Benchmarking Crude Oil

These are many different varieties and grades of crude oil. Crude Oil quality is measured by its density and sulphur content. Benchmark oils are used as references when pricing oils. There are approximately 161 different benchmark oils, of which the main three West Texas Intermediate, Brent Crude, and Dubai Crude. Their pricing is used as a barometer for the entire petroleum industry. The pricing is usually quoted based on F.O.B. (free on board, the point at what the seller transfers ownership of the goods to the buyer without consideration of final delivery costs). The delivery locations are usually sea ports close to the oil fields from which the crude was obtained.

Crude oil is considered "heavy" if it has long hydrocarbon chains, or "light" if it has short hydrocarbon chains: an API gravity of 34 or higher is "light", between 31-33 is "medium", and 30 or below is "heavy". Crude is considered "sweet" if it is low in sulphur content (< 0.5%/weight), or "sour" if high (> 1.0%/weight). Generally, the higher the API gravity (the "lighter" it is), the more valuable the crude.

Brent Crude is typically priced at about $2 over the WTI Spot price, which is typically priced $5 to $6 above the EIA's Imported Refiner Acquisition Cost (IRAC) and OPEC Basket prices. 

Crude oil is the most actively traded commodity and is bought and sold in “contracts.” A contract trades in units of 1,000 barrels of oil and benchmarks help to determine the price of a barrel of oil in a contract.

Why ?

There is always a spread between WTI, Brent and other blends due to the transportation cost. This is the price that controls world oil market price.

Types

WTI @ Cushing

West Texas Intermediate is used primarily in the U.S. It is light (API gravity) and sweet (low-sulfur) with an API gravity of 39.6 degrees.It contains 0.24% sulphur.

Brent @ North Sea (Europe) Brent Blend / London Brent / and Brent petroleum

Brent is not as light or as sweet as WTI and it is still a high-grade crude with an API gravity of 38.06.  The sulfur content is 0.37%. 

Brent Crude is a mix of crude oil from 15 different oil fields in the North Sea.

The price of Brent Crude is used to set prices for roughly 2/3 of the world’s oil. It is mostly refined in Northwest Europe and is also called Brent Blend, London Brent, and Brent petroleum. The Brent field is located in the East Shetland Basin, halfway between Scotland and Norway.

Edmonton Par and Western Canadian Select (WCS) 

They are benchmarks crude oils for the Canadian market. 

Both Edmonton Par and West Texas Intermediate are high-quality low sulfur crude oils with API gravity levels of around 40°. 

In contrast, WCS is a heavy crude oil with an API gravity level of 20.5°.

OPEC Reference Basket (ORB)

This is not a specific crude, but rather is a weighted average of petroleum that comes from OPEC countries. There are currently 11 different oils combined into the ORB. It averages an API gravity, with the present combination, of 32.7 degrees and has a sulfur content of 1.77%. It was recently changed to high density (decreased the API) and increased the sulfur content of the basket.

The OPEC basket is slightly heavier and more sour than Brent. 

Dubai Crude / Fateh

Dubai Crude is light and sour, with an API gravity of 31 degrees and a specific gravity of 0.871. Its sulfur content is 2%, It is generally used for pricing oil that comes from the Persian Gulf. Its importance comes not only from its quality, but also from the fact that it was the only freely traded oil from the Middle East until recently.

Tapis

Tapis is often referred to as the “World’s Costliest Oil” and comes from a single field in Malaysia. Its value comes from the fact that WTI and Brent Crude are difficult and expensive to export to Asia and because it is of extremely high quality. Tapis has an API gravity of 45.2 degrees and a sulfur content of 0.0343%. These are exceptional numbers, indicating that Tapis is very light and very sweet. Unfortunately, output from the Tapis field has been falling steadily since 1998.

Bonny Light

Bonny light comes from Nigeria and is a light, sweet oil.

PSV Chattering

Chattering is rapid opening and closing of a pressure relief valve. The rapid viberations may cause cause misalignment, valve seat damage, and mechanical failure of valve internal and associated piping.

Causes for chattering:
 - Excessive inlet pressure drop or PSV inlet long line ( This will result in considerable decrease in pressure by the time it reaches the PSV and the PSV shall close and not relieve the excess fluid. Then the fluid will go back to the vessel and then to the PSV and back and front and cause damage).
- Excessive back pressure
- An oversized relief valve
- A relief valve that can handle widely varying florets
- PSV inlet line is smaller than the PSV vale inlet
- Evidence of line pulling with corrosion or process materials 
 - Set Pressure is equal or very close to the Operating Pressure
 - PSV inlet and discharge pressure are very close (Inlet and outlet pressure)
- PSV inlet line pressure drop has to be well below 3 % ( very imp.)
- Multiple PSVs with same pressure setting
-For larger size of safety valves, chatter could occur because they have relatively small outlet area ratio. (the ratio is specified by API526)

Difference between chatter and flutter/simmer
Pressure Relief Valve Chatter is rapid reciprocating motion of the disc where the disc contacts with the seat in cyclic motion. Flutter/Simmer is similar to chatter except the disc does not contact with the seat during cyclic motion.

Why Chatter is destructive ?

Chatter is destructive where it repetitive and rapid reciprocating disc knock on the valve seat may cause

i) damage to the disc & valve seat lead to leakage and passing
ii) damage to bellow lead to gas leak via bonnet vent
iii) damage to the PRV itself and interconnecting piping through vibration
iv) severe noise level
v) Reliving flow rate reduction caused by insufficient valve opening due to chatter 


When the relief rate is lower than 25% of PSV rated capacity chattering will be occured because relief gas kinematic force should overcome to PSV spring force and let PSV start to opening and after PSV release if relief flowrate be lower than PSV discharge rate, PSV will be closed again and waiting for increasing of flowrate to minimum rate and after that cycling start again.

Tank Blanketing or padding

Tank blanketing, or padding, refers to applying a cover of gas over the surface of a stores commodity; usually a liquid. 

• Purpose

- Prevent volatile vapours from escaping
 If the liquid is volatile it is used to protect or contain the stored product and prevent it from harming  personnel, equipment, or the environment. It can maintain the atmosphere above a flammable or combustible liquid to reduce ignition potential.
- Prevent oxidization (oxygen entering)
If the liquid can be oxidized it protects it from oxidation or contamination through exposure to air or moisture.
- Prevent corrosion
It prevents outside air, moisture, and other contaminents from entering and causing corrosion.
It can also reduce the moisture content
- Maintaining constant pressure
If for some reason the tank suddenly cools, the vapours inside the tank will suddenly condense causing the the tank pressure to decrease.

The blanketing is typically set for as low as possible ( minimize the consumption of 2 in of H20) to minimize the wastage of blanketing gas.

• Gases 

- Gas such as nitrogen is supplied in a very pure and dry state.
- LLP Fuel gas 

• Approach

In order to effectively perform any of these functions the blanketing system must be capable of pressurizing the vapor space and accurately maintaining that pressure.

PURGING

One method involves continuous purging, whereby blanketing gas introduced into the tank as a continuous flow exits through a vent or other opening. This method is wasteful of the blanketing gas and not always effective in maintaining an inert atmosphere.

PRV

Another approach employs a simple, direct-operated pressure-reducing valve (PRV) to blanket the tank. However, these devices are the best suited to a continuous flow rate. When used to blanket a tank they must throttle over a wide flow range. Ranging from a shut-off to a full flow condition varies the controlled pressure significantly due to droop and lockup conditions.

CONTROLS/VALVES TO MAINTAIN SET PRESSURE

One of the more effective techniques relies on a blanketing system with the necessary controls and valving to sense and maintain the set pressure within the tank to as close as ± 0.25 inch w.c. [water column] (± 0.009 psi). Set pressures of 0.5 inch w.c. are possible and common.

Such a system will directly sense the tank pressure, control the inlet blanketing gas pressure if required and, through a main control valve, throttle blanketing gas into the tank.

The system may also provide a purge in the sensing and main supply piping, plus reverse flow protection to keep product out of the blanketing system. The installation may also require a pressure switch to monitor tank pressure and perform alarm or control functions in response to over- or under-pressure conditions.

http://www.spartancontrols.com/~/media/resources/emerson/ca/55_emerson_catalog.pdf

Dry and Wet Gas

The difference between dry and wet gas:
- 

• Dew Point 


wet or dry- is based on the operating temperature corresponding to its dew point. Some reference state when operating temperature 10F higher than the dew point temperature, it is considered as dry gas.To be conservative , the operating temp. should be the minimum operating temp. possible.

Dry gas --------> Min. operating temp. - dew point = at least 10F

The difference can be in % or 30-40F higher than the dew point. The more the margin the safer it is to make sure there is no condensation.

The reasoning behind it that the operating temperature shall high enough to prevent condensation due to pressure drop along the pipeline.

•  BTU

The difference is the amount of water vapor it hold and consequently the amount of BTU produce.

Dry Gas is defined as being less than 1050 BTU 

West Gas - 1050 BTU to 1350 BTU Super Rich - greater than 1350 BTU. 

The super rich will contain more condensate and high API oil. The wet gas, typically more LNG's.

•  HC content

When natural has a high methane concentration, the gas is considered dry. 

When natural gas contains ethane, butane, pentane or natural gasoline, collectively referred to as natural gas liquids (NGL) or condensates, the gas is considered wet.

Wet gas: This usually refers to unprocessed gas. Basically as it comes from the well. Even after it has gone through the gathering stations process (removing BS&W) it is still considered "wet" due to the H2O content it still has. Not all gas that comes from the well is "wet". There are places that produce very dry gas. 

Dry gas: The finished, ready for sales gas (mostly methane) at the end of the process.

• Usage

NGLs are currently more valuable than natural gas and typically follow oil prices. Wet gas ia more valuable than dry or sales gas. Sales gas is used for home heating.

To extract the NGLS, the wet natural gas is transported through pipeline to a processing plant. At the plant, the liquids are first extracted from the natural gas and then separated into their base components. The remaining portion of the gas after the methane (used for such things as home heating) has been separated out. There can be a number of different products here, such as; ethane, propane, gasolines, butanes, iso-butanes, etc... They will be processed further. 

The boiling points for the different products in the stream are different and to separate out the product you are looking for you must reach the temperatures that give you both liquids and gas (that's how they separate....then the gas goes out the top of a vessel and the liquids fall to the bottom). In order to reach the temperature necessary to keep the methane in gaseous state while putting the others into a liquid state you are reaching temperatures of roughly -140 degrees Fahrenheit. Were there to be water or CO2 in the stream you would freeze up your process. Potential catastrophic damage to equipment and piping. 

Depressuring Basic

• In case of fire, the operator will push the EDP push button in the control room. That will initiate SDV valves closing at both outlet and inlet of an equipment and BDV valves opening. The hydrocarbon fluid will be released to flare so that the pressure of the system will be depressurized to lower pressure at certain. (The pic is stolen from another blog, its too good not to steal..the link to that blog is http://process-eng.blogspot.ca/2012/05/basic-depressuring-why-15-minutes.html)

• For thickness of vessel less than 1 inch, the system is depressurized to 100 psig, and for more than 1 inch thickness can be depressurized to 50% of design pressure. The depressuring time can be longer and less than 15 minutes. The depressuring time of 15 minutes is only an example in API STD 521 which is applicable for carbon steel vessel with has thickness greater than 1 inch.

• Depressuring time depends on the vessel thickness
• Refer to section 5.15.1.2 in API 521 for graphs for steel plates and carbon steel for various thicknesses 3.2 mm, 12.7 mm and 25.4 mm thickness.

• Adiatbatic depressuring case
• Normally for most plants are shutdown for annual maintenance purpose. Such a system is depressurized to atmosphere condition. In this case, the system is depressurized in adiabatic condition, which means no heat input to the system. During depressurization, the pressure decreases, and the temperature decreases as well. The final temperature of adiabatic depressuring could be very low. As Process Engineer, we have responsibility to determine the Minimum Metal Design Temperature (MDMT) for each system zone based on this case.

• Fire case is use only for define max capacity of BDV and flare system.
• In Hysys we can calculate depressurization by: