IBP1058 09 EFFECTS OF PETROLEUM PROPERTIES ON CUSTODY TRANFER MEASUREMENT Raymond J. Kalivoda 1 Copyright 2009, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Pipeline Conference and Exposition 2009 , held between September, 22-24, 2009, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented, does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute" opinion, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Pipeline Conference Proceedings. Abstract Petroleum products bought and sold on the world wide market may be transported over thousands of miles and change ownership many times from the well head to the end user. Each time the product changes ownership, a “custody transfer” is completed and both buyer and seller expect their asset share to be accurately measured. The dynamic measurement provided by meters is a convenient and accurate means to measure valuable petroleum products. Selecting the right meter for the job with a high level of confidence is imperative to ensure accurate measurement at the lowest cost of ownership. The fluid properties of petroleum products varies widely from low viscosity refined products like LPG, gasoline, diesel and jet fuel to high crude oils with a high concentration of contaminants such as sediment & water (S&W) and entrained gas. Understanding the fluid properties of both refined produces and crude oils and how they affect the different metering technologies is important in selecting the right meter for a specific application. This paper will define the fluid properties of both refined products and crude oils, explain their effect on the different custody transfer metering technologies and evaluate these technologies for the accurate measurement of petroleum products. 1. Introduction Refined products are normally well defined and not highly viscous. Crude oils can vary widely in viscosity and quality. They can have a viscosity similar to refined products or be highly viscous. The demand of heavy or high viscous crude oil is increasing due to the price and availability. This trend is expected to accelerate with increasing demand for petroleum products due to the expansion of the world economies and the reduction in light low viscosity crude oil reserves. As a result of this trend crude oil transporters, pipeline and marine, are gearing up to handle a wider range of heavy crude oils. The accurate measurement of high viscosity crude presents new challenges, though. Each of these applications is different and no one type of meter is best for all applications. Fortunately we have a wide range of metering technologies to address low, medium and high viscosity applications: • Positive Displacement Meters, known for their highly accurate measurement of high viscosity products. • Convention Turbine Meters are suitable for most refined products and light to medium viscosity crude oils. • Helical Turbine Meters can handle light to higher viscosity crude oils and with special viscosity compensation software; the viscosity range can be increased to handle high viscosity crude oils. • Coriolis Mass Meters are especially suited for low flow applications over a wide variety of viscosities. 1 Petroleum Measurement Manager - EMC MEASUREMENT SOLUTIONS Rio Pipeline Conference and Exposition 2009 • Liquid Ultrasonic Meters can handle low and medium viscosity products and are gaining wider application experience and acceptance on more viscous products. Knowing the strengths and weaknesses of each of these technologies is important in making the best decision for a given application. 2. Viscosity Characteristics of Petroleum Products Viscosity can be expressed in many different units but for our purposes kinematic viscosity is the most suitable. The most commonly used viscosity units in the petroleum industry are: • Kinematic Viscosity - centistokes (cSt) - The base unit is a Stoke but as the value of Kinematic Viscosity is normally small, the unit is often converted by multiplying by 100, centistokes (cSt). In the metric system the kinematic viscosity unit is cm2/s (centimeter2/second). • Saybolt Universal Viscosity (SSU) - SSU can be converted to cSt using a conversion table. • Dynamic Viscosity centipoise (cP) - The base unit is Poise but like kinematic viscosity, the Poise is often multiplied by 100, centipoise (cP). Centipoise can be converted to centistokes by dividing by the specific gravity, (cSt = cP / SG). In the metric system, the dynamic viscosity unit is Pa^s (Pascal^second). Crude oils are normally defined by their API gravity, which is sometimes confused with the product ’s viscosity. API gravity is defined as the density of crude oil at a specific temperature compared to the density of water at a standard temperature, 60°F. The relationship between specific gravity (SG) and API gravity is: SG (60°F/60°F) = 141.5 / (131.5 + API). Tables 1 and 2 show refined products and crude oil characteristics, respectively. The API gravity and specific gravity are stated at reference temperature. The viscosity is also related to temperature and decreases as temperature increases. As seen in these tables the effect temperature has on viscosity is notably more significant on heavy refined and crude oils then on lighter products and crude oils. This is an import factor in the proper selection of a particular metering technology for many heavy and medium oil applications. Table 1. Refined Products Characteristics Gravity @ 60°F (15°C) Viscosity (cSt) Product £2 Specific API 30 (-1) 60 (15) 150 (66) Propane - LPG 0.51 146 0.3 0.2 0.2 0.2 Gasoline 0.77 52 1.0 0.8 0.7 0.5 Jet Fuel 0.81 43 3.5 2.6 2.0 1.1 Kerosene 0.84 37 3.7 2.7 2.1 1.2 Fuel Oil 3 (max) 0.92 22 - 11 5.7 3.5 Fuel Oil 6 (min) 1.00 10 - 828 155 46 Table 2. Crude Oil Characteristics Gravity @ 60°F (15°C) Viscosity (cSt) Crude Type Specific API 60°F (15°C) 100°F (38°C) 150°F (66°C) 0.79 48 3 2 1 Light 0.86 32.6 21 9 5 Medium 0.90 25.3 1442 243 93 0.95 17.8 2040* 340 130* Heavy 0.96 16.2 3440* 574 230* 1.00 10 5100* 1294 520* * estimated 2 Rio Pipeline Conference and Exposition 2009 3. Meter Selection Selecting the correct meter for a specific measurement task is dependant on the following operating conditions: System Characteristics - Pressure and temperature are typically specified but other characteristics such as pulsating flow from a PD pump or valve operation / location should also be considered as they may cause measurement errors for some types of meters. Product Characteristics - The basic product characteristics of viscosity, specific or API gravity, chemical characteristics and lubricating quality must be specified. Also, any contaminates such as particulates, air or water contained in the product must be identified and noted in an application analysis. Flow Range - This is the minimum and maximum flow rate over which the meter will operate. The flow range can also be expressed as the “turndown range”, which is the ratio of the maximum to the minimum flow rate (e.g., a flow range of 10 bph to 100 bph is a 10:1 turndown range). Viscosity Range - Just as the flow range can be expressed as a turndown range, so can the maximum to the minimum viscosity be expressed as a turndown range. Accuracy - The Accuracy of a liquid flow meter depends predominantly on the flow range and the viscosity range of the products over which the meter operates. The following formula relates Flow Turndown Range to Viscosity Turndown Range to yield a Measurement Turndown Range (MTR). Comparing the MTR of various meter types for specific operating conditions provides a guide to selecting the meter with the best potential accuracy for the application. Measurement Turndown Range = Flow Turndown Range x Viscosity Turndown Range or MTR = FTR x VTR 4. Types of Dynamic Flow Meters Dynamic fluid flow meters can be classified as either direct volumetric meters or inference type meters. A Positive Displacement (PD) meter (Figure 1) directly measures volumetric flow by continuously separating (isolating) the flow stream into discrete volumetric segments. Inference meters determine volumetric flow rate by measuring some dynamic property of the flow stream. Turbine meters, both conventional and helical types, fall in the latter category along with Coriolis mass meters and ultrasonic meters. PD Meters PD meters are highly versatile and have been used for custody transfer petroleum applications since their introduction in the 1930’s. Because of their high accuracy, stability, reliability, mechanical output and ease of proving, they are still widely used in the petroleum industry. Other meter technologies have displaced PD meters for high volume, low viscosity applications like refined product or light crude oil pipelines. PD meters still have measuring advantages for medium to high viscosity products. PD meters are one of the few meters that have highly stable meter factors on medium to high viscosity products. Figure 1. PD Meter Cutaway PD Meter Operating Principle PD meters measure flow by momentarily isolating segments of known volume and counting them. For example in a rotating vane PD meter, as the rotor turns, isolated chambers are formed between blades, rotor, base, cover and housing. Like a revolving door, known segments of fluid pass through the measurement chamber and are counted. 3 Rio Pipeline Conference and Exposition 2009 All PD meters have moving and stationary parts which require clearances between them.