NOZZLE IN COMPLIANCE WITH ASME PTC 6
Nozzle in compliance with ASME PTC 6 is a throttling device consisting of long radius nozzle that is together with flow straightener (downstream) built-in the meter tube. This layout represents a complete set with high degree of flow measurement accuracy. It is used for measurement of water flow in thermal performance testing of steam turbines.
The nozzle body is with elliptic inlet and cylindrical outlet, so-called nozzle throat, in which differential pressure is tapped (-). Within specification of ASME PTC 6 standard there is possibility to manufacture design of the nozzle with small diameter ratio 0.25 ≤ β ≤ 0.5. Shape of the nozzle is proposed in compliance with ASME PTC 19.5.
In most cases the nozzle body is embedded between flanges or, it can be welded into the piping, it is so-called weld-in design. Weld embedding of both nozzle body and the whole set into the piping has advantages such as material savings on the flanges and subsequently minimized leakage in flanged joints. This design is suitable especially for high pressure applications.
In both designs there are straight lengths of piping minimally 20D “downstream” the nozzle and 10D “upstream” it. Straight part of piping “downstream“ the nozzle consists of flow straightener placed minimally 16D “downstream” the nozzle body. Used type of straightener is both flow straightener with bundle of 50 pipes, and rectifier proposed in compliance with ASME PTC 6.
Axis (+) of differential pressure tapping is 1D from the inlet part of the nozzle. For differential pressure tapping (+) there have to be made four individual outlets of differential pressure, as well as for (-) differential pressure tapping in the nozzle throat. Upon agreement with customers outlets of differential pressure are always customized to various options of primary element attachment to measuring circuit.
The nozzle body with throttle tap is mostly made of stainless steel. To eliminate possible different thermal expansion of the nozzle, part of the section “downstream” the nozzle of minimal 4D is manufactured of stainless steel. The other components of the meter tube are made of carbon steel, stainless steel or of materials appropriate for parameters of measured fluid.
The whole set must be manufactured with high accuracy, especially geometry of the nozzle body and design of differential pressure tapping (-) in the nozzle throttle affecting success of calibration. Accuracy of the whole set production is proved by water calibration under conditions of flow as closest as possible to conditions under performance testing of steam turbines.