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ASME PTC 38-1980 (1985) — Determining the Concentration of Particulate Matter in a Gas Stream
🌫️ ASME PTC 38-1980 (1985) is the ASME Performance Test Code for determining the weight rate of flow and concentration of particulate matter carried in a gas stream confined in a duct or stack. Issued November 7, 1980 and reaffirmed in 1985, it continues the ASME emissions-measurement tradition that began with PTC 21 (Dust-Separating Apparatus, 1941) and PTC 27 (Determining Dust Concentration in a Gas Stream, 1957). The Code provides uniform methods for gas-cleaning apparatus performance evaluation, compliance testing with regulatory emission control and performance regulations, and verification of environmental compliance. Before the widespread use of EPA Method 5 stack-sampling practice, it was a key reference for industrial emissions measurement.
💡 The core principle of PTC 38 is isokinetic sampling: the sampling-nozzle intake velocity must equal the local duct velocity, otherwise coarse and fine particles are separated with different efficiencies and the concentration result carries a systematic bias — the first rule of all particulate sampling.
1. Object and Scope of the Test 📌
Section 1 defines the object: to determine the weight rate of flow of particulate matter carried in a gas stream confined in a duct or stack, accomplished by (a) measuring the average weight concentration of particulate matter in the gas stream, and (b) determining the total gas flow rate. The data may be used for performance evaluation of gas cleaning apparatus/systems, compliance testing under emission control and performance regulations promulgated by regulatory agencies, and similar purposes.
- Concentration of particulate matter: the Code prescribes methods for measuring the average weight concentration. Measurement on any other basis (particle count, dust spot, opacity, etc.) is beyond its scope.
- Gas flow measurement: total gas flow may be based on velocity-pressure and gas-density measurements at representative points, on actual combustion (stoichiometric) calculations, or on calibrated gas metering devices; alternate methods are allowed by agreement of all concerned parties.
- Particle size excluded: particle size distribution is not covered — those procedures are in ASME PTC 28. In-situ samplers that collect size fractions may have limited capability for total concentration, but such data should only be used for size distribution, not for weight rate of flow.
⚠️ Units decide everything: the final result of PTC 38 is “weight concentration × gas flow = particulate mass emission rate.” A report that gives concentration without flow cannot support removal-efficiency or total-emissions accounting.
2. Sampling System and Isokinetic Sampling 🔬
A typical sampling system comprises the nozzle, sampling tube, particulate collector (filter thimble or trap), dryer, flow-metering device, and vacuum pump. The Code specifies nozzle inside diameter, sampling point locations, number of points, and sampling time per point, and requires isokinetic conditions to be maintained — the suction rate must be adjusted as gas temperature and velocity fluctuate. Collected particulate is dried and weighed under prescribed temperature and time conditions to obtain the trapped mass.
| Component | Function | Key Requirements |
|---|---|---|
| Sampling nozzle | Extract a representative sample | Diameter matched to isokinetic conditions, sharp edge |
| Sampling tube | Transport the gas sample | Prevent condensation and particle dropout |
| Filter thimble / trap | Collect particulate | Collection efficiency, drying/weighing procedure |
| Metering and pump | Measure sampled volume | Calibration, leak check |
| Traverse points | Represent the duct cross-section | Multi-point grid over the full section |
3. Gas Flow Measurement 📏
Methods for determining total gas flow include:
- Velocity-pressure method: pitot traverse of velocity pressure plus gas density at representative points to build a velocity profile and total flow;
- Combustion calculation: flue-gas quantity derived from fuel consumption and stoichiometry;
- Metering devices: data from calibrated flow meters.
Concentration is multiplied by total gas flow to obtain the particulate mass emission rate. The Code requires flow measurement and concentration sampling to be performed concurrently for consistent results.
💡 Section grid: circular ducts usually place sampling points on 2–3 radial directions over equal-area annuli; rectangular ducts use an equal-area grid. More points mean less bias from a non-uniform velocity profile.
4. Engineering Practice Essentials 🛠️
- 🔍 Gas-cleaning evaluation: sample upstream and downstream of the collector simultaneously and compute removal efficiency = (inlet − outlet) / inlet; both samples must be taken at the same time and conditions.
- 🔧 Compliance testing: used to verify emission concentration limits under local environmental regulations; check mutual recognition with regulatory methods (e.g., US EPA methods).
- 📊 Site safety: stack-access work, hot flue gas and toxic-gas protection, anti-burn and anti-fall measures must be in place before sampling begins.
- 📋 Data handling: drying/weighing conditions, blank corrections, and conversion to standard conditions (dry basis, standard cubic meters) must be fully documented.
🚨 Consequences of isokinetic deviation: suction above local velocity biases the sample toward fine particles (result too high); suction below biases against coarse particles (result too low). Non-isokinetic error can reach 10–30%, so suction rate must be monitored continuously via pressure or flow indication.
5. Frequently Asked Questions ❓
❓ Q: How does PTC 38 relate to EPA Method 5?
A: They share the same methodology (isokinetic sampling, filter collection, gravimetry). PTC 38 is a general performance-test code; EPA Method 5 targets source compliance measurement. Follow the regulatory requirement for enforcement work.
A: They share the same methodology (isokinetic sampling, filter collection, gravimetry). PTC 38 is a general performance-test code; EPA Method 5 targets source compliance measurement. Follow the regulatory requirement for enforcement work.
❓ Q: Why is particle size distribution left to PTC 28?
A: PTC 28 characterizes fine particulate matter including size distribution. The total-concentration isokinetic result of PTC 38 cannot be split into size classes — the tools and purposes differ.
A: PTC 28 characterizes fine particulate matter including size distribution. The total-concentration isokinetic result of PTC 38 cannot be split into size classes — the tools and purposes differ.
❓ Q: How is accuracy maintained with high-moisture gas?
A: Heat or condense the sample in the system so water vapor does not condense in the filter and bias weighing; convert sampled volumes to a dry, standard-state basis in the calculations.
A: Heat or condense the sample in the system so water vapor does not condense in the filter and bias weighing; convert sampled volumes to a dry, standard-state basis in the calculations.
❓ Q: How can I tell whether a sampling location is acceptable?
A: Select a section in a stable flow region (straight duct, away from elbows and transitions), cover the full cross-section with a traverse grid, and keep the nozzle facing into the flow; otherwise the result lacks representativeness.
A: Select a section in a stable flow region (straight duct, away from elbows and transitions), cover the full cross-section with a traverse grid, and keep the nozzle facing into the flow; otherwise the result lacks representativeness.
