ML11186A896

NUREG Section 5.3 Recommendation

ML11186A896
Person / Time
Site:	Fermi
Issue date:	03/21/2011
From:	Cavey E Detroit Edison
To:	Office of Nuclear Reactor Regulation
Bedi G
Shared Package
ML11186A384	List: ML11186A583 ML11186A728 ML11186A791 ML11186A896 ML11186A913
References
TAC ME1150, NUREG-1482
Download: ML11186A896 (2)

Text

5.3 Allowable Variance from Reference Points and Fixed-Resistance Systems System stability, throttling variations, gage snubbing or dampening, and gage readability are just a few of the aspects that make achieving a precise, repeatable reference value nearly impossible.

Typical Operations Department instructions for reading instruments might be worded as follows:

For instrument fluctuations, "the operator shall make a quantitative assessment of the reading to record a representative value. It is expected that the value will be halfway between the high and low peaks of the needed fluctuations."

Test personnel need to have a solid understanding of the IST testing concepts, so as to ensure that the appropriate control actions are taken to achieve the reference point. All deviations from the reference value are then treated as symptoms of changes and, depending upon the degree of deviation, indicate the need for further tests or corrective action.

In the ISTB-5100 and ISTB-5200 sections, the following attributes are discussed:

• duration of the tests - they can be conducted "after pump conditions are as stable as the system permits".

• test conduct - "with the pump operating at a specified reference point."

• establishment of the reference point - vary the system resistance "until the flow rate equals the reference point."

• test analysis - "all deviations from the reference values shall be compared with the ranges and corrective action taken as specified in ISTB-6200 ISTB-3510 discusses instrument fluctuations and contains the statement: "Symmetrical damping devices or averaging techniques may be used to reduce instrument fluctuations." This is carryover from ASME XI IWP-4150 (1983 Edition) Fluctuations which states: "Symmetrical damping devices or averaging techniques may be used to reduce instrument fluctuations to within 2% of the observed reading".

Because of these issues, several licensees recently requested relief from the Code requirements for fixed reference points. Certain designs do not allow for the licensee to set the flow at an exact value because of limitations in the instruments and controls for maintaining steady flow.

The characteristics of piping systems in other designs do not allow for flow to be adjusted to exact values. Licensees have requested relief to establish a range of values similar to using a pump curve, but with a very narrow band. For example, one licensee proposed to use a reference curve with the tolerance around the selected value to be +/- 2 percent. Plant implementing procedures may instruct operators to set the flow to 1500 gpm. When this step is performed, the operator would attempt to set the flow as close as possible to 1500 gpm, with a requirement that the stable test flow must be within the range of 1470 - 1530 gpm..

ISTOG Recommendation s

The allowed tolerance for setting the fixed parameter must be established for each case individually, including evaluation of throttling capability. Licensees should consider improvements in throttling methods where system control is especially poor. A total throttling tolerance of + 2 / -1 percent of the reference value is considered as meeting the requirements of the code sections as stated in the discussion section.

For a tolerance greater than + 2 / -1 percent, a corresponding adjustment to acceptance criteria shall be made to compensate for the uncertainty, or an evaluation would be performed and documented justifying a greater tolerance. The variance and the method for establishing the variance must be documented in the IST program documents or implementing procedures.

Basis for Recommendation The ASME OM Code does not address the possibility that a flow rate or differential pressure may not be controllable to an exact value. When the Code specifies that the system resistance be varied until either the flow or differential pressure equals the corresponding reference value, it does not literally intend that the "set value" be precisely attained without any fluctuations. Licensees recognize that the reference value for certain pumps can only be achieved within a specified tolerance. Licensees shall attempt to set the repeatable parameter as close as possible to the reference value during each test.

The basis for allowing a variance of + 2 / -1 percent from the reference value is contained above in the discussion section and deals with instrument fluctuations and system stability issues. The Code allows symmetrical damping devices or averaging techniques to be used to reduce instrument fluctuations to within 2 percent of the observed reading for values specified in the implementing procedures. Greater variances must be justified and acceptance criteria adjustments made as necessary. The limitation of 1% in the negative direction reduces the non-conservative impact on the variable parameter. The total 3% allowable variance provides for a reasonable throttling control range while minimizing the impact on trendability of the variable parameter.

Licensees should ensure that performance trending of pumps is capable of detecting degradation as early as possible. Larger variances in the reference parameter will induce scatter in the variable parameter data. Techniques such as data normalization, where recorded test data is corrected by the known pressure to flow relationship, should be used when necessary to provide for accurate short term trending.

An example of an approved relief request for the use of a higher reference variable range combined with the use of data normalization can be found in ADAMS Accession Number ML102720160.