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ASME PTC 9-1970 scan — Test Code on Displacement Compressors, Rotary, and Centrifugal Compressors
🔧 ASME PTC 9-1970 is an early authoritative test standard for compressors within the family of ASME Performance Test Codes (PTC). Published in 1970 and distributed here as a scanned copy without a text layer, the Code is built around the positive-displacement machine: reciprocating (piston) compressors and rotary machines such as screw, lobe, vane, and liquid-ring types, with test methods for centrifugal compressors and blowers also provided in the same historical framework. Its purpose is straightforward and practical: under agreed operating conditions, and by means of a well-defined test arrangement, instrumentation, and computation procedure, determine the quantity of gas actually delivered (capacity), the pressure rise produced, the shaft power required, and the relevant efficiencies, then correct the measured results to the specified conditions so that they can be compared with contractual guarantees.
💡 Unlike PTC 10-1997 (reaffirmed 2014), which governs axial and centrifugal compressors and exhausters, PTC 9 is centered on the “volumetric” principle of compression — capacity is fixed by cylinder geometry or rotor interlobe volume rather than by the dynamic head developed at high impeller speed. Grasping this difference is the key to choosing correctly between PTC 9 and PTC 10.
1. Overview and Scope 📌
The object of PTC 9 is to provide a unified, reproducible procedure for compressor performance testing so that results obtained in different plants and at different times can be compared with confidence. The machines covered by the Code include the following groups:
- Reciprocating (piston) compressors: single- or multi-stage, single- or multi-cylinder units, typical of high-pressure service such as compressed air, nitrogen, and natural gas boosting; the test emphasis is on capacity, indicated power, and volumetric efficiency.
- Rotary positive-displacement compressors: screw, lobe, vane, and liquid-ring machines used mainly for medium- and low-pressure process gas, refrigeration circuits, and pneumatic conveying.
- Centrifugal compressors and blowers: test methods are included in the historical scope; for modern acceptance work, axial and centrifugal compressors are normally tested in accordance with PTC 10, while PTC 9 remains the classic reference for displacement machines.
The performance characteristics determined by the Code include: quantity of gas delivered (capacity or volumetric flow), pressure rise (discharge pressure and pressure ratio), shaft power required, efficiencies (volumetric, isentropic, mechanical, and overall), and characteristic points such as surge and choke where applicable. Beyond providing methods for calculating mechanical power losses, the Code does not cover rotor dynamics or other mechanical performance parameters, which must be confirmed from manufacturer data and the relevant machinery standards.
⚠️ PTC 9 evaluates thermodynamic performance only. It answers “how much gas the machine can deliver at a given suction condition and how much power it needs,” not questions of rotor stability or critical-speed separation. Draw this boundary explicitly in the contract before the acceptance test.
2. Machines Covered and Principal Performance Quantities 📊
The table below summarizes the machine types within the PTC 9 framework and the quantities on which each test concentrates, giving engineers a quick reference during machine selection and test planning.
| Machine Type | Compression Principle | Typical Applications | Key Test Quantities |
|---|---|---|---|
| Reciprocating (piston) | Displacement (varying volume) | High-pressure air, CNG, synthesis gas | Capacity, indicated power, volumetric efficiency |
| Screw | Displacement (rotary meshing) | Process gas, refrigeration, oil-flooded units | Capacity, shaft power, isentropic efficiency |
| Lobe (Roots) | Displacement (rotary non-meshing) | Low-pressure blowers, conveying, aeration | Flow, pressure rise, power |
| Vane | Displacement (rotary) | Small compressors, vacuum pumps | Capacity, power, leakage effects |
| Liquid-ring | Displacement (rotary) | Wet or flammable gas evacuation | Capacity, shaft power including ring drag |
| Centrifugal / blower (historical) | Dynamic (velocity head) | Large-flow, medium-pressure gas (now PTC 10) | Head, polytropic efficiency, surge point |
3. Technical Requirements and Instrumentation 🔧
PTC 9 sets explicit requirements for the test loop and the measuring instruments. Tests may be run as an open loop (suction from and discharge to atmosphere or a downstream network) or as a closed loop (recirculating gas with controlled properties). In either arrangement, straight runs of adequate length must be provided on the suction and discharge piping so that the flow is fully developed and the readings at the taps represent true cross-sectional averages.
| Measured Quantity | Method | Typical Instruments | Engineering Notes |
|---|---|---|---|
| Suction / discharge pressure | Static-pressure taps plus manometer | U-tube manometer, Bourdon gauge | Locate taps where velocity changes least; avoid eddy zones |
| Gas temperature | Thermometers / thermocouples (PTC 19.3) | Mercury thermometers, sheathed thermocouples | Sensing element must reach far into the stream; adequate well depth |
| Capacity (flow) | Orifice / nozzle meters, or volumetric check | Differential-pressure meters (PTC 19.5) | Gas composition must be measured to establish density |
| Speed | Tachometer / photoelectric counting | Mechanical tachometer, digital tachometer | Speed drives the similarity corrections and power computation |
| Shaft power | Dynamometer / torque meter, or electric-input method | Torque transducer, power analyzer | Electric method must net out motor efficiency and drive losses |
Flow measurement is the most critical and most error-prone part of the whole test. For reciprocating compressors the Code permits a volumetric check based on cylinder geometry, speed, and clearance volume, cross-checked against the measured flow; rotary machines are normally measured directly with an orifice or nozzle. Whichever method is used, the effects of seal leakage, valve resistance, and flow pulsation on the readings must be taken into account. Because pulsating flow is a fact of life on the discharge of reciprocating machines, damping devices and adequate averaging intervals are required to obtain stable readings.
🚨 Safety in high-pressure testing is non-negotiable: compressor tests frequently run at several hundred bar. Piping and connections must be designed and inspected for the applicable pressure class, the test area needs relief paths and protective barriers, and no reading should require personnel to approach a high-pressure flange closely to observe a gauge.
4. Test Procedure and Evaluation of Results 📐
The Code organizes the work into preparation, preliminary run, and the formal test. Before formal readings begin, the unit must be warmed up and stabilized at the rated condition, usually verified by pressure and temperature drift remaining within prescribed limits over a defined period. Once stable, readings are taken at the required number of intervals and averaged for every measuring point.
The heart of the evaluation is the correction to specified conditions. A test can almost never be run exactly at the contractual speed, suction pressure, suction temperature, and gas composition, so the measured values must be converted to the specified operating conditions. The conversion is based on fluid-dynamic similarity, correcting principally for speed deviation, suction-state deviation, and gas-property deviation; for reciprocating machines, the influence of clearance volume and valve-flow resistance at off-design conditions must also be allowed for. In power evaluation, the indicated power (from indicator diagrams), the shaft power (measured directly), and the overall efficiency can each be derived, with the mechanical efficiency obtained as their ratio.
💡 For contractual acceptance, never compare a “measured-at-test-condition” value directly with a “guaranteed-at-specified-condition” value. Correct the measured data to the specified conditions first; if disputes arise, add the instrument uncertainty and agree in writing in advance on the reference quantities (capacity, efficiency, or power).
5. Engineering Practice Essentials 🛠️
- 🔍 Agree everything in writing beforehand: test conditions, tolerances, instrument accuracy classes, witnessing arrangements, and the method of comparing results with guarantees must be confirmed by both parties before the test starts.
- 🔧 Calibration and traceability: pressure, temperature, and flow instruments used for acceptance must be calibrated per the Code, with calibration traceable to national standards.
- 📊 Define stability: confirm steady operation before taking data, and keep pressure and temperature drift within the Code’s limits throughout the recording period.
- 📋 Measure gas properties: for mixed gases, sample and analyze the composition and compute molecular weight and specific-heat ratio; these directly govern the accuracy of compression work and efficiency.
- 🛡️ Safety and guarding: high-pressure, high-temperature, and flammable-gas services need a dedicated safety plan, an isolated test zone, and emergency depressurizing provisions.
6. Frequently Asked Questions ❓
❓ Q: What is the division of labor between PTC 9 and PTC 10?
A: PTC 10-1997 (reaffirmed 2014) governs axial and centrifugal compressors and exhausters and offers Type 1 and Type 2 tests; PTC 9-1970 centers on displacement machines — reciprocating and rotary — with centrifugal test methods also present in the historical scope. In modern practice, choose by machine type: dynamic machines go to PTC 10, displacement machines to PTC 9.
A: PTC 10-1997 (reaffirmed 2014) governs axial and centrifugal compressors and exhausters and offers Type 1 and Type 2 tests; PTC 9-1970 centers on displacement machines — reciprocating and rotary — with centrifugal test methods also present in the historical scope. In modern practice, choose by machine type: dynamic machines go to PTC 10, displacement machines to PTC 9.
❓ Q: Why does the scanned copy have no searchable text?
A: This material is a scan of the 1970 edition with no text layer, so no body text can be extracted. The technical content above is organized from domain knowledge of the subject; specific values and tables should be checked against the official ASME publication, and this article is an engineering interpretation for reference.
A: This material is a scan of the 1970 edition with no text layer, so no body text can be extracted. The technical content above is organized from domain knowledge of the subject; specific values and tables should be checked against the official ASME publication, and this article is an engineering interpretation for reference.
❓ Q: How are test results corrected to the specified conditions?
A: First convert the measured capacity and power to the specified speed, then correct density and compression work for suction pressure, temperature, and gas properties using similarity criteria. The exponents and coefficients depend on machine type and must follow the Code — the correction parameters used for reciprocating machines must not be mixed with those for rotary machines.
A: First convert the measured capacity and power to the specified speed, then correct density and compression work for suction pressure, temperature, and gas properties using similarity criteria. The exponents and coefficients depend on machine type and must follow the Code — the correction parameters used for reciprocating machines must not be mixed with those for rotary machines.
🔍 Q: Why can capacity not simply be read from one flow meter?
A: Because delivered capacity is strongly affected by pulsating flow, valve resistance, and leakage, and a flow meter needs a stable velocity profile. PTC 9 requires measuring taps on fully developed straight runs and permits a volumetric cross-check; otherwise the measured value can deviate from the true delivered quantity by several percent or more.
A: Because delivered capacity is strongly affected by pulsating flow, valve resistance, and leakage, and a flow meter needs a stable velocity profile. PTC 9 requires measuring taps on fully developed straight runs and permits a volumetric cross-check; otherwise the measured value can deviate from the true delivered quantity by several percent or more.
