Art 13 - Internet

SAUDI ARAMCO JOURNAL OF TECHNOLOGY WINTER 2010

RECLASSIFICATION OF SRU EQUIPMENT TO ASME SECTION VIII DIVISION 1 Authors: Dhawi A. Al-Otaibi and Salamah S. Al-Anizi

ABSTRACT

This article investigates the possibility of reclassifying Sulfur Recovery Unit (SRU) reaction furnace equipment according to design rules that meet ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1, which is the standard for pressure vessel design. Currently, most SRU equipment is designed and con-structed following the design rules of ASME BPVC I, which considers them to be steam generators or boilers. Plant inspections have experienced difficulties testing and inspecting the components of a SRU reaction furnace because of the mixture of ASME BPVC design rules, which specify different inspection intervals. These difficulties include limited accessibility for inspecting the shell side of the waste heat boiler (WHB), with an inspection interval of 24 months, without removing the tubes that are designed to the ASME BPVC VIII rules, with an inspection interval of 36 months. The objective of code reclassification is to extend the Testing & Inspection (T&I) period of the WHB and steam drum in the SRU to 36 months, as allowed by standards for equipment constructed under the rules of ASME BPVC Section VIII Division 1 and the National Board Inspection Code (NBIC). The results of the evaluation and analyses revealed acceptable component configurations (thickness, stress levels, etc.) in accordance with the design rules of ASME BPVC Section VIII Division 1.

INTRODUCTION Sulfur recovery refers to the conversion of hydrogen sulfide (H2S) to elemental sulfur. Hydrogen sulfide is a byproduct of processing natural gas. Sulfur recovery units (SRUs) or Claus plants normally provide up to 98% of the sulfur recovery capability in a refinery or natural gas processing operation. To recover sulfur from sour crude, the Claus plant reaction furnace typically combusts air and H2S, which results from sulfur processing. The H2S is then partially oxidized and catalytically converted to produce high-purity molten sulfur. The Claus SRU generally consists of two distinct sections: the thermal (front-end) section, from the inlet up to the waste heat boiler (WHB) and condensers, and the catalytic (back-end) section, downstream of the condensers, Fig. 1.

The characteristics of the acid gas feed, such as H2S concentration, mainly determine the configuration of

Fig. 1. Typical sulfur recovery process with acid gas and air preheat.

the thermal section, while the desired sulfur recovery efficiency generally dictates the selection of the processes used in the back end, which includes tail gas treatment. About 60% of the conversion occurs in the thermal section. The balance, plus sulfur recovery, takes place in the catalytic stage.

DESCRIPTION OF REACTION FURNACE The reaction furnace is the most critical equipment in the Claus process. Due to the complex reactions that take place in the reaction furnace, the design criteria are heavily based on experience, rather than theoretical calculations. The reaction furnace is composed of three main pieces of equipment that include: the combustion chamber, WHB and steam drum. The reaction furnace equipment is similar in principle in different facilities. There are several minor differences in size, material and/or operating limits.

PRESENT AND PROPOSED CODE CLASSIFICATION FOR REACTION FURNACE EQUIPMENT Code classification of the reaction furnace’s three components, conforming to the standards, is shown in Table 1.

The present classification of the combustion chamber is ASME BPVC Section VIII. Therefore, its classification will stay the same.


Reaction Furnace Component

Related Standard

Combustion Chamber (CC)

ASME BPVC Section VIII Division 1

Waste Heat Boiler (WHB)

ASME BPVC Section VIII Division 1 (tube side) ASME BPVC Section I (shell side)

Steam Drum (SD) ASME BPVC Section I Table 1. Present code classification for reaction furnace equipment

The WHB is divided into two sections: the tube and

shell sides. The tube side is classified as ASME BPVC Section VIII equipment, while the shell side is classified as ASME BPVC Section I. There is no valid reason for the different classification for these WHB components. Also, the inspection units in several operating facilities experience difficulty in testing and inspecting the components of the reaction furnace. These difficulties include limited accessibility for inspecting the WHB’s internals.

Current code classification of the combustion chamber and the WHB in the reaction furnace is shown in Fig. 2 and Table 1. As shown in the Figure, the combustion chamber and the Front End Cover are designed in accordance with ASME BPVC Section VIII Division 1. The WHB shell is designed per ASME BPVC Section I, while the boiler tubes are designed as per ASME BPVC Section VIII Division 1.

Fig. 2. Current code classification of reaction furnace.

ASME BPVC I EXCEPTION The ASME BPVC Section I makes an exception to certain equipment and allows the use of ASME BPVC Section VIII design. This is stated in the ASME BPVC Section I preamble as follows: “A pressure vessel in which steam is generated by the application of heat resulting from the combustion of fuel (solid, liquid or gaseous) shall be classed as a fired steam boiler. Unfired pressure vessels in which steam generated shall

be classed as unfired steam boilers with the following exceptions: 1. Vessels known as evaporators or heat exchangers. 2. Vessels in which steam is generated by the use of

heat resulting from operation of a processing system containing a number of pressure vessels, such as used in the manufacture of chemical and petroleum products.”

Therefore, the reaction furnace is classified based

on paragraph b, which allows the construction per ASME BPVC Section I or Section VIII.

PROPOSED CODE RECLASSIFICATION FOR REACTION FURNACE EQUIPMENT Waste Heat Boilers Inspection Departments experience difficulties due to the different frequencies of inspection intervals of the two sections of the ASME BPVC Codes. Consequently, it is recommended to reclassify the WHB of the reaction furnace to a Section VIII pressure vessel. Another benefit of this reclassification is increasing the Equip-ment Inspection Schedule (EIS) of the reaction furnace equipment from 24 months of ASME BPVC Section I to 36 months of ASME BPVC Section VIII. Steam Drum It is also recommended to reclassify the steam drum from ASME BPVC Section I to a Section VIII pressure vessel. This is to unify operation classification of all equipment of the reaction furnace to ASME BPVC Section VIII Division 1, and thereby utilize the EIS span extension as mentioned in the reaction furnace WHB section.

MECHANICAL ANALYSIS Waste Heat Boilers To evaluate the reclassification, both ASME BPVC Section I and Section VIII Division 1 were reviewed as detailed.

The ASME BPVC Section I preamble clearly indicates that the subject vessels (WHB) could be classified as unfired steam boilers. The vessels are identified in the industry as WHBs. Additionally, steam is not generated in the boiler using heat produced from operation of a processing system containing a number of pressure vessels. Therefore, the Code provides the choice of constructing, or classifying in this case, the boilers under the provisions of Section VIII Division 1.

Likewise, paragraph U1-(e) of ASME BPVC Section VIII Division 1 scope states the following: “In relation to the geometry of pressure containing parts, the scope of this Division shall include the following:

WHB

SD

CC


1. U-1(g) Unfired steam boilers as defined in Section I shall be constructed in accordance with the rules of Section I or this Division {see UG-125(b) and UW-2(c)}.

2. The following pressure vessels in which steam is generated shall be constructed in accordance with the rules of this Division:

a. U1(g)(1) vessels known as evaporators or heat

exchangers. b. U1(g)(2) vessels in which steam is generated by

the use of heat resulting from operation of a processing system containing a number of pressure vessels such as used in the manufacture of chemical and petroleum products.”

The above ASME BPVC Section VIII Division 1

scope confirms the earlier conclusion of the possibility, of classifying the WHB, as ASME BPVC Section I or ASME BPVC Section VIII Division 1. Steam Drum Current code classification of the steam drum is ASME BPVC Section I. As mentioned earlier, it is proposed to reclassify this drum as a pressure vessel to utilize the EIS span extension for all the reaction furnace equipment. Therefore, the drum, shell, head, and nozzle should meet the requirements of Section VIII to be reclassified as a pressure vessel.

Thickness requirements for the shell, head, nozzles and reinforcing pads of the steam drum were calculated according to ASME BPVC Section VIII Division 1. Results of the calculations indicated that the existing thicknesses of components of the steam drum meet ASME BPVC Section VIII requirements. Therefore, the drum could be reclassified as a pressure vessel.

CALCULATIONS Nozzle reinforcement calculations were conducted according to ASME BPVC Section VIII Division 1, Paragraph UG-37 “Reinforcement Required for Openings in Shells and Formed Heads.” ASME BPVC Section I Requirements The requirements of thickness and pressure according to ASME BPVC Section I are given by the following two equations:

.........(1)

....... (2)

ASME BPVC Section VIII Division 1 Requirements The requirements of the thickness and pressure according to ASME BPVC Section VIII Division 1 are given by the following two equations (Paragraph UG-27): 1. Circumferential Stress (Longitudinal Joints). When

thickness does not exceed one half of the inside radius, or P does not exceed 0.385SE, the following formula shall apply:

or ..... (3) 2. Longitudinal Stress (Circumferential Joints). When

thickness does not exceed one half of the inside radius, or P does not exceed 1.25SE, the following formula shall apply:

or ..... .(4)

CALCULATIONS METHODOLOGY Calculations of thickness requirement for the shell, head, nozzles and reinforcing pads of the WHB and steam drum were conducted for two SRU plants using a mech-anical design software. Evaluation of the WHB includes calculating reinforcing pad and nozzle thickness of the man-way. Saddle stability is calculated to ensure saddle integrity with ASME BPVC Section VIII Division 1. A study of the steam drum includes calculating reinforcing pad and nozzle thickness of the relief valve (PZV), man-ways, steam outlet, boiler feed water (BFW) inlet and inspection openings. The thickness of the riser and down-comer are also calculated for the reinforcing pad and nozzles.

ANALYSIS RESULTS Results of the calculations are presented in Tables 2 and 3. These calculations were carried out in accordance with ASME BPVC Section VIII rules, as previously mentioned.

OPERATION RECLASSIFICATION ADVANTAGES Two SRU trains in two plants were reclassified to the design rules of ASME BPVC Section VIII Division 1. They are located at different facilities. Their EIS is


Component ASME I

Thickness (in) ASME VIII

Thickness (in) ta CA t

Shell 1.375 1.2530 1.375 0.125 0.122

Riser Forging Area

Acceptable Acceptable The area available with pad is sufficient.

Downcomer Forging Area


Manway M4 Area Acceptable Acceptable The area available with pad is sufficient.

Inspection Opening H1 and H2 Area


Table 2. WHB shell calculations

Component ASME I

Thickness (in) ASME VIII

Thickness (in) ta CA t

Shell 1.375 1.2530 1.375 0.125 0.122

Drum Head 0.8750 0.6938 0.8750 0.125 0.1812

Riser Forging Area


Downcomer Forging Area


Steam Outlet N2 Area


PSV N10-1 and 2 Area


BFW Inlet N3 Area Acceptable Acceptable The area available with pad is sufficient.

Man-way M2 Area Acceptable Acceptable The area available with pad is sufficient.

Table 3. Steam drum shell calculations

Plant Name

No. of SRUs

Previous EIS

(months)

New EIS (months)

Saving (USD)

No. 1 5 24 36 1.8 MM

year

No. 2 2 24 36 0.23 MM year

Table 4. Reclassification benefits

extended from 24 to 36 months as allowed by ASME BPVC Section VIII. Reclassification benefits are summarized in Table 4. CONCLUSION The study has revealed that the design of the WHBs and steam drums could be classified according to design rules of ASME BPVC Section VIII Division 1 instead of the ASME BPVC Section I design rules. This would entail major cost savings by extending the EIS from 24 months to 36 months. Therefore, it is recommended to reclassify the components of the reaction furnaces in SRU, based on ASME BPVC Section VIII Division 1.

NOMENCLATURE P [psi] Internal design pressure

P

* [psi] Maximum allowable working pressure

t [in] Minimum required thickness of shell D [in] Outside diameter of cylinder S [psi] Maximum allowable stress value E [-] Joint efficiency, Dimensionless y [-] Temperature coefficient c [in] Minimum allowance for threading and

structural stability ta [in] Actual thickness CA [in] Corrosion allowance

t [in] Thicknesses difference between the actual and ASME VIII values MM [-] Million

ACKNOWLEDGMENTS The authors would like to thank the management of Saudi Aramco for permission to publish this article.

REFERENCES 1. Boiler and Pressure Vessel Code, “Section I, Rules

for Construction of Power Boilers,” the American Society of Mechanical Engineers, New York, 2007 Edition.

2. Boiler and Pressure Vessel Code, “Section VIII,

Division 1, Rules for Construction of Pressure Vessels,” the American Society of Mechanical Engineers, New York, 2007 Edition.


3. Standards of the Tubular Exchanger Manufacturers

Association, the Tubular Exchange Manufacturers Association Inc., Tarrytown, New York, 9

th Edition,

2007, 296 p. 4. Sulfur Production Report, 2006. United States

Geological Survey, http://en.wikipedia.org/wiki/Amine_gas_treating.

BIOGRAPHIES Dhawi A. Al-Otaibi is a Heat Exchangers Engineer with the Consulting Services Department/ Heat Transfer Systems Unit.

He received his B.S. degree in Mechanical Engineering from King Fahd University of Petroleum & Minerals (KFUPM), Dhahran, Saudi

Arabia in 2001. In 2008, Dhawi received his M.S. degree in Heat Transfer and Thermofluid Science, also from KFUPM. Since then, he has participated in engineering activities dealing with heat exchangers.

Dhawi has authored several papers on mechanical and heat transfer engineering.

Salamah S. Al-Anizi is an Engineering Consultant with the Consulting Services Department/ Heat Transfer Systems Unit.

In 1987, he received a B.S. degree in Mechanical Engineering from the University of Texas, El Paso, TX. Since then, he has

participated in engineering activities dealing with heat exchangers.

Salamah is the chairman of the Saudi Arabian Standard Organization Boiler Standard Committee, the Saudi Aramco Heat Transfer Equipment Standard Committee, and the Mechanical Engineering Chapter of the Saudi Council of Engineers.

http://en.wikipedia.org/wiki/Amine_gas_treating

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Art 13 - Internet