Crude oil vapour pressure testing
Vapour pressure testing is an important safety check in the transport, storage and blending of crude oil
Hannes Pichler and Klaus Hense
Grabner Instruments, a subsidiary of Ametek
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It is very important for the true vapour pressure (TVP) and Reid vapour pressure (RVP) of crude oil to be tested when dealing with its production and storage. The vapour pressure indicates how the crude oil will perform during handling, highlights conditions under which bubbles are likely to build and shows where pressure build-ups of escaping light ends could happen. As such, vapour pressure measurement prevents costly damage to pipelines or vessels transporting crude oil. It also provides guidance on how transportation facilities need to be built to survive a worst-case scenario.
Pipeline operators determine TVP and bubble point before transporting the crude to a distribution point. With wellhead crude, gas has to be removed to meet pipeline, storage and tanker specifications. Excess gas can be separated or flared to regulate the vapour pressure and prevent any damage to the transportation medium. If the vapour pressure of the crude case is too high, two main safety issues can arise. The first is pumping cavitation during transfer operations and the second is vapour pressure in the pipeline, or in a vessel, which could rise because of temperature changes. Once the transportation medium is exposed to direct sunlight, the vapour pressure of the crude oil rises and, in the worst case scenario, causes damage. Hence, it is advisable for operators to be equipped with a vapour pressure analyser, to prevent costly damage and to provide evidence that the released crude is delivered according to specifications.
TVP measurement and bubble point determination
The exact definition of TVP and bubble point is a topic of wide discussion in the engineering community. First, it is worth mentioning that the bubble point refers to a temperature, whereas the bubble point vapour pressure refers to a pressure. According to the International Maritime Organisation, the TVP and bubble point pressure (BPP) are equal: “The TVP or bubble point vapour pressure is the equilibrium vapour pressure of a mixture when the gas/liquid ratio is effectively zero. It is the highest vapour pressure which is possible at any specified temperature. As the temperature of a petroleum mixture increases, its TVP also increases. If the TVP exceeds atmospheric pressure, the liquid commences to boil.” (IMO, 2006, p140). In this definition, TVP is essentially the total vapour pressure (Ptot) of the crude oil minus the vapour pressure of air and the dissolved gases in the sample (Pgas).
The resulting value is the absolute vapour pressure (Pabs or Pliquid) of the liquid, commonly referred to as TVP, as measured by ASTM D2879 (US EPA, 2006). The ASTM D2879 method should be used only for single-component substances. Crude oil in general is a multi-component liquid and thus requires a different form of analysis.
The equation TVP = BPP at a vapour-to-liquid ratio (V/L ratio) of 0/1 addresses the common practical problems for crude oil transportation. In floating roof tanks, the roof is placed directly on the liquid crude oil, while in pipelines the liquid crude oil is pressurised. In both cases, the V/L ratio is effectively zero and the TVP measurement gives a precise indication of the bubble point pressure at a specified temperature.
In some cases, the equation is not sufficient. In multi-component mixtures, bubbles tend to build whenever the vapour pressure of the liquid exceeds the environmental pressure (IMO, 2006), independent of whether the V/L ratio is 0/1 or 100/1. Three factors have an influence on the bubble point: pressure, volume and temperature. A more accurate definition takes different temperatures and V/L ratios into account when determining the bubble point: in a multi-component mixture, the bubble point is the temperature at which the first bubbles appear at a fixed V/L ratio.
One of the major risks when transporting crude oil is pumping cavitation. Cavitation happens when the TVP or bubble point vapour pressure at a V/L ratio near 0/1 is reached and usually has drastic effects. In a pumping system, the crude oil is accelerated, generating areas of low pressure. When the surrounding pressure is lower than the vapour pressure of the crude oil, bubbles build, grow, then collapse, generating high pressure and high temperatures at the bubble surface. Near a fixed surface, for instance in a pumping system, the collapse of the cavitation bubble will generate a shockwave directed to a nearby surface, which can damage the transportation system or the pump.
For correctly dimensioning new pipelines and pumping systems, and for writing standard operating procedures, it is important to know the vapour pressure of the crude oil. To prevent bubble building and pumping cavitation in an installed transportation system, it is necessary to ensure that the pressure in the transportation system is higher than the vapour pressure of the crude for any expected condition.
If the vapour pressure is too high, pressure, volume or temperature have to be modified. The pressure in the transportation system can be increased to exceed the vapour pressure of the crude oil. Also, the vapour pressure of the crude oil can be reduced, for instance by the separation or burning of excess gases, which reduces the volume. As a third option, the temperature for transportation and thus the vapour pressure of the crude oil can be reduced. Lord & Ruddin suggest oil degasification or oil cooling to reduce the vapour pressure: “The degasification program removes gas from oil in selected caverns, which reduces its bubble point pressure and gas-oil ratio, which in turn significantly increases the predicted margin of system performance under the safety criteria.”2
Crude oil producers face a complex problem here: when crude oil is extracted, it is not homogenous. The vapour pressure of the crude can change during oil production. Also, the presence of various amounts of natural gas in the crude changes the V/L ratio considerably and increases the vapour pressure dramatically. Plus, highly viscous crude oil needs to be transported at a high temperature to guarantee a flow in the pumping system and in the pipeline. Temperature changes in turn affect the vapour pressure.
Depending on the amount of light ends delivered with the crude oil, the vapour pressure of the crude oil will be significantly higher for 60°C compared to the vapour pressure at 37.8°C. Most vapour pressure testers measure crude oil only at 37.8°C (100°F) and a V/L ratio of 4/1. But crudes can sometimes be transported at 85°C and at a V/L ratio close to 0/1.
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