Feedwater heater leak detection

Feedwater heater operating performance is evaluated based upon the following items:

• Heater drain level
• Terminal temperature difference (TTD) = Extraction steam saturation temperature – Feedwater outlet temperature
• Drain cooler approach (DCA) = Drain outlet temperature – Feedwater inlet temperature
• Temperature rise (TR) = Feedwater outlet temperature –Feedwater inlet temperature

Performance degradation of the feedwater heater is often related to tube failures which increase exponentially with heater age. A leak can be diagnosed by analysis of the above-mentioned indicators trend. Some early indications of tube failure are:

• Increase in drain valve position (to maintain heater drain level, control valve position increases; concurrently feedwater flow rate and boiler feed pump motor amps also increase)
• Decrease in feedwater outlet temperature (TTD↑)
• Decrease in drain outlet temperature (DCA↓)
• Decrease in temperature rise (TR↓)
• Increase in extraction steam/drain flow at the heater outlet.

Leak discovery

Any leak will result in deviation of heater level control valve position, TTD, DCA and TR values from normal operating condition. For example, it was detected that one out of four feedwater heater level control valve positions was increasing in a coal-fired power station. Additionally, feedwater flow rate and feed pump motor amps were also increasing to near maximum, all of which suggested that the high pressure feedwater heater had tube leaks. The plant personnel isolated and inspected the heater and discovered two leaking tubes. Based on an indication of leakage during operation, the heater should be isolated and removed from service as soon as practical to prevent collateral damage from leak impingement. When a leak is discovered, following information are important and should be documented as part of heater operation history:

1. The location of the leaking tube with respect to the other tubes in the bundle. This can be done by plotting leak tube position on a tubesheet diagram i.e., tube map.
2. Length along the tube from the face of the tubesheet where the leak occurred. This can be done by probing with a tight-fitting plug or stopper while the shell side is under air pressure.
3. Types of the leak found (pinhole, longitudinal crack, ruptured tube, tube to tubesheet leak etc.)
4. Types of the plug and summary of repair methods used (plugged with welded plug, taper plug, mechanical seal plug, stabilizing rod for ruptured tubes and insurance plug around it, etc.)

Above mentioned information along with general operating condition of the unit prior to the occurrence of the leak should be part of the heater history, and they are necessary to diagnose future troubles and determine repair strategy. Inspection photos, tubesheet map updated with tubes plugged during the repair, bill of materials, and repair documents should be documented properly for future reference.

Leak detection methods

It is advised to take multiple approaches to investigate when a leak is suspected. Relying only on one method of evaluation may not provide all the required information for root cause analysis. In that case it might be difficult to determine corrective actions based on limited assessment. Once all the information regarding the leak (i.e., type of the leak, location, severity, etc.) are gathered, along with the heater’s internal drawings, archived repair information and unit operating condition, then the correct approach to the repairs (i.e. plugging) can be predicted. Upon completion of the task, all the information related to leak and repair (image taken by video probe, updated tube map, test results, plugs used, etc.) should be documented properly for future reference.

Visual inspection

Visual inspection is the primary examination in leak assessment and should be conducted in the accessible areas of the tube side while the heater is open. Early signs of failure can be easily detected by inspecting tube inlets and tube-to-tubesheet joint welds for signs of corrosion or erosion. Visual inspection can only reveal defects in the accessible area of heater. Inaccessible areas of the heater should be inspected through borescope video probes. Areas to inspect include inlet and outlet nozzles, manway area gasket seating surface, all welds, channel barrel area etc.

Tube leaks: Tube side leak test

Tube leaks can be detected by simply conducting tube side leak test:

• Leaving the tube side pressurized, and
• Once the steam and drains to the shell side are sealed, opening the shell side drains to check for leakage.

The major limitation of this simple go/no-go check is that it does not locate the leaking tube. Also, where in the span of the heater the tube has failed is critical in determining root cause of the failure. The location of tube failure and length along the tube can be quickly determined by several methods such as inserting a video probe or a tight-fitting probe.

Video probe

Video probe is the quickest way to view tube internals, determine leak location and characteristics of defects. Measuring the length of the probe inserted into the tube from tubesheet face provides the length along the tube where leak has occurred. Video inspection can easily reveal the nature of the leak as well such as whether it is a pinhole, circumferential crack, longitudinal crack, ruptured tube, etc. Instead of speculating what the potential problems within a heat exchanger may be, visual evidence obtained from video probe inspections helps to achieve better identification of the problem.

Tight fitting probe

An example of a typical tight fitting probe device is shown in Figure 2.

This probe should be inserted into the leaky tube(s) and the distance measured at the depth at which air flow pattern changes as the probe plug passes the leak location. The air pressure into the shell side should be adjusted so that only enough air is leaking through the break so that it can readily detected by ‘feel’. When probe reaches the leak, the following will happen depending upon the size and type of leak:

• Pinhole or circumferential crack: If the leak is pinhole or circumferential crack, as soon as the main body of the plug (just beyond the line of the front chamfer) passes over the leak, the flow will stop or become greatly reduced. This length to the end of the tube is the measurement to the tube leak. As the plug continues in, the flow will reverse as soon as the line of the reverse chamfer passes the failure.
• Longitudinal crack: If the leak is a longitudinal crack, the air flow will start to come out the tube end which plug was fed through initially as soon as the rear chamfer passes the crack. This is the start of the crack and should be marked on the wire or pulling snake attached to the probe. Then as the probe continues in, the air will flow out both tube ends until the line of the front chamfer passes the other end of the crack cutting off the air flow to the tube end beyond the plug. This is the other end of the crack, and the wire should be marked a second time. The distance between these two marks less the length of the straight portion of the plug body is the length of the crack. The length from the line of the front chamfer to the first mark is the depth into the tube where the leak occurred.
• Ruptured tube: If the plug stops short of the leak and gets stuck, the tube has been damaged likely due to a rupture or otherwise deformed in some way. This type of failure generally indicates an area of adjacent tube damage or leaks. If the tube is shattered the probe will go in, air will then come out from both sides of the tube ends, and the plug will then hit a solid obstruction (a broken tube end, a baffle plate, etc.). Slowly and easily withdraw the probe in that case. This is a rough measurement of rupture location. Tube stabilizing rod for the ruptured tube and insurance plugs for the adjacent tubes are highly recommended for this sort of tube failure scenario.
• If a leak occurs in the outer tubes, this plug can negotiate the ‘U’ bends and locate the leaks. However, if the leak occurs in a tube near the centerline of the bundle, the probe cannot negotiate the tight inner bends.