Spec break again
Spec break again
(OP)
A minimum flow from a high pressure pump back to a low pressure vessel is controlled with a minimum flow valve. This pressure loss over the valve is 90 bar. Upstream the valve we have #1500 piping class and downstream it will be #150. We have a spec break over the valve.
Now, we need double block and bleed valves downstream the flow valve, to isolate the pumps in case of maintenance. The equipment is placed on a skid and the skid will be shut down when maintenace is needed. The isolation valves are there to block out the downstream low pressure system.
Do we need to place the spec break downstream these isolation valves (which are locked open when the skid is in operation) or is it ok to have the spec break between the flow valve and the isolation valves?
Now, we need double block and bleed valves downstream the flow valve, to isolate the pumps in case of maintenance. The equipment is placed on a skid and the skid will be shut down when maintenace is needed. The isolation valves are there to block out the downstream low pressure system.
Do we need to place the spec break downstream these isolation valves (which are locked open when the skid is in operation) or is it ok to have the spec break between the flow valve and the isolation valves?





RE: Spec break again
RE: Spec break again
RE: Spec break again
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RE: Spec break again
RE: Spec break again
I should have added,
I really don't like locks between overpressure protection devices and ANYTHING they are supposed to be protecting.
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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Spec break again
My initial reaction was to purchase 1500# block valves, and from the response from you guys, there seem to be no way out of that.
Btw, do you have any referance to any API, ASME or other code for where and when spec breaks may be set.
RE: Spec break again
As far as locating spec breaks, you can put them anywhere you like. The trick is, as above, only insuring the lower pressure side is protected from the high side.
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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Spec break again
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdoweb/
RE: Spec break again
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Spec break again
RE: Spec break again
I didn't want to get into all of that, but as long as we're at it, why not?
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"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein
RE: Spec break again
BigInch: You say: "If I was doing this, I would place a PSV set at 275 psig right after the min FCV. That would place the PSV actually on the high pressure piping between the FCV and the DDB. This arrangement ensures that low pressure piping is always protected regardless of what happens downstream of the spec break and no need to depend on the lock for the DDB."
That would have made it possible to place the spec break at the FCV and keep all downstream piping and DBB 150#, because I would size the PSV to take the entire flow if one of the block valves were closed or if the FCV should fail. I would also need a PSV on the tank to cater for a fire case, which is the dimensioning case if the recycle is taken care of up front. No, I think it'll be better to keep the entire recycle system (FCV, DBB and piping) 1500# and keep the PSV on the tank.
The FCV is three stage valve, quite sophisticated, with a plug especially designed to take the high pressure drop. The medium is LNG, i.e a mix of light hydrocarbons which is more gentle than e.g. water wrt cavitation, since it has no sharp boiling point definition. The pipe and valve is 6" with valve inlet velocity below 5 m/s. The velocity increases internally in the valve, as it will be a two phase flow at some places in the valve.
I hope we have considered the cases correctly.....
RE: Spec break again
http://virtualpipeline.spaces.msn.com
"We can't solve problems by using the same kind of thinking we used when we created them." -Albert Einstein