Ball Valve Selection Criteria?

[pmover]

In reading a response in thread378-262877, a valid question was written regarding selection criteria for 2 different types of ball valves. For the benefit of others and to avoid "hijacking" that posting, the question is written in this posting.

What are the selection criteria between a full port "floating ball valve" and a full port "trunnion ball valve"? Please include a brief narrative.

Thanks and sorry zdas04.
-pmover

 

[rneill]

At some point in the design of a floating ball valve, the pressure acting on the surface area of the ball becomes too large to accomodate in the design of the seals, stem and operator.

The pressure results in a very high friction force on the downstream side of the valve that can not be reasonably accounted for in the design of the valve components and it would be difficult for the operator to operate the valve.

A trunnion mounted valve is pinned in position and does not move based on system pressure so the seat loads and force required to operate the valve is based purely on the design characteristics of the valve and not the differential pressure acting across the valve.

There are no hard and fast rules that I'm aware of with regards to the allowable operating load before trunnion mounted balves should be used but almost every operating company has their own guidelines with regards to this transition point.

You could look at something like the ball valve specification document published by PIP (

http://www.pip.org)

as perhaps being a reasonable guideline on this as it is the consolidated opinion of a large number of EPC and end user companies that make up PIP.

 

[zdas04]

Well, I don't have PIP's resources, but I do have a bit different spin on this topic.

A floating ball valve seats when the dP across it is able to push the ball into the downstream seat. This leaves the upstream side of the ball off the seat and the hole in the ball remains pressurized always. The dP required to seal the valve is pretty small, but it is not zero so you often find situations with a small amount of pressure (1-2 psig kind of numbers) upstream of a shut valve and the downstream side will not blow down. This can be a significant delay in a scheduled shutdown and requires finding a second valve to acheive positive energy isolation.

The trunnions (which, if you don't know are bearing surfaces forged into the body of the ball which sit in trunnion bearings, just like an old muzzle loading cannon) act to hold the ball against lateral movement. Both the upstream and downstream seats are spring loaded against the ball. These valves always come with a body bleed. Many companies (but not nearly all) will certify a single Trunnion Ball Valve as a suitable double-block-and-bleed device which can provide positive isolation without blowing down miles of pipe. They also work well in very low dP applications.

David

 

[BigInch]

Actuator torque requirements are reduced with trunnion equipped balls.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[rneill]

Unless the trunnion mounted ball valve can pass an inside out pressure test, I would not rely on it as providing double block and bleed isolation. As alluded to by zdas04, many trunnion ball valves have self relieving seats that are designed to release body cavity pressure and thus can not pass an inside out pressure test and would not meet a stringent definition of "Double Block and Bleed" as in the NORSOK standards or in the Alberta Occupational Health and Safety Code.

For true double block and bleed you would want a valve that did not have self relieving seats and these are much rarer.

Note: The NORSOK offshore standards specifically exclude trunnion mounted ball valves from use in double block and bleed services - even those that do not have self relieving seats.