Letdown valve - what is it?

[JOM]

I came across mention of a "letdown valve" in a Chem Eng article.

Can someone please tell me the function of a letdown valve?

J.

J.

 

[TD2K]

Typically a valve that you 'let-down' from a high pressure to a low pressure system.

For example, a pressure control valve to add 600 psig steam to a 250 psig header to maintain the 250 psig header pressure would commonly be referred to as a let-down valve.

 

[roker]

to complete the example of TD2K the steam at the outlet of the "let down station" is superheated and according to the use of this steam a desuperheater may be needed.

regards

 

[JOM]

Thanks for those answers. Most helpful.


J.

 

[rmw]

In addition to what has already been said, the use of the term "let down valve" often conotes that it is acting in lieu of some other device that normally reduces the steam from one pressure to a lower one, such as a steam turbine operating at a given inlet pressure and exhausting or extracting to a lower pressure.

When the turbine is off line, or is not passing enough flow to satisfy the requirements of the lower pressure header, some of the steam is "let down" as opposed to the operation of a "pressure reducing valve", whose function from day one is to reduce pressure from one level to another one.

This is not a cast in concrete definition, but just a connotative use of the term instead of just calling it a pressure reducing valve.

rmw

 

[JOM]

Thanks rmw, roker and TD2K.

I'll quote the sentence that sparked my query. It's from an AIChE publication describibg a particular accident at a gas plant.

"..a tag was placed on a faulty letdown valve instructing that a bypass must not be used."

There's nothing extra that explains the valve's purpose or the process fluid. I know that it certainly wasn't steam.

I just wanted to get an understanding of the common meaning of the term.

Thanks all.


J.

 

[JimCasey]

In some cases the letdown valve is one used to drain a batch reactor. They put all the reagents in, close the valves, usually add heat and pressure to promote the reaction, then when it is complete they open the letdown valve to allow the completed product to move forward in the process. Also called a blowdown valve.