A known damage mechanism in industry is identified as "Crevice Corrosion". A concentration of corrosive materials or combinations of substance to form an aggressive corrosion solution in which is located at a specific point to accelerate damage is known crevice corrosion.

An example of this damage mechanism is the corrosion that occurs between seals on two opposing flanges with a gasket placed in-between the seals.

Concentrations of corrosive materials collect within a crevice such as between sealing surfaces and gasket material. Because of the concentration of the corrosive material in a localized area the rate of corrosion is accelerated. Corroding or loss of sealing area can cause loss of containment therefore potential for catastrophic release with loss of assets, production and injury to personnel.

Cross Sectional View of Raised Face Flange

Testing the sealing surface of flanges has become a standard practice. This test is performed in-situ as part of a run and maintain program. Testing is also common as a pre-turnaround exercise to determine which flanges need to be repaired.

Field machining of the flange face is performed as an in-situ repair. This testing technique can determine if the flange face has been machined and if loss of sealing area occurred. In the situation where there is no more seal to repair then the flange must be replaced or possibly the seal area will be renewed using a weld build up technique.

Caution: If the flange face has been repaired by welding and machining then UT inspection might detect this weld as an interface. As the interface is detected this signal can be confused as loss of sealing area.

Phased Array Application

The two most common positions for placing the UT transducers are on the flange taper and between the bolt holes. As illustrated in the photographs below.

Taper areas of flanges are not always the same therefore geometry must be plotted for each flange. This step is difficult and can lead to error in condition assessment.

This technique is performed by placing the phased array transducer on the angled section of the flange.

Due to design requirements and normal manufacturing process the location between the bolt holes is square to the flange face. This location is also consistent in distance therefore repeated applications can be compared. In some cases two bolts must be removed at each quadrant for the UT probe to fit properly. Removing the bolts is only done if the flange is offline and depressurized. If highly hazardous materials are contained by the subject flange then removal of bolts may not be allowed.

With this technique the phased array transducer is applied between the bolts.

Raised Face Flange Photos

These photos are an example of raised face flanges as applicable to piping.

Phased Array Transducer can be placed on the angled section of the flange

Phased Array Transducer can be placed between the bolt holes

ES Beam Tool Image

Using the ES Beam Tool can help perform a setup phased array techniques

Phased array applied with the transducer on the flange taper.

Phased array applied with the transducer between the bolt holes.

Industry Applications

The primary focus for this application are manufacturers and/or users of Hydrofluoric Acid (HF). HF units are common in refineries and chemical plants. Other types of processes can attack the flange seal location including acids, steam and salt water.

Calibration Standard

A calibration standard should be used to confirm the phased array setup. Using a duplicate of the flange size and weight with targets manufactured in the raised face sealing area for UT performance demonstration is the ultimate confirmation of setup performance.

Example of calibration standard:

A = .075 in. deep X 1.0" long
B = .050 in. deep X 1.0" long
C = .025 in. deep X 1.0" long

Applicable Olympus Products

Examination of raised face flanges can be performed using the Epoch 1000, Omniscan MXU-M or Omniscan MX.

Our small Phased Array Probes are well suited for flanges with smaller distances between the bolts and nuts.

Industry has used single channel flaw detectors such as the Epoch XT with good success. The test is concluded with only numerical values illustrating loss of sealing area. No image is provided therefore only the inspectors word is left in writing.

Advantages

• Condition assessment of the sealing area without separating the flanges
• Cost savings to the owner/operator
• Increased safety doe to the reduced potential for exposure to hazardous chemicals when separating the flanges
• Inspection can be performed while the equipment is on-line
• Planning for repair before TAR commences