Ensuring a quality feedwater heater - Energy-Tech Magazine: Heat Exchangers

Ensuring a quality feedwater heater

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Posted: Friday, April 1, 2011 12:00 am

When a feedwater heater is replaced, many companies choose to replace “in-kind.” While this might be a convenient option, the utility is not taking advantage of improvements in heat exchanger technology or utilizing state of the art designs. Often, a better option is to write a new specification that reflects the current and potential operating conditions, and then allow the Feedwater Heater Vendor to come up with a new, robust design. Regardless of the method of procurement, the Purchaser should not assume that their responsibility ends once the Vendor has been selected and a delivery date agreed on. Obviously, the Purchaser is responsible for reviewing and approving the drawings prior to fabrication. If no one is familiar with heat exchanger designs or deciphering detail drawings, it might be difficult to discern whether the Vendor’s design has actually met the requirements of the specification.

One of the most overlooked portions of the replacement feedwater heater procurement process is the need to conduct Quality Assurance (QA) inspections at the Vendor’s shop. While the Vendor is ultimately responsible for the overall quality of the heater and meeting code requirements, the Purchaser should still be involved in making sure that the Vendor is performing the work correctly, as well as meeting any special requirements of the specification that might be above and beyond their standard operating procedures. Historically, feedwater heaters were purchased using low-cost procurement, where the lowest bidder was awarded the contract without regard to the overall quality. So how do you ensure quality in a feedwater heater replacement?

A replacement feedwater heater can cost a utility anywhere from $300,000 up to $1 million, especially when installation costs are considered. However, often in a feedwater heater replacement project, the utility forgets to budget or is reluctant to spend additional funds to conduct inspections at the Vendor’s facilities to ensure that they are getting the quality they need for the money spent.

Most likely, the additional cost of performing these Quality Assurance inspections is just a small part of the overall cost to replace the heater, but it is money well spent in order to protect the investment. If you also consider the cost of taking the heater out of service in order to repair a tube leak, or to correct a deficiency that could have been identified during fabrication, the cost of the inspections generally pale in comparison. Additionally, documented inspections of “as built” conditions can assist in troubleshooting or eliminate potential variables when conducting failure cause analysis of future problems.

As part of the specification, the Purchaser should reserve the right to inspect the manufacturing facilities and equipment at any time during working hours. Several fabrication processes of concern should be monitored by the Purchaser’s representatives and listed in the specification as required witness points. The Purchaser might decide to waive a certain witness point in order to allow fabrication to continue, but a later dimensional/visual examination or a record review should be conducted. The following major components are recommended for Purchaser inspection/verification during fabrication.

Tubes

The fabrication of tubing is generally subcontracted by the Feedwater Heater Vendor. However, the tube mill also should be available for inspection by the Purchaser’s representative, especially when you consider that the tubing of the heater is the single biggest material cost of the heater and is the most likely component to fail in operation.

Ideally, the inspectors should visit the tube mill while the tubes that will be used in the heater are in fabrication. The inspection should include any or all of the following:

  • Dimensional Verification
  • Witness of any required non-destructive examinations (NDE), such as eddy current tests or ultrasonic tests
  • Witness of tube bending and stress relieving
  • Witness of hydrostatic tests
  • Witness or review of mechanical tests
  • Final tube cleanliness and dryness prior to packaging.
  • Tubesheet/Pillbox

The tubesheet is generally the thickest portion of the feedwater heater and provides the boundary between the tube side and shell side pressures. It is imperative that the tubesheet drilling be done in accordance with TEMA standards to ensure the proper fit of the tubes and proper tube expansion later in the fabrication process. Any tubesheet holes that are oversized or contain defects could lead to tube-to-tubesheet joint leaks, which might result in catastrophic tube failure when the heater is in operation. Therefore inspection of the tubesheet holes is a very important QA witness point.

The inspector should conduct a 100 percent go/no-go check with an appropriately sized plug gage in order to check for any oversized holes. Additionally, a random sampling of tubesheet holes should be accurately measured using a micrometer to ensure the required tolerances are met. Other items to check during an inspection of the tubesheet are:

  • Overall dimensional verification (including tubesheet thickness)
  • Tube hole surface condition and cleanliness (free of burrs/drilling defects)
  • Quality of tubesheet overlay (including NDE)
  • Proper ligament sizing (ensuring no drill drift over the tubesheet thickness)
  • Proper hole chamfering
  • Check of pillbox corner radius (if applicable)
  • Tubesheet hole drilling layout
  • Baffles and support plates

The baffles and support plate drilling and sizing are another item that the Purchaser should consider inspecting. Often this inspection does not warrant a separate trip to the Vendor’s facility, but should be conducted as time permits as part of any other major hold point. Prior to the tube bundle cage assembly, the Purchaser’s representative should witness a stack check of the tubesheet, baffle plates and support plates in order to verify hole alignment. Solid rods, equal in OD to that of the tubing, should be used as guides and fit the full stack. Additionally, the following items also should be checked:

  • Baffle plate/support plate hole sizing
  • Freedom from burrs, sharp edges, gouges
  • Compliance with dimensional and surface finish requirements
  • Proper cut line location
  • Cleanliness of components and hole surfaces
  • Tube bundle cage assembly

We routinely tell utilities who are purchasing replacement feedwater heaters that if you can only conduct one inspection at the Vendor’s shop, this is the one. Of all the Quality Assurance inspections, this is the most important and offers the opportunity to inspect many different parts of the heater. If not done previously, the tubesheet and the baffles and supports can be inspected at this point (although at this point in fabrication, there may be little you can do if you find any problems, unless it is a major issue that will prevent further fabrication of the heater).  Additionally, this inspection will be the point at which the tubing crates are opened in preparation for loading.

This is the first opportunity to check the tubing as shipped from the mill.

At this point of fabrication, the internals of the heater will be accessible for the last time. It is important to check for general workmanship, quality of welds and cleanliness. Since the tubes will be loaded into the cage assembly immediately following this inspection, all internal surfaces should be free of dirt, grease and foreign material. Sharp scratches can be imparted to the tubes if the cage is not properly aligned and well cleaned. In certain situations, the tubes can become bent or dented if the cage is not “free running” and the tube gets hung up during insertion. In addition to the above, the other important items to verify during this QA inspection are:

  • Impact plate location/sizing/weld quality/Dye Penetrant Tests (DPT)
  • Overall alignment/perpendicularity of support plates, baffle plates and DC end plate (if applicable).
  • Ensuring the cage is “free running” (i.e. tubes can be inserted easily)
  • Verify skirt nozzles/penetrations are in correct locations
  • Vent duct assembly construction/weld quality/DPT requirements

Once the cage is inspected and approved, the Purchaser’s representative should stay and witness at least the initial stages of tube loading. Typically, if there are going to be any problems with tube loading, they will manifest themselves in the first couple of rows. This is where the bend radii are the tightest and there is the least amount of flexibility in the U-bends. Therefore, the differences in tolerances between the tubes and the supports are more critical. It also is important to ensure that the crew loading the tubes are being conscientious and handling the tubing properly – i.e., wearing gloves, not pushing the tubes in too fast or with too much force that could result in scratching or damage to the tubes.

The bundle is inserted into the shell shortly after the completion of tube loading. The shell is typically fabricated in parallel with the tube bundle. Similar to the baffles and supports, this inspection is typically conducted concurrently with another inspection and typically does not require a separate trip to the manufacturer’s facility. The inspector should verify the following:

  • Nozzle location, size and weld prep
  • Shell diameter, wall thickness and overall length
  • Support and other miscellaneous lug location
  • All welds have been properly NDE tested as required by the specification or code

Prior to the bundle insertion to the shell, both the tube bundle and the shell should be checked for any foreign material.

Tube-to-tubesheet welding/expansion

Following the bundle insertion into the shell, the tubes are welded to the tubesheet if required by the specification. Prior to expansion of the tubes to the tubesheet, these joint welds should be tested to ensure they do not leak. This is typically done by pressurizing the shell side with air or helium and then using either a soap bubble solution or helium detector to check for any leak paths from the shell side to the tube side. Additionally, all of the joint welds should be dye penetrant tested before and after expansion to ensure they did not crack during the expansion process.

Regardless of the type of expansion process used, a percentage of the tubesheet holes should be identified as “control holes” in order to quantify the amount of tube expansion and ensure that the resultant tube wall reduction is in accordance with the Vendor’s procedures or the specification.

When witnessing the above processes, the inspector should:

  • Verify pressure gages are with calibration
  • Shell pressure is maintained for the required period of time
  • All welds are free from cracks, splits and leakage at tube ends
  • Range of required tube wall reduction is achieved during expansion process
  • Hydrostatic testing

Following tube expansion, final assembly of the channel occurs. This includes installation of the hemi-head (based on the design), channel internal pass partition plates and the channel cover. 

Following assembly, the heater is subject to a final hydrostatic test on both the shell side and the tube side. While the ASME Authorized Inspector is required to witness these tests, many utilities decide to send their representative as well since it is the final test that the heater is subjected to prior to shipment. When witnessing a hydrostatic test, the inspector should verify:

  • All pressure gages are within calibration
  • The pressure gage reading is consistent with the specification/code requirements
  • The pressure is maintained for the appropriate amount of time/number of cycles
  • Hydro water temperature is well above the Nil-Ductility temperature for the materials of construction

Although the tube side and shell side hydrostatic tests are the last major witness point, several other processes occur before the heater is shipped. After the hydro, the tube side and the shell side is evacuated and dried out, then pressurized with nitrogen. If not already done, the heater must be painted if specified. Finally, the heater must be properly fastened and secured to the shipping skid mount.

Conclusion

As discussed, there are several parts of the heater fabrication process that warrant independent inspection by the Purchaser beyond the Vendor’s own QA department. The amount of time and effort expended in conducting these inspections are well worth it, however they are often forgotten or overlooked as part of the overall procurement process. Identifying and correcting a problem during fabrication that will prevent a problem in operation easily justifies the expense. As the saying goes, “You get what you inspect, not what you expect.”


Michael C. Catapano has more than 35 years experience in the operation, design, procurement and maintenance of feedwater heaters, condensers and other shell and tube heat exchangers, including 7 years with PSE&G and 28 years as president of Powerfect Inc. His current work at Powerfect is primarily devoted to consulting, troubleshooting problems and assisting utilities with feedwater heater replacement and operating and maintenance activities. Catapano is an ASME fellow and has assisted ASME and EPRI in numerous feedwater heater projects, seminars and publications. He also holds three patents pertaining to feedwater heater testing and repair. Catapano has a bachelor’s degree in Mechanical Engineering from Newark College of Engineering.

Eric Svensson graduated from the Georgia Institute of Technology in 1993 with a bachelor’s in Chemical Engineering. He joined the Naval Nuclear Propulsion program shortly after graduation, where he received training in Nuclear Power Theory and Operations. In 2000, he received a master’s degree in Operations Management from University of Arkansas. His current role as vice president of Engineering at Powerfect is devoted to consulting, troubleshooting problems, as well as operations and maintenance activities. Since joining Powerfect, he has been involved in writing the specification and conducting quality control checks for more than 20 replacement feedwater heaters. He also is a member of the ASME Heat Exchanger Committee and has co-authored several technical papers.

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