7 replies to this topic

[Golent]

Hi everybody,

 

I am a process engineer working in an engineering company and designing utilities for a Refinery.

 

I have a concern regarding the type of block valves for LPG service: block valves up and downstream the control valve, vent to flare or connection to closed drain, double-block-bleed arrangement.

 

I am very confused because sometimes I see gate valves and sometimes I see ball valves.

 

For example, in a refinery I worked in, there were Ball valves on the overhead of a Isomerate Stabilizer, which is a LPG (C3-C4) service. We also had ball valve on LPG vaporizers, where the valves where all ball valves.

 

On another project I worked on, there were gate valves everywhere, except when the line downstream is connected to atmosphere, flare or closed drain = when depressurizing could occur as I understand.

 

Is there any rule for the use of this type of valve?

 

If my post is not clear, forgive me! This is my first post and I was maybe not clear enough for a good understanding of my concern.

 

Best Regards to all of you

[PingPong]

The Piping Specification to be used for a particular pipe dictates which valves to be used in that pipe.

[rohollah]

Golent

 

Both gate and ball valve can be used for isolating.

since ball valve is quick action that is why it is recommended for isolating flammable or corrosive fluids.

Regards

Rohollah

[Art Montemayor]

When used appropriately, ball valves are far superior than gate valves - especially in LPG service.

 

If you don't have project or company piping specifications in place, then I suggest you use a ball valve.  Ball valves are considerably more leak proof than gate valves because they can be fitted with "soft" seats.  Gate valves depend solely on metal-to-metal seats that wear, corrode, and accumulate dirt and debris in the seat.  The quick, and smooth, 90 degree turn of the ball valve - as rohollah points out - makes it ideal for automatic blockage using either air cylinders or electric/hydraulic drives.

[fallah]

I have a concern regarding the type of block valves for LPG service: block valves up and downstream the control valve, vent to flare or connection to closed drain, double-block-bleed arrangement.

 

I am very confused because sometimes I see gate valves and sometimes I see ball valves.

 

Golent,

 

For LPG service and for applications you mentioned, the block valves should certainly be ball type, but:

 

It is worth to mention that for such applications there would be a sealed cavity between the ball and the valve body in closed position contained trapped LPG. The pressure rise in these cavities due to, let say, thermal expansion might overstress some valve components; then the excess fluid pressure should be relieved, normally via a hole through the ball.

[Art Montemayor]

What Fallah mentions and recommends is excellent advice.

 

For those not experienced or familiar with ball valve types and construction, the cavity introduced inherently can be an obstacle, a problem, and at worse a hazard.  If you cannot tolerate the fluid inventory captured within the ball's inherent cavity, then specify the type of hole (or orifice) described by Fallah to the valve fabricator.  This type of design now exists in the supply of most recognized and established ball valve manufacturers - but you must specify the feature or otherwise, you will not obtain it.

 

I always specify the hole oriented upstream in most cases.

[Golent]

Thank you gentlemen for your replies.

 

In our piping specification there is no ball valves, only gate valve. Maybe this will change, the piping specification may be updated later.

 

 

 

While reading your replies, I was surprised that you advise a ball valve with a hole... I understand the reason for avoiding trapped LPG. You say that the ball valve is more tight than a gate valve (due to the soft seat), however if there is a hole, the valve is not tight. I guess I am missing a point here. Can you explain me?

 

When there is a possibility to trap LPG, don't we put a safety valve?

 

Best regards,

[fallah]

_Golent,

 

In fact the hole will conduct the trapped liquid in the cavity toward valve upstream or downstream and has nothing to do with valve tightness.

 

Indeed, because the volume of the  trapped LPG is so small and routed back/forward to upstream/downstream no need to be realeased by a safety valve...