Turbo-exapnder control system

[Pro409]

Dear members, Hi
In regard to Turboexapnder control system, Recently I've read an article, turboexpander control-USPatent Aug.4 1981 (

http://www.freepatentsonline.com/4281970.pdf).

I' m doubtful about rightness of a part of it that states: "If the turbo-expander should tend to overspeed, the discharge output of the compressor is recycled to the suction inlet of the compressor to thereby increase the loading on the compressor which will result in a reduced turbine speed. More gas can then be fed through the turbo-expander which will maintain the desired pressure ".
It is should be noted that when the turbo-expander speed reduced , the gas flow to it also decreased. So, it seems there is no benefit in recycling of gas in that way article states, i mean that more gas can not be fed through the turbo-expander.
What is your opinion?

 

[dcasto]

in 35 years I've never seen a expander over speed unless there was an issue of losing control of the expander outlet pressure (the demethanizer).

We do use anti surge systems on the compressors.

 

[Pro409]

Sorry, I didnot understand the relation between what you said and my query. More clarification about your purpose can help to the discussion.
The matter is not about the dominant of Turbo-expander(TE) over speed or applications of compressor anti-surge line. If you read the whole article you will find that in order to control pressure in upstream of turboexpander ( and prevention of increase of TE speed) three steps have stated. My question is about the second resort of the article, i.e., MY View is that the decrease of TE speed ( due to the load increase on the compressor) will cause the reduction of capability of TE to receive more gas, but, the article claims "more gas can be fed through the turbo-expander". so, i think the claim of article is contradiction with this fact that the speed of TE is proportional to gas flow through it.

 

[dcasto]

First off, the expander is not a modern expander with guide vanes. The guide vanes are the key to keep from over speeding modern units.

I like to look at extremes to amplify the effects of controls, so ignore my absolute values and look at the relative changes.

Now, with that in mind, the old control system in Fig 2 will have 100,000 lb/hr at 700 psig and the expander is at 90% of speed. The pressure drop through the valve feeding the expander is 20 psig at 50% open (680 psig at inlet). If the feed goes to 120,000 pounds per hour, and the feed valve goes 100% open 2 psig drop (698 at inlet), the speed will jump to 110%. At this point, the overspeed will close the inlet valve making the inlet pressure still around 680 psig and the pressure will build up upstream and the valve going round the expander will open.

In figure 3, this time as the volume goes to 120,000 pounds per hour, the compress will recycle and more load on the compressor. Which means the expander will slow up and the inlet valve will go wide open with 698 psig on the inlet and the by pass valve will not open up.


This is an old fashion system and it is not practical any more.