Temperature control at HEX process fluid

[pastyl]

Dear all,

I have to control the outlet temperature of natural gas process fluid, which flows in the tube side of a vertical beu HEX.
At the shell side steam is condensates. By modulate steam mass flow with a throttling control valve you can control natural gas temperature. Classic temperature control method at HEX.
The problem is that in any case condensation pressure is arises from the heat and mass balance into the HEX. At very low load pressure will decreased and it may reach even 1barg. Then it is difficult for condensate to be transmitted in to the atmospheric flash tank.
In cases that a condenser shall operates at a variable loads with a significant range then HEX pressure will varies a lot. You need then a pressure control system.
The key is to install (and not the only solution) a control valve at condensation line in order to create a level at low loads and reducing thus saturation area. Then HEX pressure will increased.
The question is can you achieve with pressure control and temperature control i.e. one control valve in condensate side can handle HEX pressure and natural gas temperature ?
Or you need two control valves, one for temperature and the other for pressure control ?
Please note here that pressure and level control has the same meaning

 

[Chance17]

It seems the shell side needs at leat two control loops.
How about
1.) a pressure control loop in the shell side vapor out
to always maitain enough pressure to push liquids to
the flash drum
2.) shell side level (slave) cascaded to tube side
exit temperature (master)

 

[Renoyd]

If I understand correctly, you need 2 control valves: one is on the shell inlet line controling the tube outlet gas temperature, and the other one is on the shell condensate outlet controling the level to maintain certain pressure.

 

[Compositepro]

One control valve should be sufficient. If your heat exchanger design is appropriate to allow for controlling heat transfer by controlling condensate level, then you only need a control valve on the condensate. It may be possible to use the gas temperature to directly control the condensate valve, but it would probably be more stable to use cascade control where the gas temperature would control a condensate level control loop.

You would not use level control if there is the possibility of freezing the condensate or causing steam hammer.

 

[MikeClay]

Would an automatic pump trap be a better choice? It uses steam to push the condensate out of the trap on an intermittent basis. This assumes the condensate load is not too great.

See

http://www.spiraxsarco.com/us/pdfs/SB/apt10_apt14.pdf

for an example. Other companies also make similar products.

--Mike--

 

[pastyl]

Dear Compositepro

Controlling load with steam control valve product A*U (area*heat transfer coeff.) remains appr. constant. Then Q is directly proportional to gas outlet temperature from Q=AUMTD
But controlling load with level then A*U changes since A changes and then I think you cannot control accurate and stable gas outlet temperature. Consider and the time needs for level to be created. In fact gas is transmitted as a fuel to Gas Turbines so gas temperature is very important.
But if you think or have seen temperature control with condensate level please let me know because this was an initial though and it is very important for me to know if works.
As far as the APT solution I am familiar with it dear MikeClay but its cost is much bigger than a classic trap.
However I will propose it to my customer.
Generally speaking the real problem is weakness of client consultancy firm to understand that as HEX is designed to serve from 1 up to 4 Gas Turbines (i.e. from very min. to max. load) it is logic condensation pressure to varies.
They give me 2.5barg maximum steam pressure.
I selected area were condensation becomes at 2.3barg for normal load plus an overload they gave to me (10%).
Then at normal load and with the above area condensation becomes at 1.6barg. They demand at normal load to become at 2.3barg. But then you will no have overload because at overload HEX will demand higher then 2.5barg pressure.
Plus. at minimum load pressure will be 1.1 barg.
The above pressure arise from heat and mass balance and I cannot do anything since load varies so much.
They want to HEX design in order to serve condensation removal process ??
HEX is designed to serve natural gas load and not condensate removal process.
And at the time there are so many solutions for condensate removal as:
1) an APT
2) a pressure control valve
3) a pump
4) A bigger condensate line

 

[MJCronin]

patsy.....

Dumb question.....is a vertically oriented TEMA BEU the best configuration for your process needs ??

It's my understanding that, if you want to use steam on the shellside of a shell-and-tube HX, it is the most effective if the HX is oriented horizontally. Your orientation does not seem like the best one to me....

A horizontally-oriented HX with a generous sump on the condensate outlet can be easily controlled with a downstream trap or CV.

Was the HX used in some other service previously ?

HHHhmmmmmmmmmmmmmmmm?????

-MJC

 

[pastyl]

MJCr...

HEX cannot have been used because we have not start to manufacture it yet.
Its vertical because of the available space.
Such a comment like"HMMMmmmmmmmmmmmmmmmmm" are not valid to this forum and for professionals engineers.
We have manufacture and install more than 10 of vertical condensers to these applications with no problem.
Whats your experience of HEX? Perhaps you have seen one horizontal in a power or process plant and you think that all condensers shall be horizontal.
In this job if you do not know exactly what to say then don't say at all.

 

[EdStainless]

Vertical HXs have a legion of problems from flow distribution to 'blinding' on the underside of the top tubesheet. But there are places that they fit.

If you are trying to control the temp at the outlet on the tube side, then why don't you directly control it? Wouldn't a control valve on a bypass line do the same thing?


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