8 replies to this topic

[shan]

Hi Everybody,

We are designing a gas by-pass line with a manual globe valve (for throttling flow) and a manual ball valve (for opening and closing). Which should valve be positioned upstream and why?

Regards,

Shan

[fallah]

Shan,

Ball valve should be located upstream with a minimum distance of 600 mm from globe valve.Otherwise low temperature situation would create due to j-t effect at downstream of globe valve may affect ball valve operation (open or close stuck).

Fallah

[shan]

Thank you, Fallah. However, on the same token, J-T effect at downstream of ball valve may have negative impact on the globe valve also.

[fallah]

shan,

Thanks for your attention, but because the ball valve in drain lines is normally considered as FB (full bore) type, there is no concern about the J-T effect in its downstream.

Fallah

[shan]

Hi Fallah,

It is intersting. Then, how about lqiud service? The valve order does not matter because there is no J-T effec. Is it right?

Shan

[kkala]

In addition to the previous useful opinions, I think there is another reason to place the globe valve downstream of the ball valve, but also upstream of another ball valve: Mentioned globe valve has the role of a control valve, even though it is manual; so it had better be located between two isolation valves for maintenance reasons. This is valid for either gas or liquid service. Ball valves are actually isolation valves, they could be gate valves instead, especially for liquid service (ball valves have better sealing).
In case that the globe valve needs repair or replacement, the two ball valves are closed, with the main line (not the bypass) still in operation. Then the bypass section between the two ball valves is drained (there must be a (say) 0.75” drain line on the bypass line) and the globe valve is replaced. Otherwise the gas line (not only bypass) has to stop operation in order to replace the bypass globe valve. This cannot be avoided when you replace one of the ball valves, but these on / off valves are expected to rarely need maintenance.

[fallah]

Shan,

Generally yes, but mentioned arrangement (ball before globe with 600 mm distance) is mandatory for liquified hydrocarbons (avoiding the globe to be freezed due to low metal temperature).Of course this arrangement may be applied for all liquids for uniformity and simplification.

Fallah

[shan]

Thank you again Fallah. I am convinced.

[jklasers]

And if maintain the temperature or prevent lowering down the temperature at upstream, then what would you suggest, which have to be at upstream and which one is downstream?

Edited by jklasers, 12 February 2013 - 02:18 AM.