Innovative Onsite Technique for Proving Large Custody Flowmeters
By C. BHATASANA, A. ALABDULHAI and S. ALSHAHRANI, Saudi Aramco, Dhahran, Saudi Arabia
(P&GJ) — It is imperative to measure bulk liquid hydrocarbon accurately at the custody transfer point to avoid financial exposure and maximize revenue. The accuracy of custody flowmeters, such as turbine meters, ultrasonic meters and Coriolis meters, is affected by several factors, including wear and tear, fluid characteristic variations, process operating condition changes and the presence of contaminants. To ensure measurement accuracy and acceptable meter performance, it is well established in the industry to use a meter proving system that helps validate meter performance and quantify any shifts in custody flowmeter performance. The proving system computes an actionable corrective meter factor that is applied to a meter’s indicated volume to correct the meter’s reading and counteract any meter drift—the goal is to provide accurate and defensible volume measurement.
The custody metering system is labeled as a “cash register,” as the company receives payment for the commodity sold or makes a payment against the commodity received. This emphasis on the requirement of an available and operational custody meter proving system to reduce uncertainty, ensure accurate measurement, help protect companies against claims and decrease financial exposure is very high when the ownership of bulk hydrocarbon is transferred.
Every proving system is as unique as it is sized based on the flow capacity of a particular custody meter that must be proved. Historically, flowmeters were mechanical types with a limited flow capacity. The introduction of electronic-based flowmeters such as an ultrasonic meter, while it opens a new horizon for flow capacity, imposes a challenge in the sizing and selection of proving system. This is because the flow capacity of the evolved meters far exceeds the flow capacity of the different type of available conventional proving systems.
This limitation hinders the operational efficiency of the flow metering system; however, it inspires an innovative approach to enable onsite proving of large capacity flowmeters.
Types of provers. To validate the accuracy of liquid flowmeters in the dynamic custody measurement, various types of provers are used in the industry. These provers can be classified in three broad categories: tank provers, master meter provers and displacement provers. Displacement provers are further classified based on the type of displacer and size of provers, which are bidirectional/unidirectional sphere pipe provers, bidirectional piston pipe provers and unidirectional piston provers (also known as small volume provers).
The design of displacement type meter provers is generally guided by the American Petroleum Institute (API) Manual of Petroleum Measurement Standards (MPMS) Chapter 4.2. The design of master meter provers is guided by MPMS Chapter 4.5, but includes many other international standards and references.
Tank provers. A tank prover is typically a volumetric container that consists of a marked top neck and occasionally a graduated bottom neck. A tank prover has a certified volume between the bottom shut-off valve or bottom-neck ‘0’ level mark and an upper-neck ‘0’ graduation. Tank provers are generally used for crude oil and refined petroleum product applications, where fluid does not have a clinging effect and vapor pressure is well below atmospheric pressure.
Master meter type provers. A master meter prover utilizes a master meter to prove the duty flowmeter. A master meter can be a turbine meter, positive displacement meter, ultrasonic meter or Coriolis meter. The master meter is calibrated to a much higher standard to ensure superior accuracy of the duty meter. Selecting the master meter depends on process operating conditions.
Displacement pipe provers. Pipe provers (or displacement provers) are used extensively in the petroleum industry to provide onsite calibration of flowmeters used in custody transfer applications. There are two general types of displacement pipe provers: unidirectional and bidirectional. Both types of provers essentially consist of a calibrated pipe section with a meticulously measured internal volume [base prover volume (BPV)] and a displacer, such as a sphere/ball or piston, that moves through the pipe, displacing process fluid. During operation, the displacer traverses the calibration section, triggering detectors at both ends, which marks the beginning and end of the prover calibrated section. In a bidirectional prover, a four-way type divertor valve is used to achieve displacer back-and-forth movement within the calibration section, while a sphere exchange mechanism is used in the unidirectional prover. Both mechanisms maintain continuous flow through the custody meter and prover, and achieve the same function aside from the flow direction, which is maintained during proving cycles in a unidirectional prover—unlike a bidirectional prover.
Unidirectional piston provers (small volume provers). The small volume piston prover (SVP) is a unidirectional displacement prover that features a piston moving through an accurately machined cylindrical pipe. The piston features a special design with an extended rod that stretches outside the calibration cylinder. The rod carries a flag that triggers detectors (typically optical) to identify the piston’s location. Three fixed detectors are generally used: one detector confirms the piston’s upstream starting position, while the second and third detect measurements indicating the beginning and end of the prover calibration section as well as marking the start and end of the proving pass. In addition, the third detector signals the end of the proving pass and provides a signal to initiate the piston retraction.
The design of the piston features a special plunger, known as a poppet valve. This poppet valve opens and closes based on the piston’s direction to maintain continuous flow through the prover during retraction. The piston returns to its starting position via mechanical or hydraulic means, ensuring uninterrupted flow through the flowmeter. SVPs are proprietary, with designs emphasizing compactness and precision. The measurement rod’s flags and detectors enable high-accuracy volume tracking, while the fluid bypass mechanism ensures operational continuity. This design suits high-flow systems where the availability of space is a critical constraint.
Proving operation using a displacement prover. During the meter proving operation, the custody meter is aligned in series with the prover to enable the process fluid to sequentially and strictly flow through the custody meter and prover. It is essential that no fluid escape from or enter the line connecting the custody meter and prover. The volumetric flow measurement of the meter is compared to the prover calibrated volume to compute an actionable corrective meter factor. The meter factor is a ratio of the prover calibrated volume and the meter registered measurement, and represents any drift in the meter performance and is used to correct the meter reading. Fundamentally, to achieve an acceptable comparison, the flow capacity of the prover should be equal to or greater than the flow capacity of the meter. This type of proving operation is called “direct proving,” as it verifies the custody meter’s performance directly against a certified displacement prover.
Proving operation using master meter prover. Master meter proving follows a similar process as a displacement prover in verifying the meter’s performance. However, instead of utilizing a displacement prover, it utilizes a master meter in-series configuration with a custody meter, measuring and totaling the flow through both meters simultaneously. The master meter is first calibrated using a prover, such as a tank or displacement prover on different flow streams and/or conditions to establish its accuracy by deriving its master meter factor (MMF).
Subsequently, a master meter whose measurement is corrected using an MMF is used to ensure custody measurement accuracy. In essence, master meter proving involves using a highly accurate and repeatable master meter to correct for any inaccuracy in the custody meter. This type of proving operation is called “indirect proving,” as it verifies the meter’s performance indirectly against a certified displacement prover using a master meter. This method has a significantly higher level of uncertainty than the direct proving method.
Conversely, when a custody meter is proved using a master meter—earlier proved onsite using a displacement/tank prover with a common flowing stream within a short period (normally back-to-back)—it is called the “direct master meter proving” method. This method has a higher uncertainty than a direct method, but much more certainty than the indirect proving method. It closely approximates the direct proving method because proving is performed at the same process operating conditions, including process fluid.
To minimize measurement uncertainty, the direct proving method is always preferred; however, it has limitations. As previously stated, in the direct proving method, the custody meter is aligned in-series with the displacement prover, so the process fluid from the custody meter flows through the prover, as well. This shows that the flow capacity of the prover should be greater or at least equal to the flow capacity of the custody meter.
By design, the maximum velocity in the bidirectional prover should be restricted below certain limits to avoid hydraulic shock and reduce the chances of damaging the displacer, detection devices or receiver chamber at the end of each proving pass. API MPMS Chapter 4.2 recommends a maximum velocity of 5 feet per second (ft/sec) for the widely used conventional bidirectional sphere provers to account for the inertia in the flowing liquid when changing the direction of the flow within the calibration section.
Based on the velocity criterion recommended in the API standard, the maximum flow capacity of a large bidirectional prover with a nominal diameter of 42 in., equipped with a 24-in. divertor valve is restricted to 28,000 barrels per hour (bph) [or 4,450 cubic meters per hour (m3/hr)]. Despite the huge size of this prover, it is not suitable to prove a 20-in. custody turbine, which is capable of a maximum flowrate of 42,000 bph, to the full range, or an 18-in. liquid ultrasonic meter, which is capable of a maximum flowrate of 39,000 bph (6,250 m3/hr). This creates limitations in utilizing the custody meter up to its maximum flow capacity.
As stated previously, in the direct proving method, the displacement provers must match or exceed the flow capacity of the custody meters they calibrate, which becomes impractical for high-flow systems (e.g., marine terminals). Large provers are cumbersome, difficult to maintain and may not fit in constrained spaces. Additionally, procurement challenges arise due to the limited availability of large provers. From a maintenance difficulty perspective, the hydraulic-operated, 24-in. four-way valve weighs about 6,000 kg, a 42-in. sphere weighs more than 600 kg and a prover recalibration can take days.
The impact of not proving the full flowrate. The meter factor curve for a turbine meter tends to flatten at a higher flowrate. FIG. 1 depicts the typical meter factor curve of a 16-in. turbine meter for medium-heavy crude oil at an export terminal. The meter changes by a factor of 0.033% in the higher 20% flow range. If the flowmeter is not proven at the highest flowrate of 25,000 bph, assuming the flatness of the curve at the higher flowrate, the measurement may have a bias of 0.033%.
FIG. 2 illustrates a typical meter factor curve of a 20-in. turbine meter for crude oil applications. Though the max flowrate of the turbine meter is 42,000 bph, the meter can be proven up to 28,000 bph due to the prover size limitation. The meter factor is assumed to be constant at flowrates higher than 28,000 bph and is shown with a curve with a flat end by Series 2. The area within Series 1 and Series 3 indicates a potential bias of up to 0.033% in the meter factor based on the variation observed in the 16-in. turbine meter. This 0.033% variation may result in up to a 2,954 bbl per month bias for the refinery, which imports 300,000 barrels per day (bpd) of crude oil. At $70/bbl, the impact of bias would be $206,806 per month and $2.48 MM/yr. This example establishes the necessity of the proving meter throughout the entire operating flow range.
How to reduce prover size. Reducing the prover size relies on the direct master meter proving method. It uses multiple master meters in parallel and in-series with the custody meter. In this configuration, the entire flowrate capacity is shared through the master meter(s), resulting in the master meter needing to be smaller than the custody meter. Subsequently, the required capacity of the displacement prover must prove the master meter size is also reduced significantly.
Proving the process with multiple master meters. During normal operations, custody meters directly flow to an outlet via open outlet valves. Prover takeoff valves are closed, isolating the master meter bank.
Before proving the custody meter, each master meter in the bank is first proven individually using a displacement prover. The outlet valve of the custody meter must be closed and the prover takeoff valve must be open to redirect the custody meter’s flow to the master meter bank.
To prove each master meter individually, the outlet valve of the master meter must prove it is closed and that the prover offtake valve is open. The outlet valve of other master meter(s) in the bank remains open. This set-up effectively lines up one master meter to be proven with the displacement prover. The fluid beyond the flow capacity of the master meter that must be proven and the displacement prover flows to the outlet header via other master meters. The flow control valves in the master meter bank balance flow and the flow control valve at the outlet of the displacement prover regulates the master meter proving flowrate at a reduced rate (custody flow divided by the number of masters).
To prove the custody meter, fluid flows to the outlet through the master meter bank with the prover offtake valves, and the displacement prover remains closed. The flow control valves in the master meter bank balance flow in each master meter at the flowrate at which it was proven. Effectively, the flowrate in each master meter is in tune with the custody meter flow divided by the number of master meters. The measurement of all master meters is derived from the sum of their measured flows, adjusted by their master meter factors. The custody meter’s meter factor is calculated by dividing the total master-measured flow by the custody meter’s reading, both adjusted for the effect of pressure and temperature on the liquid. The same sequence is repeated to the prover of other custody meters by rerouting their flow through the master bank using their respective prover takeoff valves (FIG. 3).
The advantages include:
- Reduced prover size: The prover’s capacity scales inversely with the number of master meters (e.g., two masters allow a prover half the custody meter’s capacity)
- Ease of maintenance/operation: Smaller provers are easier to handle, store and service
- Space efficiency: Smaller provers fit into restricted areas such as offshore platforms
- Redundancy: Master meters with a pipe prover ensure the continuous availability of proving systems (e.g., backup provers)
- Cost savings: Smaller provers are more widely available, reducing procurement costs
- Flexibility: A single prover and master bank can validate multiple custody meters via valve switching
- Accuracy: The uncertainty of parallel master meters tends to be lower than a single master meter due to uncorrelated errors in the measurement.
Takeaway. The proving method described in this article enables the onsite verification of the accuracy of high-capacity custody flowmeters to the full flow range, using a smaller displacement prover and addressing challenges associated with traditional proving. By splitting the custody meter flow across multiple master meters, the displacement prover’s capacity requirements are reduced, enabling the use of a smaller displacement prover. As each master meter is individually validated before being used to prove the custody meter, the uncertainty of the proving operation closely resembles the lowest achievable uncertainty using the direct proving method.
LITERATURE CITED
1 API, “API MPMS Chapter 4, Section 2: Proving systems, displacement provers,” 2014.
2 API, “API MPMS Chapter 4, Section 5: Proving systems, master meter provers,” 2014.
3 API, “API MPMS Chapter 4, Section 8: Operation of proving systems,” 2014.
4 API, “API MPMS Chapter 12, Section 2: Calculation of petroleum quantities using dynamic measurement methods and volumetric correction factors,” 2014.
5 ISO, “ISO 7278-2: Pipe prover design, calibration and operation,” 2022.
About the Authors
CHANDULAL N. BHATASANA earned a BS degree in engineering and has more than 30 yrs of experience in custody/fiscal metering and instrumentation. He has been with Saudi Aramco as an engineering consultant for 12 yrs. Bhatasana has authored several papers and has received granted patents on innovative techniques related to proving systems in custody metering.
ABDULLAH A. ALABDULHAI is a liquid and gas hydrocarbon measurement engineer, with a BS degree in mechanical engineering from Oregan State University (U.S.). He has more than 14 yrs of experience with Saudi Aramco with a vast experience in fiscal measurement operations covering pipelines, truck loading and marine loading applications. Alabdulhai was part of an integrated engineering project team and specialists responsible for upgrading the world’s two largest crude measurement marine export platforms and commissioning metering systems at the world’s largest industrial gas complex. He is leading the development of a CO2 fiscal measurement program.
SALEM A. ALSHAHRANI heads the custody measurement group at Saudi Aramco. He earned an MS degree in mechanical engineering from the University of New South Wales and has more than 10 yrs of experience in hydrocarbon flow measurement for custody transfer applications. Alshahrani spearheaded the development of the first-of-its-kind, real-time uncertainty model solution, iMonitor, for crude oil metering systems. His role is developing custody frameworks for CO₂ and hydrogen, and advancing smart metering applications through innovations and cutting-edge technologies while ensuring alignment with regulatory standard requirements.