Pressure Decay Testing. Why, When, and How?

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Pressure Decay Testing. Why, When, and How?

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A simpler integrity test for less critical applications is a Pressure Decay Test. This requires the same air (or nitrogen) source and an accurate pressure gauge but does not require a mass flow meter. This type of test can be performed before and after use and is sensitive enough to detect very small leaks.

All test procedures outlined below should be followed when performing pressure decay testing of cartridge filters. The schematic below represents a cartridge integrity test setup with all of the piping, valves, gauges, and regulators needed to properly integrity test filters in place.

NOTE: This blog and the schematics in it show a filter housing with a single 10&#; cartridge installed. Different-length cartridges can be accommodated using the appropriately sized housing that will also match the end configuration on the cartridge. Contact CPF Customer Service for additional details on multi-filter housing installation.

Integrity Test Parameters

* Bubble Point exceeds maximum operating parameters

1Pressure Decay Test Formula

DP = D * T * Pa / V

Where:

DP = Maximum allowable Pressure Drop; (starting pressure &#; final pressure at end of test interval)

D = Diffusion Rate limit; (from the cartridge data sheet)

T = Time (minutes); typically, 10 minutes

Pa = Atmospheric Pressure (14.7 psi)

V = Upstream Volume of test apparatus (cc); Can be calculated by filling the upstream side with water, then draining through V2 and measuring water volume

2Diffusion specification values are shown on the Data Sheet for the cartridge being tested

 Important Notes

• ALL connections must be airtight. Any leaks in the plumbing can cause inaccurate readings. It is recommended to leak-check the system on a regular basis.

• For more Pressure Decay Leak Testerinformation, please contact us. We will provide professional answers.

  Marginal integrity failures can often be the result of inadequate cartridge wetting. As indicated in the instructions below, re-wet the module and retest.

• Gross failures could be the result of an improperly installed cartridge in the housing or a plumbing leak. Make sure the cartridge is properly installed with any O-Rings undamaged and properly seated. Check the system for air leaks.

Cartridge Installation & Wetting

1. Make sure all parts required to install the cartridge in the housing are available
2. Install the cartridge in the housing, ensuring all fittings and connections are tight
3. If a pre-wetted cartridge is being tested, skip to the appropriate integrity test section
4. To wet out a cartridge:
   1. Make sure all valves are closed
   2. Connect feed water to the &#;Water In&#; line
   3. Open valves V3 (Vent) and V4 (outlet)
   4. Slowly open V1 to allow wetting fluid to enter the housing
   5. Once the fluid is observed coming out of the vent with no bubbles, close V3
   6. Adjust V1 as necessary to feed ~2 liters per minute per 10&#; length
   7. Allow fluid to flow for ten (10) minutes
   8. Slowly close V4 until inlet pressure (P2) reads 15 &#; 20 psi
   9. Allow fluid to flow for an additional five (5) minutes
   10. Close V1
   11. Open V2 and V3 to allow housing to drain

Pressure decay procedure

1. Install the cartridge into the housing
2. Follow wetting instructions if necessary
3. Start with all valves closed
4. Connect regulated air or nitrogen line to V6
5. Open valves V2 (Drain), V3 (Vent), and V4 (outlet)
6. Set pressure regulator to 2 psi
7. Open V6, then V5
8. Allow the system to purge any upstream fluid
9. When the flow from Vent and Drain is mostly air, close V2 and V3
10. Slowly increase the air pressure (pressure reading on P3) to the value specified in the table
11. Close valve V5
    1. Allow the system to stabilize for 2 minutes
    2. Re-pressurize if necessary by opening V5
12. Read pressure on P3 at the end of test time (typically 10 minutes)
13. The cartridge passes the integrity test when the pressure drop is less than the calculated amount
14. Relieve pressure on the regulator
15. Close valves V6 and V5
16. Open valve V3 to completely relieve upstream pressure
17. Remove the cartridge from the housing

If the cartridge appears to fail the initial test, repeat the Cartridge Wetting procedure and increase the rinse time to twice the initial amount to assure that the media has been properly wetted. Then, retest. If necessary, check the cartridge seal in the housing.

Critical Process Filtration, Inc., an ISO -certified manufacturer of process filters has been helping customers for over 20 years to build and improve process filtration systems. Proven products, technical services and support, fast shipping, and very competitive pricing assure companies of getting the best value and expertise. Our comprehensive testing & analysis and validation services support your team whenever they need it. Partnering with your process team is how we deliver your company&#;s solution right the first time. For more information on Integrity Testing in your process, contact us here.

Considerations for Choosing which leak test method is right for your application.

As of late, there has been much discussion surrounding the differences between Mass Flow and Pressure Decay leak testing, specifically identifying when to use which test. While interviewing Uson&#;s leak test experts on the subject, their initial response was the same:

&#;It all depends on the application.&#;

Before we dive into why everything depends on the application to answer the question of which test to use when, let&#;s take a brief look at both test methods. Below is a great video demonstration of both test methods by Uson Applications Specialist, Steve Bencal, using our Sprint mD leak tester.

Pressure Decay Leak Testing:

The Pressure Decay leak test is also known as gauge decay, absolute pressure, leak drop or delta pressure leak test. This test method measures the drop in pressure on a gauge (transducer) during the leak test. Pressure decay testing is used to test products for leaks by trapping pressure inside a product and then measuring pressure loss.

In this test method the component is pressurized with air and the pressure transducer monitors the pressure for negative changes. Any pressure drop indicates a leak. The pressure change from the first measurement to the last measurement made during the test phase of a test is what is important.

The test is very fast, a matter of seconds, and is therefore popular for leak testing in high-volume industries such as automotive components, castings, medical devices and consumer goods.

Mass Flow Leak Testing:

The mass flow test measures the amount of air passing through an object using units of measurement of sccm or liters/time period. The major advantage of the mass flow technique is that it eliminates the need to know the volume of the product. There are two mass flow sensing techniques that have found their way in to leak and flow testing: thermal mass flow and differential sensing mass flow.

Mass flow is a direct measurement of a leak or flow making it a preferred approach when you have a targeted flow specification for your part.

Start with the Objective.

When establishing which of the two leak tests is the best choice for the application, our experts recommend starting with your objective. Do you, A) want to know IF your part has a leak? Or B) Do you want to know HOW MUCH your part leaks?

Looking for a leak?

If you are looking for a leak, chances are that the part or component is NOT supposed to leak. A leak indicates that the part is faulty. An example of a part or component that&#;s not supposed to leak would be a catheter.

Measuring how much the part leaks.

This scenario is for parts or components that are not sealed by design and intended to lack the ability to hold pressure. An example of a part that meets these criteria would be an engine block. The engine block is designed to allow oil, gas and air to flow in order for the engine to function.

Influential Factors

Is there a GENERAL guide for when one test should be considered over the other? Our experts say &#;No&#; because, once again, everything is dependent on the individual application, and you can never REALLY answer the question confidently unless you do an Application Study. But, there are some VERY GENERAL factors that can help guide you to one test or another:

Testing Flow Rate

As shown in the table above, there are many factors that can influence results. Each factor presents its own distinct challenges in setting up cycle times and repeatability. Mass flow sensors detect pressure changes to calculate flow rate. In some cases, a certain amount of pressure drop is required in order for the sensor to indicate flow. Mass flow shines when used to determine if a part will flow to specification. For instance, looking at cc per minute to 20,000 cc per minute or up to 10 to 15 cubic feet per minute, these are cases where mass flow will excel over pressure decay.

Cycle Time & Repeatability

Uson has had the privilege of working with hundreds of customers to perform application studies and develop leak specifications for thousands of parts. During our consultation process we see time and time again the concerns over test cycle times and how speed can impact the decision of which test and tester a customer chooses for their application. Why is speed a factor? The more parts you can test in less time equals more profit. Can this factor lead a customer down the wrong path? Absolutely. In rare cases where either a mass flow or a pressure decay test could be a viable option, 9 out of 10 times the customer will lean towards the option that offers the fastest cycle times and maximum throughput. In addition, pressure regulators are continually feeding mass flow which can cause flutter or variation in pressure that when detected by the mass flow sensor, can indicate that flow is occurring when it should not be, and therefore degrade repeatability and accuracy of that test. As experts, it&#;s our job to guide customers in evaluating the application study data and making an educated decision about which method(s) and tester is the best solution for their application. Based on all the data, and the objective, some compromise may be needed in order to implement the BEST solution as to which test is the more viable option.

Up Front & Ongoing Cost

Why is cost a consideration? A mass flow test requires a level of sensor sensitivity that is beyond what is available through standard supply channels. Building leak testers around the use of these sensors requires complex configurations and components, all of which are more costly than standard components. When you mix all of this together you get a piece of equipment that costs more to build and support. As mentioned earlier, the reality is that cost often influences the decision more than data. If the data indicates that mass flow is the right option for the application and will accomplish the testing objective, the budget should be calculated appropriately.

Which Test is Right for Your Application?

If you don&#;t know, or aren&#;t sure which test method is best for your application, Uson can help. Although your goals and objectives drive the path forward in test method selection, they are only one piece of the puzzle. And forgive the corny metaphor, but there are many pieces that make up the puzzle. Consulting with leak testing experts like Uson can shorten the evaluation and decision-making process, saving you hundreds in the short-term, and thousands, perhaps millions, in the long-term.

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