SURGE TANK & SURGE VESSEL
(1) To remove gas completely from the liquid.The low pressure( atmospheric pressure) maintained in surge tanks supplys gas free liquid to the pumps, outgoing metering equipment and eventually to the dehydration plant, terminal or exporting vessel.
(2) The large surge capacity permits the build up of a buffer volume in case of pump failure or other downstream interruption.
(3) To supply sufficient suction head for the transfer pumps.
(4) To give sufficient retention time for the liquid for the complete degassing.
Operations personnel should be conversant with general layout and structure of Tanks.
The basic components fitted to a tank are illustrated in Figure.
The tank inlet is located either near to the bottom (approximately ± 50 cm, 20 inch from bottom) or near the top, where it enters the tank via a down comer pipe which is usually perforated.
Both bottom or top inlets should end below the minimum operating liquid level to prevent foam formation and static electricity.
Top filling is usually preferred because it reduce turbulence and prevents loss of tank content when the filling line breaks or leaks. The top filling line usually has more flexibility and puts less load on the tank wall in the event of pulsating flow.
Since a tank can only withstand minimum over – and under – pressurization, depending on tank specifications.
- Overpressure 0.75 – 5.4 Ka(3 – 21.5 inch)
- Vacuum 0.25 – 0.63 Kpa (1 – 2.5 inch)
It must be provided with a pressure-vacuum relief valve, an air baffle or with a vent pipe. When more than one tank is installed a common vent pipe is often utilized. Care should be taken to reduce gas entry to tank. A constant purge of gas from continuous source of gas header via PCV is used to blanket the tanks. All gas from these tanks is then routed to the tank flare.
1 The level controller LRCA control the level in the Surge Tanks via low selector.
2 Standby transfer pump starts if tank show high level and stops if show low level,
3 Blanket gas is supplied from the bulk / test separator off gas header or from continuous gas supply gas header.
- There can be provision of back up blanket gas in the event of plant The pressure of the blanket gas to surge tank is independently controlled by dedicated pressure control valves (PCV )
Level switch LZA HH detects too high level in surge tank and the activation causes a PSD of the bulk and the test separators and shuts down the flare KO drum drain pumps and closes the pumped liquid outlet ESD valve. This ensures that inflow to surge tanks is stopped before the tank overflows.
Both level switches LZA LL detects very low level in the surge tanks and the activation shuts down both the transfer pumps. This ensures that the transfer pumps are not subjected to cavitation and the gas does not exit via liquid outlet line.
Pressure gauges ( PG )show the operating pressures of tank and the blanket gas header supply pressure. Local level gauges ( LG ) show the operating level in tank.
Tanks are provided with independent safeguarding elements.
The station Cause & Effect diagrams should be referred to for key protection criteria.
The surge tanks are protected by the bulk and test separator ESD valves, transfer pumps trips and station trip on power failure.
The tanks are protected against pressure / vacuum by the line to the AP flare.
Secondary protection is afforded by a pressure / vacuum relief valve (PVV) and an emergency vent.
The PVV is capable of handling the maximum calculated in and out breathing requirements of the tank. In the case of a fire outside the tank, or gas blow-by, the emergency vent will open and prevent pressure build-up which may lead to tank rupture.
A pressure vacuum valve set at 2 Kpa pressure and 0.6 Kpa vacuum is installed on surge tanks. These valves provide the final safeguarding facility to protect the tanks from either tank vacuum or tank overpressure due to blocked discharge, in case of a fire.
A fusible ring is fitted around the outside of each tank and forms part of the fire detection system.
Following are the operational problems which process has to over-come.
The fire hazard : Because tanks are usually open to the atmosphere, even though precautions may be taken, gas will breathe out of tanks and may spread out over the station. Air sucked into tanks may create explosive mixtures.
Light oils : Which may lose light fractions resulting in a lower API gravity and loss of volume, especially in hot climates.
Space : Requirements which may be prohibitive, especially on platforms.
Poor pump performance : Because of the low suction head caused when there are low liquid levels in the tanks.