Air Leak Testing

7/29/2019 Air Leak Testing

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Air Leak Testing Prior to Commissioning

Question :

"I need to commission a plant and intend to use low pressure air for leak testing. Can I

pressurise lines through a pump? In other words, if a line runs from the bottom of a tank,through a pump, and into another tank, can the whole line from bottom outlet on 1st tank

to the inlet on 2nd tank be tested at once? Or, is it necessary to test the line in twosections (before and after the pump)? Can the same be done when testing for leaks using

vacuum?"

ANSWER:

"If you're commissioning a plant, you can use clean, regulated compressed air for leak

testing. Of course, as you know, this will only show that the joints and other leak-proneareas do not leak at the testing temperature -not at the process temperatures which can be

higher. What I have done in the past is applied masking tape around the flanges' gasketedjoint with a small pin hole made afterwards. Then I use "soap and bubble" technologywith a fine brush, searching for the tell-tale bubbles that reveal an air leak. If the flange

joint leaks, the pin hole will form a soap bubble.

Be aware that before you undertake to subject your process or unit to a pneumatic

pressure, you should have a thorough and detailed knowledge of the lowest pressure

rating in your pressurized system. You must be careful not to surpass the lowest pressure

rating. For example, you may be using cast iron casings on your centrifugal pumps andthese are normally rated well below the pressure rating of the connected piping. If some

of the equipment is rated below the pipe, you can isolate the equipment and test the pipe

on its own rating, followed by testing the equipment one-by-one. I have seen what amisguided pneumatic test can cause with a ruptured piece of equipment. That is why I am

very, very cautious of pneumatic testing and would use it only if I were in control of all

the procedures. I am particularly of any cast iron equipment. Cast iron pieces orcomponents can have foundry defects or flaws and this can be devasting if they fail under

a pneumatic test because the net effect is the same as a grenade exploding. That is why I

prefer to test plant equipment hydrostatically - with water. The result of a hydrostatic test

failure is benign compared with a pneumatic one.

A vacuum test is safer but is difficult to detect leaks. The only practical measure you

have is loss of the vacuum as witnessed on a sensitive pressure gauge. This takes time

and patience.

Again, while you can pneumatically test an entire unit at one time, take time and troubleto make sure you are in complete control as to the safe, rated pressure on each component

in your system before applying air pressure. I would recommend that you use a 2-stage

air regulator to set the test pressure. This is much more accurate and is considered saferthat a single stage regulator.

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Diffusion

Diffusion is the spontaneous spreading of particles in a gas or liquid, or inside a solid material. The diffusionprocess is caused by the random thermal motion of the individual atoms, molecules or nanosized materialparticles. This process leads to a mixing of all gasses or liquids in direct contact, and is essential for all chemical

and biological processes. The speed of the process increases with the temperature. If a droplet of ink is carefullyinjected into a glass of water, the size of the coloured region will increase with the square root of the time sinceinjection (normal diffusion). In many natural processes there are some sort of memory effects which lead to anincrease in the size of the region affected by the particle mixing which grows faster than the normal square rootof time - called super-diffusion. Diffusion at a lower speed than normal is also possible, and this is called sub-diffusion. Usually, any deviation from the normal behaviour may be called anomalous diffusion, and is not sounusual as was commonly believed. Understanding the effects of diffusion is of crucial importance in manyindustrial technologies.

Diffusion and anomalous dffusion are central topics inmany of the research projects within the Complexgroup. Examples are diffusion of: - water in clayparticles, - magnetic flux vortices in superconductors,- colloidal microparticles in liquids and magneticfluids, - biopolymer in water, and - adsorbed atoms

on surfaces.

The concept ofBrownian motion is closely related diffusion. Brownianmotion is the random movements of microscopic particles in a gas orliquid. This motion is caused by the collision of the microparticle with themoving atoms of the surrounding medium. The first complete theory ofBrownian motion was formulated by Albert Einstein in 1905 but stilldiffusion and Brownian motion are active research areas. Ideas from thetheory of Brownian motion can even be used in economy.

The hazards you need know: Process Hazard Analysis (PHA)

Process Hazard Analysis (PHA) is a method to evaluate and identify credible hazardousscenarios. PHA is a thorough, orderly, systematic approach for identifying, evaluating,

and controlling the hazards of processes involving hazardous chemicals. Based on most

U.S. state and federal standards, an initial PHA must be performed on all coveredprocesses. Thereafter all PHAs must be updated and revalidated, based on their

completion date, at least every 5 years.

The process hazard analysis methodology selected must be appropriate to the complexityof the process and must identify, evaluate, and control the hazards involved in the

process. One or more of the following methods is used, as appropriate, to determine andevaluate the hazards of the process being analyzed:

What-if,

Checklist,

What-lf/checklist,

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Hazard and operability study (HAZOP),

Failure mode and effects analysis (FMEA),

Fault tree analysis, or An appropriate equivalent methodology.

The PHA must address the following:

The hazards of the process;

The identification of any previous incident that had a potential for catastrophicconsequences in the workplace;

Engineering and administrative controls applicable to the hazards and their

interrelationships, such as appropriate application of detection methodologies toprovide early warning of releases. Acceptable detection methods might include

process monitoring and control instrumentation with alarms, and detection

hardware such as hydrocarbon sensors;

Consequences of failure of engineering and administrative controls;

Facility siting; Human factors; and

A qualitative evaluation of a range of the possible safety and health effects onemployees in the workplace if there is a failure of controls.

The PHA is performed by a team with expertise in engineering and process operations.

The PHA team should include at least one employee who has experience with and

knowledge of the process being evaluated. One member of the team must beknowledgeable in the specific analysis methods being used.

In performing a PHA, the first step is to define the purpose, scope and

objectives of the study. The purpose defines why the PHA is beingperformed, e.g. to identify hazardous scenarios, to meet aregulatory requirement, etc. The scope defines the boundaries of

the process being studied. The objectives define the expectations

of the PHA results.

Next step is amass all the pertinent Process Safety Information (PSI)[see The more you know: Process Safety Information (PSI)] and

appropriate Standard Operating Procedures. To plan the PHA, the

process is divided into smaller manageable sections.

The PHA is conducted by identifying deviations from the design intent. The designintent includes values for operating conditions (e.g. temperature, pressure, flow, etc.),

equipment (e.g. materials of construction, etc.) or external events (e.g. general loss ofelectrical power, etc.). There may be one or several causes of deviations. Causes are

categorized as: 1.) Human Error, 2.) Equipment Failure or 3.) External Events. The team

brainstorms and decides the credible causes of these deviations. If one of these

deviations may occur, there are consequences that may result. The consequences mayimpact operability, quality or may be hazardous. Each scenario (deviation/cause

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consequence combination) is evaluated further, particularly the hazardous scenarios. In

evaluating these scenarios, existing safeguards are documented that prevent, detect or

mitigate the scenarios. The team then determines if a recommendation is appropriate toprevent, detect or mitigate the scenarios.

Explosion in a Salt Bath of a Synthetic Fiber Plant

Synthetic fiber plants like Nylon and Polyester use a salt bath consisting of a mixture ofsodium nitrate and sodium nitrite for cleaning metering pumps, spinnerette assemblies,

valves, and fittings coming in contact with polymer melts. These accessories are placed

in a hot salt bath for a specified time and the polymer melt sticking to the surfaces isdissolved via the oxidizing nature of this bath. After a specified time, the components are

taken out and receive a final cold water wash before being placed back in service.

The incident explained here relates to an explosion that took place while the cleaning of

these accessories was in progress.

Despite the best efforts of the management, one person involved in the accident died

while the other escaped with serious burn injuries to both hands.

What Happened?

The plant had initiated an annual shutdown to its Polymerization plant. Due to the

inherent nature of the polymer melt, jacketed steam valves are provided to ensure fluidityof the melt. One such valve was removed for cleaning and boxup before resuming

polymerization as a part of the planned shutdown activities. This large valve was

lowered into the salt bath via a thick metallic wire.

As soon as the valve contacted the saltbath, there was a loud explosion followed by

spraying of the hot salt bath liquid through the openings of the transparent shutters

enclosing the exhaust hood.

Two people, one operator and his supervisor, were in the path of the hot solution spray.Both sustained around 50% burns and the men were immediately transported to a nearby

medical facility with a burn unit.

After a week the operator expired and the supervisor had to undergo a series of plastic

surgeries to his hands for almost an year.

How It Happened

As per standard operating procedure, the piping on the jacket portion of these valves areto be blinded before cleaning. Upon inspection of this valve after explosion, the ruptured

gaskets were hanging on the flange bolts on both the sides of this valve.

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What Went Wrong?

The fitter who carried out the blinding of the inlet and outlet flanges on the steam jacket

had exhausted the metallic gaskets specified for blinding such jacketed valves. To ensure

that there was no time delay in handing over this valve, he put asbestos gaskets to the twoflanges and sent it for cleaning. The oversight was not corrected prior to cleaning the

valve.

The Investigation and Analysis

The large jacketed valve was heated with steam at 5.5 kg/cm2 (approx. 80 psig).

The trapped condensate in the jacket had not been drained fully.

With blinds on both the sides, the condensate flashed when placed in the bath. Thespecific volume changed from 915 kg/m3 to 2.67 kg/m3, increasing almost 350 times

resulting in an abnormal rise in pressure. The asbestoss gaskets could not withstand such

a huge rise in pressure and burst open. This sonic boom waves carried along with it thehot saltbath liquid spraying all over the two victims.

Lesson Learned

Never compromise on design specifications even with a minor item like a gasket. It can

mean serious consequences.

Never bypass any safety procedures. In this case,the transparent shutter was keptpartially open and the hot liquid came out through this opening.

Preparation of checklists and ensuring compliance is an important activity in any

shutdown. Such checklists prepared by an operator/technician have to be verified by asupervisor and authorized by a competent authority before proceeding.

Passivation of Stainless Steel

Passivation describes the treating of a metal with a mild oxidant (such as nitric acid) toremove surface iron or iron compounds by dissolution. This action forms a protectivepassive film on the surface of the metal. The trace iron left behind from machining and

fabrication can provide sites for corrosion if left untreated.

The process is performed by first cleaning the surface with solvents or an alkalinesolution to remove organic or metallic residues. Next, the parts are placed into

passivation solutions. The choice for this solution is usually nitric acid. The process is

controlled by three key variables: time, temperature, and concentration.

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Typical Range of Key Variables

Time: 20-120 minutes

Temperature: 60 - 160 0FConcentration: 20 - 50% Nitric Acid by volume

The key variables are typically determined by the type of stainless steel alloy beingprocessed. For example, 316 stainless steel would have different conditions than 317L

stainless steel.After the passivation bath, a sodium dichromate bath is often used to promote the

formation of a chromic oxide film. After this final treatment, the sample is placed into a

copper sulfate solution. Any remaining iron will show up as pink spots on the sheet.This would be considered unacceptable. Other testing methods can include a two hour

salt spray or a 24 hour high humidity test

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Air Leak Testing

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