Importance of Stick Testing
Stick Testing is a critical component of running a sulphuric acid plant to determine if the plant is operating optimally. All plants will experience acid mist carryover from sulphuric acid towers at some stage. The problem is that, if this situation is not detected quickly, there will be considerable damage to equipment downstream of the tower.
Some tests can be run if there is a suspected problem in the plant. These include plotting element pressure drops, monitoring acid volumes and concentration, acid flow and acid mist stick tests, dry tower stick tests, inspection through viewports and physical inspections.
Each of these testing methods is problematic for one reason or another ranging from expense, danger, or loss of production time. A dry stick test is one quick and simple way to gauge what is happening.
How the Stick Test Works
A wooden stick is inserted into the gas flow exiting the tower. The opening for the stick should be downstream from the tower in a horizontal section of the ductwork. The stick should be vertical in the gas stream. Results can be compromised if the stick is too close to an elbow in the ductwork or near the exit. Debris in the ductwork itself can also affect the stick test.
Acid particles will leave a dark spot on the wood. The size and number of spots indicate the tower and mist eliminator performance. The less acid damage to the wood the cleaner is the process.
A large number of small spots suggest problems with the mist eliminator.
An overall darkening of the stick indicates the presence of a high concentration of sulphuric acid.
A heavily spotted stick or a black wet stick is a sign of significant gas bypassing, mist eliminator flooding, or high inlet loading. This is a serious problem and must be addressed immediately.
Safety Precautions for Workers Performing the Stick Test
A harness is required if the opening for the stick is elevated or in an awkward location. Personal safety equipment must include SO2 respirators, goggles, face shield, clothing, and gloves. A ball valve for the opening is recommended as it is quickly opened or closed.
“Performing stick tests on towers that are operating under pressure can be hazardous due to the gas that escapes from the open valve,” according to Sulphuric Acid on the Web – Knowledge for the Sulphuric Acid Industry.
Some of the testers modify a device made of rubber gaskets to close the space around the stick, to reduce any acid from escaping. The gaskets are mounted on the stick using a standard valve.
Also, certain plant conditions must be recorded as well to coincide with each stick test. They are
- Amount of gas flow
- Temperature, concentration, and flow of acid
Gastemperature when exiting the tower
- Visual observations by the tester
The sticks need to be retained and documented for future comparison to determine if there are any significant changes in the tower.
Further Testing May Be Needed if there is a Problem
A stick can be placed across the top of the candles to determine which candle is faulty. The plant blower can be operated briefly for this test.
In-house Staff Testing Versus an Outside Testing Agency: Pros and Cons
Many plant operators make the decision to forego using staff for completing the stick test for the following reasons.
- Training for performing the test can be extensive.
- The staff needs to be trained to interpret the reading of the stick.
- The sticks must be retained and categorized according to date.
- The test conditions can be dangerous for staff that are not familiar with the procedures.
- Staff turnover prohibits adequate training regime.
- Scheduling the testing relies on staff to remember to complete the procedure on a regular basis.
- The staff has more experience with the procedure. The results are that the personal safety equipment will be up to date, the readings are likely to be more accurate, and the record keeping will be more precise.
- The schedule for performing the test will be more reliable.
- There is a second set of unbiased eyes regarding plant operations should any problem arise.
- The professional staff will be up to date on any new regulations or procedures.
What Do I Look for in a Professional Testing Company?
Since many companies are in the business of safety testing, it is useful to think of criteria to judge their competencies.
1) Length of time in the business and qualifications of staff
2) Accreditation in the field noted on the website or informational material
3) Links to government, scientific, or research agencies
4) Willingness to train your staff and accommodate to your needs
5) Positive testimonials from other plants, such as yours
6) A Statement of Positive Values
It will ultimately be a management decision whether to train the current staff to perform the stick test or to hire an outside agency to complete this vital task. Many decisions in business are made with the bottom line in mind, but when the safety of plant operation and workers are involved, competency needs to be the primary deciding factor.