LIQUID HAMMER ON CHEMICAL TANKERS- CAPT AJIT VADAKAYIL
On parcel chemical tankers you deliberately introduce compressed air in the cargo pipeline systems during the course of cargo work. It is important to understand the physics of liquid hammer.
Water hammer occurs as slugs of water are picked up at high speeds in a poorly designed steam main or in pipe coils or where there is a lift after a steam trap. When you heat cargo with high stern trim in cold weather , you will need to constantly drain the unlagged steam inlet line at the aftermost point.
The danger on ageing ships is going off hire or having cargo delays.
As long as you remember to start heating very slowly using the small main inlet bypass, drain all water from the main inlet pipe, and keep the steam coils full of water ( not steam ) , do not allow steam to return to engine room by controlling only the return valve ( inlet to tank 100% open ) there will not be much scope for water hammer.
In some systems, the flow may be at 120 feet per second, which is about 82 m.p.h. As the slug of condensate is carried along the steam line it reaches an obstruction, such as a bend or a valve, where it is suddenly stopped. The effect of this impact can be imagined.
It is important to note that the damaging effect of waterhammer is due to steam velocity, NOT steam pressure. It can be as damaging in low pressure systems as it can in high. This can actually produce a safety hazard, as a valve or a strainer can be blown out by the force of water hammer.
Water Hammer in Condensate Return Lines
In most installations, water hammer in condensate return lines is caused by steam pockets forming and imploding. Frequently, the cause is a rise in the discharge line from a trap or a high pressure trap discharging into a low temperature wet return line ( steam winches return expanded steam , that is why the dia of the return pipe is more than inlet , unlike tank heating systems ) . A lift in the return line after the trap will cause water hammer because the temperature of the condensate leaving the trap exceeds 100 C.
Water hammer in steam lines is normally caused by the accumulation of condensate. All equipment using a modulating steam regulator on the steam supply must provide gravity condensate drainage from the steam traps. Lifts in the return line must be avoided. In a steam system, water hammer most often occurs when some of the steam condenses into water in a horizontal section of the steam piping. Subsequently, steam picks up the water, forms a "slug" and hurls it at high velocity into a pipe elbow, creating a loud hammering noise ( which can be heard by a nearby ship ) and greatly stressing thepipe. This condition is usually caused by a poor condensate drainage strategy.
Liquid for all practical purposes is not compressible, any energy that is applied to it is instantly transmitted. This energy becomes dynamic in nature when a force such as quick closing valve or a pump applies velocity to the fluid.
Surge or water hammer, as it is commonly known is the result of a sudden change in liquid velocity. Water hammer usually occurs when a transfer system is quickly started, stopped or is forced to make a rapid change in direction. Any of these events can lead to catastrophic system component failure. Without question, the primary cause of water hammer is the quick closing valve, whether manual or automatic. A valve closing in 1.5 sec. or less depending upon valve size and system conditions, causes an abrupt stoppage of flow. The pressure spike(acoustic wave)created at rapid valve closure can be high as five(5) times the system working pressure.
Unrestricted, this pressure spike or wave will rapidly accelerate to the speed of sound in liquid, which can exceed 4000 ft/sec.
Pressure surge is caused by Kinetic energy of a fluid in motion when it is forced to stop or change direction suddenly. It depends on the fluid compressibility where there are sudden changes in pressure. For example if a valve is closed suddenly at the end of a pipeline system a water hammer wave propagates in the pipe. Moving water in a pipe has kinetic energy proportional to the mass of the water in a given volume times the square of the velocity of the water.
Water hammer can cause pipelines to break or even explodeif the pressure is high enough. On the other hand, when a valve in a pipe is closed, the water downstream of the valve will attempt to continue flowing, creating a vacuum that may cause the pipe to collapse or implode. You must have seen shore cargo hoses often getting flattened this way at the end of dicharge. This problem can be particularly acute if the pipe is on a downhill slope depending on the ships trim.
Air in the pipes can be really difficult to get rid of. The laws of physics keeps air at the highest level and liquids at the lowest . As the water moves in the pipes it breaks the pockets of air up into tiny bubbles. Then the water flows past the bubbles, leaving the air still in the pipe. These tiny bubbles moving around, and expanding and contracting, are that rapid vibrating sound you hear in Framo hydraulic systems.
Remember to get rid of air from the highest point. Of course the jockey pump does its job of continious removal of air—but not from the highest point , which can sometimes be on top of the crane.
-------CAPT AJIT VADAKAYIL
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