Q & A > Question Details
In certain gas processing installations, we find that the Pressure Safety Valves (PSVs) on demethaniser, deethaniser and ethylene towers vent directly to the atmosphere. Is this acceptable practice or should PSVs always be connected to flare systems? What is best practice for routing of safety valve discharges of such columns handling lighter hydrocarbons?
26/05/2010 A: Shailesh Paranjpe, Toyo Engineering India Ltd, shailesh_paranjpe@rediffmail.com
The reasons mentioned like metallurgy constraints due to low temperature as a result of auto-refrigeration during relief is valid.
Another, more important reason why typically, demethanizer overheads relief to atmosphere might be considered safe is that methane gas disperses very fast in atmosphere. Therefore, the demethanizer top upon relief (predominantly CH4) would immediately flash to vapors and disperse and dilute so quickly that any untowardly incident of hazard may not be obvious. Further, the demethanizer upset case one of the largest relieving case in Naphtha Cracker units and could be a governing case for flare sizing, if connected to flare making flare header large size and of expensive MOC.
In any case, economic considerations is the only reason of not connecting the demethanizer overheads to flare, in some cases. But it involves some compromise with safety.
14/05/2010 A: Prabhat Choudhary , IOCL, prabhatchoudhary26@gmail.com
As per the normal design all hydrocarbon service PSVs is vented to flare burning but as far as demethaniser column is concerned which operates normally at -110degC and 15kg pressure, PSV vent goes to atmosphere because such high volume of flow nearly 7-8 MMSCMD (9MMSCMD gas processing plant) and at such cryogenic temp can not be handled by flare due to metallurgy consideration of piping.Such vent is provided with u/s RD and steam dilution near vent.This PSV popping can lead to cloudy atmosphere in large area.
29/07/2008 A: Rajesh Wadhwa, National Fertilizers Limited, rkwadhwa@nfl.co.in
It is the general Industrial practice to connect the vent from any process equipment to a flare header. But this is also seen in some cases that sometimes vents have been open to atmosphere due to complexity or the temperature problems in connecting those vents to a flare header. One must use separate flare headers in case streams are of different nature, but that also adds to the cost.
06/12/2007 A: Roger Haglund, Eprom, r_haglund44@hotmail.com
In general all PSV should be connected to a flare system. If not there is a major risk for site wide explosions and fires.The Mexico disaster shows this.
One problem with lighter hydrocarbons routed flare system is the resulting lower temperatures that can/will freeze up the flare. This can be taken into consideration in design but complicates and makes the solution more costly. Flare header segregation can also be a solution for the same problem. One can frequently see PSV routed to atmosphere, often on older sites and even for LPG storage. This is however not an ideal solution and should not be allowed, particularly if the distance between the process site and storage isn't enough.
06/09/2007 A: Graham Bennett, DNV , graham.bennett@dnv.com
In principle wherever it is possible, flammable vents should be routed to a safe location such as a flare. We saw the tragic results in Texas City of flammable material being routed to lower level atmospheric vent stacks. It is difficult to guarantee that even lighter hydrocarbon gases or vapours will not contain some degree of liquid droplet which could subsequently "rain out" of an atmospheric vent, particularly in a process upset, or loss of control situation. Atmospheric conditions and building effects can also have the undesired effect of making even a buoyant gas act like a neutral or even dense gas in some circumstances. There are several modelling tools that can help you simulate the impact of such atmospheric vents. I would suggest you consider some further analysis. ......
04/09/2007 A: SUNDEEP GUPTA, BECHTEL, sundeepgupta1@rediffmail.com
Generally all pressurized equipments should be connected to flare, if one has a flare system.
Low pressure storage tanks, like cryogenic tanks are vented to atmosphere.
There is no reason why ethylene tower, demethanizers should not be hooked up to the flare system.......