CHEVRON-Maintenance-Heat-Exchanger

1000 1000 Main Mainte tena nanc nce e Abstract This section discusses the major maintenance requirements for heat exchangers. This includes repairing and replacing body flanges, body flange gasketing and torquing considerations, bundle replacement and repair, tube leak repairs, shell repairs, and on-line leak repairs.

Contents 1010 Replacing Versus Repairing Body Flange Leaks 1011 1011 Cause Causess of Flan Flange ge Leak Leakss 1012 Criter Criteria ia for Flange Flange Replac Replaceme ement nt

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Heat Exchanger and Cooling Tower Manual

1070 References

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1010 Replacing Versus Repairing Body Flange Leaks 1011 Causes of Flange Leaks Exchanger flange leaks are a major environmental, safety, and economic concern. Flange leaks are generally caused by one or more of the following:

Wrong Gasket Selection •

Gasket is too wide, there is not enough bolting to properly compress the gasket.

•

Gasket is too narrow, causing gasket alignment and seating problems.

•

Gasket seating surface is not compatible with the gasket. It can be either too smooth for gaskets such as composition asbestos, or too rough for gaskets such as solid metal or metal jacketed.

•

Gasket is the wrong material for the application (i.e., stock, temperature, and pressure.)

Poor Flange Design •

Flanges do not have enough thickness to withstand the operating and hydrostatic test pressures without leaking. (See Section 530 for more information in this area.)

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Fig. 1000-1 Flange Alignment Problems (1 of 3)




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•

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Gasket material is not resistant to the process fluid.

Process Upsets •

Excessive temperatures or pressure surges can unseat the gasket and stretch the bolts.

1012 Criteria for Flange Replacement Consider replacing flanges under the following conditions: •

Flange is near the ASME tmin for the operating conditions.

•

Flange is so badly corroded the repair costs are greater than or equal to the replacement costs.

•

Flange is rotated causing the gasket not to seat properly.

•

The right gasket cannot be used because of flange configuration problems.

•

Flange is a chronic leaker. ASME flange design may not be adequate.

1013 Analyzing Existing Flanges The reasons for flange leakage and the decision to repair, insulate, or replace the

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Fig. 1000-2 Determining the Cause of Leakage by Examining the Gasket Observation

Causes and Possible Remedies

Gasket badly corroded

Select replacement material with improved corrosion resistance.

Gasket extruded excessively

Select replacement material with better cold flow properties; select replacement material with better load carrying capacity, i.e., more dense. This could also indicate excessive bolt load or insufficient gasket width.

Gasket grossly crushed

Select replacement material with better load carrying capacity; provide means to prevent crushing the gasket by use of a stop ring or re-design of flanges. This could also indicate excessive bolt load or insufficient gasket width.

Gasket mechanically damaged due to overhang of raised face or flange bore.

Review gasket dimensions to insure gaskets are proper size. Make certain gaskets are property centered in joint.

No apparent gasket compression achieved.

Select softer gasket material. Select thicker gasket material. Reduce gasket area to allow higher unit seating load.

Gasket substantially thinner on O.D. than on I.D.

This is indicative of excessive “flange rotation” or bending. Alter gasket dimensions to move gasket reac-


Fig. 1000-3 Exchanger Flange Tolerances (Courtesy of TEMA)

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flanges. See Section 530 and Appendix G for more detailed discussions of the design methods. •

Use integral body flanges if possible.

•

Follow ASME Code procedures for replacing and inspecting the flanges.

1015 Repairing Existing Flanges If flange leakage is caused by damage to the gasket surface or a partially corroded gasket surface, the flanges can normally be repaired by weld build-up and/or skim cutting. Flanges that are close to ASME t min should be built up with weld material before skim cutting the surface in order to avoid losing flange thickness. Flanges that are designed in accordance with Appendix G can be skim cut down to tmin as determined by Appendix G without a problem because these flanges are usually much thicker than ASME flanges. However, skim cutting to thicknesses below Appendix G tmin may result in leakage. The type of skim cut depends on the gasket selected. For example, composition asbestos gaskets work best with a 250 RMS surface because the deeper grooves “bite into” the gasket better. Refer to Section 540 and Figure 500-15 for the surface requirements for various types of gaskets. S

f

D i

ti

A


•

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They prevent cocking the flange faces or pinching the gasket due to uneven bolt loading.

1023 Torque Calculations Figure 1000-4 is a torque calculation procedure. Using this procedure, you can determine the torque required to pass hydrotest and reseat in service if the flange does not deform.

1024 Developing a Torquing Procedure Refer to Appendix G for flange design and Section 532 for boltup procedures for large flanges. After initial torquing, the exchanger should be retorqued to 100% of Tf (see Figure 1000-4) within 4 hours at ambient temperatures to compensate for any joint relaxation. High temperature applications or applications that have had a history of chronic leakage, should be retorqued at 100% of Tf after 24 hours at operating pressure and temperature. This is to compensate for bolt, flange, or tubesheet relaxation that may occur. 

Caution

The allowable bolt/flange stress will decrease with increasing tempera-

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Fig. 1000-4 Torque Calculation Procedure (1 of 2) A. Nomenclature Ab

=

Cross sectional root area of one bolt (in2) (see Figure G-4, Appendix G)

bo

=

Gasket seating width (N/2) (inches) (see Figure G-3, Appendix G)

b

=

Effective gasket seating width (inches) (see Figure G-3, Appendix G): b = b o if bo 0.25"; b = (bo)/2 if bo 0.25"

D

=

Nominal diameter of bolt or stud (inches)

Dr

=

Bolt diameter at root of threads (inches)

G

=

Diameter at location of gasket load reaction (inches): G = (O.D. + I.D.)/2 if bo 0.25 G = O.D. - 2b, if bo > 0.25

Pd

=

Design pressure for your application (psig)

Ph

=

Hydrotest pressure for your application (psig)

Lp

=

Total length of pass partition gaskets (in.) (Lp = G for one pass partition)

m

=

Gasket factor (Refer to Figure G-2, Appendix G or ASME Code, Section VIII, Division 1, Appendix 2, Table 2-5.1)

n

=

Number of bolts


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Fig. 1000-4 Torque Calculation Procedure (2 of 2) B. Calculating a Torque Value: Find bo

bo

= (in)

Find b (see Nomenclature)

b

= (in)

Calculate G (see Nomenclature)

G

= (in)

Find Lp

Lp

= (in)

Find m

m

= (in)

Find y

y

= (psi)

W1 = 0.785 G2Ph + 2b(3.14 G + Lp) m Ph

W1

= (lb)

W2 = 0.785 G2Pd + b(3.14 G + Lp) y

W2

= (lb)

W = Larger of W 1 or W2

W

= (lb)

Calculate bolt stress, S = W/(A b ⋅ n)

S

= (psi)

T

= (ft-lb)

T

(ft-lb)

Calculate W1 and W2:

Calculate Torque T = 0.013 S(Dr)3 Round to nearest multiple of 25 for fi

lt

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1032 Considerations for Retubing, Repairing, or Replacing the Bundle Assuming the bundle is leaking or, based on past experience, that it will not last until the next turnaround, then some repairs will need to be made. The following areas need to be considered before deciding on the extent of repairs. •

Is bundle nearing its historical life? If a bundle is leaking and it is not approaching its historical life, then a failure analysis should be performed to determine if design changes are required. The best way to determine the cause of failure is to pull the leaking tubes and inspect them.

•

Can tubesheet be reused? Normally, a tubesheet can only be used two or three times before it can no longer have a tube rolled into it. If a tubesheet cannot be reused, then a new bundle will be necessary.

•

Can leaking tubes be plugged or replaced? Individual leaking tubes can be the sign of a much larger problem. Simply plugging or replacing leaking tubes may be setting yourself up for another failure before the next shutdown. The leaking tube should be pulled and inspected to determine the cause of


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•

Change shell side pass configuration and baffling to improve heat transfer and decrease fouling.

•

Use different materials that will increase the bundle reliability and life. (As a general rule, even in corrosive services such as sea water, a bundle should last at least two operating runs. Contact the CRTC Materials and Equipment Engineering Unit for help in this area.)

•

Change from floating head to U-tube design. (See Sections 450 and 520.)

1033 Bundle Repair Worksheet Figure 1000-11 is a worksheet that can be used prior to taking an exchanger out of service to make a sound decision as to the extent of repairs required. Note

Figure 1000-11 is an 11 × 17 foldout at the end of this section.

1040 Tube Leak Repairs Roll leaks should be repaired by rerolling (never by driving a drift pin or pin wrench in the tube.) Caution shall be taken not to overexpand the tubes. See Section 520 for guidelines on rolling tubes. Defective tubes should be plugged off. The inside surface of the tubes should be

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1061 Types of Online Repairs Online repairs that can be made include: 1.

Retorquing flanges This usually involves tightening all of the bolts. Follow appropriate torquing procedures to prevent worsening the problem. Belleville Spring Washers have sometimes been used to solve leakage problems caused by differential thermal expansion. It is usually necessary to replace the bolts one at a time when doing this. Appendix H discusses the use of Belleville Washers in more detail. Note that torquing the bolts over their code allowable stress may stop a leak temporarily. However, this may cause permanent deformation of the flange such that, (1) the flange may leak on the next thermal cycle, and (2) the flange may need major repairs or replacement on the next shutdown.

2.

Tapping the flange and pumping in sealant between the flange faces at high pressure.

3.

Installing a ring around two flanges and pumping in sealant at high pressure.

Repairs 2 and 3 are only temporary fixes and can be very expensive. Permanent repairs must be made during the next turnaround. The work required to remove the


Fig. 1000-5 Example of a Leak Sealing Checklist (1 of 2)

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Fig. 1000-5 Example of a Leak Sealing Checklist (2 of 2)



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is sometimes recommended to replace studs with Teflon coated “blue bolts” to resist caustic cracking. •

B-7 studs are also subject to sulfide cracking (H2S sour water) at temperatures below 200°F. Stainless steel bolts are subject to chloride cracking at temperatures above 150°F and caustic cracking above 140°F. Consider using B7M bolts in these situations. Consult CRTC Technical Standards Team’s Materials Division for more information if necessary.

•

Extra caution should be exercised if excessive external pressure can be applied by sealant injection on cylindrical or spherical shapes. Collapsing pressures of thin-walled cylindrical members, based on an empty line, can be calculated by formulas in the Standard Handbook for Mechanical Engineers. All calculations should be approved by the appropriate engineering supervisor.

Repair Procedures Figures 1000-6 through 1000-9 give the general work procedures to follow depending on the specific situation. These procedures can be adapted to a specific job and attached to the work order. •

Figure 1000-6 Procedure for Flange Joints with Less than 3/8" Gap Using Drill and Tap Method

•

Figure 1000-7 Procedures for Sealing Flange Leaks with Use of Injection Ring

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Fig. 1000-6 Procedure for Flange Joints with Less than 3/8" Gap Using D rill and Tap Method 1.

When possible, change out studs one at a time with the use of boiler clamps.

2. Drill 3/16-inch diameter holes between studs approximately 4 inches apart from outer circumferential flange surface at an angle to break through into gap area at bolt circle. (Outside stress area of flange.) – On full surface gaskets and metal to metal joints, drilling is done into stud clearance areas. 3. Drill out hole to 5/16-inch diameter by 1/2-inch deep. 4. Tap out holes to 3/8-inch NC thread. 5. Install shut-off adaptors in tapped holes. 6. Insert tight fitting wire into gap around flange. 7. Lightly peen lock edge of flanges over wire with a bull nose peening chisel approximately 1/8-inch over gap size. 8. Starting 180 degrees from leak blow area, inject thermosetting compound around flange in both directions until gap area and stud clearance areas are full with final injection on shut-off adaptor directly over leak area. 9. After appropriate cure time depending on the temperature for steam, water, and air services under 600 psi, shut-off adaptors are removed and set screw plugs installed. 10. On pressures above 600 psi and on chemical services, shut-off adaptors are left in place.

Fig. 1000-7 Procedures for Sealing Flange Leaks with Use of Injection Ring Adaptor (1) 1.

Change out studs one at a time along with installing ring adaptor under the nut of each stud. One ring adaptor on thin, n

gap flanges


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Fig. 1000-9 Procedures for Sealing Flange Joints with Gap Width in Excess of 3/8”(1) 1.

Contractor will engineer and fabricate flange clamp.

2. When possible change out stud one at a time. 3. Install shutoff adaptors in flange clamp. 4. Install clamp in gap of flange. 5. Peen lock both flanges to clamp joints. 6. Inject thermosetting compound through shutoff adaptors starting at the point farthest away from leak working around the flange in both directions until flange joint is completely full and leak is stopped. 7. On steam, water, and air services with pressures below 600 psi: – After proper compound curing time shutoff adaptors are removed and set screw plugs installed. 8. On pressures over 600 psi and chemical service, shutoff adaptors are left in place. 9. Depending on flange dimensions, temperature, and special circumstances, ring adaptor or drill and tap techniques may be used in conjunction with this procedure. ( 1) Ref er to Figure 1000-10, Type I

1070 References 1.

Appendix G, “Heat Exchanger Body Flange Calculations.”

2.

Appendix H, “PCFlange Program User’s Guide.”

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Fig. 1000-10 Methods for Sealing Flange Leaks



Fig. 1000-11 Bundle Repair Worksheet

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