TurnDownPlant Turndown OperationDears,At this topic I would like to discuss about plant turndown operation case such that everybody share their experiences.Turndown operation as one of the design cases of every plant can potentially involve many challenges. This mode of operation can cause interruption at normal operation, damage to equipment, process shutdown and increased process downtime.For turndown case as you know all the times designers are faced with challenges to prevent/minimize the plant suffering from turndown, to keep plant productivity & availability and to prevent plant shutdown.Some queries in this regards are as follows:1. How to decide and plan about a reasonable value as turndown value of a plant? How much turndown have you experienced at your projects? I have seen values like 20%, 30% and 50%.2. In reality how much turndown you could achieve at your plants?3. Based on your experience with different equipment and packaged units such as instrumentation (control valve, flow meter, etc) , pump, compressor, column, desalter, gas sweetening package, TEG gas dehydration and etc, how much turndown is achievable?4. Which control & shutdown options do you propose for smooth operation of the plant in case of turndown?5. Which design options do you propose to cope with turndown operation difficulties?Some options in this regard:5.1. Compressor Equipment: Recycle operation via anti surge/capacity control system, consideration of low suction pressure over-ride5.2. Pump Equipment: Recycle operation via minimum flow line5. 3. Column Equipment: Some vendors offer special internals that can help in case of turndown5.4. Implementing training concept for example considering 2x50% trains for packaged equipment.5.5. Consideration of surge tanks can be regarded as short term solutionLet me know your idea and expeiences.All The Best,Mojtaba19 days ago

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Follow S MS M Kumar Mojtaba: I am afraid to touch this on account of follow-on queries that may come. I have never seen turn-down an issue to warrant such a detailed analysis. Generally we have 2 trains shutting down one and affordable turndown of individual pieces of equipment in the still running single train usually gets what one wants.1. Equipment are in series. While one may allow 20% turndown, one next to it may demand minimum 60%. Hence it is difficult to give a generic response.2. See first para and item 13. Control valve: 20-30% You can go for reduced Cv seats if so required. Flow meter: Depends on type. At best you may lose a bit on accuracy within calibration error should not matter if it is NOT custody transfer. Pump: Does not matter with minimum flow; large head and large MW pumps may heat up liquid recycle via min flow; again shutdown a parallel pump. Compressor: Does not matter with recycle + speed control. Column: Depends on internal and how close the design case itself to weeping. Usually column and fired heater may impact entire trains turndown. Desalter: A vessel as long as its PCV/FCV/LCV can open and close it is OK; You can go for reduced Cv seats if so required. Gas sweetening package: Its column may limit the show. TEG gas dehydration: Its column may limit the show.Fired Heater: Gas burner may allow 20%. Liquid burner 40~50%. Here again, you can cut out a few burners in a multi-burner unit as long as heating symmetry is possible. Two phase flow regime viz annular, slug kind of things, inside coil 60%. Slurry or sand borne line: Min velocity required to avoid sedimentation and line plugging.4. Depends. Shutdown of a spare train. Will help rotating machineries.5. See 3.Final: Not a big deal as long as you consider each piece of equipment in series and go by the governing one. 1 day ago Like

Follow SaeidSaeid Rahimi Mofrad, P.E. The lower turn down you specify, the higher cost you need to bear. In reality, in oil and gas production plants, the plant can be designed for any turn down ratio. However, there are few points to consider:The lower turn down ratio demands for specific flow meters to satisfy the required accuracy for such a wide range of operation.You may need two control valves in parallel (one small and one large) to handle turndown case.Equipment are all explained by Kumar. Columns have normally 50% turndown but they can be operated at very low capacity if you over-reflux the column or circulate part of the product back to the inlet and run the reboiler above what is needed according to turndown case simulation. It is just waste of energy but can make the system work.Reactors are the only equipment that I have not seen any flexibility in terms of turndown when I discussed with licensors or technology providers.In short, if you have proper justification for turn down, the plant can be designed for it but everything has its own cost.I have designed a condensate stabilizer unit for 6% turn down because there were four condensate producing trains upstream, each one designed for 25% turndown. So when one train was working on turndown the condensate stabilization unit was on 6% (25% divided by 4).Keep in mind that most of the times, the operating headaches and cost of running at such a low turndown is so significant that operating companies do not ask for less than 15-20% turndown ratio. They are ready to shut down the plant instead of keeping it running with extra trouble and operting cost.

pv2PSV inlet line sizingDears,For PSV inlet sizing normally 2 items shall be checked;1) Pressure drop between equipment and the PSV < 3% of PSV set pressure, this item shall be checked in order to avoid chattering phenomena.2) V.Would you please let me know the reason of checking 2nd item (V2)?ThanksMohammad reza24-Jan-12

LikeCommentFollowFlagMore14 commentsFollow SampathSampath Kumar R Dear Mohammad,Rho V2 criteria is to be followed for entire flare system. PSV inlet line is part of flare system, hence this has to be met apart from 50 bar g."Generally, RhoV2 criteria serves as a measure for typical line sizing to ensure that there is no excessively high velocities in the piping, which can lead to excessive pressure drop or vibrations. January 31, 2012 Like 1Follow JimmyJimmy Lee So far, i understand only Technip and Total is using V2 and 3% as PSV Inlet Line criteria. January 31, 2012 Like Follow MohammadrezaMohammadreza Ebrahimi Based on most standards and practices Checking of 3% set pressure in PSV inlet line is mandatory. I saw only in some gas plants and upstream project (regarding to process design criteria) checking of RohV2 is necessary.What will happen if we dont care about RohV2 limitation? January 31, 2012 Like Follow MojtabaMojtaba Habibi Based on section 7.3.1.2 of API 521 standard:"In addition to flow considerations, the vessel nozzle and other inlet piping should be designed to withstand thermal loadings, reaction forces resulting from valve operation, vibration, dead weight and externally applied loadings.The strength of the inlet piping is less than that of the valve because the inlet piping has a smaller section modulus. Any moments created by loads applied to the outlet flange and by the reactive force of the discharging fluid transmit bending stresses and rotational forces to the inlet piping."So I think not only valve chatter but also mechanical strength of PSV inlet line are design concerns which have standard base. January 31, 2012 Like 1Follow SampathSampath Kumar R Yes, I agree with Mr. Mojtaba. However, these activities (i.e.) thermal loadings / reaction forces resulting from PSV opening shall be carried out by Piping stress team. As a Process engineer, we need to carry out 3% pressure drop criteria. Also, RhoV2 to be checked as PSV inlet line being part of flare system. It could be client's requirement too. I have checked both in my past projects.Kind RegardsSampath Kumar R February 1, 2012 Like Follow DhirajDhiraj Joshi Hi ,I understand that this discussion has happened quite sometime ago. However, in addition to the rhov2 criteria, there is also rhov3 criteria that i have observed in some of the client line sizing philosophies. As I understand, the physical significance of rhov2 criteria is the limiting criteria for pressure (drop). However I am unable to understand the same for rhov3 criteria.Appreciate the forums' thought on the rhov3 criteria.RegardsDhiraj 5 months ago Like Follow HoomanHooman Tabaraei I think V criterion, in addition to 3% pressure drop criterion, serves as a measure to maintain the pressure drop across the nozzle of protected equipment below the allowable figure to ensure that there is no chattering in PSV. 5 months ago Like Follow AmarAmar Naik How is the 3% pressure drop criteria applied to the PSV inlet line, especially when the PSV is installed on a piping portion and not on an equipment? The case I want to know of is the case where PSV is installed on the discharge of a reciprocating compressor. Should the PSV inlet line for which the 3% criteria needs to be applied be considered for the non-flowing portion of the inlet pipe (after the TEE point) or for the entire piping upto the pressure source (in this case Compressor) needs to be considered? Please advise. 12 days ago Like Follow JeremyJeremy Goldbloom If I remember correctly, if you are using a pilot operated PSV, the sensor can be upstream to avoid chattering and the 3 % limit on pressure is then not so important. 12 days ago Like Follow ASHOKASHOK KUMAR Dear allI have a query.The velocity in rho v2 is calculated based on inlet line size or the psv inlet nozzle size.Mostly the psv inlet nozzle size will be smaller than the psv inlet line.ThanksRegardsAshok 11 days ago Like Follow PaulPaul Frey I have to agree with not calculating the pv2 value for the RVinlet line, the reason will become apparent when you consider that pv2 calculated by Process Engineers is a 'screening tool' for piping to analyse the downstream piping for Flow Induced Vibration, which is a low frequency oscillation of relief piping which can cause localised stresses at branch connections with the flare header. In severe cases, the relief valve tailpipe can rupture. To answer Ashoks question, pv2 is calculated at each change of diameter, starting with the Relief valve outlet flange.

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