How to Use a Pressure Decay Test to Check for a Leak in a PTFE Exchanger Shell?

Sometimes a PTFE exchanger is thought to have a very small leak in the external steel shell that is frequently too small to be visually seen or found with traditional bubble testing. A pressure decay test is a very sensitive, clean and quantitative approach for diagnostics. The detection and measurement of even very small rates of leakage, on the order of only a few molecules per minute, can be achieved without the need of liquids or risk of contamination by employing a stable, dry gas and a precision digital pressure device.

The PTFE exchanger shell leak method of pressure decay test is considered as one of the most dependable and most used methods to confirm shell integrity in field diagnostics in a controlled and repeatable manner.

Working Principle of Pressure Decay Method
The pressure decay technique is based on a simple physical principle: a sealed compartment holding gas will retain constant pressure if it is leak-tight. The hole in the containment boundary permits the escape of gas molecules and a quantifiable decrease of the pressure over time.

Unlike liquid-based test procedures, dry nitrogen provides a stable, inert medium, lowering corrosion risk and eliminating condensation effects.

The pressure gauge becomes a slow, silent, honest clock, counting off the molecules lost through the smallest break.

Preparing PTFE Exchanger Shell for Testing
Isolation and Drying
Before testing commences, the suspected shell side should be fully isolated from any systems connected to it. All process lines are bled and locked out.

The inside volume is then thoroughly dried to ensure no moisture remains. False indications are produced by moisture and evaporation or temperature expansion affect pressure readings.

Proper preparation consists of:

Full mechanical isolation of shell side.

Drainage of leftover process liquid

Nitrogen purge dried

Closed valve and fitting verification

Stable Temperature
Results are accurate only at a stable temperature. Gas pressure varies greatly with change in temperature . This might simulate or disguise genuine leaks .

Pressure decay test: Reliable PTFE exchanger shell leak results:

Protect the exchanger against direct sunlight

Drafts and air currents should be kept to a minimum

Measure ambient temperature

Stabilization of the confirmation gas temperature prior to measurement

Procedure for Pressurization
Nitrogen filling controlled
The isolated shell side is connected to a clean, dry regulated supply of nitrogen. Pressure is then slowly raised to the desired test pressure.

Some important restrictions are:

The test pressure shall not exceed the MAWP (Maximum Allowable Working Pressure).

Increase the pressure in a methodical manner

Avoid sudden pressurization to prevent mechanical stress

When the desired pressure has been obtained the system is disconnected from the nitrogen source.

Stabilization Phase
After pressurization, a stabilization period is necessary to allow:

Equalization of compressed gases using heat

Shell minor mechanical expansion

Pressure redistribution within exchanger volume

You may see a slight initial dip in pressure during this interval, which is not to be mistaken for a leak.

The formal data logging should be started only after the stabilization is finished.

Pressure Decay Test
Monitoring with digital equipment
The apparatus is connected to a high resolution digital pressure gauge and a timed measuring period starts. Typical test times range from an hour to hours depending on system volume and sensitivity requirements.

The gauge takes measurements of pressure at set points and produces a time-decay profile.

A leak path may be indicated by a slow and steady decrease in pressure:

Defects in welding

Defects in flange gasket

Tube sheet leakage problems

Tiny fissures in shell material

Interpretation of pressure drop
When pressure decrease exceeds permitted threshold accounting for valid leak notification is confirmed:

Effects of temperature change

Instrument accuracy tolerance

Environmental variation in the ambient

Gas pressure is very temperature sensitive so even slight temperature changes need to be properly connected with the pressure data.

Data Accuracy and Environmental Control
The Importance of Thermal Stability
Pressure decay testing is quite sensitive to temperature changes. A change of 1 °C in gas temperature can cause a detectable change in pressure which could lead to misleading results.

For correctness:

Ambient circumstances should be steady throughout the test

Direct heating or cooling sources should be avoided

Temperature should be noted together with pressure measurements

Instrument Resolution Specifications
High sensitivity pressure decay testing often requires:

Fine resolution digital meters

Calibration status: stable

Systems for measuring low drift

Adequate sampling frequency

Without suitable instrumentation extremely minor leaks may be missed or misconstrued.

Estimate Leak Size from Pressure Decay
You can estimate how serious the leak is by how fast the pressure falls with time. The steeper the slope, the bigger the leak aperture, and the flatter the curve, the more likely that there is no leak or the fault is very small.

The quantitative evaluation requires more modeling . The pattern of pressure degradation, however, gives useful comparative diagnostic information for maintenance decisions .

Advantages of Pressure Decay Method
The approach of PTFE exchanger shell leak pressure decay test has a number of practical advantages:

Non-destructive testing method

Liquid contamination not introduced

Extreme sensitivity to little leaks.

Digital and reproducible measurement

Permanent electronic data recording

Pressure decay testing is different from soapy water or dye testing since it may be performed on sealed systems without dismantling them.

Safety and Operational Considerations
Overpressure Protection
All testing must be strictly below the MAWP to prevent mechanical damage to the shell of the exchanger. Pressure relief devices or controlled supply systems should prevent unintentional overpressure.

Gas Safety -
Dry nitrogen is inert; however, adequate ventilation in confined areas is required to prevent the risks of oxygen displacement during pressurization and venting.

Summary
The pressure decay test provides the ultimate, clean and very sensitive approach to locate and quantify leaks in a PTFE exchanger shell. A method is shown permitting the detection also of very small leakage rates with a high reliability. The isolated shell is pressurized with dry nitrogen, thermal and mechanical stabilization is permitted and the pressure fall is carefully monitored over time using a high resolution digital gauge.

The temperature conditions must be carefully controlled and the maximum permitted operating pressure must be strictly observed to provide accurate results. Done right, the technique offers clear, quantifiable proof to back up smart judgments about repair or replacement.

It's in sophisticated exchanger diagnostics where the most elusive leaks are identified not by dramatic events, but by the careful observation of a steady and measured drop in pressure over time.