SAFETY HAZARD IN NAPHTHA BASED GAS TURBINE POWER PLANTS
Innovation and technology in Gas Turbine design during last 5-6 decades have resulted in designs where multiple fuel can be used for Gas Turbine. Due to support given by power authorities to Naphtha based Gas Turbine plants esp. in India coupled with low international price of Naphtha resulted into sharp rise in Naphtha based power plants during last decade.
Naphtha being highly volatile, inflammable and hazardous fuel, it has its own problem for using in power plants. Though power plants are running with Naphtha successfully. There is a high degree of safety involved.
Here is a brief summary of safety involved with using Naphtha for Gas Turbine. Part-1 of this article deals with naphtha unloading up to storage tank.
HAZARD CLASSIFICATION OF NAPHTHA:
Division 1, Group II B.
¨ Highly volatile
¨ Distillation IBP 10% 950F
¨ Flash point 1400F
¨ Specific Gravity 0.6 – 0.75
Normally Naphtha is transported through rail, tankers to the project site. Naphtha is then pumped through unloading area to the storage vessel. Depending upon the quality, it may be required to be purified in the centrifuge. After purification, naphtha is stored in the pure Naphtha tank.
From pure naphtha tank, naphtha is pumped by the naphtha forwarding pumps to the Gas Turbine. Normally a ring header is formed with branch lines given to individual Gas Turbines and then balance naphtha is returned to the Purified Naphtha tank through a header pressure control valve. This header pressure control valves ensures that naphtha pressure is sufficient up to the last branch in the ring header.
Naphtha going to individual Gas Turbine is further pressurized, by Naphtha injection pumps provided in the Fuel Injection skid, to around 40-50 bar (depending upon Gas Turbine requirement). This fuel goes to fuel nozzles in the gas turbine. Fuel nozzle sprays naphtha with required air and atomizing air in the combustion chamber where naphtha is burnt. Naphtha has got very low lubricity. So, to increase its lubricity, a small quantity of additives is added, as recommended by the OEM of Gas Turbine.
Since calorific value of naphtha is low, it is normally used as a running fuel only. For start-up of naphtha-based plant, some other fuel such as High Speed Diesel or natural gas is used. Once Gas Turbine Generator breaker is closed, changeover to naphtha may be initiated.
A lot of precautions are required to be taken, as mentioned below, while handling naphtha. Major hazard involved with naphtha is fire hazard. So, all possible measures to be taken to avoid fire hazard.
¨ Tanker or rail wagon is required to be earthed properly, to avoid building up of static electricity.
¨ Short link to be provided over each flange joint, to avoid building up of potential difference across flange which may be induced by velocity of the naphtha in the pipeline.
¨ Like any other fuel, bare flame is to be strictly prevented in the region.
¨ For any hot work like welding and gas cutting, proper safety permits to be ensured to avoid accident. Before doing any hot work area, it should be ensured that area is clear of any combustible gases.
¨ Fire water system for; naphtha unloading, forwarding, storage and injection area; to be checked on regular basis and system healthiness at all conditions to be ensured.
¨ Foam system for the naphtha storage tanks to be provided and its healthiness to be checked periodically.
¨ Potable fire extinguishers in sufficient quantities to be provided wherever naphtha is being used.
¨ For fixed roof naphtha storage tanks, nitrogen blanketing to be provided, to prevent naphtha vapor escaping from breather valve.
¨ For large size storage tanks, floating roof tank is desirable.
¨ All electrical fittings and lighting fixtures shall be explosion-proof.
¨ A spillage tank may be provided to collect unloading, storage tank drain and forwarding area leakage.
¨ All electrical motors and mechanical equipment shall be earthed properly with earthing strip.
¨ Resistance of the earth-pit to be measured regularly and earth-pit to be maintained properly.
Instrumentation provided for naphtha area shall be either intrinsically safe or explosion-proof confirming to above indicated hazard classification.
Typically following instrumentation will be required for naphtha unloading and storage area.
¨ Combustible gas detector in the naphtha unloading area, to detect naphtha leakage. Location of the gas detector is very critical. If point type gas sensor is used, it should located in such a way that it receives maximum naphtha vapor when leakage occurs. For unloading area open to wind velocity, location should be strategically chosen considering wind direction round the year.
If Infrared line type detectors are provided then they should be located in such a way as to cover the complete area. Annunciation of the gas leakage shall be provided in the central control room.
Gas detector shall be checked for sensitivity periodically. Also, it should be calibrated with standard gas at an interval specified by the manufacturer of the detector.
¨ Heat detector shall also be provided to detect fire in the area. Heat detector shall operate around 1080C. In addition to fixed temperature heat detectors, rate of rise heat detector shall also be provided for early detection of the fire. On heat detection, automatic fire protection system shall start and command to be given to start water spraying.
¨ HAZOP valves to be provided in the line going from unloading pump to storage tank. This valve shall trip on detection of fire. This valve shall be design as failsafe to close type. HAZOP valve shall be provided without handle. HAZOP valve shall be of fire safe design and quick closing type. Closing time for the valve shall be less than 3 seconds preferably.
¨ Apart from above instrumentation, interlocks for the unloading pump shall be provided. Typically these interlocks include-
STORAGE TANK AREA:
¨ Storage tank level measurement is very important. It is desirable to measure tank level by two independent methods. Two independent methods are desirable since in no case failure of one method left with no measurement of the level.
One method may be non-contact type like ultrasonic type or Radar type. Ultrasonic type has been found working satisfactorily and it is cheaper to radar type. Other measurement may be with Float and Tape or Float and Board type instrument. A local indication of the measurement may also be provided. For fixed roof tank, even differential pressure type level measurement, provides reasonable accuracy.
If storage tank level is to be used for the custody transfer then highly accurate Servo type instrument may be used.
¨ A combustible gas detector shall be provided on the inlet and outlet flange connection of storage tank to identify even a minor leakage.
¨ Combustible gas detectors shall also be provided on storage tank roof. These detectors shall be strategically located on the tank roof for proper detection.
¨ Heat detector and rate of rise heat detector shall also be provided on the tank roof for detection of the fire.
¨ A heat sensing cable shall also be laid on the tank roof to detect early fire.
¨ On detection of combustible gas leakage, foam system shall start to cover tank roof with foam and preventing fire chances.
¨ On detection of fire by rate of rise detector or fixed heat detector or heat sensing cable, immediately water spraying shall start. Fixed heat detector operates on a fixed value of temperature hence it operates when fire builds up the required temperature. Where as rate of rise detector responds to fast fire and it immediately gives alarm the moment temperature rises faster then detector setpoint. Linear heat sensing cables are of two types, analog and digital. Just like a transmitter and a switch. Normally heat sensing cables are laid on the roof periphery to sense temperature rise along the periphery.
¨ Solenoid valves for the deluge valve shall be of fail safe type. I.e. on power supply failure to the deluge valve solenoid valve, it should open and water should start spraying. This is to ensure that in any worst condition, water is available for extinguishing fire.
updated by Brian Smith on August 6, 2002.