Drums are cylindrical vessels that are adopted in the process units for multiple functions. These consist of intermediate liquid collection, product storage, and separation of two contrasting products of different densities, surging, reflux accumulation, de-aeration etc. For the above-mentioned functions, internals required are curbed to baffles, demister pads, vortex breakers, distributor piping and draw off piping.
Types of drums
Drums are segregated into two categories, hinging on the orientation of the cylindrical axis i.e. horizontal and vertical drums. For most cases, inclined drums are considered as horizontal drums.
Locating the Horizontal drums:
The piping designer should cut down piping interconnections between the drum and its adjacent equipment when situating the drum. The accompanying documents are required to locate the drum on the plot plan.
· Process Vessel Sketch including General arrangement drawing, fabrication drawing etc.
· Plot plan
· Piping & Plant Layout Specification.
· The drum is located on the plot plan as per the process sequence dictated by the P&ID.
· Drums can be installed on standalone structures or grouped together with related equipment in a confined structure. Sufficient space must be administered around the drum for operator movement and upkeep access.
· The drum elevation is fixed by the P&ID. The same may be stepped up to facilitate piping and equipment layout in consultation with the Process group.
· Keeping the axis of the drum perpendicular to the pipe rack reduces the space obliged for locating the drum. Locate close to an connection road to reduce maintenance efforts.
· Underground (U/G) drums should be preferably discovered in open and comfortably accessible areas. Nothing should be spotted above the U/G drum as it may be required to be taken out and replaced.
· Underground drums are used to gather gravity draining liquids like slops. The elevation of the drum should be monitored properly as it is presumed to be the lowest point in the collection system. Slope of the underground line, starting point elevation, and the estimated length of piping should be known before the vessel elevation is cleared. The elevation can be dropped below the one specified in the P&ID if the end elevation works out to be below the one given in the P&ID.
• U/G drums commonly have vertical pumps and motors escalated on top. Both should be accessible from HPP for plant operation and routine maintenance.
• Drums with mixers and agitators should have a minimum expulsion space above them equal to length of mixer or agitator inside the drum plus one meter.
After the drum has been situated on the plot plan, the following jobs are carried out.
· Vessel Elevation review
· Nozzle orientation – Location and orientation of nozzles on vessel
· Vessel support location planning
· Platform and access requirement
· Support cleat location detailing
· Lifting lugs and earthing lugs location planning
· Finalizing Vessel Name Plate location
Vessel Elevation review:
Drum elevation set by the P&ID is the minimum requested elevation from NPSH (Net Positive Suction Head) point of view. This can be stepped up to suit supporting arrangements.
The following documents are vital for orienting the nozzles. Nozzle orientation drawings are used to find the nozzle
· Process vessel sketch
· Level co-ordination diagram
· Plant layout specification
· Nozzle summary
· Insulation requirements
· Plot plan
General considerations for locating nozzles:
Generally, the following nozzles are present on all drums.
· Product Inlet
· Product Outlet
· Vapour Outlet
· Instrument Nozzles
· Steam Out Nozzle
· Access Manway
Orienting the nozzles:
While orienting these nozzles the following points are to be examined.
· Product Inlet and Vapour Outlet will be on the top of the drum, as far from each other as possible for proper dis-engagement of liquid and vapour.
· Product Outlet will be on the bottom and as far as possible form the Product Inlet.
· Level Instrument nozzles should be as far from the inlet as possible to avoid turbulence at the inlet end.
· Pressure tapping for vapour pressure to be on the top of the drum, near the Vapour outlet.
· Temperature tapping for liquid temperature to be in the lower liquid region. It is to be insured that sufficient space is usable for removal of the temperature element. When multiple temperature elements are involved, they are best placed at the same elevation along the axis of the drum.
· Steam out connection to be opposite to the access manway and vent.
· Drain to be located at the lowest point, on the bottom of the drum.
· Inaccessible Instrument nozzles to be located near ladders (location of ladder and Instrument nozzles to be decided concurrently)
· Access man way can be located at the following places, depending on the type of access required :-
v On the top of the vessel.
v On the cylindrical portion of the drum (radially or hill side) or at either head for side entry.
v On the bottom of the drum.
· It should be verified that the davit swing area of the manhole cover does not obstruct the movement of maintenance personnel and does not hit any instruments or instrument nozzle connections. The center line of the manhole should be between 600mm to 1000mm (ideally 760mm) from the top of service elevation of the vessel.
· Special concern is to be taken for bottom nozzles. They should be situated so that they not only clear the saddle support (R/F pads should clear), they should also clear the concrete or structural support on which the saddles supports rest. A minimum of 150mm should be accessible between the edge of the flange and the edge of the civil support for easy maintenance.
·Goose neck nozzles should be regarded when piping layout is fixed and requires an elbow immediately at the nozzle.
·In addition to the regular nozzles, vessel boot may be administered for liquid accumulation. This is given at the bottom of the vessel. The boot should be located as per guidelines of outlet nozzles. The boot has a draw off nozzle at the bottom and a minimum of an instrument nozzle for level measurement. This has to be aligned in conjunction with the upper level nozzle (on the drum).
Nozzles on the top of the drum should have their flange a speck of 180mm and a maximum of 1000mm from the TOG of the access platform. Hill side nozzle standouts should be anticipated so that there is no clash (hard or soft) between the insulation of the drum and standpipe. The flange and bolts should also be outside the vessel insulation. Standouts of Nozzles on the heads and on the bottom should be calculated so that the flange and bolts are outside the vessel insulation so that the flange joint is smoothly reachable for maintenance.
Supporting the Drum:
Horizontal drums are strengthened on saddles welded to them. These are to be evenly spaced from the center of the drum. Specific prerequisites on spacing can be passed on to the Mechanical group. For very long drums, supplementary saddles may be instructed to prevent sagging of the drum at the center. The saddle towards the pipe rack is commonly made the fixed end and the other support is made the sliding end. This is done in conjunction with piping growth calculation. Slot hole dimensions when contributed by vendor drawing should be cross-checked with manual calculation to diversify the actual size of slot instructed. An additional 5 mm should be added to take care of minor civil dimensional variations.
Preparing the Nozzle Orientation Document:
This document should show the plan, elevation and if needed, the side view of the vessel, requested location of saddles and the location of nozzles. Nozzle designing should be from the centerline and one tan line. Nozzle projections should be from the vessel center line. A nozzle summary table pointing out the Nozzle number, service, size, rating, flange type, flange face, standout and remarks is to be incorporated in the drum.
Miscellaneous Data to be included in Nozzle Orientation Details:
Commonly, drums can be raised with two lugs welded on the top of the vessel. The favored locations should be signified on the nozzle orientation drawing
Earthing Lugs/ Earthing Boss
Two earthing lugs, mostly one on each saddle support should be indicated on the nozzle orientation drawing.
Name plate should be placed at a prominent location and indicated on the nozzle orientation drawing. Care should be given that the name plate projects outside the vessel insulation.
Vessel Insulation Clips
Stipulate the insulation clips/rods requested for holding the vessel insulating bands.
Platforms and Access Ladders
Platforms are compelled for operational access to valves, instruments, and for upkeep. Horizontal drums are to be provided rectangular platforms.
Calculating the TOG elevation
Round off to the next higher multiple of 10.
A platform should involve all the nozzles that need entrance for operations and conservation. Ideally a space of 750mm should be administered around 3 sides of a nozzle. This may be dropped at the discretion of the piping lead.
Also, the shape of the platform should favor side entry from the access ladder. Platforms longer than 6 m should have two access ladders.
Access ladder is administered for entry to equipment platforms. It is also to be used for access to instrument connections that are unattainable from working level. Ladder should be side entry type, wherever probable.
Preparing the Platform Input Document
Platform and Access ladder input is spreaded to Civil via a platform input drawing. This should distinctly indicate the TOG, dimensions, and its location w.r.t. the vessel centerline and one tan line. Grating cutout stipulations (indicating size, shape and location) need to be signified on the same drawing.
Supporting Piping from Drums:
Piping should be supported from the vessel or its platform when it is problematic to construct a civil support from grade or adjacent structure at the instructed location. Vessel support may also be taken to take advantage of lower differential thermal growth between vessel and piping, as contrasted to piping and civil support. Judicious selection of support location can eliminate the demand of springs.
Thumb rules for supporting from drums:
· Small loads can be carried directly to the platform members. These incorporate rest, one way stops, two ways stop or hold down supports and the piping layout should be done subsequently
· Large loads should be moved to the vessel shell and the piping layout should be done such that the platform members do not obstruct with these independent supports.
· Piping support should not cause any interruption to movement of personnel.
· Vessel growth should be taken to check clash of a piping support with any adjacent piping or structure.