1" Bypass around Large Bore Valves 1

[DGrayPPD]

We have several P&ID's for a project that call for 1" bypass piping around large bore valves (6", 12", 14" NPS). These are not control valves, they are just regular hand and gear operated block valves.

I'm just curious what purpose or situations call for a 1" bypass around large bore valves?

If the question is too vague or more information is needed, just let me know.

Thanks

 

[SwinnyGG]

There are many reasons you might want a bypass around a valve- safe starting or shutdown of equipment, repair of the main valve, etc etc.

 

[Gator]

Pressure equalization.

 

[LittleInch]

It's interesting that you get some projects where you can't move for bypasses and others where you get none.

Very dependant on how often you need to shut and equalise pressure and the sensitivity of your valves to opening under pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[DGrayPPD]

Can someone explain the pressure equalization for me? So I can better understand it.

 

[SwinnyGG]

Certain types of valves can be very difficult to open if there is a pressure differential across the valve mechanism.

Think about a sliding gate valve. If the valve is closed and pressure on both sides of the gate are equal, the valve is easy to open because the equalized pressure allows the gate to be centered within its housing and seal assembly. The pressure on both sides could be 1 psi or 10,000 psi- the valve does not really care, as long as the pressure is in balance.

Take that same sliding gate valve, and apply 10,000 psi to one side and 1 psi to the other- you have 9,999 psi of pressure driving the gate valve against the low pressure side of the housing, creating a potentially HUGE friction force holding the valve in place, making it very difficult to open.

In this case, you would open the ball valve in the bypass and allow pressure to equalize on either side of the gate, making it possible to open the gate valve with much less force.

 

[DGrayPPD]

Thank you jgKRI for your very detailed and easy to understand answer. It makes much more sense to me now. Thank you to everyone else as well for pointing out the pressure equalization for me.

 

[racookpe1978]

Warmup, venting after or during filling, purging, cleanout or flushes, and recirculation of the process fluid is much simplified with the smaller valves.

 

[MJCronin]

http://www.eng-tips.com/viewthread.cfm?qid=96667

MJCronin
Sr. Process Engineer

 

[georgeverghese]

While a bypass across large bore manual valves may be a good idea, also note than these bypasses should preferably be located on a side tap (and not a bottom tap) off the main pipe to avoid localised low point corrosion. In one large OpCo, these sundry small bore bypasses are required to be a minimum 2inch nominal bore, and constructed out of some CRA material. Reasons for this are obvious.