re: How does an equalization line work 1

[TurbineBlade]

Hi,

While I was reading a flowsheet, I came across with a gate line with an equalization line? While the gate valve is closed, how does it equalize the upstream and the downstream pressure without bypassing any fluid?

Thanks

 

[JLSeagull]

Many large valves have a small bypass valve to pressureize. You open the valve and bypass a small flow until the pressures are equalized. With a low pressure differential you are permitted to open the main valve.

 

[dcasto]

Some larger valves will not open or will have seal damaged if there is a large differntial pressure across them. I also put a bypas to repressure up catalyst beds, filters, and othe equipment to keep from damaging the equipment.

 

[749KDV]

The small bypass equalization line is a product of an industry problem with gate valves for thermal binding and presure locking. This is especially true in the Nuclear industry where gate valves were used for sump recirculation isolation valves. These valves would be under large differential temperature when required to open. The pressure locking phenomenon occurs when some fluid seeps into the bonnet, filling it solid. With the increase in temperature, pressure in the bonnet increases substantially causing the valve to "lock" closed. The equalization line prevents the pressure buildup in the bonnet. There is no connection made between the upstream side and the downstream side, just between the fluid end and the bonnet.

 

[priya17385]

And what about hole in the Disc? Even it is used fr pressure relieving right?
I also came across one more point that mostly Globe valve is a preferred Bypass valve.But I could not find any other reasons except that it can be used for throttling purpose.

 

[749KDV]

The hole in the disk is used in "parallel disk" type gate valves to equalize pressure from between the disk(s). This is to prevent the fluid that becomes trapped in between the disks from causing an outward force when heated thereby preventing the disks from binding against the seating surfaces. So, yes, the drilled disk also helps prevent thermal binding of gate valves.

If the valve application that you need is for throttling of the system fluid (e.g., a bypass line around a larger isolation valve, then a Globe valve is a better choice. Gate valves are chosen strictly for "isolation" valves because when fully open they provide very little system resistance. Globe valves will give more pressure drop through the system, however that is a property from their design because they are good throttle valves.