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RE: Delta Valve Steam Purge

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Nick George

You seem to have two different issues here:

(i) Typically, we maintain the delta valve bonnet steam pressure at a differential of 25 psid above the bottom feed line pressure. We do this by measuring the feedline pressure and then adding 25psig to it using a DCS controller/algorithm block and then sending the resulting output signal to the control valve on the steam line to the DV bonnet.

(ii) For the steam purge to the pressure transmitters on the top & bottom outlet and feed lines we feel that the best design is using a Kates automatic flow rate controller to maintain a flow rate such that the flow velocity at the tapping point is always 6 ft/sec or more as this is a solids application. For a 3/4″ tapping point, this would result in a typical steam flow of only 6 lbs/hr into a feed line at 40psig, and 10 lbs/hr into a top line at 80 psig. Almost always, if you use fixed restriction orifices, the steam flow is way in excess of what you need for meeting the traditionally accepted solids purge velocity, and cannot be controlled as downstream pressure varies unless you are always in choked flow conditions. Hammering is almost inevitable due to the high steam flow through an RO – mainly because you cannot size one less than say an 1/8″ and not worry about blockage. With the adjustable controllers you can leave the steam purge on all the time and the issue of turning them on & off doesn’t arise. A caveat here – we are looking for long term reliability data on this type of purge design/equipment.    

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