Why Vacuum Column Condensers are Installed so high ?

Vacuum Column have ejectors to maintain the vacuum in the column. Downstream of ejectors condensers are installed to condense the vapour coming out of the ejector. Many people ask why the condensers are installed so high while other condensers in different services are installed at much lower height? The following discussion will answer the question.

 

First lets see a typical arrangement of a vacuum column overhead circuit with operating pressures in the following diagram.

 

From the pressure values, it can be seen that the pressure in the hot well chamber is around 0.16 Kg/cm2(g), while the pressure at the Booster Condenser outlet is 70 mm Hg (a). Since the condenser outlet is at a lower pressure than the hot well vessel, the liquid will try to travel up the boot leg of the condenser.

 

But how far the liquid go up? The liquid will go up, upto a level where it will balance out the pressure difference. Now lets calculate the pressure difference:

 

Pressure in Hot well = 0.16 kg/cm2 (g) or 1.013 + 0.16 = 1.173 Kg/cm2 (a)

Pressure in Condenser Outlet = 70 mm Hg (a) or 70 / 760 = 0.0921 Kg/cm2 (a)

Pressure Difference = (1.173 - 0.0921) kg/cm2 = 1.0809 kg/cm2

Liquid (water) height in dip leg = 10809 mm (1 kg/cm2 = ~10000 mm water column)

 

Similarly height of water in 2nd Stage condensers can also be calculated. Try yourself. The level in 3rd stage condenser dip leg will be negligible since it is floating with the hot well pressure.

 

Now the question arises, what will happen if the length of the condenser dip leg is less than the calculated value of 10809 mm?

 

Look below:

 

As you can see the condensers will be flooded with water and there will be no heat transfer are available to condense the vacuum column overhead vapours and you can not pull vacuum at all, making the ejector ineffective. That is why the condensers are placed high above the hot well drum, so that even when the column pressure goes to a very low level, the condensers does not get submerged with water.

 

One more question before you go to the plant and see the condenser boot leg. What is the purpose of the smaller baffle (left side one) inside the hot well vessel? Since the larger baffle is of more height, the smaller baffle will always be submerged in liquid and it is also not helping in separating oil and water. Then what is the purpose?

 

The purpose is to maintain a seal of water even if the sour water pump level control malfunctions and empties out the vessel, it will empty out the liquid between the small baffle and the large baffle but liquid level up to the small level will always be maintained with out allowing vapour to go up along the condensers boot legs.

Purpose of the inverted "U" loop from the seal drum (V-014) is also the same, to maintain a liquid seal.

 

So the next time you enter the hot well drum for inspection in the shutdown, ensure that the integrity of the smaller baffle as well as clean the common header at the inlet of hot well. Chokage of the header will also cause liquid level build up and may flood the condensers.