LNG BOIL-OFF RE-LIQUEFACTION PLANTS AND GAS COMBUSTION UNITS

Transcription

1 CASSIFICATION NOTES No NG BOI-OFF RE-IQUEFACTION PANTS AND GAS COMBUSTION UNITS MAY 2006 Veritasveien 1, NO-1322 øvik, Norway Tel.: Fax:

2 FOREWORD (DNV) is an autonomous and independent foundation with the objectives of safeguarding life, property and the environment, at sea and onshore. DNV undertakes classification, certification, and other verification and consultancy services relating to quality of ships, offshore units and installations, and onshore industries worldwide, and carries out research in relation to these functions. Classification Notes Classification Notes are publications that give practical information on classification of ships and other objects. Examples of design solutions, calculation methods, specifications of test procedures, as well as acceptable repair methods for some components are given as interpretations of the more general rule requirements. A list of Classification Notes is found in the latest edition of Pt.0 Ch.1 of the Rules for Classification of Ships and the Rules for Classification of igh Speed, ight Craft and Naval Surface Craft. The list of Classification Notes is also included in the current Classification Services Publications issued by the Society, which is available on request. All publications may be ordered from the Society s Web site Motives New propulsion systems for the latest generation of NG Carriers introduce reliquefaction plants and gas combustion units for handling the boil-off. This is as an alternative to the traditional arrangements where the boil-off is burned as fuel in boilers. Several NG carriers now on order will be fitted with re-liquefaction and gas combustion units (incineration). DNV has felt a need to establish guidance for these installations in the form of a new Classification Note (No 61.2). In the not too distant future introducing requirements for these installations in the Rules for Classification of Ships will be considered, but only after having gained more experience with such installations. Thus, in the meantime applicable DNV expectations on new ships to re-liquefaction plants and gas combustion units for handling of NG "boil-off" are outlined in the Classification Note Comments may be sent by to rules@dnv.com For subscription orders or information about subscription terms, please use distribution@dnv.com Comprehensive information about DNV and the Society's services is found at the Web site Det Norske Veritas Computer Typesetting (FM+SGM) by Det Norske Veritas Printed in Norway. If any person suffers loss or damage which is proved to have been caused by any negligent act or omission of Det Norske Veritas, then Det Norske Veritas shall pay compensation to such person for his proved direct loss or damage. owever, the compensation shall not exceed an amount equal to ten times the fee charged for the service in question, provided that the maximum compensation shall never exceed USD 2 million. In this provision "Det Norske Veritas" shall mean the Foundation Det Norske Veritas as well as all its subsidiaries, directors, officers, employees, agents and any other acting on behalf of Det Norske Veritas.

3 Classification Notes - No CONTENTS 1. GENERA Introduction Scope ARRANGEMENT Re-liquefaction systems Gas combustion unit MATERIAS, MANUFACTURE, INSPECTION AND TESTING Materials Piping design and manufacture Certification of components Strength testing MONITORING AND CONTRO General Extent of monitoring of re-liquefaction plants Monitoring of GCU... 7

4 4 Classification Notes - No General 1.1 Introduction An alternative to using NG boil-off gas as fuel in steam boilers, gas turbines or combustion engines is to re-liquefy the vapour and return the NG to the cargo tanks. The IGC Code requires two means for disposal of the boil-off gas and in the case of re-liquefaction is the primary means the secondary means is typically a Gas Combustion Unit (Thermal Oxidizer) where the boil-off gas is burned and the combustion gas cooled and exhausted to atmosphere. 1.2 Scope This class note outlines requirements for classification of reliquefaction installations and gas combustion units. Since the installations may differ in arrangements and equipment the requirements outlined may be subject to adjustments to suit the particular installations. 2. Arrangement 2.1 Re-liquefaction systems Fig.2-1 and Fig.2-2 show two examples of re-liquefaction systems. The system in Fig.2-1 (Moss Maritime / amworthy) uses a nitrogen refrigerant cycle. The nitrogen is compressed in three stages with inter-cooling, then pre-cooled in the cold box and expanded through a rotary expander from about 57 bar to 14 bar. By this expansion the temperature of the nitrogen falls to 163 C (Joule-Thompson effect). The cold nitrogen is lead to the cold box where it cools the boil-off gas which has been compressed to about 4.5 bar of in the low duty compressor. The condensed boil-off gas is lead to a liquid receiver where the NG is collected and the non-condensable gases, mainly nitrogen, is vented. The vented gases are either returned to the vapour main or directed to the gas combustion unit (GCU) for incineration. Venting to the atmosphere is normally not permitted since the contents of methane gas in the vented gas is 20-25% at 4.5 bar of in the liquid receiver. Figure 2-1 Re-liquefaction system (Moss Maritime / amworthy)

5 Classification Notes - No Boil - Off Gas CW Vent BOG Compressor Skid Cold Box Flash pot NG oad heat exchanger N 2 heat exchanger CW N 2 rejection column CCNR Skid Nitrogen (make - up) Figure 2-2 Re-liquefaction system (Air Products) process flowsheet The system in Fig.2-2 (Air Products) is also utilizing a nitrogen refrigerant cycle. The main difference is that while the system in Fig.2-1 achieves the temperature reduction through expansion of the nitrogen alone, the system in Fig.2-2 utilizes the J-T effect both by expansion of the nitrogen and by expansion of the boiloff gas. For this reason the boil-off gas has to be compressed to about 40 bar. After cooling in the cold box (load heat exchanger) the gas is expanded into the N 2 rejection column at a of 20 bar. In the N 2 rejection column, cooling of the gas mixture in the packed bed by liquefied gas brings the methane content in the vented gas down to about 5%. With this low methane content the gas may be vented to atmosphere. 2.2 Gas combustion unit Disposal of boil-off gas may be done by incinerating the gas in a gas combustion unit (GCU). Examples of GCUs for marine use are shown in Fig.2-3 and Fig.2-4. Mixing device Self checking Flame detection system Spark igniters (2) Man ole Diaphragm Air room UV Funnel Full metallic Air cooled combustion chamber Gas burner Block & Bleed Valves Control Valve BOG Combined combustiondilution air fans (3 or +) Constant flow Figure 2-3 Gas combustion unit - (Snecma Moteurs)

6 6 Classification Notes - No Examples DIUTION AIR FAN COMBUSTION AIR FAN (2) amworthy Combustion AMOxsafe GCU International Patent Number PCT / GB 2005 / pending Figure 2-4 Gas combustion unit - (amworthy Combustion) BOG The unit consists of an outer and an inner steel cylinder. The inner cylinder is the combustion chamber with gas burners and igniters. Some designs may use pilot burners for ignition while others, as in the example, use direct spark ignition. The combustion chamber has no refractory but is cooled by air flowing in the space between the inner and the outer cylinder. The supply of air for cooling and combustion may be by combined fans, or by separate fans for combustion air and cooling and dilution air. The bulk average exit temperature of the combustion gas must be kept well below 500 C in order not to ignite any possible leakages of methane gas and also to avoid excessive heat radiation to surrounding installations. ocalised maximum peak temperatures must not exceed 535 C. The reduction of gas temperature is achieved by diluting the gas by air before exiting the funnel. A GCU may be accepted to meet the IGC Code requirement for both primary and secondary means for disposal of boil-off gas on condition that redundancy is arranged for fans (combustion and dilution), burners, igniters, gas flow control valve(s) and control system. A GCU control system includes both combustion control and burner management. A redundant GCU control system is a system with dual processors and single I/O or a single processor with an independent manual/remote control system. Vent AIR JETS FOR MIXING DIUTION AIR FAN Double Block & Vent Valves 100% STAINESS STEE, TWIN SE CONSTRUCTION REGISTER BURNER WIT DUA SWIRER TECNOOGY Control Valve Propulsion Primary Secondary Capacity and redundancy Diesel Re-liquefaction Re-liquefaction 2 x 100% design boil-off gas capacity re-liquefaction units. Common cold box may be accepted. Diesel Re-liquefaction GCU 1 x 100% design boil-off gas capacity re-liquefaction unit. 1 x 100% design boil-off gas capacity GCU. Diesel GCU GCU 1 x 100% design boil-off gas capacity GCU with redundant fans, igniters, burners, gas flow control valves and combustion control systems; or 2 x 100% design boil-off GCUs; or 3 x 50% design boil-off GCUs Dual fuel diesel electric GCU GCU 1 x 100% design boil-off gas capacity GCU with redundant fans, igniters, burners, gas flow control valve and combustion control system; or 2 x 100% design boil-off GCUs; or 3 x 50% design boil-off GCUs The design boil-off capacity is normally the maximum boil-off rate. The amount of boil-off gas consumed by the dual fuel engines during harbour operations is defined as the base load (hotel load). This base load may be subtracted from the maximum design boil-off rate to establish the design boil-off capacity of the gas combustion units. 3. Materials, Manufacture, Inspection and Testing 3.1 Materials Materials for the refrigerant piping and cargo vapour piping are to be suitable for low temperature service. In general material should be selected for a temperature of 165 C. Material selection for warmer temperatures in parts of the system may be considered. Requirements for materials, documentation and testing, see Rules Pt.5 Ch.5 Sec.2 D and E. Materials for the steel cylinders of the GCU are to be suitable for the high temperatures. Material quality is to be documented by Works Certificates. 3.2 Piping design and manufacture Piping shall be joined by butt welding. The number of flanges shall be kept to a minimum. Valves should preferably be of weld-in type. Requirements for design, welding and testing, see Rules Pt.5 Ch.5 Sec.6 C500-C800. For gas combustion units located in enclosed spaces the gas piping within the space shall be of double walled type or fully enclosed in a ventilated duct with arrangements complying with Rules Pt.5 Ch.5 Sec.16 B100 and 200.

7 Classification Notes - No Certification of components Component Certification Rule reference Valves DN 75 mm, DNV Pt.5 Ch.5 Sec.6 DN < 75 mm, W Pressure vessels DNV Pt.4 Ch.7 eat exchangers DNV Compressors DNV Pt.4 Ch.5 Sec.4 Pumps DNV Pt.5 Ch.5 Sec.6 G Gas combustion unit W W = Works (makers) certificate 3.4 Strength testing Piping and heat exchangers shall be hydrostatically tested to 1.5 times the design. Testing by using gas instead of liquid may be accepted on special considerations of safety implications. 4. Monitoring and Control 4.1 General Monitoring and control systems shall in general comply with the requirements of Rules Pt.4 Ch Extent of monitoring of re-liquefaction plants Item/Parameter Alarm Shut down Comment BOG compressor, gas outlet temp. BOG compressor, gas outlet BOG compressor, gas inlet / BOG compressor, gas inlet/outlet temp. diff. BOG compressor, gas inlet temp. BOG compressor, vibrations BOG compressor, lube oil press BOG compressor, bearings For shaft power above kw BOG pre-cooler level Vapour header / NG separator level / NG separator, / N 2 compressor gas inlet N 2 compressor gas inlet temp. N 2 compressor 2.stage gas inlet temp N 2 compressor 2.stage gas outlet N 2 compressor 3.stage gas inlet temp N 2 compressor 3.stage gas outlet temp. after cooler Item/Parameter Alarm Shut down Comment N 2 compressor/expander, vibrations N 2 compressor/expander lube oil press N 2 compressor, bearings For shaft power above kw N 2 high press/low press X Outside differential normal range Cold box enclosure, gas concentration methane oxygen Cooling water press. Cooling water outlet temp. N 2 compressor room, O 2 concentration N 2 compressor room, loss of X ventilation N 2 rejection column, level N 2 rejection column, / BOG compressor room O 2 concentration BOG compressor room C gas concentration BOG compressor room fire detection BOG compressor room, loss of ventilation ESD activated Cargo tanks overfill danger oss of power supply to control and monitoring system Instrument air, 4.3 Monitoring of GCU X SD may be time delayed X X Blow down of gas X X Activate at levels above existing X X Item/Parameter Alarm Shut down Comment Flame failure X X oss of combustion air supply X X oss of cooling air/dilution air X X supply BOG inlet temperature Combustion gas exit temperature Methane gas concentration in gas pipe duct oss of ventilation in gas pipe X X duct, Alternatively loss of N 2 Fire detection in GCU space X X