A Better Black Liquor Process

Finding Vacuum Leaks in a Multi Effect Evaporator

So many industrial processes depend on creating a stable vacuum, but system leaks impair process efficiency and, if left unchecked, will shut it down. Finding these leaks can be challenging in noisy plant environments and reliability engineers must weigh the balance between the costs of downtime versus the cost of continuing production with a leaky, inefficient system.

One method of detecting vacuum leaks is to use airborne ultrasound detection, a technology already widely used for positive leak detection in compressed air systems. But finding vacuum leaks is not as straightforward as finding pressure leaks, and often times, the method is abandoned in frustration.

One problem here is the quality of the ultrasonic instrument which can vary significantly from one manufacturer to another. Lesser quality detectors cannot function well in high noise situations. They simply have difficulty differentiating a leak sound from ambient plant noise. Since vacuum pumps already generate a lot of background noise, rarely will an inspector perform vacuum leak inspections in a quiet atmosphere. Another problem is lack of inspector training which really plays a role when searching for vacuum leaks in high noise environments.

Just like positive pressure leaks, vacuum leaks produce a rushing, whooshing ultrasonic signal with peaks around 35-40 kHz. The ultrasound is caused by turbulent flow of air molecules at the leak site. Positive pressure leaks, such as those found in compressed air systems, push the turbulent flow outward making them easily detectable from several feet with a quality ultrasound tool. Vacuum leaks behave quite the opposite, drawing the turbulent flow inward, decreasing the distance of detection as compared with positive pressure leaks. Most of the telltale leak sound is contained within the body which means inspectors must diligently trace an entire installation leaving no stone unturned in the search for ingress.

The amplitude of a vacuum leak is less in comparison to a pressure leak, so proper shielding and positioning techniques are paramount to success. As leading manufacturers of ultrasound equipment continue to innovate, new methods for finding vacuum leaks are resulting in more successful inspections and less frustrating project abandonment.

One such innovation is the improvement of contact sensors. That’s right; contact sensors can play a huge role in finding vacuum leaks. When we think of leak detection, most of us draw our point of reference to airborne sensors only. If you only rely on airborne sensors for leak detection you will miss potential wins. Think about it. In a vacuum leak much of the turbulence is contained inside the vessel. A contact sensor that is accurate, repeatable, and super sensitive within a confined bandwidth can be a very effective tool. One area where vacuum leak detection improves efficiency and throughput is applied to multiple effect evaporators used in the processing of sugarcane, the desalination of water, and the production of black liquor in the pulp and paper industry to name just a few applications. The case study focus of this paper addresses significant wins for black liquor production at a large Pacific Northwest pulp and paper maker.

In the pulp and paper industry, vacuum serves a key role in several processes, not the least of which is the production and recovery of black liquor. Black liquor is a by-product of the kraft process which is the stage in the production of paper pulp where wood chips are digested into pulp cellulose. The black liquor contains a majority of the energy potential of the wood so its recovery and reuse has value for pulp mills. They use recovery boilers to burn the black liquor they produce, generating steam while reclaiming spent chemicals that can be re-purposed by the digestion process. During wood digestion chemicals and heat are used to cook the wood into cellulose fibres. Lignin pieces and chemical agents are recovered through evaporation.

Evaporators are large vessels used to produce black liquor and recover cooking chemicals. Steam is fed through tubes inside the calandria. Water is boiled out of the black liquor and removed as condensate. The condensate is sent to the boiler for reuse after purification. Meanwhile the liquor is further concentrated and becomes more viscous at each stage of the evaporation.

Multi-Effect Evaporators

In the pulp process, multiple effect evaporators provide more efficiency than single effect for production of black liquor. Multiple Effect Evaporators are more efficient than single-stage evaporators because the energy they consume in the first effect is re-used in the proceeding effects. The temperature in the steam chest is higher in the first effect than the second effect and so on. So, in order for the steam provided by first effect to boil off liquid in the second effect, the boiling temperature point in the second effect must be lower. For this to happen, the second effect must be under lower pressure than the first effect. Each proceeding effect will be at a lower pressure than the previous effect. In some cases, the first effect may be above atmospheric pressure so the second effect could be at atmospheric pressure. Usually the third and later effects must be put under vacuum. In a forward feed evaporator, the vacuum serves two purposes. The first purpose is to keep the boiling temperature of the concentrate lower than the previous effect. The second purpose is to move the concentrate forward in the process without the need for pumping. Backward feeding evaporators work in reverse but require pumping of the viscous concentrate. In either case, adding multiple effects reduces the energy consumption of the evaporator. Adding a second effect reduces energy by 50%. Adding a third effect reduces it to 33%, and each effect thereafter reduces the energy consumption even further.

Of course after a certain number of effects are added, the energy savings is displaced by the capital cost of the additional effects. For pulp and paper, the magic ratio of energy consumption to capital cost equates to seven. So it would be unusual to see more than a seven effect evaporator in any pulping process. Each effect consists of a heat transfer surface, a vapor separator, a vacuum source, and a condenser. Multiple effect evaporators evaporate more water per kg of steam by re-using vapors as heat sources in subsequent effects. They also improve heat transfer due to the viscous effects of the black liquor as it becomes more concentrated. But they also require efficient vacuum to move the liquor on through the process and maintain differential pressure from effect to effect.

As explained above, each effect operates at a lower pressure and temperature than the preceding one. The lower pressure creates a temperature difference across each effect. Since vapors are removed from the preceding effect at the boiling temperature of the black liquor, the difference in temperature cannot exist in the proceeding effect without increasing its vacuum. The operating cost of evaporation is relative to the number of effects and the temperature at which they operate, all of which hinges on the tightness of the system, otherwise expressed as its ability to pull and hold a vacuum.

It should be noted here that black liquor, for all its energy potential, is also corrosive. Stress corrosion cracking of stainless steel is more likely to occur in heavy black liquors where the solids contents are above 70%. This is due to the high process temperatures required to both concentrate the liquor solids and to also keep the viscosity of the liquor low enough for pumping. Very high service temperatures combined with corrosive products have been known to impact stainless steel tubes in heat exchangers and are now being replaced with high chromium ferrite stainless steel, which provides better resistance to corrosion from liquors and high temperatures. Herein was the problem on the number four effect stack at our customer.

Black Liquor Evaporator Vacuum Leak Survey

In November, 2009 SDT Ultrasound Systems received a phone call from a pulp and paper plant in the Pacific Northwest asking if our ultrasound technology could find vacuum leaks on evaporator stacks. Several leak detection service companies had been approached already but none seemed willing to risk the expense of visiting the plant with an uncertain outcome. SDT is lucky enough to have a sound technical representative situated near the Pacific Northwest willing to embrace risk in exchange for providing customer solutions and satisfaction. Karl Hoffower, of Failure Prevention and Condition Monitoring Solutions, Inc took the call and scheduled a visit to the mill. He filed this report.

Vacuum Leak Inspection on Multiple Effect Evaporator at major Pacific Northwest Pulp & Paper Mill

A report submitted by Karl Hoffower

On December 14th & 15th, 2009, a vacuum leak survey was completed on the black liquor evaporator at a major pulp and paper producer in Idaho. Evaluation of the black liquor process numbers indicated the most likely area of the vacuum leak was somewhere in the 4th effect piping.

Using an SDT170 ultrasound listening device coupled with the new SDT RS-1 (resonant sensor 1) contact needle probe, contact measurements were obtained at various locations along the 4th effect stack. These readings were taken through the insulation and outer steel wrapping. The highest external reading noted was 51db. This location also correlated with the thinnest section of the stack recorded by ultrasonic thickness testing already done by their NDT crew.

Switching to airborne ultrasound detection mode revealed a jump from the ambient levels of 18db to a strong 32dBμV -33 dBμV with the tell-tale whooshing sound of a vacuum leak. When the insulation near the bottom of the access door was moved the airborne ultrasound levels rose to 38dBμV.

Cutting away a large section of the sheet steel and insulation revealed numerous points where the metal had been breached by the corrosive black liquor. The breaches had created a loss of vacuum in the 4th effect stack.

Rubber sheets had been pre-cut in preparation for discovering the locations of the leaks. These sheets were placed over the areas of the holes as a temporary repair and to avoid a complete loss of vacuum on the evaporator system when the insulation was removed.

A repair was carried out to help allow the system to function properly and continue until a planned outage in March 2010 can have the stacks replaced. Gor-tex sheets were wrapped around the stack and sealed with silicone caulking.

The OSI PI process monitoring software showed an immediate change in the correct direction by the application of the rubber sheets and an even better improvement with the Gore-Tex sheets and silicone caulking.

A “morning report” on 12/17/09 stated: “Vacuum improvements on the evaporators resulted in the best solids throughput tons per day we have achieved on the set in the recent past.”

Testing procedure

Plant personnel explained that the evaporator stacks had several access ports covered by rubber stoppers. These pre-made ports had been constructed to allow easy access for periodic thickness testing. The procedure discussed was to access these ports on the evaporator and contact or “touch” the point with the RS-1 needle probe. The decibel level would be recorded and mapped out.

Maintenance personnel also told us of additional access doors created on the stacks for thickness testing. These access doors were sealed with silicone and screws with 4”-6” of insulation in-between. These additional doors helped make our survey successful. By mapping the decibel levels at various points around and along the stacks, an area or areas of potential leak could be determined for further investigation. (See Figure 3.)

Challenges

There were several challenges to completing this survey, access being the primary hurdle. Four rubber access points were easy to get to while standing on the roof of the evaporator building. A man-lift was then employed to gain access to more points (see Figure 4). But if the leak was occurring in a location inaccessible by the man-lift, then staging would have to be employed to gain access to the other stacks. Flow is made more turbulent at twists, angles and bends, like 90 degree elbows. The evaporator stacks had numerous 90 degree elbows. Differentiating between excessive turbulent flow caused by a 90 degree elbow and a vacuum leak was another potential hurdle that was overcome by my extensive inspector training. This training prepared me with the necessary skills to differentiate between internal turbulent flow and turbulent flow that is the result of a leak.

Survey

The black liquor evaporator uses a 7-effect counter flow method to concentrate the mixture. Using the OSI PI monitoring software, they determined the most likely location was in the 4th effect stack.

Figure 3 shows the maps of decibel levels after we conducted the survey. The decibel levels taken on the inside wall area of the stack (Figure 3, at right) dropped as we moved away from the high of 51db, down to 32db.

The survey required the use of a man-lift to access all of the doors. While the contact ultrasound measurements were made, RCM Tech Jim Storey also conducted contact ultrasound thickness testing. Mr. Storey said that the last thickness survey conducted on this stack had been about 5 years ago.

With airborne ultrasound leaks, a large leak usually registers 65-75db at or around 15’ away from the source. Since no other external points had anything higher than 51db we decided to return to that location and investigate further.

Readings rose when we opened the access door (Figure 5). There was a strong correlation between the locations of the leaks and the thinning of the wall of the stack. At the location of the highest ultrasound, 56db, the wall was found to be 0.091” thin, which was the thinnest area found on our initial survey.

With the ultrasound level increasing as we recorded contacts close to the bottom, I changed from the contact probe to the airborne sensor. Ambient ultrasonic db levels were 18db around the stack. When I brought the airborne sensor near the 51db access door, the airborne sensor jumped to 32db – 33db and gave the whooshing sound of rapidly moving air. When I used my hand to separate some of the insulation away from the metal flange( Figures 6 and 7), readings jumped even further, as the airborne sensor levels rose to 36db.

We decided that we would need to have a larger area opened for inspection. Mr. Storey and I went back to confer with other personnel about how we could gain access to the stack below the retaining ring.

As shown in Figure 8, Mr. Storey marked out the approximate dimensions we wanted to have cut away in chalk.

We went back to the Maintenance shop and were introduced to Mr. Jim Rose. He discussed how he would cut the sheet steel away and remove the insulation to give us better access. We also figured out that we would need to have some type of blocking material to immediately cover the holes we expected to find. If we did not, the probability of losing the vacuum was great, thus potentially causing the entire process to shutdown.

Corrosion Damage Found

As soon as Mr. Rose removed the sheet steel (see Figure 9) we could see where the insulation had collapsed around the vacuum pull of the stack. As the insulation was peeled away, the holes were immediately visible. We placed two rubber sheets over the large holes to prevent the loss of vacuum. The sheet on the left covered the largest area of corrosion, about 8” wide. The 2nd major area of damage was about 1½ inches wide. Several other smaller holes were noticed along the underside of the metal flange area (see Figure 10). The exact number of holes were too numerous to count. Also many of the holes had blended together from the corrosion.

Mr. Rose recommended applying Gor-Tex sheets sealed with silicone to effect a strong, yet temporary repair. The decision was to strengthen and seal the holes so that the process could continue until a planned outage in several months later. At that time a complete repair/replacement could be implemented. As they removed more of the sheet metal and insulation for the final repair, additional thickness testing was performed. The areas below where the holes were discovered clearly show how the stack is wearing out. 0.077” was the thinnest area found without actually being a hole.

Conclusion

After the Gore-Tex sheets were wrapped and sealed, the process monitoring software validated the repair. The amount of vacuum began returning to levels not seen for quite a while. The control valve also moved dramatically into the correct direction right as the repair was being finished. By 4pm on December 15th it was apparent the system had been returned to normal. The survey is considered ended and a success.

I was sent an e-mail that noted, the “morning report” on 12/17/09 stated the following: “Vacuum improvements on the evaporators resulted in the best solids throughput tons per day we have achieved on the set in the recent past.”

The report filed by Mr. Hoffower illustrates just how complex the job of locating vacuum leaks can be. The complexity in this case was magnified by several conditions, including insulation material wrapping the stack, sheet metal covering the insulation, primary and secondary air gaps between stack, insulation, and sheet metal, high elevations requiring a lift and platform, ambient noise in the ultrasound frequencies related to non-leaking turbulent flow, and of course, the discomfort of high temperatures which also pose a safety risk.

The report also illustrates how rewarding the job can be. The win for this paper company is a reduction in energy costs through more efficient vacuum level maintenance and better thermal transfer from effect to effect. Additionally they have the best throughput of black liquor in years. Make no mistake here; these are trying times for paper makers. The difference between a profitable quarter and a losing quarter may well be decided by the efficiency of a single process such as black liquor production.

Many leak surveys are abandoned due to frustration, which is the product of poor quality equipment ill suited to the task. It is also the product of training. Without ultrasound training, an inspector will be overcome by the hurdles of the task. Your investment in an ultrasound program must be threefold. Invest in quality ultrasound equipment, quality personnel to carry out the inspection, and equally as important, but often overlooked, inspector training. Training must address the unique place ultrasound holds for reliability and plant maintenance, ensure good transfer of knowledge between teacher and inspector, and return the inspector to the field with the confidence to succeed in the most trying inspections.

Allan Rienstra is the President of SDT North America Ultrasound Systems. He has been involved with airborne ultrasound methods for nearly two decades and has helped thousands of ultrasound inspectors achieve inspection greatness through his unique coaching techniques. He is founder of the SDT certification training and implementation guide and co-author of two certification training manuals, and the newly published book Hear More, a Guide to Using Ultrasound for Leak Detection and Condition Monitoring. His writing appears in maintenance journals around the world. He lives in Cobourg, Ontario Canada with his wife and two sons. Allan can be reached at allan@sdtnorthamerica.com or 905-377-1313 x 221

Karl Hoffower is the founder of Failure Prevention Associates (FPA), Houston, TX. FPA helps Predictive and Condition-based Maintenance programs become a reality for many more companies. When first introduced to the concept of PdM/CBM, Dr. Karl realized this was a great marriage between his interest in science and real-world applications. He has been working in industrial reliability since 2007. You can reach him at
Karlh@failureprevention.net