Confirm Best Practices for Compressed Air in Refineries
on Wed, 12/12/2018
Compressed air is an important element for many processes within the petrochemical and oil refining industries. First, compressed air acts as a medium for the transfer of energy, which is critical to power equipment such as drills, jack hammers and air-powered tools, among others. Compressed air is also used elsewhere in refineries to move material such as paint, oils, cylinders for industrial and automation, and brakes on large equipment.
Compressed air is also utilized in a refinery for breathing gas when operations face confined space requirements. Additionally, it controls precise measurements and calibration in the production of gases and liquids during operation of specific machinery and instrumentation.
For more than 140 yr, compressed air has been a critical tool used to help drive the industry. Due to its nature, compressed air is a stable process for process equipment needed for drilling, pneumatic tools, power machinery and generators that produce light without combustible fuels. Types of compressors found in refinery and petrochemical facilities include:
- Compound compressors
- Rotary screw compressors
- Rotary vane compressors
- Scroll compressors
- Turbo compressors
- Reciprocating compressors
- Centrifugal compressors
Refineries most commonly use electrically-driven reciprocating, rotary screw and centrifugal compressors based on the specific application for which they are being utilized. These compressors are then classified by the amount of pressure they produce:
- Low-pressure compressors produce 150 psi or less
- Medium-pressure compressors produce 151 psi–1,000 psi
- High-pressure compressors produce more than 1,000 psi
These compressors are generally used for smaller applications within the refinery. Several types of reciprocating compressors exist:
- Single-stage, < 5 hp range
- Two-stage, 5 hp–30 hp
- More stages are added to increase hp range
A piston-stroke type air compressor pumps air into a tank to a required psi, distributes the air as required and maintains the maximum psi within the tank. These positive displacement compressors force air into a chamber that decreases by the demand of compressed air.
These compressors (FIG. 1) use positive displacement compression by matching two helical screws that guide air into a chamber when turned, decreasing the volume as the screw turns. Alternatively, rotary vane compressors use a slotted rotor with varied blade placement to guide air into a chamber and compress the air. This results in fixed air at high pressure.
Centrifugal compressors are most commonly used in very large applications within refineries. Rotating components impart kinetic energy to the air, which is eventually converted into pressure energy. A centrifugal force is generated by the impeller to capture and then pressurize the air.
Two types of centrifugal compressors are used in a refinery environment: oil-lubed compressors, which are very common, and oil-free compressors, which are louder and more expensive. Oil-free compressors come in different forms: oil-free centrifugal compressors, oil-free screw compressors and oil-free tooth compressors.
When an application calls for 100% oil-free air, an oil-free centrifugal compressor is appropriate. Due to the ultra-high-speed electric motors used to drive impellers, it contains a compact compressor without a gearbox and associated oil-lubrication system, making it oil-free. These types of compressors are an optimal solution for an engineered skid combined with dryers, controls and receivers. If the priority is superior reliability and efficiency, often due to very harsh conditions, oil-free screw compressors are ideal. Furthermore, these compressors can be engineered with a fire-proof canopy and additional controls on an engineered skid.
When the efficiency in energy consumption is important, oil-free tooth compressors are often selected. This compressor utilizes a symmetrical, dynamically-balanced rotor that facilitates efficient and oil-free compression.
Finally, low-pressure blowers and compressors are generally used for applications that require low-pressure air. This can be in the form of aeration, pneumatic conveying and sulfur recovery, among others.
The heat that is generated from compressing the air, air-water or oil (as required for rotary type compressors) is used to cool the compressors. When the air is compressed, it contains more water vapor than high-pressure air can hold. This relative humidity (dewpoint) plays a large role in the quality of the compressed air. When air is compressed at temperatures above its dewpoint, it becomes saturated with moisture. When the temperature of the compressed air is below the dewpoint, condensation will occur.
Coalescing should be done at the lowest possible temperature to preclude moisture precipitation. At low temperatures, compressor oil varnish accumulation and large accumulations of dirty finger prints on the coalesce elements can all cause high differential pressure and lead to lower coalescing efficiency and, eventually, premature element failure.
Compressed Air Filters
Compressed air filters are an important element in a refinery. These compressed air filters come in various forms, including coalescing, water separator, vapor removal and particulate removal.
Water separators remove bulk liquid at any point in a compressed air system. They provide protection to the pre-filtration train prior to membranes and desiccant dryers. Additionally, water separators remove liquid from compressor intercoolers and aftercoolers, and provide liquid separations within refrigeration dryers.
Vapor removal compressed air filters are useful for odor removal, breathing air, high-purity gases and hydrocarbon vapor removal.
Compressed air filters should always be placed downstream of the receiver. If no aftercooler is in use or the aftercooler is inefficient, a coalescing filter should be installed as close to the point of use as possible. Air saturated with water vapor leaves a compressor at 240°F–400°F (116°C–204°C). Without an aftercooler, the air cools close to ambient in the distribution lines, and water condenses throughout the air distribution line. Two-thirds of the total water content of the air will be condensed when it has cooled to 100°F (38°C). A filter located just before the main airline will branch into smaller distribution lines and remove most of the water load from the system.
Many refineries use compressed air filters that have polycarbonate bowls. These are suitable for use in normal industrial environments but should not be used in areas where they could be subjected to direct sunlight, an impact blow or temperatures outside the rated range. Also, they should not be exposed to chlorinated hydrocarbons, ketones, esters and certain alcohols, nor used in air systems where compressors are lubricated with fluids such as phosphate ester and diester types.
In all applications where a polycarbonate bowl is in use, a properly fitted metal guard should be attached to the bowl to protect against any issue resulting in a rupture. Metal bowls are useful where ambient and/or media conditions are incompatible with polycarbonate bowls. Metal bowls can resist the effects of most solvents but should not be subjected to salt laden atmospheres nor used where strong acids or bases are present.
All safety precautions must be followed when working with and sizing compressed air filters, as safety is paramount to an efficient process. A responsible person must be available to supervise the regular inspection and maintenance of compressed air filters. For extensive amounts of filters within the refinery, some form of written system of planned maintenance should be used, and records of tests, repairs and modifications should be kept. In addition to any maintenance required by governmental requirements, daily, weekly, monthly, quarterly and by-yearly procedures should be implemented.
- Daily—The procedure should include daily checks of the compressed air filters for leaks and repairs that must be carried out, as necessary. At the end of each day, any accumulation of water should be drained from the drain ports. The drain valves should be inspected to make sure they are not choked with sediment. Those responsible for compressed air quality should be notified if the water contains hydrocarbons that do not drain to a controlled environmental sump.
- Weekly—At the end of each week, airline lubricators should be filled, as needed, with the correct grade of oil and checked to ensure they are providing adequate but not excessive lubrication. Compressed air filters should be checked; those that are clogged or causing unacceptable flow restrictions (dp) should be replaced.
- Monthly—All connections should be inspected for signs of cracking or deterioration and be replaced, as necessary. Every 3 mos, the valves and leaks should be replaced. New vessels may be required, and filter cartridges may require replacement.
- Twice per year—The coalescers, pre-filters and final filters should be replaced, and the air supply must be checked so it operates freely.
Compressor Oil Filters
Another critical piece to keeping a compressor running efficiently is the use of oil, which is primarily used to cool anything mechanical within the compressor. This oil will have a specific ISO code that is used to identify the machine’s capabilities according to the manufacturer. This code is important, as it points to the cleanliness level standard. Attention to this detail is gaining wide acceptance in the industry. The struggle comes in maintaining the code due to the amount of particulate in the atmosphere. While in the compressor, this particulate can be difficult to maintain. Leakage seals are utilized. However, in doing so, the potential of particulate in the water from the seals breaking down increases.
If ISO codes with compressors dictate that particles within the oil must be less than a specific number of microns, proper filtration must be implemented. One method is to properly filter the oil as it goes into the compressor. In this scenario, the oil sits in a pump as it is being circulated through the compressor. These filters can either be paper or synthetic.
The second option is to install a filter that not only handles the problem with particulates but is also designed to properly filter out water. In these situations, a kidney loop system can be installed on the compressor’s oil reservoir. These filters are known as oil conditioners.
As the compressor ages and begins to leak, humidity can become an issue, caused by factors that include the type of tank and where it is located. Many of these tanks are submerged underground. In a dry area, fewer problems arise. Humidity breaks down the oil, diminishing its quality. If it is protecting equipment and keeping heat dissipated, the present water can turn into steam. This can affect the metals it is supposed to be protecting. Hot water can crack a bearing. To avoid these kinds of issues, this oil should be checked daily.
Static can also be an issue with oil filters. Often, static can occur in the oil, causing the filters to break down quickly. Fortunately, available filters are designed specifically to handle static.
Servicing a main compressor requires a planned shutdown in a refinery. However, as these shutdowns can sometimes take years to be scheduled, some temporary workarounds can be performed. Rental equipment can be used to address any water issues a plant may be experiencing with a compressor. In these situations, oil can be recirculated until it reaches an acceptable cleanliness level and then maintained until the next planned outage.
Planning for routine maintenance during a planned outage is critical to avoid complete failures in refineries. Those failures come with a significant cost. Emergency, unplanned shutdowns can often cost a refinery up to five times as much in repairs than during a planned outage.
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