Understand Petroleum Refining for Better Process Control Support

Oil refineries can have different specialties and function

The petroleum refining industry provides an expansive market for process measurement and control instrumentation and equipment, valves, and process analyzers. Having a basic understanding of the industry can help purveyors of instrumentation and equipment properly address customer needs, as well as recognize where opportunities may lie. Here is a summary of the types of plants and processes.

Petroleum refineries produce liquefied petroleum gases (LPG), motor gasoline, jet fuels, kerosene, distillate fuel oils, residual fuel oils, lubricants, asphalt (bitumen), and other products through distillation of crude oil or through re-distillation, cracking, or reforming of unfinished petroleum derivatives.

There are three basic types of refineries:

• Topping refineries
• Hydroskimming refineries
• Upgrading refineries (also referred to as “conversion” or “complex” refineries). 

Topping refineries have a crude distillation column and produce naphtha and other intermediate products, but not gasoline. There are only a few topping refineries in the U.S., predominately in Alaska.

Hydroskimming refineries have mild conversion units such as hydrotreating units and/or reforming units to produce finished gasoline products, but they do not upgrade heavier components of the crude oil that exit near the bottom of the crude distillation column. Some topping/hydroskimming refineries specialize in processing heavy crude oils to produce asphalt.

The vast majority (75 to 80 percent) of the approximately 150 domestic US refineries are upgrading/conversion refineries. Upgrading/conversion refineries have cracking or coking operations to convert long-chain, high molecular weight hydrocarbons (“heavy distillates”) into smaller hydrocarbons that can be used to produce gasoline product (“light distillates”) and other higher value products and petrochemical feedstocks.

Figure 1 provides a simplified flow diagram of a typical refinery. The flow of intermediates between the processes will vary by refinery, and depends on the structure of the refinery, type of crude processes, as well as product mix.

Figure 1 - Refinery Flow Diagram
Wikipedia - www.en.wikipedia.org/wiki/Petroleum_refining_processes

The first process unit in nearly all refineries is the crude oil or “atmospheric” distillation unit. Different conversion processes are available using thermal or catalytic processes, e.g., delayed coking, catalytic cracking, or catalytic reforming, to produce the desired mix of products from the crude oil. The products may be treated to upgrade the product quality (e.g., sulfur removal using a hydrotreater).

Side processes that are used to condition inputs or produce hydrogen or by-products include crude conditioning (e.g., desalting), hydrogen production, power and steam production, and asphalt production. Lubricants and other specialized products may be produced at special locations.

Temperature Measurement: Thermistors, Thermocouples, and RTDs

This post explains the basic operation of the three most common temperature sensing elements - thermocouples, RTD's and thermistors.

A thermocouple is a temperature sensor that produces a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and maintained, the voltage produced by the sensing junction can be measured and directly applied to the change in the sensing junctions' temperature.

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, sufficiently accurate for many uses, have a nearly linear temperature-to-signal output curve, come in many “types” (different metal alloys) for many different temperature ranges, and are easily interchangeable. They require no external power to work and can be used in continuous temperature measurement applications from -185 Deg. Celsius (Type T) up to 1700 Deg. Celsius (Type B).

Common application for thermocouples are industrial processes, the plastics industry, kilns, boilers, steel making, power generation, gas turbine exhaust and diesel engines, They also have many consumer uses such as temperature sensors in thermostats and flame sensors, and for consumer cooking and heating equipment.

Coil wound RTD element
(image courtesy of Wikipedia)

RTD’s (resistance temperature detectors), are temperature sensors that measure a change in resistance as the temperature of the RTD element changes. They are normally designed as a fine wire coiled around a bobbin (made of glass or ceramic), and inserted into a protective sheath. The can also be manufactured as a thin-film element with the pure metal deposited on a ceramic base much like a circuit on a circuit board. 

Thin-film RTD element
(image courtesy of Wikipedia)

The RTD wire is usually a pure metal such as platinum, nickel or copper because these metals have a predictable change in resistance as the temperature changes. RTD’s offer considerably higher accuracy and repeatability than thermocouples and can be used up to 600 Deg. Celsius. They are most often used in biomedical applications, semiconductor processing and industrial applications where accuracy is important. Because they are made of pure metals, they tend to more costly than thermocouples. RTD’s do need to be supplied an excitation voltage from the control circuitry as well. 

The third most common temperature sensor is the thermistor. A thermistor functions similarly to a RTD in that it exhibits a change in resistance associated with a change in temperature. A difference between the two is that, instead of using pure metal, thermistors use a very inexpensive polymer or ceramic material as the resistance element. The practical application difference between thermistors and RTD’s is the shape of the resistance curve for the devices. The RTD is linear, whereas a thermistor is non-linear, making it useful only over a narrow temperature range. 

Thermistor bead with wires
(image courtesy of Wikipedia)

Thermistors however are very inexpensive and have a very fast response. They also come in two varieties, positive temperature coefficient (PTC - resistance increases with increasing temperature), and negative temperature coefficient (NTC - resistance decreases with increasing temperature). Thermistors are used widely in monitoring temperature of circuit boards, digital thermostats, food processing, and consumer appliances.

How To Protect Wired Equipment From Lightning Strikes

There are methods to protect process control
equipment from lightning strikes

Lightning, though visually and audibly fascinating, is a high risk environmental factor for process measurement and control equipment. Recent advancements in the study of this natural occurrence have significantly increased general understanding of the source and path of a lighting strike, granting some additional insight into mitigating the risk to electronic equipment associated with lightning activity.

The abstract included below provides a useful synopsis of grounding, surge protection, and other methods for providing levels of protection from lightning strikes. Of particular interest is the concept of lightning ground potential rise (L-GPR) and how it can be used to predict an impending lighting event.

The predictive technology, combined with automated disconnection of the protected equipment from the AC power service can provide superior protection against damage caused by lighting strikes. Browse the paper below, as it is concise, illustrated, and will improve your understanding of lightning and how it impacts industrial process control equipment.

More information and consultation on enhancing your level of lightning protection is available from Analynk Wireless.

Lightning Protection Considerations for Wired Systems from Analynk Wireless, LLC

Practical Considerations for Wireless Transmission of Industrial Process Control Signals

Industrial process signal transmitter

Rigging up the proper gear to establish wireless transmission of process measurement signals is generally a straight forward task. There are, however, a vastly different set of considerations than those for a wired transmission of the same signal. In order to select the right equipment for the job, some general comprehension of radio signals can be useful.

Radio wave frequencies are below the infrared range on the electromagnetic spectrum, thus their wavelengths are comparatively long. Three things can happen to electromagnetic radiation (radio waves) when encountering a barrier. 

• Reflectance: The wave bounces off the barrier.
• Transmittance: The wave passes through the barrier.
• Absorbance: The wave is stopped.

Which of the three possibilities will occur depends upon a number of factors relating to the signal and the barrier, some of which include:

• The wavelength of the radiation
• The intensity of the radiation hitting the barrier
• The chemical composition of the barrier
• The physical microstructure of the barrier
• The thickness of the barrier

Here is a conglomeration of knowledge items pulled together from a number of public sources that can be applied when considering a wireless installation.

Milliwatts (mW) are the common measurement unit of radio frequency (RF) power. A logarithmic scale of decibels, referencing 1 mW as the zero point, provides a useful way to express the comparative strength of RF signals. Using decibels, a signal strength of 1 mW is registered as 0 dBm. RF power attenuates according to a logarithmic function, so the dBm method of expressing RF power enjoys widespread use.

Industrial wireless communications applications in North America predominantly operate in either the 2.4 GHz or 900 MHz frequency range. Higher frequency will provide more bandwidth, but at the cost of reduced transmission distance and obstacle penetration. Lower frequency can require a larger antenna to attain the same signal gain.

Transmission power is not the only solution for delivering a signal. Low power signals can be successfully received by sensitive radio equipment. Reducing the data transmission rate can increase the functional sensitivity of the receiving equipment, too.

Be mindful of the existence or potential for RF background noise in your communications environment. A higher level of background noise can hamper the effectiveness of your equipment. The "noise floor" varies throughout the frequency spectrum and is generally below the sensitivity level of most equipment. Industrial environments can sometimes provide unusual conditions which may warrant a site survey to determine the actual noise floor throughout the communications area.

Weather conditions can impact signal transmission

Radio transmission is susceptible to environmental elements on a variable basis. Since the environment can change without notice, it is useful to know the fade margin of a wireless installation. Fade margin expresses the difference between the current signal strength and the level at which the installation no longer provides adequate performance. One recommendation is to configure the installation to provide a minimum of 10dB of fade margin in good weather conditions. This level can provide sufficient excess signal strength to overcome the diminishing effects of most weather, solar, and interference conditions.

There are a number of simple methods to determine whether an installation has at least a 10 dB fade margin. Temporarily installing a 10dB attenuator on the system antenna, or installing a length of antenna cable that yields 10dB of attenuation will allow you to determine if the installation can accommodate 10dB of environmental impact on the signal. If the system operates suitably with the attenuation installed, you have at least that much fade margin.

RF signals attenuate with the square of the distance traveled, so if transmission distance is to be doubled, then the signal power must increase fourfold. 

True “line of sight” signal paths are found in a limited number of installations. The number, type, and location of obstacles in the signal path can have a significant impact on the signal and contribute to what is referred to as path loss. Probably the simplest way to reduce the impact of obstacles is to elevate the antennas above them.  Obstacles, in almost every case, are affixed to the earth, so their interference is reduced by elevating antennas to “see” over the obstacles.  

Wooded areas can be a significant barrier

When the signal path extends through an outdoor area, weather conditions have an impact on the path loss, with higher moisture levels increasing the loss. Large plants, most notably heavily wooded areas, can impose substantial reduction on RF signals and may require elevating antennas above the trees or using repeaters to route the signal around a forested area.

Industrial installations routinely present many reflective obstacles in the signal path. The transmitted signal may reflect off several obstacles and still reach the receiving antenna. The received signal strength will be the vector sum of all the paths reaching the antenna. The phase of each signal reaching the antenna can impact the total signal strength in a positive or negative way. Sometimes relocating the antenna by even a small amount can significantly change the strength of the received signal.

Antenna cable 

Antenna cable contributes to signal attenuation. Use high quality cable of the shortest length possible to minimize the impact on performance.

Analynk Wireless has the equipment and expertise to help you deliver wireless process signals across the room, across the street, or across the globe.