Measurement of Steam Flow by Average Pitot Tube 2

[swk]

Is it acceptable that the measruement of steam flow (secondary flow) by Average Pitot Tube for following conditions?

1. Steam condition : 105 barg, 500 deg.C
2. Measuring point :
- Steam Turbine inlet / exhaust
- Several secondary flow measurement

ASME PTC 6 recommends Flow Nozzle for the measurement of primary flow and it recommends Nozzle, Orifice and Venturi for the measurement of secondary flow.
But the problem is that ASME PTC 6 or 19.5 does not mentioned about Average Pitot Tube.

 

[hacksaw]

The performance test codes cover just that: a recommended (and agreed upon) basis for performance testing in the shop and in the field, and only installed on a temporary basis.

You are not prevented from using other devices for permanent (non-PTC) monitoring.

 

[ApC2Kp]

To swk,
The averaging pitot tube should be adequate for normal control purposes within 3:1 turndown range or more. As hacksaw has pointed out the PTC establishes the basis for performance test and should not limit your choices for normal operation controls.
The design of pitot tube needs to have robust construction to resist fatigue failure due to flow induced vibration. Failures of thermowells at turbine inlets have resulted in destruction of turbine blades by the ingested thermowell.
A similar geometry is used for steam sample probes and the ASTM D 1066 standard has suggested designs. The probe is recommended to be supported at both ends to resist vibration and fatigue failure.

 

[hacksaw]

Such probes have done more than damage a few blades, sad to say. They have penetrated impact plates and wrecked entire machines in less that a few hours after startup. Seems that they only fail when you can least afford it...