API 6D Gate Valve Selection - Slab vs Expanding

[Gorman3]

In a situation where the valve size, class and trim are the same why would I choose an expanding over a slab. Not talking about metal seated. when would you use a 600# expanding gate over a 600# slab gate, let's say both were PEEK seated for high temp.

I have assumed the expanding gate's mechanical seal would be well suited for gas applications when there line pressure fluctuates. Is that right?

Any other info appreciated.

 

[ricklts]

Gorman3

Expanding gates will give you, at least are supposed to, 1. an upstream seal against line pressure, and 2. reduced operating torques at full pressure differential when opening.

Rick

 

[LittleInch]

You would choose an expanding gate valve when you need true double isolation and possible bleed as well in a single valve. The sealing o the upstream seta is by means of the mechanical action closing the valve, not the fluid pressure. They also claim to be more resistant to particle wear as the seats pull away before sliding up inside the valve. As ricklts says, because the slab often disengages and is then held off the seats, the opening torque can be reduced.

As the seating force is not affected by the fluid pressure then any application where line pressure is not high would be a good use.

Places where you have isolation for entry - things like pig traps - and you would like really good isolation over a long period of time / usage like station isolation valves are where you might go the extra mile in terms of cost and size to install an expanding gate.

As with DPE seated ball valves you need to allow for liquid expansion in the cavity if exposed to solar effect or other heat input for an expanding gate valve.



My motto: Learn something new every day

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

 

[Gorman3]

Than both of you. This is just the kind if answer I was looking for.