Api Mpms Chapter 8.2 Pdf Free Download
Nayra Waddles
There are two internationally recognized standards covering tank gauging in atmospheric tanks. From the American Petroleum Institute, there is API MPMS chapter 3.1B and API MPMS chapter 7, and from the International Organization for Standardization, ISO 4266-1, ISO 4266-4, and ISO 11223. This article is based on API MPMS chapter 3.1B and API MPMS chapter 7, because these two documents provide detailed requirements for inventory applications. API MPMS chapter 3.1B mandates the following requirement for automatic tank gauges (ATGs):
This solution is the reference for all other solutions, and it utilizes a dip tape. Manual gauging uses either the innage (from the top flange to the bottom of the tank) or ullage (from the top flange to the top liquid layer) method to measure the total liquid level inside the tank. API MPMS chapter 3.1A covers the procedure and requirement for manual gauging, which is normally three consecutive readings with a difference not exceeding 3 mm.
This solution uses a float attached to a spring via a perforated tape. The spring provides constant tension, which balances the float on the liquid level. The perforated tape is connected to a mechanical counter assembly.
The servo gauge uses the displacement measurement principle. A small displacer (weight) on a measuring wire from a drum is accurately positioned and balanced in the liquid medium using a servomotor. These devices can meet the inventory and custody accuracy requirements.
Radar tank gauging (RTG) is the most common solution for tank gauging. It used to be the Saudi Aramco standard tank gauging solution for inventory tanks. This technology is microwave-based, which measures the distance from the top connection to the liquid surface. The two available techniques are frequency modulated continuous wave (FMCW) and time-of-flight time domain reflectometry (TDR). These devices can meet the inventory and custody accuracy requirements.
A hydrostatic tank gauge (HTG) system has up to three pressure transmitters and one temperature transmitter. Two pressure transmitters are installed close to the bottom of the tank and are used to calculate the density. A third transmitter measures the vapor pressure at the top of the tank to increase the accuracy.
Guided wave radar (GWR) or time domain reflectometry is a two-wire microwave level instrument that transmits a radar signal down a metallic wave guide (single-rod/cable, twin rod, cable, or coaxial rod). The instrument principle of operation is time of flight.
A plan was set to pilot four GWR level instruments from two different manufacturers on different types of tanks with different refinery products, and to monitor the performance over one year. Over this period, the operation team conducted frequent manual gauging (three times per test) to check the installed accuracy of the GWR.
Over one year, the refinery operation team conducted manual dipping to check the reading accuracy of the installed GWR instruments. The data over the year has clearly demonstrated that the accuracy is within the required 25 mm for inventory tank gauging applications. A sample of the data collected is in figure 12.
The successful results of piloting GWR for the inventory tank gauging application revealed a new and suitable solution for Saudi Aramco's Riyadh Refinery project. The project scope was revised to mandate GWR for the project and eliminate all the complexity and constructability challenges. The vendor selection was biding based, since the GWR instruments from both vendors demonstrated a superior and equal performance. The project's final scope of work was revised as follows:
The journey of the Saudi Aramco tank gauging project has resulted in a simple, reliable, and highly economical system for inventory tank gauging. This solution demonstrated full compliance with API MPMS chapter 3.1B in terms of accuracy and performance. Moreover, it provides a versatile solution, which allows different hardware and communication protocols to be used and allows DCS as a platform for software inventory calculations.
It is important to highlight that GWR has fundamental rules set by the manufacturers for selection, installation, and commissioning. Following these rules will guarantee successful performance. Deviating from these requirements can generate unpleasant challenges and errors in measurement.
The GWR success story has also opened another opportunity for using two-wire noncontact radar for inventory tank gauging, as these noncontact radar instruments have the capability to meet the requirements of API MPMS chapter 3.1B. They can be very useful in applications like molten sulfur and asphalt.
This success story had an impact not only on the Riyadh Refinery project, but also on Saudi Aramco standards, which have been revised to specify two-wire GWR for inventory tank gauging, as long as the vendors demonstrate a full compliance to API MPMS chapter 3.1B requirements.
The author would like to acknowledge the support of the Saudi Aramco Riyadh Refinery management, and to acknowledge Riyadh Refinery automation engineers Fawaz AlHadlaq and Khalid Batoq for their assistance in preparing this article and their commitment to conducting the pilot testing and building a new milestone.
Prior to 2016, the methods for measuring and evaluating oil during custody transfer from a wellhead site were described in API MPMS, Chapter 18.1: Measurement Procedures for Crude Oil Gathered from Lease Tanks by Truck. This standard was published in 1990 and became widely recognized as the accepted method for this type of custody transfer. API MPMS, Chapter 18.1 was updated over the years before being supplanted in 2016 by API MPMS, Chapter 18.2: Custody Transfer of Crude Oil from Lease Tanks Using Alternative Measurement Methods, which is now the recognized standard and offers significant advances over its predecessor. At the time of its release, API issued this explanation:
The new standard still allows for the old practices to be retained, but it offers mechanisms to use better methods able to make oil volume measurements more accurate, and the process of transferring oil much safer.
Under API MPMS, Chapter 18.1, large production sites with frequent deliveries could be outfitted with a lease automatic custody transfer (LACT) skid. A LACT uses a sophisticated flowmeter to measure the volume of oil transferred along with other parameters, such as density, but it is not economical for smaller sites and volumes. The manual practices under API MPMS, Chapter 18.1, were designed to work anywhere, including wellhead sites that had effectively zero instrumentation. All the measurement methods could be carried out by a receiving truck driver using a kit of simple manual tools to take readings from the top of the tank. Here are the steps:
Volume transferred can be measured in one of two ways. First, a flowmeter can be used without the need for implementing a full LACT skid. API MPMS has chapters covering a variety of flowmeter technologies used for this purpose. Second, the opening and closing gauge measurements are still used, but with the measurement performed using a level instrument rather than manual measurement. Choosing the most suitable level instrument becomes a critical question. The application calls for several key capabilities:
Meeting these three requirements with one technology narrows the field quickly: Radar instruments that mount from the top not only minimize the need for mechanical modifications to a tank, but they also have no moving parts. This approach is ideal since it provides the precision and resolution needed. API MPMS, Chapter 18.1, called for three consecutive manual readings to agree within 0.25 inches. The right radar level gauge can provide reliable readings with accuracy better than 0.125 inches, meeting the first and second requirements.
The third requirement is the most difficult to meet, as few technologies other than guided-wave radar (GWR) (Figure 1) are capable of detecting and measuring the position of an oil-water interface. Magnetostrictive level instruments can be set up to capture an interface measurement, but effective operation depends on consistent densities of the liquids and free movement of the floats. If tar or other material from the oil accumulates on the rod it can interfere with movement, and the reliability of a reading will be lost. There is no way to tell this is happening from the reading data, short of the float freezing in one position.
A GWR probe can also accumulate buildup, but it takes a lot of material to interfere with the reading. Moreover, the nature of the echo curve can be monitored to indicate if material is building up, allowing appropriate maintenance action to be taken. Additionally, if there is an emulsion layer between the two liquid layers instead of a definite interface, the magnetostrictive device will always float at a point in the emulsion layer. If an operator relies on that measurement for the separation, some of the material identified as oil will be an emulsion of oil and water. Radar, on the other hand, will not measure well with an emulsion, indicating that separation is not complete.
Absent a situation where a LACT skid is available, the manual actions of custody transfer under API MPMS, Chapter 18.1, did not lend themselves to automated data collection. Accurate record keeping depended on the fastidiousness of the driver either writing down the figures legibly or entering them without error into a laptop or tablet. API MPMS, Chapter 18.2, was developed specifically to allow for replacement of many of the manual measurement methods with the additional allowance for measurements to be made outside the more dangerous tank zone.
Using a GWR level gauge along with temperature transmitters and other electronic instrumentation provides the ability to tie readings directly to a data-gathering platform, greatly reducing the potential for errors. Additionally, the use of GWR for level measurement, instead of float-based technologies, can improve measurement reliability and help to monitor separation. The growing availability of WirelessHART instrumentation makes this easier to implement since these require no wired infrastructure for power or data transfer from the instruments. All the instruments necessary to perform a transfer operation under API MPMS, Chapter 18.2, are available as battery-operated wireless units, including GWR level gauges (Figure 2). Wired instruments are also available to perform all the necessary tasks.
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