why flanged gate valve? 2

[simplemath2]

In one of refinery revamp , both hot air and steam service, the piping spec calls out valves 3" and above with no exception flanged gate valve.

The service is 500F - 800 F and 40 psi. Class 150.

I wondered. I am thinking of butt-weld would be superior to flange considering from weight, leak path or reliability.
I am also thinking of ball valve or butterfly valve (over 8")as of better block valve options for these two services than gate valve. Is gate cheaper?

Am I missing something?

 

[pennpiper]

"the piping spec calls out"....
If this is a fact then why are you questioning it.
a) Normally the assigned Piping Material Engineer would do a lot of study to determine the best (technical vs. cost) valve and other material.
b) The piping Specs are then sent to the Client for review and approval. I assume this was done.
c) Piping design is responsible for routing the pipe and making the fabrication and installation drawings using the material in the specs.
d) The purchasing department is responsible for buying what is in the spec. in the quantities that are on the Material Summary.
e) Construction is responsible for installing what is on the drawings with the material from the specs.

Now, who are you? If you are not the piping material engineer or the client then you need to just do your job.

 

[BigInch]

Probably the client.

Balls & gates are better for on/off service, butterflys for throttling, plugs for blowdown, check valves work best with one-way service, however, its customary to refer to your spec to see which one to use, and when and where to use it.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gerhardl]

At 40 psi you will have saturated steam at about 130 deg C.

I assume that the temperatures 500 - 800F ( corresponding to 260 - 427deg C) either are wrong, or given for hot air in connection with smokestack gases or some other special purposes. (Superheated steam at higer preussure classes could reach this temperature)

Such temperatures will require special valves and shut-off devices, and I will limit my answers to the given pressure class, based on steam at given pressure and temperature.

1. Buttweld is normally OK and widely used for steam for smaller diameter valves. In Europe normally limited to 2". Above 2" to 3" the commercial availability of valves with buttweld and threads are more limited, but exists. For this pressure class and size ball-valves with full bore and carbon reinforced teflon seat is in my opinion a better solution than gate or globe. Gate or globe could be cheaper.

2. For smaller sizes three-piece ball valves with swing out and changeable mid section could be used both for threaded and buttweld, and in any case you would with a good quality valve, any type suited, expect a long lifetime at least for sizes up to 2".

3. If you buttweld for larger sizes you will normally have very large costs when changing a valve, and you will for a refinery, even for steam- carrying purposes, probably have to requalify and test both welds and pipeline.

4. Both ball and globe-valves are widely used for larger sizes in addition to gate. In addition comes metal seated butterfly valves for on/off. (Ask for reson of limitation to gate).

5. Throtteling : Metal seated butterfly not to be recommended, if not sized, built and guaranteed for the exact purpose and regulating amount and pressure.

Special built regulating valves recommended for steam. For 'common sizes' are normally used globe-valves or other type of special designed hand-operated throtteling valves. For automated throtteling valves often pilot operated for pressure reduction, or motor or air operated.

Please remember that steam is a gas, often with high speed, in this case also mixed perhaps with condensate. In either case: if a valve is left with small or hair-crack opening you will see the result as metal thorn away with cavitation and/or abrasion.

Note: any steam application need to be laid out and equipped with steam traps etc. by a steam expert. Consequences are always an increased safety risk and severe loss of energy and shortened lifetime if done wrong.