Opening A Valve with Pressure on one side 1

[nSey]

Pretty novice question. Which of these type of valves have limitation in opening against full pressure on one side (due to potential for seal or other damage):
- Ball Valves
- Gate Valves
- Butterfly Valves
- Globe Valves

I was told that you had to have a bypass line to balance the pressure for some of these types at school, but cannot remember now!

 

[MJCronin]

nSe621,

It is standard practice to equalize pressure on both sides of closed steam valves with a small valved bypass.

There are a few reasons for this:

- Make the larger steam isolation valve easier to turn.
- Slowly pre-heat the downstream piping
- Enable the downstream piping traps to actuate with the gradual start-up condensate load.

http://books.google.com/books?id=o2...onepage&q=valve bypass steam -turbine&f=false

Also, the bypass is mandatory in Control valve installations....

IMHO, for pumped liquid systems, the smaller bypass is not necessary, but may be a desired luxury by the operations staff.

Once valves get large enough, they typically have operators to open. If a large isolation valve in liquid or vapor service has repeated seal damage, I would consider the bypass.

MJCronin
Sr. Process Engineer
Venture Engineering & Construction

http://www.VentureEngr.com

 

[Kreot]

The main issue is the delta p, what is the max the valve will be open at? Not all valve manufacturers has a good seat design.
For a valve with sound softseat design, you should avoid opening the valve with more than 60barg DP. For higher DP you should use metalseat, or for high pressure a equilising line. I have seen many broken seats because of too high DP.

On a general basis i would rate the valves that withstand dp best in the following order:
- Chokevalve
- Control/Globe
- Butterfly
- Rising stem ball valve
- Wedge gate¨
- Slabgate/Double expanding gate
- Ball valve metal seat
- Ball valve soft seat

 

[LittleInch]

Stiafs reply is pretty good. You more or less have them in order of vulnerability. Gas / steam is worse than liquids as the velocities are higher, temperature can be low and the time for the downstream section to pack up is often longer.

Having said that ball valves are opened against DP all the time all over the world and survive. Seat design is important and use of soft seats or soft inserts in metal seats suffer more than most but if you start adding bypasses around all the valves your costs will increase a lot so you need to pick and choose which ones you go for. Globe or plug valves are often used as the throttling valves in the bypasses so are not normally bypassed themselves.

In response to MJCs post, I normally remove any isolation type bypasses I find around control valves. If the valve is there to control something, you don't want some lunatic opening the bypass, thus removing the control function. Therefore I don't know where the "mandatory" comes from.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way