Air Compressors for Chemical Plants

Chemical plant compressed air systems are different from regular factories.

Chemical Plant Special Risk Factors

Flammable and Explosive Environments

Chemical production areas are classified by explosion protection rating per IEC 60079 standard. Zone 0, Zone 1, Zone 2, the whole system. Before siting the compressor station, go to safety department and grab the plant explosion zone map. Figure out which areas are what classification. This map, some plant safety officers can't even explain where they put it. Gotta dig through archives.

Safety Critical

Chemical Plant Environment

Vast majority of chemical plants build compressor stations in non-hazardous areas. Reason is simple. Explosion-proof equipment is way too expensive. 100 hp explosion-proof screw machine quoted at $40,000 and up. Standard model, $18,000-20,000 handles it. When compressed air piping crosses hazardous areas, wall penetrations sealed tight with fireproof sealant. Piping gets anti-static grounding. Ground resistance controlled under 10 ohms. Grounding is where construction crews frequently fake it. During acceptance, take a megger and actually measure. Don't just look at the paperwork.

Atlas Copco's GA VSD series has Zone 2 explosion-proof version. This model is seen pretty often. Buying the explosion-proof machine isn't the end of it. Cable, junction boxes, control cabinet all need to be swapped to explosion-proof versions. Bits and pieces add up to another $25,000. Some factories balk at the price, want to relocate the whole compressor station to safe area. Calculate civil work plus piping modification costs, ends up higher than going explosion-proof equipment. End up buying the explosion-proof machine after all.

Corrosive Gases

Trouble with acid gases is they don't break equipment immediately. They work slow. Intake air with even just a few ppm of hydrogen sulfide or hydrogen chloride, after a year or two rotor coating starts peeling. Bearings covered in rust spots. Oil circuit system corroded and leaking everywhere. Chlor-alkali plants especially watch out. Trace chlorine gas floating in the air is the norm. Can smell that irritating odor with your nose. Equipment has problems and you call the manufacturer. Manufacturer pulls out warranty terms. Black and white writing says intake must not contain corrosive components. Repair costs on you. Arguing is useless. Contract really does say that.

One factory took a shortcut, built compressor station less than 100 feet from the wastewater treatment station. Hydrogen sulfide drifted over. Compressor needed major overhaul in under two years. Later spent over $30,000 rerouting the air intake pipe to the other end of the plant. If they'd just walked a few extra steps to look around the surroundings during site selection, wouldn't have had all this hassle afterward.

Intake pretreatment depends on what you're dealing with. Organic solvent vapors, activated carbon works well. Acid and alkaline gases need chemical media. Some situations, two stages in series. Filter differential pressure needs watching. Rises to 2 inches of water, change it. Don't feel sorry for that filter cost. Some people think the filter still has life left, tough it out without changing. Result is differential too high, intake volume insufficient. Compressor loading rate goes up. Extra electricity cost is way more than the filter.

Cooling water often gets overlooked. Chemical plant circulating water systems have all kinds of treatment chemicals. Water quality is far from tap water. High chloride content, copper coolers won't last long. Sample and test every quarter. Watch pH, chloride, total hardness. Testing cost isn't much. Few hundred bucks a year. Way cheaper than replacing a cooler.

Challenge

Dusty Environment

Challenge

High Temperature

Challenge

Corrosive Gases

Dusty Environments

Chemical plant dust isn't the same as regular dust. Catalyst dust contains metal oxides. Hard. Gets into the airend and it's like a grinding wheel. Some raw material dust is also corrosive or hygroscopic. Tougher to deal with than regular dust.

Compressor station building as positive-pressure sealed enclosure is basic practice. Fresh air filtered then pushed in. Maintain indoor pressure 0.1-0.12 inches of water above outdoor. Outside contaminants can't get in. Fresh air filter select F7 grade or higher bag filter. Check filter bags every three months.

Compressor's own air filter in high dust environments has noticeably shortened life. Normal conditions, change every 3,000-4,000 hours. Dusty locations, gotta change at 1,000-something hours. Some maintenance people don't take it seriously. Air filter clogged so bad intake resistance is up and they're still using it. Airend intake insufficient, long-term running in high loading state. Bearing and rotor life both affected. During oil change maintenance, take an oil sample for testing. Particle count and acid number are the focus. Oil degradation rate more than double normal means dust has gotten in. Need to check where the intake system is leaking air.

High Temperature Conditions

Chemical plants have heat sources everywhere. Reactors, distillation columns, heat exchangers, all putting out heat. Summer noon, outdoor 100°F. Close to these equipment, local temperature can reach 115°F and above.

Screw compressors are sensitive to intake temperature. Manufacturer catalog output capacity is calculated at specific intake temperature. Actual ambient temperature higher, output capacity needs a discount. Selection must be calculated at the hottest conditions. Using annual average temperature is digging yourself a pit. Come summer, not enough air, adding units, money wasted.

Compressor station ventilation volume needs to be sufficient. 75 hp screw machine running full load puts out roughly 45 kW of heat to dissipate. Station room with several units, heat output stacks up. Ventilation system scales accordingly. Inlet and outlet follow low-intake, high-exhaust pattern. Takes advantage of hot air rising. Some factories didn't consider this when designing the compressor station. Air inlet and exhaust at same height on the wall. Hot air swirling inside the room, can't get out. Summer, units take turns high-temp shutting down.

Process Requirements for Air Quality

Instrument air has strictest requirements. Whether pneumatic control valves and emergency shutdown valves can actuate reliably directly relates to production safety. Pressure dew point, particles, oil content all have targets. Meeting this standard requires desiccant dryer. Difference between heatless and micro-heated regeneration is purge air loss. Heatless regeneration has high purge consumption. Micro-heated saves air but equipment is complex and expensive. Large flow systems, micro-heated is more economical. Small flow, heatless is enough.

Process air, each application has its own requirements. Some material conveying only needs -4°F dew point. Some purging operations are oil-sensitive but dew point can be relaxed. Where problems most commonly occur here is design firms using one-size-fits-all approach. Configure everything to highest standard. Pile of aftertreatment equipment. Operating costs scary high. Actually many applications don't need that level. Figure out each use point's requirements before deciding on the scheme. Can save a lot of money.

Chemical Plant Compressed Air Main Uses

Instrument Air

Instrument air supply stability matters more than anything. Large chemical plant with a thousand-plus pneumatic valves waiting for air. Supply cut for few seconds and process parameters might go out of control.

Instrument air system set up independently. Separate from general production air. Supply pressure 75-85 psi. Doesn't need to be too high. Receiver capacity sized at minimum three minutes of maximum consumption. Buffer during compressor switchover or brief fault.

Dew point control is the core of instrument air. Cold northern areas, outdoor piping and valves with moisture inside will freeze. Valves stick. Quite a few plants up north require dew point -40°F or below. Online dew point meters, install two. One at dryer outlet, one at main header position. Dew point exceeds limit, alarm interlocks to start backup dryer. Vaisala dew point meters have good stability. Many plants use them.

One plant had instrument air consumption of 530 CFM. Receiver only 175 cubic feet. Equals twenty seconds of buffer. During compressor switchover, pressure frequently dropped through floor. Pneumatic valves going haywire. Process parameters fluctuating. Operators on edge every day. Later added a 350 cubic foot receiver and problem was solved. Whoever designed it originally never did this math. One more receiver costs how much. Accident happens, how much is the loss.

Process Air

Pneumatic conveying systems are sensitive to compressed air flow fluctuation. Conveying dense phase material, flow suddenly drops and you might plug the pipe. Plug once and clearing it is a real hassle. Sometimes means production shutdown to deal with it. Surge tank on supply main can alleviate this problem.

Applications requiring oil-free, choose oil-free machine. No other way. Coatings, food additives, electronic-grade chemicals production process, trace oil contamination will affect product quality. Small flow use oil-free screw or oil-free scroll. Large flow use centrifugal. Oil-free screw machines cost more than double oil-flooded screw. But where it's needed, can't cut corners. Some factories try to save money using regular oil-flooded screw with aftertreatment oil removal. Theoretically oil content can meet spec. In actual operation, aftertreatment equipment has slightest issue and oil runs downstream. Product has problems before anyone notices. Loss is way more than the equipment money saved.

Breathing Air and Safety Air

Breathing air for emergency escape, confined space entry, positive pressure suit supply. Entire system configured independently. Has its own compressor, aftertreatment equipment, and high-pressure cylinder banks.

Cylinder bank pressure 2,200 to 4,400 psi. Capacity calculated based on maximum number of people needing air in emergency and duration of use. Chemical plant safety codes require guaranteeing at least fifteen minutes of emergency evacuation air for all plant personnel. This system normally goes unused. But must have it. And must regularly inspect to keep it ready at all times. Some plants install breathing air system then nobody touches it for years. When actually needed, discover bottles have long since leaked down to zero pressure. This kind of thing has happened.

General Utility Air

Air tools, cylinder drives, equipment blowdown. These applications don't demand much. Dew point -4°F, oil content below 1 mg/m³ is sufficient. Main header pressure 100-115 psi, a bit higher than instrument air. Control pipe network pressure drop. End points with insufficient pressure, add local boosters. Nothing special here. Similar to regular factories.

Explosion-Proof Compressors

When They're Mandatory

Compressor station in Zone 1 or Zone 2, no avoiding explosion-proof machine. Zone 1 requires equipment protection level ExdIIB T4 or higher. Zone 2 requirements slightly lower.

Explosion-proof certification is mandatory requirement. Imported equipment needs ATEX or IECEx certification. Domestic equipment uses domestic certification bodies. Contract must clearly stipulate providing original certification certificates. Copies won't be accepted for safety bureau filing. Some small manufacturers can't produce certificates or certificates are fake. Only discovered during acceptance. Equipment can't be returned, money can't be recovered. Lawsuits drag on endlessly.

Technical Approach

Motor is the focus. Flameproof motor encloses parts that might produce sparks in a housing that can withstand internal explosion pressure. Increased safety motor prevents spark generation by raising insulation class and limiting temperature rise. Flameproof has higher protection level and higher price. How to choose depends on zone classification requirements.

Control system, two approaches. One is field control box uses explosion-proof enclosure. VFD also explosion-proof model. Whole set stays in hazardous area. This approach, high equipment cost. Other approach moves control system to safe area. Only start-stop buttons and indicator lights at field location use explosion-proof models. Saves considerable money. If the second approach works, use it.

Wiring workmanship can't be sloppy. Cable entering explosion-proof enclosure must use explosion-proof cable glands. Armor layer reliably grounded. Exposed conductor ends tinned or crimped with lugs. After wiring, check terminal tightening torque. Loose terminals easily arc. Many electrical incidents in explosion-proof areas, investigated all the way back, end up being loose terminal connections. This kind of basic mistake happens every year.

Maintenance Requirements

Explosion-proof compressor maintenance must be performed by personnel with explosion-proof electrical operation certification. Regular electricians aren't qualified. Work permit before maintenance. Site combustible gas concentration tested and passed before starting work. After shutdown, can't immediately open covers. Wait for equipment temperature to come down.

Compressor Station Siting

Safety Distances

Distances between compressor station and various hazardous sources are regulated by NFPA and API codes. Fire separation distance from Class I/II hazardous process equipment not less than 80 feet. From flammable gas storage not less than 50 feet. Design firms know these numbers. Problem is some old plant sites simply can't meet these distances. Can only do risk assessment then take compensating measures. Like enhanced ventilation, combustible gas detectors, using explosion-proof equipment, and so on.

Orientation selection looks at wind frequency statistics. Air intake placed upwind of prevailing direction. Northern winters mostly north and northwest wind. Compressor station positioned on north or northwest side of plant is appropriate. This principle sounds simple. In practice, frequently conflicts with overall plant layout. Ideal location occupied by other equipment. Can only settle for second best and work harder on the intake system.

Pipe Network Pressure Drop

Compressor station far from use points means high piping investment and high pressure drop losses. This math needs long-term view. Piping is one-time investment. Operating energy consumption comes every year. Some plants to keep compressor station in safe area, run piping several hundred feet. Pressure drop loss of a few psi. Compressor discharge pressure needs to be raised accordingly to guarantee end pressure. One year, tens of thousands of extra kWh.

Widely distributed plant sites aren't uncommon. One central compressor station can't reach everywhere, either set up several distributed small stations, or add boosters and receivers at the far end. Which approach is suitable requires total cost calculation. Equipment investment, civil work cost, operating energy, maintenance cost all need to go in.

Station Building Design

Compressor station buildings mostly single story. Ceiling height based on equipment height. Roof exhaust skylights for natural ventilation using thermal buoyancy. Large units might also need mechanical ventilation. Floor treated for oil penetration resistance. Oil collection trench around compressors. Noise control: sound enclosures on equipment, wall cavity filled with acoustic absorption material, silencers on air inlet and exhaust openings.

Equipment surrounding maintenance aisle width not less than 4 feet. This frequently gets squeezed. Some plants stuff compressor stations packed full. Changing an air filter requires squeezing in sideways. Major overhaul, crane can't get in, can only use chain hoist to slowly drag things out. Efficiency is terrible. Leaving an extra couple feet of aisle during original design, how much trouble would that have been.

Piping

Materials

Carbon steel pipe is cheap. Normal environment, works fine. Weakness is poor corrosion resistance. Humid environments or corrosive media, it rusts. Rust flakes ride compressed air downstream and clog valves and equipment. Galvanized pipe is slightly more corrosion resistant than carbon steel. But can only use threaded or flanged connections, can't weld. Large diameter threaded connections have poor sealing.

Stainless steel pipe is corrosion resistant, smooth inner wall, low pressure drop. Instrument air and breathing air piping, recommend stainless steel. Material 304 or 316L. 316L has better chloride corrosion resistance. But noticeably more expensive. General utility air, carbon steel pipe is sufficient. No need to go all stainless steel.

Aluminum alloy pipe has been used more in recent years. Light, corrosion resistant, inner wall doesn't rust, push-in fittings install fast. Downside is can't handle strong acids or bases. pH too low or too high, can't use. Price more than carbon steel, less than stainless steel. Overall value is good.

Pipe Diameter

Flow velocity too fast, big pressure drop. Too slow, waste pipe material. Main header velocity 25-33 feet per second. Branch lines can go a bit wider. Selecting diameter, check a flow-velocity-diameter reference table, or calculate with formula. Not complicated. Key is don't take shortcuts and just pick a random diameter. Discover later pressure drop is too big, changing it then is trouble.

Construction

Welding is critical. Stainless steel pipe during welding needs argon purge inside the pipe. Otherwise weld seam inner wall oxidizes and turns black. Corrosion resistance greatly compromised. Some construction crews don't understand this or skip it because it's a hassle. After welding, can't tell from outside. Used a year or two and weld seams start leaking.

After installation, do strength test and leak test. Strength test uses water. Pressure is 1.5 times design pressure. Hold thirty minutes without leaking. Leak test uses dry air or nitrogen. Pressure equals design pressure. Check all weld seams and joints with soap solution or leak detector. Both tests must be done. Don't think about skipping to save effort. Leaking pipe that goes into service, repairing it later is ten times more trouble than during installation.

Pipe support spacing per code. Carbon steel pipe horizontal installation, one support every 10-16 feet. Some construction crews space supports too far apart. Pipe hangs unsupported in the middle for long stretches. Over time pipe sags, water accumulates. Winter it freezes and cracks. This kind of problem can't be seen during construction. Shows up after a year or two. By then tracking down the construction crew, they're long gone who knows where.

Redundancy Design

Unit Configuration

Chemical production has high continuity requirements. Compressed air interrupted even for seconds can cause problems. Pneumatic valves lose air source and go to fail position. Safety interlock valves' fail position is typically closed. Under some conditions this triggers chain reactions.

Unit configuration basic principle is N+1. Beyond the N units meeting production demand, add one standby. Production needs 1,750 CFM. Two 1,050 CFM units theoretically enough. Safe approach is three units. Standby machine is normally off and needs time to start. Receiver capacity must ensure during switchover time pressure doesn't drop below critical value.

Instrument Air Independence

Instrument air has highest importance. Recommended independent configuration separate from general production air. Production air has problems, instrument air keeps supplying. Safety interlock systems unaffected. Instrument air compressor, two units as mutual backup. Dryers also two, one running one standby, automatic failover.

Receiver capacity sized at three to five minutes of instrument air maximum consumption. Many plants don't hit this number. Configure one or two minutes' worth and call it enough. Normal times looks fine. Real switchover happens and discover it's not enough. Pressure drops through, valves go haywire. Operators scrambling.

Power Supply

Compressor power fed from two bus sections. Automatic transfer switch between them. Either bus loses power, auto-switch to the other. Transfer time controlled in fractions of a second. Control system CPU modules with dual redundancy. Main CPU faults, standby CPU bumps takes over. Communication links dual network. One goes down, other picks up.

These redundancy measures look like they cost money. Total math, it's worth it. Compressor system down once, how much does that cost. Safety incident consequences, don't even need to mention. Invest a bit more to make the system reliable. Sleep better at night.

Emergency Measures

Nitrogen is common medium at chemical plants. Compressed air cuts out, can temporarily substitute. Emergency tie-in valve between instrument air main and plant nitrogen network. Normally closed. Emergency, open it. Before using, confirm nitrogen pressure is sufficient. Nitrogen pressure is usually lower than compressed air. Some pneumatic equipment is pressure sensitive and may not actuate properly.

High pressure cylinder banks are another approach. Bank of bottles through regulator valve stepped down then fed into instrument air main. Can hold for few minutes to tens of minutes. Bank pressure checked periodically. Low, replenish promptly. Some plants install cylinder banks and call it done. Years go by, nobody checks. When actually needed, discover bottles lost pressure long ago.

Critical pneumatic valves get handwheels. Lose air source, can manually operate. Handwheel is only the last line of defense. Operation takes time, emergency conditions may not be fast enough. Main reliance is still on front-end redundancy design to keep air supply uninterrupted. Don't count on handwheels to save the day.