TEST METHODS

LDS uses various test methods to address multiple plant scenarios. Test methods are selected based on multiple criteria, including (but not limited to) safety, accessibility, and accuracy.

The Signature Comparison Method

This method is used when differential pressure across the valve can be removed. The Signature Comparison Method requires valve manipulation. As a result, it requires a test plan to avoid injury or mistakes.

Two or more acoustic signatures (Amplitude vs. Frequency), with and without pressure, are recorded with a transducer. The signatures are then compared to determine if the valve leaks. All items within the valve group are also tested to identify the presence of crosstalk.

Signature Comparison Method

The Differential Signature Method

This method is used when the comparison method cannot be used for various reasons. It relies on the signal level of a leaking valve being higher than the signal levels on either side.

When using this method, multiple signatures are taken and analyzed as follows:

Signatures are recorded at the valve, upstream, and downstream of the valve.

The background noise on the pipe is subtracted from the signal on the valve.

A positive difference indicates that the valve is producing a stronger signal than the connected piping, and is therefore leaking.

Differential Signature Method

The Direct Comparison Method

This method is used when the objective is to determine, with the least difficulty, which are the worst leakers among a group of essentially identical valves at identical differential pressures. This method is used only when the test time is limited, as in the case of very high temperatures or radiation.

One signature is recorded from each valve under the same pressure and then all are compared. No Background Signatures are recorded.

Direct Comparison Method

The Time Sweep Method

This method is used to evaluate the condition of traps. It demonstrates whether or not a trap is functioning properly. It can also show if a trap is stuck open or shut, oversized, or if it opens too frequently or too long.

A pressure signature is recorded using a 2-minute scale. It displays the opening and closing of the trap with the block valves open. Next, a background signature is recorded with the trap block valves shut. Finally, a differential test is made on the trap bypass valve which identifies potential crosstalk.
All signatures are compared to permit a diagnosis.

Time Sweep Method