Black Liquor Evaporator

Evaporator performance can be negatively impacted by various mechanisms such as fouling, surface corrosion, stress from thermal cycling, or tube damage from vibration.

Leaking evaporator tubes allow cross contamination. In order to minimize this impact, a simple temporary solution is to plug both ends of the tube and remove it from service.

The number of tubes removed from service effectively reduces the thermal capacity of the heat exchanger. When reduced efficiency is detected in one or more of the evaporators, the overall system performance is impacted, resulting in one or both of the following outcomes:

  • Reduced Black Liquor Production
  • Increased Steam Consumption
Black Liquor Evaporator Process Figure 1 - A typical position of black liquor evaporators in the overall process. Black Liquor Evaporators take Weak Black Liquor (typically 15% solids) and condense to 70-80% solids based on evaporator design. This process provides Heavy Black Liquor as fuel to the Recovery Boiler. Black Liquor Evaporators also reduce the waste water stream by recycling it to the process.

Because the recovery process is critical to the efficient operation of the overall system, proactive maintenance is a requirement.

If the quality or quantity of Heavy Black Liquor production is reduced, then the boiler may not have sufficient fuel to maintain steam production without a supplemental fuel source. Additionally, the production of White Liquor to the digester will either be reduced or made up with consumption of additional chemicals. When evaporator steam consumption is increased, the overall system steam availability is reduced. This reduction in performance may impact power generation, if the facility produces power, and reduce steam to other processes.


CMS Evaporator Inspection Services

CMS has over 30 years of experience inspecting and repairing heat exchangers in wide variety of industrial operations, including plants in the Pulp & Paper Industry. CMS’ expertise includes process analysis, inspections and customized repair of heat exchangers that can be a valuable cost effective component of your maintenance strategy.

It is becoming increasingly difficult for the Paper Industry to maintain profitable and competitive operating margins due to environmental and energy demands. Recognizing and identifying the existing operating conditions and the appropriate maintenance techniques are critical steps in any effort to minimize energy consumption and maximize quality production.

One difficult area to assess in evaporators is the external corrosion damage that can occur near the top from trapped non-condensable gasses, or other corrosion elements near the bottom of the evaporator. This issue may be overcome through the application of the proper test equipment to evaluate the total wall thickness.

Multiple Effect Evaporator Black Liquor Evaporators are often arranged as Multiple Effect Evaporator (MEE) Systems. Weak black liquor enters in the last stage, and the steam enters in the first stage. As the Black Liquor flows through the system, water is removed by evaporation to drive the next evaporator stage. The solids content of the liquor is increased through each stage. Each evaporator effect stage is a separate heat exchanger that operates at increasing vacuum as the stages progress.
The most popular configuration for these MEE heat exchangers is the shell in tube configuration.

A regularly scheduled inspection routine can:

  • Locate damaged tubes
  • Identify tube sheet damage
  • Apply methods to return damaged tubes to service
  • Replace tube sheets or repair tube sheet damage
  • Removed damaged tubes from service
  • Install full length replacement tubes
  • Install full length or partial tube liners
  • Can improve operating efficiency
  • Locate tube corrosion, erosion, or cracking
  • Assess damage and provide a recovery plan

Visual Inspections

Often, a visual inspection is the first step in identifying damaged tubes. This is performed from the top and bottom tube sheets. Using an external light or other equipment, the inside walls of the tube can be inspected over about the first 18 inches of the tube.

This inspection technique is sufficient at times, since damage is predominantly near the tube sheet interface. As damaged tubes are identified, these tubes may require no further inspection at this point.

Repair Ready

Where failures are identified within 12 inches of the tubesheet, CMS inspectors can install airtight sleeves to capture the failure behind the sleeve and keep the tube in service. In other cases, the tube can be removed from service by installing mechanical plugs.

CMS Inspector performs a pressure test Using an external light or other equipment, the inside walls of the tube can be inspected over about the first 18 inches of the tube. Some tubes can be identified as damaged from visual inspection.

Both methods are a quick way to temporarily improve efficiency and can add up to measurable energy cost savings, while allowing time to plan for a more permanent solution.

As with all CMS inspections, a detailed report is provided that includes a tubesheet map, documenting damaged tubes as well as any necessary repairs.


Pressure Testing

Once the visual inspection is complete, a pressure test can be performed. This is conducted by having access to both the top and bottom tube sheets. A pressurized airline is placed at one tube end. A pressure indication allows the inspector to detect the presence of a leak. Anything less than a specified pressure is considered a tube failure.

During this test, a CMS inspector pressurizes each tube and records any drop in tube pressure and the associated time. The speed of the pressure change is an indication of severity of the air leakage. If a leak is detected, it may be possible to pinpoint the location by using the more advanced Acoustic Pulse Reflectometry Inspection System (APRIS) tube test to determine the location and damage mechanism (hole, crack, or obstruction). Knowing the location of the leak may provide a clue in determination of the failure mechanism and provide guidance in the repair technique selection.

CMS Inspector performs a pressure test A CMS inspector pressurizes each tube and records any drop in tube pressure. CMS’s repair specialists are often able to conduct field repairs on plugged or damaged tubes.

Repair Ready

Where failures are identified within 12 inches of the tubesheet, CMS inspectors can install airtight sleeves to capture the failure behind the sleeve and keep the tube in service. In other cases, the tube can be removed from service by installing mechanical plugs. Both methods are a quick way to temporarily improve efficiency and can add up to measurable energy cost savings, while allowing time to plan for a more permanent solution.

As with all CMS inspections, a detailed report is provided that includes a tubesheet map, documenting damaged tubes as well as any necessary repairs.


APRIS Inspections

Using portable acoustic pulse reflectometry equipment (APRIS), CMS inspectors direct one-dimensional acoustic waves at tubular systems to detect defects in along the contours of the tube inner diameter.

If the pulse encounters a discontinuity in cross section, a reflection is created. The shape of the reflection enables inspectors to define the damage mechanism, which they then verify using a borescope. CMS APRIS inspections are able to accurately identify the exact location, type, and size of tube defects, without impacting outage schedules.

Illustration showing APR Acoustic Pulse Reflectometry - APR

Our APRIS inspection reports are translated into a tubesheet map that indicates the pattern of damage detected. In addition to the tubesheet map, sample plots from the APRIS are provided to illustrate the information used. The combination of this information enables the customer to plan tube repairs and replacements for improved operational efficiency and cost savings. For more information on Acoustic Pulse Reflectometry technology, you can visit the Talcyon APRIS website.


CMS Evaporator Repair Services

A number of repair solutions can be applied based on the findings of CMS Inspectors and the client's input. CMS will work with site representatives to provide the data and options for development of the best approach for repairs. The repair process in most cases will have different options based on site objectives.

Whether you are looking for a quick fix to bridge the gap between outages/maintenance intervals or a more permanent solution, turn to Corrosion Monitoring Services to solve your problems before they turn into performance issues.

Tubesheet Map As the inspection process is completed, a tube sheet map is generated to document the findings.

There are several primary repair solutions:

  • Install partial or full length tube sleeves
  • Full tube replacement
  • Plug leaking or damaged tubes detected by the testing
  • Repair or replace tube sheet
  • Correct vibration issues that are detected by tube damage assessment

Sleeves

Locating damaged areas that have not yet failed is critical to determining the right repair approach. If the tube damage is located near the tube sheet, CMS may be able to install a sleeve on the inner diameter of the tube to capture the tube failure or crack. This process places a liner section of tube inside a fully cleaned parent tube.

The sleeve has a 360° seal towards the bottom of the liner tube and is then rolled into the tube sheet plate. Depending on inspection results, this process is often recommended to be applied proactively to increase the service life of evaporator tubes. In some cases, where a tube has severed completely from a tube sheet plate, CMS can install a sleeve and return these tubes to service. Installing tube sleeves is ideal for repairing damaged tubes and extending the service life of evaporator tubes. A critical step when installing partial or full length sleeves is that the parent tube must be clean to establish metal to metal contact for sealing and for heat transfer.

Plugs

While plugging evaporator tubes is often seen as the simplest solution, it should be evaluated for overall thermal performance based on the number of tubes out of service. Once a tube is plugged, it no longer adds to the evaporator performance, the total surface area is reduced and pressure drop across the bundle will increase if enough tubes are taken out of service.

CMS has been able to remove plugs and repair the damage and return these tubes to service. This cost effective repair method can restore significant operational gains and extend the service life.

Full Tube Replacement

As a complete repair option, tubes can be replaced or full length liners installed.

For more information on how CMS can evaluate and repair your evaporator damage, ask to speak to one of our engineers at (800) 637-6592 or contact us at inforeq@cmsinc.us Our proprietary tools and analyses techniques can help you avoid unexpected downtime and expenses.