ASME PTC 20.3-1970 (1979) scan — Pressure Control Systems Used on Steam Turbine-Generator Units

28 10 月, 2026
ASME Standards

📐 ASME PTC 20.3-1970 (reaffirmed 1979) is the ASME Performance Test Code covering pressure control systems used on steam turbine-generator units. The code lineage traces to the 1921 committee on speed-responsive governors: reorganized in 1940 with scope expanded to speed-, temperature-, and pressure-responsive governors of prime movers, the committee issued PTC 20.1 (speed and load governing) in 1958 and PTC 20.2 (overspeed trip systems) in 1965; work on this third part began in December 1965, the committee was renamed Performance Test Code Committee No. 20 in 1967, and the code was approved in August 1969 and issued in 1970. Alongside PTC 20.1 and PTC 20.2, it completes the ASME trio of turbine-generator control system test codes, and is applied wherever steam pressure regulation quality matters — extraction/heating units, industrial turbines, and cogeneration plants.

💡 A pressure control system governs the turbine “as a pressure source”: automatic regulation of extraction pressure, main steam pressure, or reheat pressure. Its quality directly determines the stability of heating/process steam and, in turn, the safety and operating economy of downstream plants.

1. Object and Scope 📌

This code establishes standard test procedures for pressure control systems used on steam turbine-generator units, structured in mirror fashion to PTC 20.1:

Systems and actions, components, and definitions (Section 2): precise definitions of terms such as “pressure control system,” “pressure set point,” and control actions, plus a description of system composition.
Guiding principles (Section 3): advance planning and preparation, items on which agreement shall be reached, tolerances, preliminary tests, operating conditions, instruments and records, and evaluation of results.
Instruments and methods of measurement (paras. 4.01–4.10): pressure, time, speed, motion or travel, power output, steam flow, temperature, and special instruments.
Test procedures, evaluation, and computation (paras. 4.11–4.24 and Section 5): from pressure set point range through a full set of static and dynamic characteristics.

⚠️ The pressure loop and the speed loop are inherently coupled on a turbine: an extraction-pressure control action simultaneously changes the steam flow through and work done by the unit, necessarily disturbing speed/load. The code treats “system interactions” as a distinct characteristic, so tests must account for the combined response with the governing system.

2. Pressure Control System Components 🔧

Component Class	Typical Elements	Function
Pressure sensing/transmitting	Pressure transmitters, bellows, pressure switches	Convert the controlled pressure into a standard measurement signal
Set point mechanism	Pressure setter, adjusting spring, electronic set-point unit	Establish the target pressure reference
Controller/amplifier	Hydraulic amplifier, PI/PID controller, relay circuits	Compare measured value with set point and output the control signal
Actuators and control valves	Servomotors, extraction control valves, bypass valves	Change steam flow and the pressure/energy balance
Feedback and damping	Mechanical feedback, oil dashpots, electrical damping networks	Ensure stability and regulation accuracy

The code emphasizes that a pressure control system may comprise several parallel or series regulation branches (e.g., extraction pressure and main steam pressure regulated separately), and the interactions among these branches must also be included in testing and evaluation.

3. Key Characteristics and Their Determination 🔥

Characteristic	Essentials of the determination method
Pressure set point range	Determine the upper and lower pressure limits reachable by the set-point mechanism, verifying they cover operating and heating demands.
Rate of change of pressure set point	Change the set point at a prescribed rate and observe following and overshoot to assess reference tracking.
Steady-state pressure regulation	After a load change, the steady deviation between the controlled pressure and the set point.
Steady-state incremental regulation (Method I)	Determination of incremental regulation between adjacent operating points under small load steps.
Hysteretic error	The deviation between regulation curves obtained on load increase and load decrease, reflecting mechanical/hydraulic lag.
Dead band	The no-response interval found by driving the operating condition slowly in both directions.
Control velocity saturation	Maximum rate of actuator/controller action and its effect on regulation quality.
Step response — control / system	Apply steps to the controller input and to the plant condition respectively; record response, overshoot, and settling time.
Stability band	The range over which the controlled pressure fluctuates about the set point during stable operation.
System interactions	Examine the coupling effects between pressure control and other loops (governing, overspeed protection).
Frequency response — control / system	Apply sinusoidal disturbance; determine amplitude ratio and phase shift versus frequency to assess dynamic stability margin.
Incremental regulation (Method II)	Statistical evaluation of incremental regulation under continuous small disturbances, complementary to Method I.

💡 Step and frequency response are the two main dynamic tests: step tests give a direct view of response speed and overshoot; frequency response gives bandwidth and phase margin. Together they characterize the stability and rapidity of the pressure control system.

4. Instrumentation and Test Arrangements 📏

Pressure measurement: pressures at the controlled point, extraction, main steam, and exhaust sides must be measured with transmitters or precision gauges meeting the prescribed accuracy class; representative tap locations are essential.
Time and speed: a time base and speed signal are used for synchronized recording and for correlating the speed–pressure coupling.
Travel and power: servomotor travel, control valve position, and generator power help analyze the response path.
Steam flow and temperature: heating-extraction flow acts as both disturbance source and measured quantity.
Recording: all channels are recorded synchronously at sufficient speed to resolve step and frequency-response data.

🚨 Pressure tests involve high-pressure steam and fast-acting actuators: verify pressure-part ratings, relief provisions, and personnel protection before testing; during step and frequency sweeps, operators must not stand facing valve bodies or flange joints.

5. Engineering Practice Essentials 🛠️

Extraction/heating units: extraction pressure regulation quality determines heating-steam parameters; contracts often include steady-state deviation, stability band, and step recovery time as acceptance items.
Tuning coordination: PID settings of the pressure controller should be tuned against the governing-system response to avoid mutual excitation between the two loops (system-interaction tests provide the basis).
Condition coverage: tests should be conducted in typical operating modes (pure condensing, extraction/heating) covering the upper and lower ends of the set point range.
Reporting: tabulate each characteristic’s measured results, curves, and agreed deviations per Section 6, stating operating conditions and instrument information.

6. Frequently Asked Questions ❓

❓ Q: Which pressures does PTC 20.3 cover?
A: Any steam pressure automatically regulated by a control system on a turbine-generator unit — typically heating-extraction pressure, main steam pressure, and reheat pressure. The exact boundary follows the “pressure control system” agreed in the contract.

❓ Q: Why is hysteretic error important?
A: Hysteresis makes the response to load increase and load decrease asymmetric, worsening pressure fluctuation and regulation quality — a direct manifestation of wear and friction in mechanical/hydraulic elements.

❓ Q: How does it coordinate with the PTC 20.1 governing test?
A: Since the pressure and speed loops are coupled, run the PTC 20.1 governing characteristics first, then the PTC 20.3 pressure characteristics, and explicitly evaluate “system interactions,” with combined-disturbance tests where necessary.

🔍 Q: Is the 1970 edition still applicable?
A: The edition (reaffirmed 1979) remains industry-cited; its methodology — set-point, steady-state, dynamic, and interaction testing — matches current turbine control-system acceptance practice. Specific limits and tolerances should follow the specifications agreed by purchaser and manufacturer.