PTC-6 primary flow elements

Highly accurate primary flow sections for ASME PTC-6 performance tests


Our primary flow sections are specifically designed and manufactured to meet the requirements of ASME PTC-6, 1996 Performance Test Code for Steam Turbines. They are intended to be the primary flow measurement device during steam turbine acceptance tests. Test sections consist of a low-Beta series ASME throat-tap flow nozzle that is installed in a section of pipe approximately 30 pipe diameters long. The upstream section of pipe, which is approximately 20 diameters long, is carefully honed on the inside to insure cylindricality/finish for a minimum of 4 pipe diameters upstream of the nozzle. The downstream section of pipe is approximately 10 pipe diameters long. A flow conditioner is built into the upstream pipe section at a distance of approximately 16 pipe diameters upstream of the flow nozzle.


Nozzle:
The nozzle is available in two styles. The flanged style is intended to be clamped between two mating flanges. Flanges may be flat faced. ring type joint, large groove or tongue and grooved. Fig-l shows a typical flanged style nozzle.


The weld-in style is permanently welded in place between the upstream and downstream pipe sections. Fig-2 shows a typical welded-in style nozzle.


Either style can be furnished with pipeshell ends beveled for butt-welding or with flanged ends.


Both styles are equipped with four integrally machined, equally spaced throat pressure taps. The interior surface of the nozzle is precision machined to insure roundness and cylindricality then polished to achieve a hydraulically smooth surface.


Flow conditioner:
The flow conditioner may be either the perforated plate or 50-tube bundle style, at the purchaser's option. The code-preferred flow conditioner is the perforated plate with a nonuniform hole placement pattern. This style of plate has been shown to be able to remove both swirl and distortion from the upstream flow.


Inspection port:
For weld-in type flow nozzle assemblies, a code-recommended inspection port with a removable, contoured pin oriented plug can be furnished at the purchaser's option.


Diffuser cone:
Differential pressures for PTC-6 nozzles are typically high with ranges often exceeding 1,000 inches of water column. In order to reduce the permanent pressure loss of the flow section, a diffuser cone can be installed at the exit of the flow nozzle. The diffuser may be integral with the flow nozzle or may be supplied as a separate component coupled to the nozzle. Permanent pressure losses through the section can be reduced by as much as 70% through the use of diffuser cone attachments.


Design standards
All our primary flow sections are designed in accordance with the applicable sections of the following standards.

  • ASME PTC-6 1996
  • ASME PTC 19.5
  • ASME Research Committee Report on Fluid Meters-6th.Edition


Fabrication standards

  • ASME Section I
  • ASME B31.l - Power piping
  • ASME B31.3 - Process piping


Flow measurement accuracy All our primary flow sections are guaranteed to meet the requirements for fabrication and the laboratory calibration acceptance criteria as set forth in ASME PTC-6, 1996 and the PTC-6 Code Committee's further explanation of those criteria as published in "Mechanical Engineering," Feb 1999, pages 81-83. Specifically, these acceptance criteria are:

  1. The average value of Cx must be within the range 1.0029<= Cx <=1.0079.
  2. The confidence interval of the slope of the Cx's must include 0 (zero) or the absolute value of the slope must be less than 2.72 x 10-7.
  3. The confidence interval of the Cx's must not exceed 0.0006. In the special case when the range of Reynolds numbers achieved during the calibration matches or exceeds the range of Reynolds numbers that will be encountered during the acceptance test, the above criteria are modified as follows:
    • The average value of Cx need not be within the range 1.0029<= Cx <=1.0079.
    • The confidence interval of the slope of the Cx's need not include O (zero) nor must the absolute value of the slope be less than 2.72 x 10-7.
    • The confidence interval of the Cx's must not exceed 0.0006.
    • The calibration data must be repeatable. This can be verified by a minimum of three repeat points in the calibration. The repeat points must agree to within 0.05% in order to satisfy this criterion.
    • The calibration must be reliable. This requirement is considered to have been met if the calibration is done at a facility with an established reputation for performing this type of calibration correctly. One such laboratory is ARL, Inc of Holden, MA.

Please contact our sales engineering department for assistance with your particular application.