the measurements is affected by noise and turbulence. Normally Summary - Meter Station Design Checklist occurring turbulence will result in standard deviation values between 3% and 5%. Increases This paper has discussed some aspects of meter in standard deviation often indicate the presence station design from the flow measurement of trash blocking part of the flow conditioner. perspective. It does not present a complete design guide. In the interest of being thorough, The analog electronic circuitry reports signal to below is a list of other meter station design noise ratio (SNR) and gain. The SNR value can requirements and considerations: indicate excessive noise from a source such as a valve. • Site location, buildings, fencing etc. • General piping Custody transfer meters have multiple acoustic • Overpressure protection paths to allow for averaging velocity across the • Pressure and temperature transmitters flow area. The individual path velocity • Flow Computer measurements will roughly follow a velocity • Tubing profile shape. Deviations can indicate profile • Gas sampling distortion or swirl. In the case of thermal • Gas chromatograph convection discussed above the temperature • Odorization gradient can be detected based on speed of • Gas Quality sound measurements from the individual paths. • Valves • Wiring Over time each of the four meters in a typical • Communication meter station will be exposed to the gas for a • Corrosion different length of time. One meter will always • Material Specifications be open; another will only be open during period of high flow. If dirt builds up on the surfaces of the meters it will not be equally References distributed on all four meters. Changes in SNR and gain can detect the differential buildup and 1. Kegel, T., “Run Switching,” Western trigger the need for an on-site inspection. Gas Measurement Short Course, 2017. 2. AGA Report No. 9, “Measurement of The general approach to diagnostics is to rely on Gas by Multipath Ultrasonic Meters.” observed changes rather than absolute values. The following process is recommended: Obtain 3. Miller, R. and Hanks, E., “Gas a “fingerprint” upon calibration, confirm the Ultrasonic Meter Installation Effects and fingerprint upon installation in the field, and Diagnostic Indicators,” International monitor over time. A fingerprint is the unique Symposium on Fluid Flow Measurement, 2016. 4. Images from CFD analysis courtesy Canada Pipeline Accessories. 5. Private communication, Danny Sawchuck, Canada Pipeline Accessories. 6. ASME PTC 19.3, Temperature Measurement Instruments and Apparatus. 7. Kegel, T., “Ultrasonic Meter Station Design”, Western Gas Measurement Short Course, 2015.