Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME PTC 24-1976 (1982) scan — Ejectors
📐 ASME PTC 24-1976 (reaffirmed 1982) is the ASME Performance Test Code devoted to ejectors — above all the steam jet ejector. An ejector differs from other compressors in having no moving parts: the work of compression is done by the kinetic energy of the motive fluid issuing from a nozzle at high velocity. The basic unit consists of a nozzle, a suction chamber, and a diffuser; a single assembly is a “stage,” and two or more stages in series (with interstage condensers as required by the compression ratio) form a multi-stage unit. The motive fluid most commonly used is steam, though air, hydrocarbon gases, and other fluids with completely and reliably known physical properties may be used. Suction fluids are predominantly air, water vapor, and air-vapor mixtures; suction pressures are usually below atmosphere and occasionally extend to the very low absolute-pressure ranges measured in microns. Recognizing the need for a code on steam jet ejectors, ASME organized PTC Committee No. 24 in 1948; the original code was approved in May 1956. This 1976 edition, prepared after the committee was reorganized in 1969, was approved in September 1975 and extends coverage to non-steam motive fluids.
💡 Ejectors are widely used for condenser air removal in power plants and for vacuum distillation and evaporation in the chemical industry. PTC 24 provides a common terminology and procedure for establishing the capacity-pressure-stability relationships so that design calculations and site acceptance speak the same language.
1. Object and Scope 📌
The code provides standard directions and rules for conducting and reporting tests on single- or multi-stage ejector units. The primary object of the test measurements is to establish:
- Ejector capacity in relation to suction pressure;
- Discharge pressure in relation to suction pressure;
- The flow rate of the motive fluid in relation to a stipulated pressure and temperature;
- Ejector stability — the relation of motive-fluid pressure or discharge pressure to the breakdown and recovery of the pumping action.
Important limitations of scope:
- If the motive fluid is steam or another condensable vapor, it must be dry (without moisture); the code does not cover two-phase fluids.
- Instructions and capacity measurements cover cases where the suction fluid is air, water vapor, or other gases whose physical and thermodynamic properties are known; a procedure is provided for gas mixtures whose components can be separately measured and controlled.
- The code is limited to ejectors having suction pressures that permit accurate measurement with available instrumentation and technique.
- The procedures and instrument specifications of Section 4, the formulae and methods of Section 5, and the reporting form of Section 6 are mandatory; only tests conducted strictly in accordance with the mandatory provisions may be designated as complying with the ASME Test Code for Ejectors.
⚠️ The test problems peculiar to ejectors are the measurement of relatively small flow rates and low absolute pressures, plus the critical relation between motive-fluid pressure and stability. Establishing acceptable test conditions therefore requires a specific procedure; the methods of ordinary compressor testing cannot simply be carried over.
2. Terminology and Definitions 🔧
| Term | Definition |
|---|---|
| Absolute pressure | Pressure measured from absolute zero (= atmospheric pressure plus gage pressure) |
| Static / total / velocity pressure | Static pressure is measured so that no effect of gas velocity enters; total pressure is stagnation pressure (impact tube); velocity pressure = total − static (Pitot differential) |
| Suction pressure | Static pressure at the suction inlet, absolute units |
| Discharge pressure | Static pressure at the discharge of the ejector, absolute units |
| Motive-fluid pressure | Static pressure at the nozzle inlet, absolute units |
| Stability | If discharge pressure is too high or motive-fluid pressure too low, the suction stream may momentarily reverse — the ejector is unstable; stable operation means suction pressure is not decreased by further lowering discharge pressure or raising motive-fluid pressure |
| Breaking / recovery pressure | Pressure of the motive fluid or discharge that causes instability (breaking), and the pressure at which the ejector recovers to stable operation (recovery or pick-up) |
| Capacity | Weight rate of flow of the fluid compressed and discharged by the ejector — the gas stream pumped through the suction inlet |
| Motive-fluid consumption | The nozzle flow of steam or other working fluid |
3. Test Arrangements and Measurements 🔥
- Motive-fluid flow: measured with flow nozzles, orifices, or similar throttling devices, with dryness verified for steam.
- Suction flow: capacity depends on the physical properties of the pumped gas, so suction pressure, temperature, and flow must be measured simultaneously.
- Low absolute pressure: use calibrated precision vacuum gauges or transmitters; pay attention to the leak-tightness of the measurement lines.
- Interstage measurement: for multi-stage systems, measure each stage’s suction and discharge pressures; record interstage condenser conditions (cooling water flow and temperature).
- Reference to PTC 19 Instruments & Apparatus: general instrument requirements follow the I&A supplements; the specific directions of this code prevail where they differ.
💡 The essential measurements are “three pressures plus two flows”: motive-fluid pressure and temperature, suction pressure and temperature, discharge pressure, plus motive-fluid flow and suction capacity. Any capacity-versus-pressure characteristic curve rests on this synchronized data set.
4. Stability Determination Procedure 📏
| Step | Operation |
|---|---|
| Establish condition | Reach a stable operating condition at the design suction pressure and motive-fluid pressure |
| Find breaking point | With motive-fluid pressure fixed, slowly raise discharge pressure (or lower motive-fluid pressure) until the pumping action breaks down |
| Find recovery point | Then slowly lower discharge pressure (or raise motive-fluid pressure) until stable operation returns |
| Record | Record breaking and recovery pressures; their difference is the instability hysteresis band |
🚨 Near the breaking point the suction stream may momentarily reverse and pressures fluctuate severely: the measurement system must have adequate dynamic response and surge resistance, and operators must approach the critical point slowly under close supervision to avoid equipment damage.
5. Computations and Reporting 🛠️
- Computations: compute capacity, motive-fluid consumption, compression ratio, and related quantities per Section 5, and adjust test results to design conditions as agreed.
- Inaccuracy: the code does not consider an overall tolerance or margin (which may be made applicable by agreement); allowances for measurement inaccuracy are recognized per para. 3.10.
- Reporting: report test data, characteristic curves (capacity versus suction pressure), the stability band and breaking/recovery pressures, and all agreed items in the Section 6 format.
- Acceptance use: for ejector acceptance, verify capacity at the design suction pressure, motive-fluid consumption, and stability boundaries against the specification.
6. Frequently Asked Questions ❓
❓ Q: How do ejectors compare with compressors?
A: Ejectors have no moving parts, simple and reliable construction, long life, and low maintenance — ideal for large-capacity, low-pressure-vacuum duty such as condenser air removal — but relatively low efficiency, so they are usually reserved for low-pressure services where compressors are uneconomical.
A: Ejectors have no moving parts, simple and reliable construction, long life, and low maintenance — ideal for large-capacity, low-pressure-vacuum duty such as condenser air removal — but relatively low efficiency, so they are usually reserved for low-pressure services where compressors are uneconomical.
❓ Q: Why must the motive fluid be dry steam?
A: Wet steam is a two-phase medium whose physical and thermodynamic properties are not reliably known; moisture disturbs flow calculations and critical-flow conditions, so the code requires dry (moisture-free) steam and its dryness measurement.
A: Wet steam is a two-phase medium whose physical and thermodynamic properties are not reliably known; moisture disturbs flow calculations and critical-flow conditions, so the code requires dry (moisture-free) steam and its dryness measurement.
❓ Q: How are multi-stage ejectors tested?
A: With interstage condensers, test as a system: measure each stage’s suction and discharge pressures, record interstage condenser conditions, and compute and report the overall characteristics by the multi-stage series relationships.
A: With interstage condensers, test as a system: measure each stage’s suction and discharge pressures, record interstage condenser conditions, and compute and report the overall characteristics by the multi-stage series relationships.
🔍 Q: Why is the stability characteristic so important?
A: Once an ejector breaks down it suddenly loses pumping action and condenser vacuum is destroyed, threatening unit operation. The breaking/recovery band must therefore be stated as the operating boundary and alarm set-point basis.
A: Once an ejector breaks down it suddenly loses pumping action and condenser vacuum is destroyed, threatening unit operation. The breaking/recovery band must therefore be stated as the operating boundary and alarm set-point basis.
