Blowdown valving

[Andyallovertheplace]

Good Afternoon All,
Just a question, I'm trying to get a drawing right, and having moved companies, I'm having trouble laying hands on some of my resources.
My question is: What is the better arrangement for a manual blowdown station in a gas plant, assuming a globe valve and ball valve in series? Should the ball valve be upstream or downstream? It seems that having the ball valve downstream would be best if the globe valve undergoes hydrate formation, since you can close the ball valve, and the ice plug is now back under pressure, with a denser, warmer media around it to melt the plug.
Of course, the opposite has it's advantages, such as the fact that the ice plug is no longer under pressure, so it should dissipate relatively quickly, and the hydrates are forming downstream of the ball valve, so they won't interfere with the operation of the ball valve.

Thanks to anyone who can give a answer on this.

Andrew

 

[mbt22]

Andrew,

Probably worth looking at the Feyzin disaster.

Globe valve -> ball valve, if as you slowly open the globe valve you get hydrate formation, which may stop you closing your isolation ball valve. If/when the globe valve passes, you do not have any way of shutting off the supply.

Ball valve -> globe valve, the ball valve remains upstream of any phase transition where the fluid is throttled, so you have a much better chance of getting shut-off when you need to.

Regards,

Matt

 

[mfelzien]

Check ASME 31.3 Appendix M. If plugging is a concern consider use of a plug valve "if the fluid conditions meets the criteria" otherwise go with 31.3.

Also, ask a piping designer if you have no reason to change standards.

If your company doesn't have access to these, ask them why they don't have those available. Or get a copy of the standards.

-MF

 

[mfelzien]

My error

 

[Owlish48]

My experience has been to install a ball valve both upstream and downstream of the globe valve, even in manual blowdown situations. mbt22 is correct that hydrates could interfere with getting a good seal on a downstream ball valve, and an extra valve in a blowdown line is likely a 2" or smaller. Eventually, the globe valve trim will cut out and the upstream valve is a shut-down saver.

~OvO~

 

[SNORGY]

I guess I have a different take on this.

Presumably, the blowdown is required for an isolation or maintenance activity. In the event you are actually operating the blowdown, is it the normal operating intent to *complete* the blowdown? If so, then after depressurization, I would prefer to be able to isolate the blowdown valve (like any "control" or "quasi-control" valve) from the flare header, which would put the ball valve downstream of the globe. Judicious placement of methanol injection points might address the hydrate formation on the low pressure side, if the lines aren't very big (i.e., you might not want to launch a 12"-D x 36" long hydrate plug into or down the flare header). The last bit said, judicious placement of pressure gauges is also probably in order; maybe Owlish48's answer is the best option.

Regards,

SNORGY.

 

[Andyallovertheplace]

Thanks to all who have responded, first off.

I think that, given the shortage of information provided by the customer, that I'll take the route of having the ball valve upstream. That way they can shut the line off, regardless of hydrate formation. The inconvenience of having to wait for a plugged valve to thaw out at low pressure is better than the possible inability to shut off the valve at all.

I should have mentioned earlier that this is a bypass around an actuated blow down valve installed to provide the function on, say, loss of power, or an alarm condition. The manual operation should only be used as a back up when the BDV is isolated.

I didn't know that matters such as this were covered in B31.3. While I do have access to this, it's in another building, so this is considerably quicker.

Thanks again for all your help.

Andrew