Spark testing is a standard procedure used for inspecting glass-lined equipment. During testing, the entire glass-lined surface is examined, and any chips, cracks, pinholes and other defects are documented and marked for repair.
There are two apparatuses available for spark testing - DC and AC spark testers. DC spark testers are the preferred method for vessels and any equipment that can be grounded, as they are portable and less hazardous. AC spark testers are required when examining removable agitators and other equipment that are completely glassed and can’t be grounded.
In this post we’ll give a high-level overview of how spark tests are performed as well as best practices for safely operating a spark tester.
General Safety Guidelines for Spark Testing
When guidelines are followed, spark testing is a safe and reliable way to inspect your glass-lined equipment for damage. Regardless of what type of spark tester you use, there are certain common-sense safety measures that should be followed. Spark testing is considered a destructive test due to the nature of the procedure, so we recommend that testing is only done if there is a particular concern that something is damaged (incident or visual clue) or if it is part of a routine preventative maintenance. Generally speaking, once a year is a suitable frequency.
First and foremost, you must always refrain from using a spark tester in any combustible atmosphere or on any unclean glass surface. Both types of units produce a spark of up to 6000-volts that can result in an explosion if not operated according to standard operating procedures or in unsafe conditions. It is recommended that that only trained, responsible personnel should operate the tester and that plant safety personnel should be consulted for permission to operate before using the device.
While the spark discharge is nonlethal, direct contact with the discharge may produce an involuntary reaction resulting in possible injury. Stored high voltage charge on the tested glass surface may also discharge to the operator causing an involuntary reaction. Due to the residual charges that build up during this procedure, spark testing should be the last test conducted before exiting a vessel.
Certain glass surfaces are fabricated for visual glass inspection only; the spark testers should not be used to test linings with this specification. Additionally, repaired areas should be avoided during this procedure as the voltage from the spark tester can damage the PTFE (instead, repair areas are inspected thoroughly during the visual examination).
Additional safety guidelines include:
- Never enter a glass-lined vessel wearing unprotected shoes
- Never carry or wear any metal objects (e.g., large belt buckles, tape measures)
- Remove all loosely worn objects to avoid scratching and/or chipping of the glass
- Only hold the test probe by the insulated handle
- Keep the test probe away from other personnel and away from metal and other electrically conductive surfaces that may be in contact with other personnel
- Make sure the electrode brush is clean and free of broken or bent wires (this can create sharp edges which may cause scratching of the glass during spark testing)
Preparing your Vessel
To prepare your equipment for spark testing, all vessel instrument probes that may be damaged by high voltage should be disconnected and all surfaces to be tested must be thoroughly cleaned and dried – even humidity can give false readings.
It is important to make sure glass-lined equipment is electrically grounded. In the case of AC spark testers this is preferable for safety reasons, but for DC testers it is critical. Failure to properly ground may cause an electrical shock to the operator and interfere with the proper operation of the test unit.
With respect to DC spark testers, you’ll want to touch the test probe to the metal substrate of the unit to be tested to ensure that the alarm circuits are functional, and brush/electrode creates a spark. If electrical ground on the equipment is inadequate, you must ground the equipment to water pipes or a solid rod driven into the earth.
Additional Maintenance Tips
It’s important to periodically check the resistance between the metal end of the right-angled probe handle plug and the installed test brush using an ohmmeter. If there is a measurable resistance, the high voltage lead should be replaced. Failure to do so could result in erratic tester performance and possible injury to the operator.
The portable DC spark tester operates using a rechargeable battery. Best practices for maintaining battery health include not running the battery down to zero charge, recharging it after every use, and recharging the battery at least once every 3 months to avoid long term storage damage.
Operation of Spark Testers
The following instructions give a general overview of how to operate an AC or DC spark tester. For complete detailed instructions on the DC and AC spark testers contact DDPS for our engineering bulletins that outline the operating procedures for each type of unit.
The power switch should only be turned on when the spark tester brush is safely held away from personnel and electrically conductive surfaces. At this point the power on indicator light will illuminate and the tester should buzz signifying that the probe is energized. To test the probe, touch it to a metallic structure such as the substrate of unit to be tested and check for sparking. Keep in mind that 6000 volts is the recommended voltage for our normal lining thickness in the field. If you have a thinner lining the voltage should be lower.
Once operation has been confirmed, place the probe brush in contact with the glass and move it gently across the glass surface as shown in the figure below:
It’s important to always hold the probe at the correct angle. If the full length of the probe brush is laid flat against glass lining, the voltage may be reduced below the potential required to indicate a discontinuity. Conversely, if only one or a few brush wires are placed in contact with the glass lining the applied voltage may be substantially increased, possibly damaging the lining.
A visible spark at the end of the probe will indicate a defect. With respect to the DC tester, there will also be a red light alarm signal, a drop in output voltage, as well as an audible signal. If you are using an AC model, a blue corona will normally be seen when brush is passed across a continuous lining. It’s important to note that moisture or residual surface impurities can also produce a slight discharge. The area of a suspected discontinuity should be thoroughly cleaned and rechecked to assure the existence of the discontinuity. Any damaged areas should be marked with a grease pencil for later repair.
While it’s good to be thorough, you want to make sure you don’t go over the same area repeatedly or you can actually damage the lining. Once you have spark tested all glass-lined surfaces (it is important to carefully inspect nozzles and other internal attachments, as these areas are extra susceptible to damage), turn the spark tester off and wipe the glass completely with the tester probe to discharge any high voltage charge that may have built up on the glass surface. At this point all vessel instrument probes can be reconnected, and if no repairs are needed the vessel can be put back into service.
Routine inspections of your glass-lined equipment are a critical part of any preventative maintenance regime. De Dietrich Process Systems has a network of field service technicians located throughout North America who can perform these tests, but spark testing is a basic procedure that can also be handled in-house by trained personnel. Not only will it help to locate areas of the lining that are in need of repair, but it will help to extend the life of your equipment by identifying minor issues before they become more serious problems. For more information about spark testers and other inspection procedures download a free copy of our Glass-Lined Equipment Inspection eGuide or contact our aftermarket department online at 908-317-2585 and speak to a technical sales representative. If you’d like a copy of our engineering bulletins that outline the step-by-step process for operating these spark testers, ask for Bulletin 185 (AC models) or Bulletin 883 (DC models).
