8 replies to this topic

[jakjak]

Throttling has been mentioned as a constant enthalpy process in Thermodynamics book. But i was wondering how does the industry actually uses/implements this technique, in term of getting to a desired state point in a system ?

From h-s (or even T-s) diagram, I can see how pressure is dropped and you reach another equilibrium point. But you can, in theory, go from, subcooled to a saturation, or from saturation to superheated, if you know where you are to begin with and how much pressure to drop.

How does the industry implement this kind of throttling process? and what is the best practise/guideline?

Appreciate any comments or feedbacks.

Thanks.

[pleckner]

In industry, we use control valves to control different parametrs, most typical being flow and pressure. When using a control valve you do just as you describe, throttle a fluid from a higher pressure state to a lower pressure state. For example, if I have to control the flow into a vessel (tank, reactor, distillation column) I would use the control valve sized for a specific flow at some known pressure drop (upstream of the valve and downstream). Because the control valve is an isenthalpic process, it doesn't mess up my heat balance.

[Art Montemayor]

jakjak:

You ask:

..."how does the industry actually uses/implements this (throttling) technique, in term of getting to a desired state point in a system ?"

Probably the most outstanding example (in my opinion) of throttling used in industry to obtain a useful end is: Mechanical Refrigeration.

The refrigeration effect is obtained by using a saturated liquefied gas (such as Ammonia) - also called a "refrigerant" - and throttling it from its saturated condition (of approximately +30 ^oC & 1.1672 MPa) to a pressure of 0.13151 MPa (which yields a mixed refrigerant of -28 ^oC). The throttling process is controlled either manually or better yet, with a control valve. The thermodynamic process is an isenthalpic one.

This simple throttling technique is the reason we can have preserved, refrigerated foods, air conditioning, industrial cooling, and also enjoy such items like Ice Cream.

[Adriaan]

"But you can, in theory, go from, subcooled to a saturation, or from saturation to superheated, if you know where you are to begin with and how much pressure to drop."

Not just in theory matey and not just on purpose either. I have experienced problems because of subcooling; waterhammer in steamlines being the result (of combining high pressure boilers in a supersaturated steam network with the last boiler linked in not actually having reached equilibrium).

In industry the most common throttling problem is one of condensate flashing (in my experience anyway); the condensate line being much larger than the aperture of the controlling valve (or Steamguard ™ etcetera). As a result the best guideline is to over-design the condensate network to allow for the excess volume.

[Alzack]

Throtling is usually by using a control valve (which is normally a gate valve). If you want to restrict the flow of steam, you can use orifice as well. You can also reduce or restrict the pressure of steam by using pressure reducing valve (PRV).

Alzack
Chemical Engineering World
http://chem-eng.blogspot.com

[gvdlans]

"Throtling is usually by using a control valve (which is normally a gate valve)."

What is this statement based on? I normally use globe valves for control valves, not gate valves. See for example http://www.engvalves...nessentials.pdf

[pleckner]

To add to gvdlans, a gate valve is probably one of the last choices you want for control. I'd take a ball valve for control before using a gate valve. The globe valve is probably the most common type of valve to use in control situations. There are others but gate is not one of them as a primary choice.

[Art Montemayor]

Alzack:

Do we have a language or a typo problem here? Perhaps you mean something else by the word "gate" - it is somewhat similar to "globe".

I've used just about every conceivable type of pneumatically actuated control valve made - and then maybe some others. But I have never seen, read, or even heard of anyone trying to use a gate valve type as a throttling device. And I dare to say that anyone trying to sell such a valve for throttling purposes would starve to death. A gate valve will ultimately shatter itself (or its seat) to pieces while going through the throttling action. The destruction of the gate will be even more pronounced if the gate is of the "wedge" type. The wedge pieces will literally wear out or simply break apart.

The best way to ruin a good gate valve is to place it in throttling service.

As Guido and Phil correctly point out, the globe design is the valve of choice in throttling service. It is only surpassed in this application by the needle valve - which is simply a basic type of globe valve itself.

Never, never employ a gate valve in throttling service.

[Alzack]

Ops....sorry everybody....what I meant was actually a globe valve....not a gate valve.....