After correctly selected inlet treatment is installed, corrosive gas loading stays below the compressor supplier’s accepted limits, deposit-driven rotor unbalance is reduced, and inspection data can confirm whether the media is protecting the machine. The number that matters is the contaminant dose reaching wet metal surfaces, especially during intercooling and shutdown—not the inlet concentration alone.
Contaminant Load and Moisture State
An atmospheric-suction compressor draws the local concentrations of hydrogen sulfide (H2S) and sulfur dioxide (SO2) into its first stage. Compression raises gas temperature, while each intercooler removes heat. If a cooled surface falls below the gas mixture’s dew point, water condenses and converts a dry-gas exposure into a wet-corrosion environment. This is heat, moisture, and chemistry—not logic.
For a dilute contaminant, calculate partial pressure from p_i = y_i × P, where y_i is the mole fraction and P is absolute pressure. When concentration is reported in ppm by volume, use y_i = ppm / 1,000,000. Partial pressure increases as the air is compressed unless removal, reaction, or condensation changes the composition.
Relative humidity alone does not define the risk. Record contaminant concentration, pressure, gas and metal temperatures, water content or dew point, and the conditions during operation and standstill. A shutdown machine can remain cool and wet after airflow stops, extending the contact time between retained contaminants and metal.
| Quantity | Limit or decision | Where to obtain it |
|---|---|---|
H2S and SO2 concentration |
Use normal, peak, and credible upset values | Representative inlet-air sampling or a site environmental survey |
| Contaminant partial pressure | Compare the calculated value at each pressure state with the applicable material assessment | Measured concentration and compressor pressure data |
| Moisture state | Identify stages and shutdown periods where liquid water can exist | Dew-point measurement, intercooler outlet data, drain observations, and shutdown temperature records |
| Allowable inlet chemistry | Use the compressor manufacturer’s written limits | Proposal data, operating manual, material review, or official technical support |
| Particle loading | Meet the manufacturer’s erosion and fouling criterion | Compressor inlet-filtration specification |
| Adsorber breakthrough | Keep outlet concentration below the selected machine limit through the service interval | Media supplier’s sizing and breakthrough data for the actual gas mixture |
Protection Approaches Compared
| Approach | Primary function | Strength | Limitation |
|---|---|---|---|
| Particle filtration | Removes airborne solids and protects against erosion or deposit accumulation | Controls a direct cause of fouling and rotor mass imbalance | Ordinary particle media does not establish removal of gaseous H2S or SO2
|
| Chemical adsorption | Captures or reacts with target gases before compression | Reduces corrosive-gas dose when media chemistry and residence time match the service | Finite capacity, humidity sensitivity, pressure-drop growth, and breakthrough require monitoring |
| Material selection alone | Uses compressor materials accepted for the measured environment | Avoids reliance on consumable media where materials are qualified | Does not prevent deposits, particle erosion, or contamination of downstream instrument air |
| Combined particle and chemical filtration | Controls solids and corrosive gases separately | Addresses both rotor cleanliness and wet-corrosion exposure | Requires staged housings, pressure-drop allowance, media surveillance, and planned replacement |
A chemical filter and a particle filter solve different failure mechanisms. Rotor deposits can create mass eccentricity and vibration, while corrosive gases can attack casing internals or influence the suitability of stressed rotating parts. A single unexplained “chemical filter” line item does not define which gas it removes, its capacity, or its breakthrough criterion.
Recommended Selection Basis
For a three-stage integral-gear instrument-air compressor with intercoolers and 17-4 PH impellers, base the decision on the highest credible wet contaminant exposure across operating and shutdown states. Give the first-stage inlet particular attention because it sees untreated atmospheric air. Then assess later stages where higher pressure raises contaminant partial pressure and intercooling can produce liquid water.
Use NACE MR0175 as a material-screening reference when the defined environment falls within its scope. It is not an inlet-filter performance specification and does not supply an SO2 removal target for this compressor. Confirm the applicability of the document, the condition of the 17-4 PH, the stresses, and the environmental limits with the compressor manufacturer or a qualified materials engineer.
The preferred arrangement is combined particle and chemical filtration when measurements show meaningful gas exposure and the existing materials review does not independently accept that exposure. Specify each element by function. The particle stage must satisfy the manufacturer’s erosion criterion; the chemical stage must have documented performance for both named gases at the actual humidity, temperature, flow, and competing-contaminant load.
Filter Specification Procedure
- Sample suction air at the proposed intake location. Capture normal operation and credible high-emission periods rather than relying on a single spot reading.
- Record atmospheric pressure, temperature, relative humidity, and dew point with the gas samples. Obtain compressor pressures, intercooler outlet temperatures, drain behavior, and shutdown temperature history.
- Calculate contaminant partial pressures for relevant stages using absolute pressure. Mark every condition where condensation or retained liquid can place contaminants in contact with metal.
- Request written allowable limits from the compressor manufacturer for inlet gases, moisture, particles, and corrosive exposure. Ask for a material review covering the
17-4 PHimpellers, first-stage inlet components, casing internals, coolers, and drains. - Give the filtration supplier the full flow range, gas concentrations, humidity, temperature, allowable clean and terminal pressure drop, required outlet concentration, service interval, and competing contaminants. Require separate removal or capacity data for
H2SandSO2. - Define outlet sampling points and a replacement trigger based on breakthrough, pressure drop, or both. Include safe isolation and media handling requirements from the filter supplier.
- Check the compressor performance calculation with the added inlet loss. The inlet system must remain within the compressor manufacturer’s permitted suction conditions at maximum flow and terminal filter pressure drop.
Commissioning and Verification
Establish a baseline before loading the new media: inlet and outlet gas concentrations, filter differential pressure, compressor flow, stage pressures, intercooler temperatures, vibration, and drain condition. Repeat the gas measurements after startup to prove that the filter—not dilution or changing weather—is producing the reduction.
Trend differential pressure and outlet contaminant concentration over the media life. Rising differential pressure indicates accumulating restriction; contaminant breakthrough can occur without a large pressure change because adsorption capacity and airflow resistance are different properties. Inspect drains and accessible wetted surfaces for discoloration, corrosion products, or deposits, and compare vibration with the clean baseline.
A successful installation meets three tests: outlet chemistry remains below the manufacturer-approved limit, terminal pressure drop does not move the compressor outside its permitted inlet condition, and vibration or inspection results show no progressive contamination-related deterioration.
Recurring Specification Pitfalls
- Treating ppm as the complete severity measure while ignoring absolute pressure, humidity, dew point, and exposure time.
- Using a particulate efficiency rating as proof of gaseous
H2SorSO2removal. - Sizing media from average concentration while omitting peak releases and competing gases that consume capacity.
- Checking running conditions but missing wet standstill periods after an intercooler or casing cools.
- Accepting generic “acid gas” media without individual capacity or breakthrough data for both target compounds.
- Adding filtration without accounting for clean and terminal inlet pressure loss.
- Using
NACE MR0175as a universal permission for an overload or as a substitute for the compressor manufacturer’s material assessment.
FAQ
What happens if H2S concentration is low but moisture condenses?
The corrosion risk can rise sharply because the contaminant contacts metal in a liquid-water environment. Calculate partial pressure, locate condensation points, and include wet shutdown duration in the material review.
What happens if I install only a particle filter?
It can reduce erosion and deposit-driven unbalance, but it does not demonstrate removal of gaseous H2S or SO2. Use media with documented gas-specific capacity when chemical removal is required.
What happens if the chemical media becomes saturated?
Outlet contaminant concentration rises as breakthrough develops, sometimes without a significant differential-pressure increase. Monitor downstream chemistry and replace media at the defined breakthrough or pressure-drop trigger.
What happens if filter pressure drop becomes too high?
The compressor suction condition moves away from its approved operating basis and available capacity or stability margin may change. Compare both clean and terminal pressure drop with the manufacturer’s permitted inlet conditions.
When should I stop and escalate an H2S or SO2 filter decision?
Stop when no written contaminant limit is available, condensation status is unresolved, outlet breakthrough cannot be verified, or the suitability of 17-4 PH remains undecided. Send the measured chemistry, dew point, pressure and temperature profile, filter pressure-drop data, vibration trend, and material details to the compressor manufacturer’s official support channel for a documented disposition.