Review the pipe isometric. Identify a straight run meeting the 20-diameter rule. Avoid flanges, welds, or external heating jackets.
Use a thread-o-let or weld-o-let. Back-purge with argon to prevent slag on the inside. After welding, ream the inside to remove any internal weld penetration. asme ptc 192 fixed
But what exactly does "ASME PTC 19.2 fixed" refer to? Why do engineers specify this particular standard for fixed pressure measurement installations? And how does adhering to this code improve your plant’s heat rate and profitability? Review the pipe isometric
Place the pressure transmitter (e.g., Rosemount 3051 or Yokogawa EJA) at a fixed elevation. For liquid service, the transmitter should be below the tap. For gas service, above the tap. Use a thread-o-let or weld-o-let
| Mistake | Consequence | ASME PTC 19.2 Fixed Requirement | | :--- | :--- | :--- | | Using a threaded tap with a protruding fitting | Creates a cavity or protrusion, causing reading errors >1% | Tap must be flush, drilled after the pipe is in place | | Placing a tap downstream of a partially open gate valve | Asymmetric velocity profile leads to unpredictable static pressure | Minimum 20 diameters from any disturbance | | Forgetting to correct for impulse line fluid head | Zero shift of up to 10 psi in a vertical line | Requires explicit calculation of leg height (Lh) | | Using a transmitter range too wide for the fixed tap | Loss of resolution; uncertainty increases | Transmitter should be ranged so the operating pressure is 50-80% of full scale | If you are planning to install a new fixed pressure tap for a performance test:
For , ASME PTC 19.2 remains the controlling document. The "fixed" installation is acceptable but auditors will expect to see the uncertainty budget. Real-World Case Study: The Importance of ASME PTC 19.2 Fixed A 500 MW combined-cycle power plant struggled with unexplained 1.5% variance in gas turbine output between spring and fall. The culprit? Fixed pressure taps on the compressor discharge.
| Standard | Focus | Fixed Tap Allowed? | | :--- | :--- | :--- | | | General pressure measurement for performance tests | Yes (primary focus) | | ASME PTC 6 | Steam turbine testing (refers back to PTC 19.2) | Yes, with additional requirements | | ISO 5167 | Differential pressure flow meters (orifice plates) | No (requires corner taps or flange taps) | | API 551 | Process measurement (not performance testing) | Yes, but less stringent |