Production • Processing • Handling
Understanding the limits of ultrasonics for crude oil measurement
Raymond J Kalivoda outlines benefits of using ultrasonic flowmeter technology. L
iquid ultrasonic flowmeters (LUFMs) are gaining acceptance in the petroleum industry for a wide range of applications. Initially they were used for non- custody applications. Today, advances
in microprocessors, transducers and electronic technology allow multipath LUFMs can provide highly accurate custody transfer flow measurement. Tey are now accepted and routinely used in many European countries, and API Standard 5.8 Measurement of Liquid Hydrocarbons by Ultrasonic Flowmeters Using Transit Time Technology recognises this technology in North America. High accuracy and low maintenance are key
features that are driving this technology. Ultrasonic meters, like turbine meters are inference meters. Tey infer the volumetric through-put by measuring the velocity over the flow area. As with all velocity meters, they are Reynolds Number dependent, that is, they are more or less affected by the relationship between velocity and viscosity. Tey may also be affected by entrained solids, water, gas and wax. Tese characteristics can affect both the short term accuracy and long term stability of an ultrasonic meter.
Ultrasonic flowmeters for crude oil Crude oil measurement, unlike refined products, defines a wide range of applications from light condensates with a viscosity of less than 0.5cP to heavy crude oils over 2000cP. Te quality of the crude oil, that is the amount and type of containments, also varies widely. Viscosity can be expressed in many different
units. For our purposes kinematic viscosity, which is expressed in centistokes (cSt), is the most suitable. Te other commonly used viscosity units in the petroleum industry are: dynamic viscosity; centipoise (cP), which can be converted to centistokes by dividing by the specific gravity, (cSt = cP / SG); and Saybolt Seconds Universal (SSU) viscosity, which can be changed to centistokes with a conversation chart. Crude oils are normally defined by their API
gravity, which is sometimes confused with the product’s viscosity. API gravity is defined as the
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density of crude oil at a specific temperature compared to the density of water at a standard temperature of 60°F. Te relationship between specific gravity (SG) and API gravity is:
SG (60F/60F) = 141.5 / (131.5 + API)
API gravity is loosely related to viscosity. For light crude oils there is a fairly close relation between viscosity and API gravity. For medium crude oils and heavy crude oils, it is important to obtain the viscosity from the assay or from a specific viscosity test.
Te viscosity of all liquids varies with temperature as Table 1 illustrates. Te effect of temperature for medium and heavy crude oils can significantly change a meter’s performance due to the considerable change in viscosity. For this reason, it is important when evaluating any meter application that the viscosity of each product must be specified over the operating temperature range.
Table 1. Effect of temperature on the viscosity of selected products. API gravity for selective crude oils
60 (15)
48 API 32.6 API 25.3 API 17.8 API 16.2 API 10 API
* estimated
Fluid properties Sediment and Water (S&W) is a collective term for non-hydrocarbons found in crude oil. In API MPMS Chapter 1, S&W is defined as: “A material, coexisting with yet foreign to a petroleum liquid ... may include free water and sediment (FW&S) and emulsified or suspended water and sediment (SW&S).” Since all pipelines regulate the amount of S&W they will accept, normally less than 1 per cent, a crude oil within these requirements is termed
2.7 21
1442 2040* 3440* 5100*
Viscosity in cP @ °F (°C) 100 (38)
1.7 9
243 340 574 1294
150 (66)
1.1 5
93
130* 230* 520*
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