Friday, November 26, 2010

BLOCK AND BLEED ON CHEMICAL TANKERS-- CAPT AJIT VADAKAYIL

   
 BLOCK AND BLEED ARRANGEMENT ( IN LIEU OF DECK SEAL )



ON CHEMICAL TANKERS WHICH CARRY WATER SENSITIVE ( LIKE MEG FG )  AND WATER AGGRESSIVE CARGOES ( SULPHURIC ACID )--  IT IS OBVIOUS THAT SHIP CANNOT HAVE A DECK SEAL WITH WATER MEDIUM.

If cargo vapours enter the nitrogen membrane system/ CMS system -- the plant can be kissed goodbye!


Hence Nitrogen plants piping system will have a Block and Bleed arrangement.

Company policy requires that the Block and Bleed valves, be require properly  maintenaned at regular intervals and all the valves (Regulating valve, Block valves, Bleed valve, Non-Return valve and the Deck Main Isolating valve) be regularly tested for proper operation and tightness.

Unlike the fool proof deck seal, this arrangement requires close monitoring and testing alarms for safety.

When chemical tankers with a Nitrogen plant carries both chemical and Annex 1 cargoes, a BLOCK AND BLEED arrangement is used instead of a deck seal, to prevent backflow from cargo tank into IG plant and safe non-cargo spaces.

 It is an arrangement consisting of two shut-off block valves in series with a venting to atmosphere bleed valve in between (double block and bleed).  It is between the DMV and the GRV . The operation of the valves is to be automatically executed. Signals for opening/ closing is to be taken from the process directly, e.g. inert gas flow or differential pressure. Alarm for faulty operation of the valves is to be provided, e.g. the operation status of “Blower Stop” and “supply valves open” is an alarm condition”. 

The size of the bleed valve should be such that it will not allow pressure build-up from back-flow of hydrocarbons in the event that the block and bleed arrangement and the non return valve) fail to achieve their function. Block and Bleed arrangement is triggered by zero flow condition. The differential pressure arrangement will sense the difference between the pressure in the line upstream of the pressure regulating valve and the pressure in the IG main.

 If the pressure in the IG main is higher than the pressure upstream of the control valve, the system will close the gas regulating valve and shut down automatically. Indication of this condition should be provided in the Engine Control Room and the Cargo Control Room. The measuring points should be arranged to preclude bypass of hydrocarbons from the tanks into the engine room via the sensing lines by, for example, using secondary 3 to 15 p.s.i. loop signal in lieu of direct signal or by using other suitable electronic sensors. A timing mechanism, providing a few seconds delay, should be included into the starting sequence to permit the blower to start while the pressure in the tanks is higher than the pressure at the blower discharge.

The effectiveness of the Block and Bleed devices depends on their ability to sense the “process” fluid (Inert Gas) pressure or flow. In some instances, deficient systems have been built with Block and Bleed arrangements which do not sense the “process” fluid but rely on the blower operational “on” / “off” status to operate the Block and Bleed valves and achieve the sealing function.

For Block and Bleed arrangements which do not have differential pressure or flow sensing devices, it is possible that, during periods of cargo discharge operation with the blower operational, the tank pressure may increase to a point higher than the inert gas generator discharge pressure.

In this case, cargo gases back-flow into the machinery space could occur if the Non-Return valve was defective. Thus, when the Block and Bleed arrangements depend on the blower “on” / “off” condition, there is effectively only one protective non-return device in the system, namely the Non-Return valve. Such arrangement does not conform as an alternative arrangement providing a measure of safety “Equivalent” to that of a water seal.

The minimum hardware recommended for proper Block and Bleed arrangement are indicated below.

1. Block and Bleed control to be based on signals from a differential pressure or flow sensing detector of the “process” fluid, Inert Gas or Nitrogen.

2. Provision of an audible and visual alarm for low differential pressure or low flow if the block valves remain open.

3. Provision of an audible and visual alarm for faulty operation of the block and bleed valves. Blower “stop” and supply valves “open” should be an alarm condition.

4. Audible and visual alarm to be provided if valve position is mismatched ((i.e. if Block Valves and Bleed Valve are closed) – this function to be provided with a time delay.

5. Provision of indication in the Cargo Control room of “open” and “closed” status of the block and bleed valves, the Gas Regulating valve, and Bleed to Atmosphere valve.

6. Provision of control air for the Block and Bleed Valves to be via an independent solenoid valve arrangement from the solenoid supplying the Regulating Valve (Bulkhead Valve).

7. The pressure sensors used for the block and bleed control should be independent from the pressure sensor required by SOLAS Regulation 62.16.1.1 (equivalent Regulation 14.1.1 on Reference A)

8. Provision of interlock to ensure that Blower shutdown will close the Block and Bleed arrangement and that the Block and Bleed arrangement cannot be opened unless blower power is on. Also, the blower should be arranged so that it cannot start unless the block valves are closed.

The bleed ( vent ) to atmosphere is best operated by power.

Flow diagram schematics:

N2 plant— valve-- -vent to atmosphere--- pressure transmitter--- GRV/ safe area to dangerous area/ bleed to atmosphere with flame screen ---   block and bleed arrangement ( in lieu of deck seal )--- NR valve— pressure transmitter of differential pressure device.


CAPT AJIT VADAKAYIL ( 28 years in command )
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