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PiNPOiNT Colormetric Leak Detectors
Leak Testing Large Containers
Leak Testing Primer

 

 
  PiNPOiNT Colormetric Leak Testing
A Technical Paper by Gerald L. Anderson
 
Introduction
Colormetric Leak Detection, the primary topic of this report, is a relatively new, reliable, economical method of Leak Detection. Such Leak Detection is an outgrowth of industries need for better methods of leakage testing. Before launching into the specific qualities of this method, I would like to briefly comment on the total Leak Detection field.

The Alaskan Pipeline is a dramatic illustration of the need for more attention to the field of leakage testing. Even the extensive flaw testing performed for the Alaskan Pipeline was not enough, leaks still developed. The Alaskan Pipeline is not the only project that has suffered, sometimes at great expense from failure to understand the important role of Leak Detection. As a result of such leakage problems as experienced by the Pipeline, quality control experts are demanding that leak testing as well as flaw testing, be in their control.

Leak Testing can be as refined as flaw testing. Each company must require of their supplier, quality leak testing products. Since leak detection, in the past was assigned a minor role, it often remains a branch of alchemy to many people. Often companies are not sure how to derive a leakage standard for their product which adequately balances the need for legal liability protection with the demands for economic production.

The rush to know more has brought a mixture of information on the market, sometimes to the point of confusion. Much of the confusion could be eliminated by a clear set of terminology, some of which I will set forth here.

In its simplest form, all leakage rates are expressed in mass loss per unit of time, flowing across a pressure barrier. The variety of ways of expressing this leakage is large and a uniform means of communication is necessary. It is easy to see how confusing this can be when leakage is defined in an unfamiliar standard. For instance, I think of leakage in terms of standard cubic centimeters per second. However does a standard cubic centimeter look like? If I say to you that a leakage rate of 10-4 std. Cc/sec. Is the equivalent of the loss of 1 cc. of air over 3 hours, this sounds like a large leakage rate. However, if I say that it is the equivalent of losing a pound of freon over 27 years, you will probably think the leakage quite small.

Some of the equivalents to standard cc. per seconds are illustrated here:

EQUIVALENT LEAK SIZE ( to Std. cc/sec)
Std.cc/sec= One cubic centimeter of gas flow per second at 14.7 psi of pressure and a temperature of 77o.
 
Std cc/sec Time for lb of
freon to leak
Torr
Liters/sec
Time for
cc to leak
Bubble Time
in Water
10-1 10 Days 7.6x10-2 10 seconds 1.3 seconds
10-2 3 Months 7.6x10-3 100 seconds 13.3 seconds
10-3 2.7 Years 7.6x10-4 16.67 minutes  14.5 minutes
10-4 27 Years 7.6x10-5 2.78 hours 24 minutes
10-5 270 Years 7.6x10-6 27.8 hours 4 hours
10-6 2,700 Years 7.6x10-7 11.57 days  
10-7 27,000 Years 7.6x10-8 3.86 months  
10-8 270,000 Years 7.6x10-9 3.22 years  
10-9 2,700,000 Years 7.6x10-10  32 years  
10-10 27,000,000 Years 7.6x10-11 320 years  


Equal in importance to an understanding and uniformity of Leak Detection standards is an understanding of the leakage testing products. A multitude of Leak Detection and leakage measurement devices exist. Some devices are pneumatic. Some are electronic. Some are chemical. Each device has advantages and disadvantages. To use any method effectively, a careful analysis of the device is necessary.

The device I will attempt to analyze today is one form of chemical leakage testing. This device, "Colormetric Developer System", is a two component leak locator, consisting of a tracer-activator and a developer-indicator.

The first component, the tracer, can be either gaseous or liquid; such as ammonia, chlorine, hydrogen chloride, carbon dioxide, hydrogen sulfide, nitrogen dioxide, hydrazine, water and liquid hydrocarbons. Obviously, in some cases, adding tracers is unnecessary since the tracer is inherent in the product being tested. The most important characteristic of a tracer is its ability to rapidly penetrate through a leak.

The second component, the developer-indicator, must have the ability to react with the tracer and to indicate colormetrically minute leakage of the tracer. The developer should enhance the visibility of the leakage indication by drawing the tracer to it, by absorption. This process should cause the indication to spread. Also, a good developer creates a sharp color contract between its reacted and unreacted appearance. Ideally, the color reaction should be the compliment of the original developer color. The developer should be such that it is easy to remove by rinsing or wiping. And obviously, the developer should not be harmful to the material being tested.

How Colormetric Systems Work
To locate a system leak, a thin coating of the indicator developer should be applied to the outer surface of the object being tested. The application can be made by spraying or dipping the test object. Or, the developer can be incorporated into a dry paper which can be taped on to the test object. In most cases, only a small area of the test object need developer coating such as seams, welds, couplings and joints; these areas are often the susceptible areas.

Next, the tracer-activator should be introduced into the test object at a predetermined concentration and pressure. The tracer-activator must be given a specific soak time so that it may effuse through any leaks and thus react with the developer.

For any given Colormetric System, sensitivity is affected by four factors - the physical properties of the tracer, the differential pressure across the leak, the concentration of the tracer and the soak time. These four factors, which I will outline, make significant differences in the test results and they need to be accurately calculated.

1). Physical Properties of the Tracer. The viscosity and molecular weight of the tracer determine its ability to rapidly diffuse throughout the test object and transverse a leak. Usually, the tracer is a combination of two gases; one is the activator, one is inert. For maximum effectiveness, the molecular weights of the activator and inert constituent should be similar.

Graph #1 shows the relationship between viscosity of the tracer and its ability to diffuse. This graph illustrates that the lower the viscosity of the tracer, the more rapid will be the rate of diffusion. Likewise, low molecular weight increases the rate of diffusion.



2). Differential Pressure Across the Leak. Generally, the higher the pressure across the leak, the more sensitive the test.

Graph #2 shows the relationship of Colormetric sensitivity as a function of pressure and soak time. The curve itself represents a spot of 1mm diameter produced by the colormetric interaction between the tracer activator and indicator developer. As the pressure differential is increased, the time required for a visible Colormetric reaction decreases. In many systems, as this graph indicates, at a certain point, further increases in pressure will only marginally reduce the development time.


Leak Rate 1x10-4 std cc/sec
Tracer Activator: 1% NH3 in 99% N2
Indicator Developer: Pinpoint ADP-119


3). Tracer Concentration. Changes in the concentration of the tracer-activator will vary the Colormetric reaction time with the developer. By increasing the concentration of the tracer-activator, Colormetric development time decreases.

Graph #3 shows the relationship between tracer-activator concentration and development time (in seconds) to produce a 1mm spot on ADP-119.

4). Soak-Time. Soak time is the time allowed for the tracer to diffuse through the system, transverse the leak and react with the developer. Soak time must be long enough to allow sufficient tracer to transverse the leak. The smaller the leak, the longer soak time required.

By increasing the pressure differential, tracer concentration and soak time, the maximum sensitivity for any given Colormetric System can be achieved. However, the smallest leak which can be found with a Colormetric System is limited by the diameter of the tracer molecule. Obviously, if the leak size is smaller than the size of the tracer molecule, the tracer cannot transverse the leak and cause a reaction. For practical purposes, the theoretical limit of sensitivity is reached when the leak size is one decade larger than the molecular size of the tracer.

Colormetrics In Use
Having given an overview of Colormetric Systems, I will describe one system is use in the marine industry - The Colormetric System now being used to test the tightness of integrated LNG tanks. This is an ammonia-tracer system, which is being used because of the relatively high sensitivity, which can be achieved with ammonia as a tracer. Ammonia, has a molecule small enough to transverse leaks with a minimum diameter of 10-3 microns. It has low molecular weight and low viscosity. Because of these physical characteristics, ammonia Colormetric Systems can be satisfactorily used for leakage rates as small as 10-7 std. cc/second. Chart #2 compares the physical characteristics of ammonia with two of the most used Non-Colormetric tracers - Freon and Helium. As can be seen from the chart, the physical properties of ammonia make it an excellent tracer.

 

Chart #2 Tracer Gas Comparison
  Viscosity in Centipoise Molecular Weight Molecule Diameter in Microns (M)
Ammonia .0094 17 3x10-4 M
Freon .0118 121 6.96x10-4 M
Helium .0178 4 2.18 x 10-4 M


The formulas used to precisely calculate are described in the ASTM Specification E1066
Standard Method for Ammonia Colorimetric Leak Testing.
Refer to Section 11.1.2.

Application of Colormetrics
Once the variables are calculated, the application of the ammonia Colormetric System is fairly straightforward. The recommended process for testing pipes and containment tanks with an ammonia system is:

1). Clean the inside and outside of the tank thoroughly.

2). Dry the outside wall of the container with a hot air gun.

3). Coat the tank seams and other points of suspected leakage with a thin layer of developer.

4). If the atmosphere is humid, dry the developer with a hot air gun.

5). When the developer is dry, in some instances a vacuum should be drawn on the inside of the tank to reduce the moisture and provide for an even tracer mixture in the tank.

6). Meter in the ammonia-nitrogen mixture until atmospheric pressure has been achieved. Large leaks will develop immediately.

7). If no leaks are apparent, slowly increase the pressure of the tracer mixture until test pressure is achieved. Maintain vigilance for the development of large leaks.

8). At the end of the soak time, mark any leakage indications, evacuate the tracer, and rinse with nitrogen.

The ammonia tracer process just described is one of many Colormetric Development Systems. Each Colormetric System though different in specifics, has several common features. One of the common features is that system sensitivity can be adjusted on a formula basis to meet any given leakage standard. Once the calculation is done, unlike electronics, no further calculation or adjustment is required.

Colormetric Systems are easy to operate. They do not require highly trained personnel slowly investigating each seam. Color reactions can usually be seen quickly in a wide visible range. Since personnel time and initial system cost is low, Colormetric Leak Detection is relatively inexpensive. Colormetric Systems are operator independent. Colors readouts are clear and repeating eliminating decisions based on personal opinion.

Because of these features, colormetric systems have assumed a major role in the rapidly advancing field of leak location.
 

  
 

 
 

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