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Characteristics of Crude Oil | API Gravity, Flash Point, Fire Point, Octane Number

Written by Partha Pratim Panja in Piping Design Basics,Process

The important characteristic of crude oil is governed by the properties of pure hydrocarbon in terms of viscosity, density, and boiling point curves. These properties are very important for designing almost every type of equipment in the industry as well as these properties are also indicative of the quality of the product as well as the feed.

Important Characterization Properties

Below are the main characterizations parameters used in the refinery,

  • API gravity
  • Watson Characterization factor
  • Viscosity
  • Sulfur content
  • True boiling point (TBP) curve
  • Pour point
  • Flash and fire point
  • ASTM distillation curve
  • Octane number

What is API gravity?

API gravity is mainly used to define heavier and lighter crude petroleum products. It is a measure of the density of the crude. The API gravity is formulated as

oAPI (API Gravity) = 141.5/S.G– 131.5

Here, S.G means specific gravity is measured at a temperature of 60 oF. At the same time, specific gravity can be calculated from a known API gravity.

An oil of S.G 1 has an API gravity of 10, i,e 10 oAPI. So, API gravity is inversely proportional to the specific gravity. So, the higher the values of the API gravity of an oil lower will be its specific gravity. Thus the oil will be lighter and vice-versa. As far as crude oil is concerned, a lighter API gravity value is more important for lighter crude oil than heavier crude oil as more amount of gas fraction, naphtha, and gas oils can be achieved from the lighter crude oil than with heavier crude oil. So, crude oil with higher API gravity is expensive to procure due to its good quality.

What is Watson’s characterization factor?

The Watson characterization factor is usually expressed as

K = (TB)1/3/S.G (at 15’C)

Here,

  • S.G. is specific gravity.
  • TB is the volume or average boiling point in degrees R (Rankine).

Watson’s characterization factor varies between 10.5 and 12.9 for various crude components. A k factor of 12.9 indicates highly paraffinic crude. On the other hand factor of 10.5 indicates highly naphthenic crude. Therefore, the Watson characterization factor can be used to measure the aromaticity and paraffinic of the crude.

What is Sulfur content?

Since crude oil is excavated from reservoirs, sulfur is present in the crude oil. Organic and inorganic sulfur both usually exist in crude oil. Based on the sulfur content (less than 0.5% wt) crude oil is classified into two categories. One is sour crude which contains more sulfur and the other is sweet crude which contains low sulfur. The normal range of sulfur content of crude oil is 0.5 – 5 wt %. It is observed that almost 80 % of the total crude oil across the globe is sour.

As we know the presence of sulfur is responsible for SO2 emission which is not desirable as well as for catalytic reactions sulfur reduces the efficiency of the catalyst. There are several hydro-treating operations conducted in the refineries to remove sulfur from crude oil. Presently, India is heading towards the generation of diesel with Euro III standards that indicate that the maximum sulfur content is about 500 ppm in the product. This indicates that large quantities of inorganic sulfur need to be removed from the fuel. Typically, inorganic sulfur that is normally found in various intermediate product streams is removed by the hydrogenation process using hydrogen as hydrogen sulfide.

What is True boiling Point (TBP)/ASTM distillation curves?

The TBP/ASTM distillation curve is the most important property of the crude and intermediate product streams. These TBP & Distillation curves are determined at atmospheric pressure. The boiling points of different volume fractions are determined. However, the main difference between True Boiling Point & ASTM distillation curve is that the TBP curve is generated using a batch distillation setup containing less than 100 trays with a higher reflux ratio while the ASTM distillation curve is plotted in a single-stage apparatus and the absence of reflux. Therefore, the TBP curve does indicate a good separation of the different components while ASTM does not indicate a good separation of a different component.

What is the Viscosity of Crude Oil?

Flow properties of refinery streams are measured by viscosity. The unit that is used to define viscosity is centistokes or say bolt seconds or redwood seconds. Normally, the measurements of viscosity are carried out at temperatures of 100 oF and 210 oF. For heavy products obtained from crude oil, viscosity is a very crucial property to measure as well as viscosity plays an important characterization property in the blending units, especially for a heavy product e.g. bunkers fuel.

What are Flash Point and Fire Point?

Flash and fire points are important characterization properties that are related to the process safety and transmission of refinery units. Flashpoint is the lowest temperature at which the crude oil flashes into a vapor that is able to ignite on exposure to an open flame. The flashpoint is an indication of the combustibility of a liquid. Flashpoint is normally measured in the “open cup” or “Closed Cup” apparatus. Some examples of flashpoints are as below,

  • automotive gasoline, −43 °C (−45 °F)
  • ethyl alcohol, 13 °C (55 °F)
  • automotive diesel fuel, 38 °C (100 °F)
  • kerosene, 42–72 °C (108–162 °F)
  • home heating oil, 52–96 °C (126–205 °F)
  • SAE 10W-30 motor oil, 216 °C (421 °F)

The fire point of crude oil is the temperature above the flashpoint where the crude oil produces sufficient vapor to catch fire.

Pour point

The pour point is the temperature below which crude oil will not flow; i.e, it will become plastic. During the cooling of a petroleum product, a cloudy appearance occurs at a specific temperature. This temperature is called the cloud point. On further reduction of the temperature of the product, the product stops the flow. Both pour and cloud points are important characterization properties of the refinery product streams as far as heavier components are concerned. For heavier fractions, they are specified in a certain range & achieved by blending proper amounts of lighter intermediate components. Click here to explore more about the significance of Pour Point.

What is an Octane number?

The octane number is an important characterization property used to define many intermediate streams that undergo blending to produce gasoline, diesel, etc. It is often called the Antiknock Rating which is used to check the ability of a fuel to prevent knocking during the ignition of a mixture with air in the cylinder of a combustion engine. The knocking tendency of a fuel is defined as the maximum compression ratio of the internal-combustion engine at which the knock occurs. Therefore, high-quality automotive gasoline tends to knock the internal-combustion engine at higher compression ratios and vice versa. However, for comparative measurement purposes, one needs to have a pure component that has a known compression ratio. Iso-octane is eventually considered as the yardstick for octane number comparison. If iso-octane is given an octane number of 100 then n-heptanes is given a scale of 0. So, the octane number of a fuel is defined as an equivalent mixture of iso-octane and n-heptane that provides the same compression ratio in an internal-combustion engine. Thus an octane number of 80 signifies that the fuel is equivalent to the performance characteristics in a combustion engine fed with 80 (vol % ) of iso-octane and 20 (vol%) of n-heptane.

Octane numbers are much related to the reforming, alkylation & isomerization processes of the refining industry. The above processes do the reactive transformation to produce side-chain paraffin that possesses high octane number compared to the feed component which does not contain higher quantities of straight-chain paraffin and non-aromatics that are called naphthenes.

Crude chemistry

Generally, Crude oil contains 84 – 87 wt % of carbon (C) , 10 – 14 % of hydrogen(H), 0 .5– 5 wt % of sulfur(S) , 0 – 2 wt % of oxygen(O) , 0 – 0.6 wt % of nitrogen(N) and some metals like copper, lead, sodium, zinc, vanadium, etc ranging from 0 – 100 ppm. It is very important to understand thoroughly the basics of crude chemistry.

Depending on chemical analysis and the existence of various functional groups like –OH,-CH3, =CH2, etc refinery crude can be classified into five main categories as mentioned below,

Paraffins: Paraffin rates the alkanes such as methane (CH4), ethane (C2H6), propane (C3H8), n-butane, iso-butane, n-pentane, and iso-pentane. The below compounds are primarily obtained as a gas fraction from the CDU (crude distillation unit).

Olefins: Basically, it is alkenes such as ethylene (C2H4), propylene (C3H6), and butylenes (C4H8) are highly unstable and susceptible to a chemical reactions. They are not obtained in a sufficient amount in crude oil but are encountered in a few refinery processes such as alkylation.

Major Crude Oil Types
Fig. 1: Major Crude Oil Types

Naphthenes: Naphthenes are the cyclo-alkanes such as cyclo-propane, and methylcyclohexane present in crude oil. Naphthenes are not aromatic and so it does not possess a higher octane number. Therefore, during the reforming reaction, naphthenes are encountered to produce aromatics that possess higher octane numbers compared to naphthenes.

Aromatics: It is the chemical compound that consists of one or more rings with delocalized double bond pi electrons) such as benzene, toluene o/m/p-xylene. Aromatic compounds are available in crude oil. The aromatic compound possesses a higher octane number and it is the objective to maximize its quantity in a refinery.

Naphthalene: It is a polynuclear aromatic consisting of two or three or more aromatic rings.

Click here to explore more about Crude Oils

Online Video Courses on Crude Oils

If you have plan to become a master of Crude oil, its production, and its basics, then check out the following online video courses:

The Rise of Electric Vehicles and Their Impact on Oil Demand

Written by Anup Kumar Dey in Mechanical

The automotive industry is undergoing a revolutionary transformation, primarily driven by the rapid adoption of electric vehicles (EVs). As governments and consumers increasingly prioritize sustainability and energy efficiency, the shift from internal combustion engine (ICE) vehicles to EVs is reshaping not only transportation but also global oil demand. This article delves into the details of electric vehicles, their benefits, the factors driving their adoption, and the subsequent impact on oil demand.

Although the passenger vehicle sector accounts for only about a quarter of total oil demand, it receives much attention from governments and the media. It is believed that a quick transition away from conventional oil-powered vehicles will benefit. Switching from autos to electric vehicles (EVs) is achievable and necessary to minimize greenhouse gas emissions. Emissions and improve the quality of urban air. There have been a lot of studies looking into the influence of electric vehicles on the environment. The oil demand has been made public. The excitement surrounding the potential for EVs to reduce fossil fuel consumption is justified, but it is difficult to derive insight from comparing these published studies.

Sector-wise Global Oil Demand
Fig. 1: Sector-wise Global Oil Demand (Source: energypolicy.columbia.edu)

Metals like copper and lithium have benefited from the growth in the electric car sector. It’s also sparked fears about the long-term prognosis for oil demand, which some analysts argue is unfounded. The electric-vehicle sector has already had a considerable impact on commodities markets, most notably supporting rapid price growth in metals such as copper, platinum, and palladium, vital to the auto manufacturing business. Global miles driven are expected to increase to 32 trillion by 2030, up from 11 trillion now, with emerging nations driving much expansion. They claim that the oil and gas industry outlook is positive for electric automobiles, light trucks, and gasoline-powered vehicles.

The Electric Vehicle Landscape

1. Definition and Types of Electric Vehicles

Electric vehicles are defined as vehicles powered entirely or partially by electricity. They can be categorized into several types:

  • Battery Electric Vehicles (BEVs): These run entirely on electric power stored in batteries and have no internal combustion engine.
  • Plug-in Hybrid Electric Vehicles (PHEVs): These combine an electric motor with a gasoline engine, allowing them to run on electricity or gasoline.
  • Hybrid Electric Vehicles (HEVs): These use both an internal combustion engine and an electric motor, but cannot be plugged in to charge.
  • Fuel Cell Electric Vehicles (FCEVs): These generate electricity onboard through a chemical reaction between hydrogen and oxygen, emitting only water as a byproduct.

2. The Evolution of Electric Vehicles

The history of electric vehicles dates back to the 19th century. However, they gained significant attention in the late 20th and early 21st centuries, spurred by advancements in battery technology, environmental concerns, and governmental support. The introduction of models like the Tesla Roadster, Nissan Leaf, and Chevrolet Volt marked pivotal moments in this evolution.

Benefits of Electric Vehicles

1. Environmental Impact

EVs produce zero tailpipe emissions, significantly reducing air pollution in urban areas. The potential for reduced greenhouse gas emissions depends on the energy sources used for electricity generation. If the electricity is sourced from renewable energy, the environmental benefits are even more substantial.

2. Economic Advantages

  • Lower Operating Costs: EVs generally have lower fuel costs compared to gasoline vehicles. Electricity is often cheaper than gasoline on a per-mile basis.
  • Reduced Maintenance Costs: Electric vehicles have fewer moving parts, which leads to reduced wear and tear and lower maintenance costs over time.

3. Energy Independence

Increased adoption of EVs can lead to decreased reliance on oil, contributing to energy independence for countries heavily dependent on oil imports.

Growing Electric Vehicle Market

The Electric Vehicle (EV) Sector is quickly Expanding:

Since 2014, annual EV sales have doubled to over 1.5 million units. Despite this rapid growth, electric vehicle sales still account for barely 2% of new car sales and less than 0.3 percent of the world’s 1.1 billion passenger automobile fleet. China is. Therefore the largest EV market globally, accounting for over half of global sales in 2018. As Chinese officials aim to address pollution-related issues, large government incentives, both to producers and customers, have contributed to the rapid adoption of electric vehicles.

Over the next few decades, electric vehicle sales are forecasted to skyrocket. The International Whether Energy Agency (IEA) estimates that 120 million electric vehicles will be on the road in 2030, accounting for around 7% of the worldwide passenger vehicle fleet in that year. EVs are expected to become even more cost-competitive by 2030, thanks to technological advancements in battery production. Although government incentives will continue to play an important role in the medium term, these cost reductions should reduce the need for government subsidies. Future EV sales, on the other hand, are fraught with uncertainty. Importantly, the future viability of EVs is contingent on the ambition of future government environmental regulations.

Government subsidies currently cover nearly a fifth of the cost of every new electric vehicle sold worldwide. While several governments have set ambitious goals for electric vehicle adoption in the coming decades, the direction of environmental policies can quickly change the current key EV targets of several major economies). For example, the IEA forecasts that by 2030 there could be as many as 300 million EVs (19 percent of the global fleet) under a more ambitious set of environmental policies, or as low as 57 million (4 percent of the global fleet) under a more modest set of environmental policies.

Transition from Petroleum Cars to Electric Vehicles

Converting to Electric Vehicles Reduces Petroleum Use:

The conversion to electric vehicles (Click here to learn about electric vehicle technology) predicts reducing gasoline usage in passenger vehicles. Electric Vehicle adoption has resulted in a small reduction in this usage. However, as the number of electric vehicles (EVs) increases, the shift from internal combustion engines (ICEs) to EVs may significantly impact gasoline consumption. One-third of EVs on the road are battery electric vehicles (BEVs), and two-thirds are plug-in hybrid electric vehicles (PHEVs) based on IEA predictions (PHEVs). PHEVs are equally powered by gasoline and electricity; thus, they use half as much gasoline as an ICE over the same distance. The degree of gasoline displacement would be higher if we believed BEVs made up a larger share of the EV fleet or PHEVs had a lower requirement for fuel.

According to BP (British Petroleum) forecasts, the typical ICE’s fuel economy will improve from roughly 8 to 5 liters per 100 kilometers traveled between now and 2030. This equates to a nearly 3% annual increase through 2030, faster than the 1.5 percent increase seen during the previous decade. This assumption is based on the increased use of existing technology in emerging-market economies rather than on further big technological breakthroughs. If we assume a lower degree of efficiency, each EV will reduce gasoline usage further. By 2030, the average number of kilometers driven per vehicle will only grow by 4%. If we assume a longer distance per car, the amount of gasoline consumed by each EV will be higher.

The Impact of Electric Vehicles on Oil Demand

Increased use of electric vehicles will cut oil prices, but the forecast for oil demand is dependent on how other industries respond:

Using a structural vector autoregression (SVAR) model for the global oil market, the influence of gasoline savings due to EVs on oil prices. Using a structural vector autoregression model for the global oil market, the influence of gasoline savings due to EVs on oil prices. Oil supply shocks, oil-market-specific demand shocks, storage demand shocks, and global economic growth stocks are the four known structural shocks that affect oil prices. Gasoline savings from EVs can be implemented as an oil-market-specific demand shock inside the scope of the SVAR. The shift to electric vehicles indicates an unanticipated shift in oil demand not influenced by exogenous factors. Oil prices in 2030 might range from US$85 to US$93, depending on the rate of EV adoption, compared to the IEA’s base-case prediction of US$90. In its base-case scenario, the IEA predicts that oil prices will be around $90 (actual 2017 US dollars) in 2030.

Assume 120 million electric vehicles and 23 million metric tonnes of fuel are consumed per day by passengers. Whether oil prices are $5 lower or $3 higher, the oil price might be $5 lower or $3 higher. Compared to the base case, environmental policies are more ambitious or moderate. Even in the most optimistic scenario, we considered, electric vehicles account for less than 20% of the worldwide fleet. If countries only adopt small environmental policies, this number drops to 4%.

Battery Prices:

A fundamental underlying driver of EV adoption is the expectation that battery pack costs will exceed $100/kWh, making EVs competitive with conventional vehicles without government or industry subsidies. Automobile industry forecasters asked to predict the year battery costs will rise the most. The cost of a battery pack is currently estimated to be over $200 per kWh.

Conclusions:

The increased use of electric vehicles is expected to impact the global oil market, but changes are expected to be gradual. According to the IEA, 120 million electric vehicles will be on the road by 2030, but the fleet size could range from 57 million to 300 million, depending on how ambitious environmental policies are. Given the range of reasonable scenarios for the size of the EV fleet, analysis shows that oil prices in 2030 could reasonably move between US$85 and US$93 per barrel, compared to the IEA’s base-case projection of US$90 per barrel. Because EVs will account for less than 20% of global passenger traffic, reducing oil prices associated with their adoption will be modest. So, the major point can be summarised as follows:

As passenger and light vehicle sector only constitutes roughly 25% of the total oil consumer market. Even with aggressive pushing by the government, the effect on oil demand will be less. Also, this impact will be slow and will take at least 10-20 years to reach its peak. So, overall crude oil is here to stay as it will contribute as a significant energy source for other sectors.

Would you like to learn more about electric vehicle technologies? Then just check out the following online video courses:

References:

1. Mukul, J., & @. (2020, November 9). 30% EV Share May Cut Oil Bill By Rs 1 Trn, But Hurt Tax Revenue Too: Study | Business Standard News. 30% EV share may cut oil bill by Rs 1 trn, but hurt tax revenue too: Study. https://www.business-standard.com/article/automobile/30-ev-share-may-cut-oil-bill-by-rs-1-trn-but-hurt-tax-revenue-too-study-120110901210_1.html.

2. How Is EV Charging Going To Transform the Oil And Gas Industry?. (2020, February 25). Driivz. https://driivz.com/blog/ev- -gas-industry charging-transform-oil /.

3. Alternative Fuels Data Center: Electric Vehicle Benefits And Considerations. (n.d.). Alternative Fuels Data Center: Electric Vehicle Benefits and Considerations. https://afdc.energy.gov/fuels/electricity_benefits.html.

4. BADE, G., & @. (2019, September 16). The Oil Industry Vs. the Electric Car. The oil industry vs. the electric car – POLITICO. https://www.politico.com/story/2019/09/16/oil-industry-electric-car-1729429.

Further Studies

If you wish to learn more on the subject I recommend studying the following thesis paper on The Impact of Electric Cars on Oil Demand and Greenhouse Gas Emissions in Key Markets by Jonatan J. Gómez Vilchez available on https://d-nb.info/1188427881/34

Guidelines for Slurry Piping and Pipeline System Design

Written by Anup Kumar Dey in Piping Design Basics

The slurry is defined as a mixture of suspended solid particles and liquids. Various factors like the size and distribution of particles, level of turbulence, temperature, the concentration of solids in the liquid phase, size of the conduit, and viscosity of the carrier decide the physical characteristics of the slurry. In a slurry mixture, a carrying fluid held the solid particles in suspension. The common carrying fluids are water, crude oils with milled coal, and air in pneumatic conveying.

A slurry piping or pipeline system consists of pipelines, valves, and most importantly slurry pumps. A slurry is non-homogeneous, unlike the liquid or gas phase. Hence, the flow of slurry inside a pipe or pipeline system is very much different and they need special considerations. To flow inside a pipeline, the slurry has to overcome a deposition critical velocity. So if the flow velocity of the slurry is not sufficient, the suspension of solid particles will not be maintained. In a broader sense, the flow of slurry inside a pipe can be grouped into two categories:

  • Homogeneous Slurry Flow where solids are distributed uniformly in the liquid carrier, and
  • Heterogeneous Slurry Flow where solids are not uniformly mixed.

However, sometimes slurry flow can have intermediate or mixed flow regimes.

Examples of Slurry Piping System

In general, the slurry is liquid with solids suspended in it. Some of the industrial examples where slurry piping is used for slurry handling are:

  • In coal washeries where coal particles are suspended in water.
  • In the wax manufacturing process, where crystals are suspended in the solvent during the crystallization process.
  • Feed for common filtration equipment.
  • Pulp suspension is encountered in papermaking.
  • Sludge in the effluent treatment process.

Factors Affecting Slurry Behaviour

The main design parameters that affect the behavior of slurry in slurry piping are:

  • The solid concentration that is expressed in wt. % of solid
  • Size and nature of solid particles (soft, hard, abrasive)
  • Density, viscosity, chemical nature, and other properties of liquid

Slurry piping system design is governed by the following considerations:

  • Solid deposition in the system should be avoided.
  • Design should aim that there is no change in slurry composition from the inlet to the outlet of the system.
  • Minimum wear and tear or erosion.

In the following section, we will discuss, the slurry piping system components in brief.

Line Sizing And Pressure Drop

The basic design steps for a slurry system are:

Identify slurry characteristics: Homogenous, heterogeneous, or mixed behavior.

Select slurry concentration: Solid concentration must be known as the specific gravity or density and the rheology of the slurry is dependent on solids concentration which in turn decides the pumping cost. At certain solid concentrations, Slurry will be unstable and very difficult to flow. In such a scenario, proper solids concentration must be known for proper design for transportation. A solid concentration 10-15% below the static settled Slurry concentration is considered stable and convenient for easy handling.

Select trial pipe size: In the next step, a trial pipe size needs to be selected to determine the design velocity using the following formula:

Volume flow rate = design velocity × area of cross-section.

The design velocity is important with respect to the critical velocity.

Critical Velocity:

For slurry systems, when the design velocity is less than the critical velocity of the slurry, the solids may start to separate from the mixture. So, the slurry piping design must aim for a velocity greater than the critical velocity for the given slurry. The critical velocity for a specified Slurry is dependent on various parameters like:

  • Size and specific gravity of solids,
  • Solids concentration viscosity of liquid and
  • Degree of turbulence.

For homogeneous Slurry, the critical velocity is calculated as follows:

A) If the Slurry shows Newtonian behavior, then critical Reynolds’s No. is considered as 2100 and critical velocity is calculated using the following formula:

Critical Velocity =(Reynolds Number x Viscosity)/(Density x Pipe Diameter)

B) When the Slurry is the non-Newtonian type and exhibits Bingham plastics behavior, then critical velocity is calculated by finding the critical Reynolds Number from a chart.

In normal practice, the design velocity (Vd) should be greater than the critical velocity (Vc) by more than 0.3 m/s i.e (Vd-Vc)>0.3 m/s. If this criterion is not met, change the trial pipe size selection and follow the above-mentioned steps again.

Calculate design friction loss: When the selection of trial pipe size is satisfactory, calculate the pressure loss.

Calculate Pump Discharge Pressure: In the next step, calculate the pump discharge pressure, considering the elevation change of the system.

The slope of pipelines: The Slope provided in horizontal pipelines must not exceed the angle of repose for Slurry. There should be provisions for flushing and draining of pipelines and manual cleaning.

Slurry Piping and Pipeline Design Considerations

In general, the following design considerations can be followed while designing slurry piping systems:

  • The use of Short and Direct Routes is preferred for slurry piping systems. It will ensure minimum pressure drop.
  • Elbows shall be 5D or greater.
  • Use Y connections in place of Tee branch connections.
  • Minimize directional changes to avoid Wear.
  • Remove all dead spots where solid accumulation is possible.
  • Valves shall be installed horizontally. For manifolds, locate the valves as close to the manifold as possible.
  • Use Eccentric Reducers Flat on Bottom type.
  • Provide steam and cleaning oil bleed connections from the top of the slurry lines.
  • Allow provision for cleaning to clear the solid build-up.
  • Provide drain on the end-points of long lines.
  • Ensure proper cleaning of such lines.
  • Valves to be used shall be of maximum port size. For ball valves, it shall be full-port.
  • Locate flushing connections at a minimum distance from the origin of the Slurry pipeline.
  • Steel pipes lined with abrasion-resistant lining can highly improve the service life of slurry pipes.
  • Due to greater continued exposure to the slurry, the bottom of the slurry pipes can wear quickly. So, if feasible rotating the pipe periodically will solve this problem.

ASME B 31.11 code provides guidelines for the design of Slurry transportation piping systems.

Slurry Pipelines

Slurry pipelines are used to move slurries for long distances. The main concern with slurry pipelines is pipe abrasion and the associated erosion loss. Various materials like carbon steel, stainless steel, abrasion-resistant lined pipes, alloy steel, hardened steel, non-ferrous pipes, HDPE, GRP, etc are used for transporting slurries. However, slurry pipeline material is decided based on the application, material being pumped, and cost. In recent times, there is a surge of non-ferrous slurry pipelines around the world.

Supporting and Design Consideration of Small-bore Piping

Written by Anup Kumar Dey in Piping Supports

Small-bore piping is defined as pipes that are 2 inches or less in size. In the piping industry, pipes are categorized into two groups; small-bore pipes and large-bore pipes. Any pipe having a size of more than 2 inches is termed a large bore pipe. Whereas piping systems carrying pipes of two inches or less are considered small-bore piping systems. Some organizations consider a 2″ pipe size as large-bore.

Small-bore Pipe Sizes

The usual pipe diameters that are frequently used in small bore pipe connections are

  • 1/8″
  • 1/4″
  • 3/8″
  • 1/2″
  • 3/4″
  • 1″
  • 1-1/4″
  • 1-1/2″ and
  • 2″

Most of the time these lines are instrument connections, drain connections, sampling connections, or vent connections. Almost in all large bore piping systems, there will be some small bore pipe connections. In general small-bore piping is considered non-critical and usually does not require detailed pipe stress analysis. Small-bore pipe sizes usually branch off from the large-bore main pipes.

As very little attention is provided for the design and support of small bore pipe connections in both the process and power piping industry, these pipes sometimes become the cause of pipe vibration and failures impacting the reliability of the complete piping system.

Small-bore piping supports

Small-bore pipes are generally site-supported. In general, they are supported by taking support from nearby large-bore lines. Clamp supports are the most frequently and widely used small-bore piping supports. In absence of nearby parallel large-bore piping, simple steel members are used for support. As small bore pipes do not impose large support loads, the supports are not designed specifically. The following guidelines can be applied for supporting small-bore piping systems:

  • Overhang and unsupported spans for small-bore pipe connections need to be minimized.
  • Support shall be provided near the mass and stress concentration points to reduce the fatigue failure potential.
  • Supporting from the main pipe is preferred such that the small-bore connection moves along with the parent pipe during start-up, shutdown, thermal transients, etc.
  • Neoprene pad, PTFE pad, etc can be used to dampen vibration when supporting vibration-prone lines.
  • Large vertical runs should be avoided and a guide needs to be provided to reduce vibration tendency.
  • The bracing connections if provided between the small bore pipework and the parent line should be in two planes. Also, the bracing shall never be taken from local structures.
  • If the geometry of small bore piping is difficult to support, it must be re-routed to provide support easily.

Small-bore pipes connected to high-temperature large-bore pipes

Small-bore pipe connections that are connected to high-temperature large-bore main pipes must be stress analyzed. For example, small-bore drain piping from high-pressure steam lines needs to be stress analyzed to provide proper flexibility that will be arising from the high temperature of the main pipe. Enough flexibility needs to be provided so that the branch connection interface does not fail due to excessive stress generation.

Vibration in Small-bore pipe connections

Due to the geometry and mass, it carries, small-bore piping connections are prone to vibration. Even a vibration of a very low amplitude vibration on the main piping can lead to excessive vibration in the small-bore branch connection and eventually may break due to fatigue failure.

Design Considerations for Small-bore Pipe Connections

Considering the flexibility and vibration potential of small-bore pipe connections, their sizes are sometimes limited during application as branch connections. The following guidelines can be used in general:

  • Process team to review if unnecessary small-piping can be avoided in the plant.
  • As the failure tendency for small bore piping increases with an increase in cantilever arm length, small bore piping must be routed to have an as small arm as possible.
  • Larger piping is preferable for mechanical strength consideration instead of using support bracing for small bore piping.
  • For direct mounted instrument connections, the minimum size of branch connections to run pipes shall be 3/4″ from 3/4″ through 2″ run pipe, 1″ from NPS 3 through 12, and NPS 1-1/2 for run pipes greater than NPS 12.
  • The location of small bore tappings shall be decided as close to rigid supports on the main pipe as possible.
  • To reduce vibration tendency and increase the mechanical strength of instrument connections bracing can be provided to the run pipe.
  • Non-direct mounted instrument connections from small-bore piping need to be minimized as possible.
  • Pipes with long branch connections are required to be braced.
  • Small bore pipe connections shall be avoided in piping directly connected to compressors, nozzles, or pulsation bottles.
  • The small bore piping connected to the flare tip must be designed to withstand the flare radiation temperature and flare tip thermal movement.
  • While providing stiffeners in small bore piping the differential expansion must be checked. For example, while supporting small bore piping that is connected with pipes or equipment with large thermal movement the differential thermal movement and stress generated must be evaluated.
  • Sufficient rigidity must be provided for small bore piping systems which are prone to vibration.
  • It is preferable to avoid socket welded fittings for small bore connections as those weld points create a weak link.
  • After installation of the plant, a site review must be planned to check all small bore pipe connections to find any problems with support.

Online Video Courses on Piping Support

To learn more about piping support design and engineering you can opt for the following video course.

What is Pipe Routing? | Concepts of Pipe Routing

Written by Anup Kumar Dey in Piping Design Basics

Pipe routing is an engineering technique applied for selecting a proper piping layout fulfilling code and standard requirements, economic considerations, and most importantly safety. It is the responsibility of piping designers and piping design or piping layout engineers to design the most economic and safe piping route considering all the engineering requirements. 

Pipe routing is one of the most complex activities and the core of piping engineering for oil & gas, chemical, and power piping plants. While pipe routing the piping engineer has to comply with various engineering requirements like process requirements, accessibility, safety, constructability, maintenance, and operational requirements. To comply with all such engineering requirements and develop a techno-economical, safe, and cost-effective pipe routing the piping professional must be able to manage the following four major piping components:

Skills Required for Designing a Good Pipe Routing

To develop an economic and safe pipe routing, the responsible piping design engineer should possess the following skills:

Input Reading and Interpreting Skills: The piping design engineer must be competent in reading and understanding P&IDs, Piping Material Specifications, Project Specifications, Engineering guidelines, Code and Standard requirements, Equipment GA drawings, and any other design requirements.

3D and 2D software skills: the piping designer must be efficient to handle the 2D and 3D software.

Piping Skills: The concerned engineer or designer must be conversant with pipes and fittings, instrument items, valves, other inline items, piping supports, Piping Support Spans, and other piping accessories to build a good piping design.

Pipe Routing Concepts

Pipe routing is not a single activity. While doing pipe routing, the engineer must do the support placement and support selection. Without support placement or without deciding the pipe support locations if a pipe layout is decided there is no meaning of that pipe routing. The piping designer must decide on the first support location and then based on the pipe support span decide on the other support locations. If there is a group of lines running together over the same structures then usually the smallest large bore pipe is decided as the basis for support locations.

The usual parameters that must be considered for finalizing the pipe routing of the industrial piping to make it techno-economic and safe are

  • Simple and Straight routing as much as possible.
  • Grouping lines together to reduce the number of structures required for pipe support.
  • Minimize fitting to reduce cost.
  • Provide sufficient flexibility (as per pipe stress analysis recommendation for critical lines).
  • Space optimization.
  • Considering and optimizing expansion loop requirements. Avoid unnecessary piping expansion loops.
  • Headroom clearance, proper accessibility of valves, and other engineering items.
  • Bridge crossing clearance.
  • Road crossing clearance.
  • Keeping provision for future piping.
  • Optimizing pipe spacing or gaps between pipes
  • Considering operational and maintenance space requirements
  • Clash checking considering the worst thermal movements of pipes.

What is Stress Corrosion Cracking (SCC)? Mechanism and Prevention of SCC

Written by Anup Kumar Dey in Engineering Materials,Mechanical,Piping Design Basics

Stress Corrosion Cracking or SCC is a slow failure mechanism of engineering materials in a corrosive environment. Many ductile metals and alloys fail each year due to stress corrosion cracking which starts with a crack initiation, propagation, and growth of that crack to a damaging limit in exposure to a corrosive environment. Stress corrosion cracking is alloy and environment-specific which means the mechanism varies widely depending on the material and environment. A metal that shows SCC tendency in one environment may not be under SCC attack in a different environment. However, the exact mechanism of stress corrosion cracking is not yet fully developed.

Stress corrosion cracking is a slow and delayed failure process. SCC can initiate and propagate with little or no outside warning of corrosion. Cracks usually start at surface flaws by corrosion, wear, or other processes.

In the steel industry, stress corrosion cracking (SCC) is a form of intergranular corrosion that results in crack formation in a corrosive environment. As steel is the most common industrial material, stress corrosion cracking poses a significant threat to industrial systems such as pipelines, power plants, chemical industries, bridges, and so on.

What Causes Stress Corrosion Cracking?

There are three major factors that contribute to the failure following stress corrosion cracking methodology:

  • Tensile Stress (usually because of operational applied stress, thermal stress, or residual stresses from welding and fabrication)
  • Corrosive Environment and
  • Susceptible material in a certain metallurgical condition promotes premature failure in a component.

Other elements for stress corrosion cracking to occur are temperature and time. Fig. 1 below shows a typical example of stress corrosion cracking.

Stress Corrosion Cracking Example
Fig. 1: Example of Stress Corrosion Cracking

The fracture processes in SCC are accelerated by the presence of cracks and other defects on the components. The failures caused by SCC are sudden and catastrophic in nature and are usually caused at much lower stress levels than yield stress. Some of the typical examples of SCC are listed below:

  • Seasonal cracking of brass in an ammonia-rich environment
  • Sensitization and stress corrosion cracking of stainless steels in presence of caustic, chlorides, and polythionic acid.

Types of Stress Corrosion Cracking

Depending on the actual SCC mechanism, various types of stress corrosion cracking are found.

  • Chloride Stress Corrosion cracking– prevalent in austenitic stainless steels in combination with tensile mechanical stress at a high-temperature condition in the presence of chloride ions and oxygen.
  • Caustic Embrittlement-Prevalent in stainless steels in the presence of a high hydrogen concentration under caustic environments.
  • SCC cracking of steels in hydrogen sulfide environment in oil and chemical industries.
  • Sessional Cracking-Cracking of brass in ammonia environments.
  • Craze Cracking-cracking of polymeric materials due to applied stress and environmental reaction.

Characteristic Features of Stress Corrosion Cracking

SCC has the following characteristic features:

  • Stress corrosion cracking failure occurs at stress levels much lower than the material yield stress.
  • The materials subjected to SCC are ductile but the failure mechanism is brittle.
  • The cracks in stress corrosion cracking are in general caused by corrosion.
  • At microscopic levels, intergranular and transgranular cracks are the major features of stress corrosion cracking. Intergranular cracks grow along grain boundaries but transgranular cracks proceed across the grains.

Materials Susceptible to Stress Corrosion Cracking

The following materials are prone to SCC attack:

  • Stainless steels (In the temperature range of 415°C to 850°C in chloride, caustic, and polythionic acid environment)
  • Carbon Steel (In carbonates, strong caustic solutions, nitrates, phosphates, seawater solution, acidic H2S, and high-temperature water environment)
  • Copper and copper alloys (In an environment containing ammonia, amines, and water vapor)
  • Aluminum and aluminum alloys (In environments containing moisture and NaCl solution)
  • Titanium and titanium alloys (In exposure to seawater, fuming nitric acid, and methanol-HCl environment)
  • Polymers (In aggressive acid and alkali environments)
  • Ceramics

Stress Corrosion Cracking in Welding

The major cause attributed to stress corrosion cracking is the residual stress generated during welding and fabrication processes. In the stress corrosion cracking of metal alloys, the residual stress due to welding plays a crucial role. Stress corrosion cracking in welding is caused by non-uniform temperature changes during welding. Also, while welding certain steel grades, the solid-state transformation of austenite to martensite during cooling generates a significant amount of residual stresses. For carbon and low alloy steels, while quenching the austenite containing carbon atoms at a fast cooling rate, martensite is formed. In this situation, the carbon atoms do not get a chance to diffuse out from the crystal structure and form cementite. This increases the metal volume which results in significant residual stress.

Mechanisms of Stress Corrosion Cracking

Depending on the type of material and environment, various different Stress Corrosion cracking mechanisms are prevalent in the industry. Some of the well-known SCC mechanisms are:

Mechano-electrochemical model

As per this mechanism of stress corrosion cracking, in an alloy microstructure, there are pre-existing regions that become sensitive to anodic dissolution.

Film rupture model

For alloys having a passive layer on their surface, the film rupture SCC mechanism is well-known. In this mechanism, the corrosion starts following plastic deformation. The plastic strain disrupts the film which discloses the bare metal to the corrosive environment. Soon, a localized SCC attack starts in those areas and the process is repeated resulting in the growth of the cracking.

Adsorption phenomenon

The SCC mechanism considers the material embrittlement in the vicinity of a corroding area.

Pre-existing Active Path Model

Intermetallics and compounds are formed in the already existing paths like grain boundaries which are prone to SCC attack.

How to prevent Stress Corrosion Cracking?

As the mechanism of stress corrosion cracking is not yet fully understood, the prevention methods are based on empirical experiences. In general, one or more of the following methods can reduce the possibility of SCC:

  • As tensile stress is one of the major elements helping stress corrosion cracking, lowering the stress levels in components will reduce the potential of an SCC attack. By providing annealing treatment of the component, residual stress can be eliminated to a great extent.
  • Eliminating or decreasing aggressive species from the environment where the component is installed will serve as one method of reducing SCC attacks. For example, in the case of austenitic stainless steels, maintaining chloride content below 10 ppm significantly reduces the probability of SCC.
  • Selecting more stress corrosion cracking resistant materials will protect the product from stress corrosion cracking.
  • The application of cathodic protection reduces failures from stress corrosion cracking.
  • In mildly corrosive media, adding phosphate and other organic and inorganic inhibitors can reduce the stress corrosion cracking effects.
  • Sometimes, applying a protective coating can be effective.
  • Using the shot-peening method to produce residual compressive stress in the component surface can prevent stress corrosion cracking.
  • Controlling the temperature and the electrochemical potential reduces the SCC possibility.