4 replies to this topic

[abhijit-tce]

Hi,
Am doing flarenet analysis for a hydrotreater unit. Regarding tail pipe sizing of psv I need some clarification. Is it required to do tail pipe sizing of psv only for controlling case or is it to be verified for rated flow of all applicable scenario. If is to be done for all applicable scenario then problem which I am facing is that for noncontrolling cases rated flowrate is much higher than rated flow of governing cases. Because reqiuired orifice area of noncontrolling scenario is much less than
required orifice area of governing scenario. Selected orifice size is highly oversized for non controlling scenarios. So for this higher rated flow back pressure in tail pipe will be much higher. So in flarenet analysis, I am doing flare header sizing for global scenario i.e. ppf case and at that time I have disabled the option "rated flow for tail pipe sizing" as ppf is not governing for some psvs and rate d flow rate is much higher for those. Correct me if I am wrong.

[fallah]

Hi,

 

In PSV sizing the governing case is always the case with largest required relief area. The largest required relief load as per calculations will not necessarily lead to the largest required relief area, because the relieving fluid and relieving conditions might not be similar for all relieving scenarios such that it might the relieving fluid to be liquid, vapor or even two phase flow. On the other hand, if a relieving scenario will result in largest required relief area among all credible scenarios it will normally lead to largest size of the PSV tail pipe...Then to size PSV tail pipe you should use the rated flow corresponding to largest required relief area...

[abhijit-tce]

Thanks for your reply. But rated flowrate of non controlling scenario for my case is higher than rated flow of governing case. Because psv is highly oversized for this case.

[fallah]

Hi,

 

Appears you did use scale up method to obtain the PSV rated capacity for each scenario. Obviously, for a nongoverning scenario with very low relief load the calculated rated capacity based on such method might lead to a value much higher than the one for governing scenario. But, what you need to check the friction loss if 3% rule is met in PSV inlet line; is the certified or rated capacity of the valve which already specified and limited by the manufacturer and you are not allowed to use that valve for higher capacity. Then for a nongoverning scenario the valve might release the valve's rated flow which is much higher than required relief load for moments, but after a very short time the valve would reseat and repeating this cycle will lead to valve chattering. In such case it is better to use a dedicated small PSV in addition to the main larger one...

[abhijit-tce]

Thanks.