Heater Revamp Guidelines

Table of Contents

1. Purpose

This procedure provides basic guidelines for Design Engineers to use for preliminary reviews of fired heaters for revamp conditions.

2. Information Needed for Heater Evaluation

2.1 Data Sheets and Drawings

To perform the heater evaluation, provide the Design Engineer with the vendor design data sheets, drawings and inspection reports for all of the heaters included in the scope of the project.

2.2 Existing Heater Operating Data

For Platforming, Oleflex, Pacol, Cyclar, Crude, Vacuum and Unicracking units, field operating data shall be collected and provided to the Design Engineer before evaluating the heater for the revamp. Blank data collection sheets, which list the required data, are available from the Heater Specialists.

3. Heater Design Limitations

When the design engineer is reviewing a heater for a revamp, it is important to know the type of limitation for the original design, and for the revamp conditions. For capacity-increase revamps, the type of limitation for the revamp is usually the same as for the original design. In conversion revamps, one type of process technology is converted to another (i.e., Platforming to Penex). Thus, in conversion revamps, a heater designed for one service may be used in a new service. Therefore, the type of heater limitation may be different for the new service.

The three types of heater design limitations are:

• Heat Flux
• Tube Wall Temperature (TWT)
• Bridgewall Temperature (BWT)

Heaters limited by heat flux are characterized by high process P (> 20 psi). Most general service heaters fall into the flux-limited category.

TWT - limited designs are characterized by low process P (2-6 psi). The low P heaters have low tube mass velocities, which result in low heat transfer coefficients, and thus high tube wall temperatures. Examples of TWT-limited designs include Oleflex, Platforming, Pacol and Cyclar reactor heaters, and Unicracking recycle gas heaters.

BWT – limited designs are usually encountered when a heater with conventional burners is replaced with Low NOx / Ultra Low NOx / Next generation burners or the revamp requires higher turndown ability. Flame instability and flameout can occur at low BWT.

4. Revamp Design Criteria

This procedure gives guidelines for reviewing a heater for revamp suitability based on the following items:

• Radiant Flux Limit
• TWT Limit
• Bridgewall Temperature
• Metallurgy Review
• Coil Thickness
• Coil Pressure Drop (P)
• Mechanical Review - Burners
• Mechanical Review - Stack

5. Radiant Flux Limits

5.1 Single-Fired Heaters

Inflection Point Engineering designs use new single-fired heaters for 10,000 Btu/ft²-hr radiant flux. Small heaters (< 10 MM Btu/hr process duty) typically have lower radiant flux rates.

For revamps, Inflection Point Engineering limits single fired heaters to a radiant heat flux of 12000 Btu/ft²-hr or 20% greater than the original design heat flux, whichever is lower.

Estimate the revamp radiant flux for the revamp process duty:

Revamp radiant flux = existing flux x revamp process duty

existing process duty

5.2 Double-Fired Heaters

Double-fired heaters are usually TWT-limited. Nevertheless, there are flux limits for revamps. Radiant flux revamp limits for two types of heaters are as follows:

• For double-fired Platforming reactor heaters, the revamp radiant flux limit is 22,000 Btu/ft2-hr.
• For double-fired Unicracking recycle gas heaters, the current policy is not to exceed the design duty.

See the Heater Specialists for other double-fired heater radiant flux limits.

6. TWT Limits

TWT-limited heaters usually occur in high temperature processes such as Platforming, Oleflex, Pacol, Unicracking and Cyclar. The Heater Specialist calculates the TWT. Table 1 shows Inflection Point Engineering’s TWT limits for new designs.

Table 1 - TWT Limits for New Unit Designs

Metallurgy	TWT Limit, °F
Killed Carbon Steel	800
1¼ Cr	1025
2¼ Cr	1100*
5 Cr	1150
9 Cr	1175
Stainless Steel	**

The killed carbon steel (KCS) TWT limit is based on preventing graphitization in the tubes. The chrome (Cr) limits are based on inhibiting tube oxidation. These values may be exceeded on revamps; however, doing so shortens the coil life.

* Use a limit of 1075-1100 F per recent data.

** For stainless steel (SS), process temperature limits usually occur before reaching the TWT limit. An exception may be the Unicracking Recycle Gas heater, which operates at high pressure.

7. Metallurgy Review

7.1 Changes in Design Practice

Inflection Point Engineering design practices have changed over the years. Thus, when revamping a heater, consideration must be given to upgrading the coil metallurgy to meet current design practices. Examples of changes in design metallurgy are as follows:

• Older Naphtha Hydrotreating coils were 1¼ Cr. Now, 9 Cr is standard.
• Older Platforming heater coils were 2¼ Cr. Now, 9 Cr is standard.
• Older Isomax heater coils were 9 Cr. Today, the standard for Unicracking heater coils is Type 347 S.S.

Consult the Heater Specialists for further information on changes in metallurgy design practices.

7.2 Tube Corrosion Rates

Corrosion rates greater than 15 mils/yr cause plugging and pressure drop problems due to scale deposition in downstream equipment (such as reactor beds). If the corrosion rate exceeds 15 mils/yr, upgrade the coil metallurgy to austenitic stainless steel.

The following sections give guidelines for predicting corrosion rates for the revamp conditions.

a. General Guidelines for Estimating Corrosion Rate

For many heater designs, the Inflection Point Engineering sulfur curves govern the corrosion rate.

Experience has shown corrosion rates predicted by the sulfur curves are much greater than actual corrosion rates seen in commercial heater coils. Thus, when estimating the corrosion rates for revamp conditions, check the customer heater inspection reports for actual corrosion rates rather than assuming the corrosion rates predicted from the Inflection Point Engineering sulfur curves.

b. Hydrotreating Services with Cr-Mo Heater Coils

H2S is the most important corrosion consideration for hydrotreating services with Cr-Mo heater coils. The corrosion rate is a function of maximum TWT and H2S level.

Reference Procedure . Estimate the maximum TWT by adding 100°F to the process outlet temperature. Use this temperature and the process fluid H2S level to determine the corrosion rate.

c. Austenitic Heater Coils

Except for the Unicracking recycle gas heater, most austenitic heater coils have very low corrosion rates.

8. Tube Thickness

The required tube thickness is a function of the pressure in the tube, the TWT and the corrosion allowance. This section gives two methods for finding the required tube thickness for the revamp conditions.

The first method involves the use of a spreadsheet and the API 530 / ISO 13704 procedures. This method shall be used by Design Engineers working in the office.

The second method involves using look-up tables to estimate the required tube thickness. This method may be used when access to the heater spreadsheet is not readily available (i.e., on business trips).

For either method, compare the required tube thickness to the actual tube thickness shown in the heater vendor design data sheets or from the customer’s inspection records. It is the responsibility of the Revamp Contractor to determine the remaining life of the existing coil.

8.1 Tube Thickness Using Spreadsheet and API 530 / ISO 13704

Check the tube thickness using API 530 / ISO 13704 Petroleum and natural gas industries - Calculation of heater-tube thickness in petroleum refineries. This calculation is available in .

Apply the following guidelines when using API 530 / ISO 13704:

• API 530 / ISO 13704 and Inflection Point Engineering use design pressure (API 530 / ISO 13704 Elastic Design Pressure) as the basis for tube thickness for low temperature heaters (< 1000°F TWT).
• API 530 / ISO 13704 uses operating pressure (API 530 / ISO 13704 Rupture Design Pressure) as the basis for tube thickness for high temperature heaters (>1000°F TWT). Inflection Point Engineering uses design pressure. There may be times where the heater specialist may use their judgment and evaluate an existing coil using the maximum operating pressure instead.
• Beware of alloy heaters designed before 1978. The second edition of API 530 / ISO 13704 switched from average stress rupture data to minimum stress rupture data (40% lower values). As a result, most older alloy heater coils are one tube schedule number less than the thickness calculated by the current methods.

8.2 Tube Thickness Using Look-up Tables

Tables 2, 3 and 4 may be used to quickly check the required tube thickness for the revamp conditions. Tables 2 and 3 assume a 1/8 inch corrosion allowance, and Table 4 assumes a 0 to 1/16 inch corrosion allowance.

In these tables, AW is average wall tube, with mill tolerance of -12.5% to +12.5%. MW is minimum wall tube, with mill tolerance of -0% to + 28%. Due to the -12.5% tolerance, AW tubes have an actual wall thickness that is 12.5% less than that of an MW tube of the same schedule number. Thus, for the same schedule number, the MW tubes can withstand a higher pressure.

Consult the Heater Specialists for heaters that are not represented in these tables.

Table 2 - Estimated Pressure Limits for CS Heater Tubes

Thickness	Pressure Limit, psig
CS Sch 40 AW	500
CS Sch 80 AW	1000

Note: The limits in Table 2 are at metallurgy design temperature of 800F for carbon steel.

Table 3 - Estimated Pressure Limits for Chrome Heater Tubes

Thickness	Pressure Limit, psig
2¼ Cr Sch 40 AW	250
2¼ Cr Sch 80 AW	410
9 Cr Sch 40 AW	150
9 Cr Sch 80 AW	250

Note: The limits in Table 3 are at metallurgy design temperature of 1125F for 2¼ Cr and 1175F for 9 Cr.

Table 4 - Estimated Pressure Limits for Type 347 S.S. Heater Tubes

Thickness	Pressure Limit, psig
Sch 40 AW	880
Sch 40 MW	1050
Sch 80 AW	1550
Sch 80 MW	1850
Sch 120 AW	2150
Sch 120 MW	2550
Sch 160 AW	2900
Sch 160 MW	3400

Note: The limits in Table 4 are for 6 inch tubes at 1120-1125F TWT.

9. Coil Pressure Drop

9.1 Reactor Circuit Heaters

Pressure drop (P) is important for heaters in a reactor circuit.

a. Single-Phase Heater Pressure Drop

Calculate single-phase circuit drops with the Platforming Heater spreadsheet or line sizing programs.

b. Two-Phase Heater Pressure Drop

For two-phase heaters, multiply the existing P by the ratio of the mass flow squared (Gnew/Gexisting)2.

c. New Number of Passes (Single or Two-Phase)

For heaters modified for a new number of passes, multiply the existing P by the ratio of number of passes cubed (original number of passes / new number of passes)3.

9.2 Reboiler Pressure Drop

Most reboilers designed for 50 wt% vaporization do not need additional flow to achieve an increased duty requirement. The percent vaporization may be increased up to 85% by volume to achieve the increased duty. Estimate the new P by multiplying the old P by the ratio of the outlet vaporization rates (new vapor rate / existing vapor rate).

9.3 Detailed Pressure Drop Analysis

Heater Specialists use FRNC5 software for detailed P analysis. This program calculates accurate two-phase pressure drops by performing frequent stepwise flashes. Heater specialists report the pressure drop as F*FRNC5 calculated value to account for manifolding and for safety margin. F is 1.2 for new heaters and 1.1 for revamps.

10. Review - Burners

Burners are inexpensive to replace unless the floor/casing also needs modification, and usually represent only 10% of the total heater cost. On revamps, new burners are often required to meet new emission requirements or increased duty.

10.1 Flame Length

Review the flame length to confirm the flame will fit within the existing firebox. This is extremely important on opposed fired U-tube heaters. Table 5 gives estimated flames lengths per heat released by one burner.

On opposed fired U-tube heaters, the flames from opposing burners are not expected to run into each other in the middle of the heater. Minimum clearance from flame tip to flame tip is 4 feet. On other heaters, the flame length is not expected to exceed 2/3 of the tube length. Consult with the heater specialists after completing the flame length calculations.

Table 5 - Estimated Flame Lengths

Burner Type	Flame Length (ft) / Heat Release (MMBtu/hr)
Gas	1.5
Oil	2.0
Low NOx Gas	2.0
Low NOx Oil	2.5
Ultra-Low NOx Gas	2.5
Next Generation Gas	2.5

Calculate the heat release of an individual burner for the revamp conditions per the following equation:

Burner heat release =                    Process duty                     

Heater efficiency x Number of burners

Use the chart in Attachment 1 to estimate the heater efficiency.

10.2 Heat Release and Emission Requirements

Heater Specialists shall check the suitability of the burners for heat release and emissions requirements.

11. Bridgewall Temperature Limits

Heater Specialists shall check the burner spacing and bridgewall temperatures at normal and turndown operations on the stability and operation of burners. For fuel gas firing with Ultra Low NOx and Next Generation burners, BWT should be greater than 1200°F. For Low NOx burners, the BWT should be greater than 1000°F. For oil firing in combination burners, the BWT shall be greater than 1200°F.

12. Review - Stack

12.1 Revamp Studies

For revamp studies, it is not necessary for the Design Engineer to check heater stack hydraulics.

12.2 Revamp Schedule A’s

For revamp Schedule A’s, the Heater Specialist shall check the stack hydraulics.

12.3 Revamp Recommendations for Stacks

Stack draft is directly proportional to ambient temperature. Stack height for new designs is set by the high ambient temperature. However, most plants do not operate at or above the high ambient temperature for more than 5 % of the year. Thus, additional draft is available much of the year.

It is extremely expensive to modify an existing stack due to structural and foundation modifications. Thus, for revamps, Inflection Point Engineering recommends taking advantage of below-design ambient temperatures. A typical comment, which the Heater Specialist may include in Project Specification 207, for a stack that is short on draft (not tall enough) during maximum ambient temperature, is:

"Stack height may not provide sufficient draft at all operating conditions and high ambient temperatures. An additional __ feet (meters) of stack height would be needed."

Attachment 1