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ASME Y32.2.6-1950 — Graphical Symbols for Heat-Power Apparatus
🔥 ASME Y32.2.6-1950 (originally ASA Z32.2.6-1950, reaffirmed 1956) is the American standard defining graphical symbols for heat-power apparatus used on engineering drawings. It is the sixth of a group of standards developed by the ASA Sectional Committee on Standardization of Graphical Symbols and Abbreviations for Use on Drawings (Z32), under the joint sponsorship of the American Institute of Electrical Engineers and The American Society of Mechanical Engineers, and it represents a revision and expansion of the earlier Z32.2-1941. In preparing the symbol list, many leading power companies throughout the United States and their representatives were consulted, and careful study was made to avoid conflicts with symbols being assembled by committees in allied fields. The Standard provides 44 symbols as a minimum requirement for thermal-power and power-plant diagrams.
💡 The coverage centers on thermal power and power-plant equipment — from boilers, turbines, and condensers to feed pumps, deaerators, heaters, and heat exchangers — providing a unified graphical language for complete power-plant heat-balance diagrams.
1. Overview and Scope 📌
The basic principles are the same as those of the companion Z32-series standards:
- Clarity and simplicity: symbols are kept simple, drafting is simplified first, and identification letters are used only where similar shapes could cause error;
- National consensus: only symbols with nationwide agreement are included; regionally used symbols are omitted.
The 44 symbols are stated to be a minimum requirement: several other heat-power equipment symbols were in use at the time but did not have national acceptance. The proposal was designated as an American Standard on February 23, 1950.
⚠️ Letter codes must be applied carefully: on the pump symbol, letters denote service (F-boiler feed, S-service, D-condensate, C-circ. water, A-air, G-oil); on prime movers, letters denote the driver (T-turbine, E-steam engine, G-gas engine, C-diesel engine). Do not mix or omit them.
2. Symbol Families and Classification 📐
The 44 symbols are organized by thermal-system function. The table below lists the principal categories and typical symbols:
| Category | Typical Symbols | Letter / Annotation Convention |
|---|---|---|
| Compressor (1) | Rotary, reciprocating, centrifugal | M-motor drive, T-turbine drive |
| Condenser (2) | Barometric, jet, surface | Distinguish by condensing method |
| Cooler / heat exchanger (3), cooling tower (4) | Cooler or heat exchanger, cooling tower | Drawn per equipment form |
| Deaerator (5) | Deaerator, deaerator with surge tank | Note whether a surge tank is included |
| Drainer / liquid-level controller (6) | Drainer or liquid-level controller | Distinguish drainer from level control |
| Engine (7) | Steam engine, diesel engine, gas engine | S-supercharger; distinguish fuel type |
| Evaporator (8) | Single-effect, double-effect | Distinguish by number of effects |
| Fan-blower (10), filter (11), flow nozzle (12), fluid drive (13) | Fan-blower, filter, flow nozzle, fluid drive | M/T mark the drive |
| Heater (14) | Air heater (plate/tubular, rotating), desuperheater, direct-contact feed-water heater, feed heater with air outlet, flue-gas reheater, live-steam superheater or reheater | Distinguish by type and position |
| Precipitator (17) | Electrostatic, mechanical, wet | E-electrostatic, M-mechanical, W-wet |
| Pump (18) | Centrifugal and rotary, reciprocating, dynamic (air ejector or eductor) | Letters denote service and drive (see above) |
| Separator (19), steam trap (21), strainer (22) | Separator, steam trap, strainer (single/double) | Strainer distinguishes single and double |
| Tank (23) | Closed, open, flash or pressure | Distinguish by pressure form |
| Turbine (24) | Condensing turbine, steam turbine or axial compressor | Condensing type has its own symbol |
| Venturi tube (25) | Venturi tube | For flow measurement |
💡 The pump letter convention is the key usage point: F (boiler feed), S (service), D (condensate), C (circulating water), A (air), G (oil); drive letters T/E/G/C denote turbine/steam engine/gas engine/diesel engine. Correct service letters let one system diagram show many different pumps unambiguously.
3. Engineering Practice Essentials 🛠️
- 📋 Build a power-plant symbol library: freeze the 44 symbols into power-plant heat-balance templates with a unified legend and letter conventions so steam/water, flue-gas, and oil-system expressions stay consistent.
- ✏️ Distinguish similar symbols: drainer vs. liquid-level controller and steam trap vs. strainer are close in shape — draw to the standardized geometry and pair with letters.
- 🔄 Cross-standard consistency: heat-power diagrams usually accompany Y32.4 (plumbing), Z32.2.3 (pipe fittings and valves), and Z32.2.4 (HVAC); the symbol systems share the same origin, making a unified set easy.
- 📐 Drawing organization: arrange symbols along the thermal cycle “boiler — turbine — condenser — feed-heating” to highlight the main energy-flow line.
🚨 The pump symbol is a generic circle with letter annotation. If the letters are omitted, feed pumps, condensate pumps, and circulating-water pumps become indistinguishable on the diagram — a frequent source of misreading in system drawings.
4. Frequently Asked Questions ❓
❓ Q: What is the difference between Y32.2.6 and Y32.2.4 (HVAC)?
A: Y32.2.4 serves building heating, ventilating, and air-conditioning systems, while Y32.2.6 serves power-plant and thermal-power equipment. Both belong to the Z32 series and their symbol forms resemble each other, but the served equipment differs.
A: Y32.2.4 serves building heating, ventilating, and air-conditioning systems, while Y32.2.6 serves power-plant and thermal-power equipment. Both belong to the Z32 series and their symbol forms resemble each other, but the served equipment differs.
❓ Q: Why only 44 symbols?
A: The Standard includes only symbols with nationwide consensus as the minimum requirement; other symbols in use (e.g., regional ones) were not yet nationally accepted.
A: The Standard includes only symbols with nationwide consensus as the minimum requirement; other symbols in use (e.g., regional ones) were not yet nationally accepted.
❓ Q: Must the letters (F, S, D, etc.) be marked on pump symbols?
A: Yes. These letters identify the service of the pump — the key identifying information of the symbol family; omitting them makes pumps indistinguishable on the same diagram.
A: Yes. These letters identify the service of the pump — the key identifying information of the symbol family; omitting them makes pumps indistinguishable on the same diagram.
🔍 Q: What does “dynamic” mean in the pump group?
A: It refers to pumps working on fluid kinetic energy (such as air ejectors or eductors), standing alongside positive-displacement (reciprocating) and rotodynamic (centrifugal/rotary) pumps by operating principle.
A: It refers to pumps working on fluid kinetic energy (such as air ejectors or eductors), standing alongside positive-displacement (reciprocating) and rotodynamic (centrifugal/rotary) pumps by operating principle.
