Engine cooling system

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Big Pete
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Engine cooling system

Post by Big Pete »

I have just completed a trip as a temporary Chief on a Diesel Electric ship, with 4 generator sets cooled by one running electric HT pump and one running Electric LT pump. There was a an electric standby pump for each.
Below are some observations on the system:-

Each engine was fitted with an isolating valve on the HT & LT inlets and outlets and bypass valves around the engine for HT & LT.

As normal, the HT circuit was the engine jacket water and supplied heating for the evaporator, cooling of the HT circuit was achieved by the control thermostatic valve discharging hot water to the LT circuit and drawing cold water from the LT circuit to replace it.

The LT circuit cooled the charge air and Lubricating Oil.

The sensor for the LT thermostatic valve was placed in the common inlet manifold.
The sensor for the HT thermostatic valves was placed in the common outlet manifold.

All 4 HT bypass valves were fully open as well as the inlet & outlet valves.
Three LT bypass valves were fully open, the 4th was fully closed, the inlet & outlet valves were all fully open.

There were 2 x 6cylinder engines and 2 x 9 cylinder engines. According to the drawings each engine should have been fitted with a variable orifice plate in the water outlet.
These were fitted to the two 6 cylinder engines and consisted of a sliding orifice plate with elongated bolt holes which acted similarly to a gate valve, placed between two pipe flanges. They were missing from the 9 cylinder engines, instead an extra thick gasket was fitted between the pipe flanges. The orifice plates for the 9 cylinder engines were shown in the spare parts inventory.
When one of the 9 cylinder engines was run its jacket water temperature was much hotter than the other engines.

Has anyone got any thoughts???

BP.
It is always better to ask a stupid question than to do a stupid thing.
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JK
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Re: Engine cooling system

Post by JK »

It sounds like they overheated the system at one time and opened all the valves. Does the ship go in ice by chance?

With all the bypass valves open and the orifice plates out, the HT and LT systems pressure and flow rates must be below the design values. The heat balance is way out on the central cooling. The vap must work like crap.

Your one engine is running hotter because the other engines are robbing the CW, maybe because of the location in the system or because it's CW pump is not as efficient as the others.

I would suggest closing the bypasses and re-installing the orifice plates. Then you will have to run everything up and adjust the sliding orifice plates so each engine is being cooled equally and not robbing the system. Did the central cooling system drawing indicate the design pressures and temperatures?

Does the LT central cooling also cool auxiliary systems like the refrigeration and air compressors?
Big Pete
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Re: Engine cooling system

Post by Big Pete »

Re JKs points:-

1) The ship works as a Research ship from Arctic to Antarctic.

2) The Fresh water pressures are actually too high, but there is a massive reduction in pipe diameter at the inlet to the central coolers, and the pressure drop has always been assumed to be across the coolers.

3) All the engines are supplied by a single electrically driven cooling water pump, common water inlet & outlet manifolds to all engines..

The Vap DOES work like crap, there is actually an electric heater fitted in the jacket water system between the engine outlet and the vap inlet which has to be used to boost the JW temperature. One problem being the Laval evap is designed for a JW inlet temp of 85 while the engine outlet design temperature is 77.
Also the original motor for the Vap sea water pump burnt out and the identical replacement motor is run with the pump SUCTION valve throttled to limit the motor current!! . The vacuum gauge was U/S and there was no spare so I couldn't measure how this affected the vacuum.

I assume that all the bypass valves should normally be shut, and their function is to be opened when the engine cooling system is isolated for maintenance, in order to maintain a constant prssure drop and hence flow across the engines in use.

I estimate that if the system was set up correctly, with the bypass valves shut, with all engines running at full load the engine inlets manifold should be 67C and all the engine outlets 77 (once the flows have been balanced). Opening all the bypass valves would half the flow rate through the engines, so there would be double the temperature rise across them. ie engine outlet now 87C. 50% of the water would be going into the outlet manifold directly from the inlet manifold, through the bypass valves at 67C. Therefore the average temperature at the thermostatic valve sensor would still be 77C and the thermostatic valve would not move. Therefore, for the engine outlet to be 77C now, the set point of the thermostatic controller has been changed to 67. (Which is the temperature going to the Vap)
There are Doppler flow sensors fitted to each individual engine, wired into the alarm system, I suspect that these should be in alarm, but the calibration/ set point has been adjusted.

If all engines were running correctly as in the initial case above, and 2 engines were stopped, the situation would be similar, half the flow being heated by running engines and half going through stopped engines, acting as radiators, and again, the actual running engine outlet temperature would rise by 5C . ie. Running engine outlet 82C, all engines inlet 72 temperature at thermostat 77C.

If the ship then went down to 1 engine, 3/4 of the cooling water would be going into the control thermostat at the inlet temperature and only 25% going through an engine. The running engine temperature would then rise a further 2.5C.
Running engine outlet temperature 84.5C, all engine inlet temp 74.5C, temperature at thermostat still 77C.

NOT a very good plant design.

I have suggested that the HT thermostatic valve sensor should be re-located to the engines inlet manifold. However, although that would stop the variation in outlet temperature caused by changing the number of engines running, there would then be a change according to load. Because the Power Management System stops the engines running near full load or light load the temperature variation would be much less.

The operating profile is 3 engines running during transit, 2 engines running during Survey/ sampling operations, 1 engine in Port.
The ship is 40 years old and due for scrap in 3 years so no major modifications will be done.

My first concern that with only half the designed cooling flow through the engines, the Reynolds numbers would be too low for turbulent flow. laminar flow would permit hot surfaces to be "blanketed" by steam bubbles, leading to localised overheating and cracking, especially in the cylinder heads.
Fortunatly this did not happen while I was on board.

AS JK suggests my initial thought was to shut all the bypass valves as soon as possible, however, the long serving Third Engineer was most insistent that closing the bypass valves would cause the engines to overheat, and although I could not see any way that this could possibly happen I had to take notice of this, he was also convinced that the ship had been operating like this for 6 months. The Second Enginer was also determined not to change anything, so in the end I kept my fingers crossed and hoped. I would have liked to have experimented further but I was Chief for only one Sea passage before my relief joined.

Big Pete
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JK
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Re: Engine cooling system

Post by JK »

You sure put a huge amount of info in your posts. I am still trying to digest it.

How high was the header tank in the ship?

I would put money on the fact they lost the SW cooling in the ice at one time and the quick fix was to open everything up. The engineers really have to understand how the system will operate in the ice to adjust it correctly.
If the SW recirc temperature sensor is in the wrong spot it creates absolute havoc and causes huge swings in the system temperatures.
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offshoresnipe
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Re: Engine cooling system

Post by offshoresnipe »

I have read and reread this post many times Big Pete, boy it sure has made my head go to work. Great posts from you and JK. Keep them coming. Can I sail as your 2nd?
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