Piping Layout: Drums And Vessel Layout Nozzle Orientation And Platform

The process vessels and drums are cylindrical hollow vessels used in process plants as intermediate containers. They are often used to provide surge volumes for liquid-vapour separations on distillation columns or separating mixtures of immiscible liquids. Other uses are flash drums, condensate and other process liquid collectors and holding drums for additives and chemicals.

The piping study on drums shall consider the general requirements for drum plant layout and provide information required to locate nozzles, instruments, piping and controls for plat forming and operation / maintenance access.

In the first study of plot plan, platform levels and details of vessel elevations are set from process requirements (net positive suction head - NPSH, gravity feed, barometric legs etc.) and from considerations of access for safe and convenient operation and maintenance.

Methods of supporting vessels and operating platforms are detailed. Access for lifting equipment or overhead hoists and trolley beams is arranged for removal of motors, mixers and internal heat exchangers from process vessels. A platform should always be provided for the removal of such heavy items of equipment and for access to manholes, sight glass, light glass etc.

In absense of any specific process requirement, the drum should be located at a minimum height depending on the valves and fittings that are below the drum. Platforming should be kept to the minimum necessary to provide safe and suitable access to manholes and operating valves.

A typical vertical drum and horizontal drum are illustrated in sketches

Types Of Drum:

Drums are categorised as horizontal or vertical drums.  Drums internals are normally demister pads, baffles, vortex breakers and distribution piping.

Steam drums and deaerators are usually proprietary items. Knock-out drums are used to separate condensate in the relieving discharges before going to the flare stack. Headers to flare stacks from relief and blow down valves and vents are run first to knockout drum. It is recommended that the lines should run overhead to the top of the drum and must slope at minimum 1:400 so that all condensate drain into the drum.

The leg-supported drum is illustrated in sketch

The saddle supported drum is illustrated in sketch

The lug-supported drum is illustrated in sketch

Leg-supported drums should not be used in reciprocating compressor circuits

Location Of Drums:

Drums are located within a process unit either adjacent to related equipment (e.g. the reflux drum) or as a stand alone unit (e.g. a condensate collection drum)

When operating within process sequence of related items (e.g. pumps, condensers and towers) the drum should be positioned to facilitate an orderly and economic piping interconnection between itself and those items.

Within a conventional inline process unit, drums and their related items are generally located on either side of a central piperack serviced by auxilliary roads for maintenance access.

In certain cases (e.g. flash drums and deaerators), drums can be located above the piperack. In chemical plants, drums are located at all levels of enclosed or open structures.

Similar to towers, drum elevations are dictated by the NPSH

Drums for chemical collection systems are generally located below ground level inside concrete pits.

To locate a drum, the following information to be collected so that the level of drum can be finalised.

Drum dimensions 

heads Support detail

NPSH requirements of pump Bottom outlet size

Minimum clearances Location

Nozzle Location:

The following information is required to position the drum nozzles.

Process vessel Sketch

Instrument vessel sketch

Piping and Instrumentation diagrams Plant layout guidelines

Nozzle schedule or summary Insulation requirements

Unit Plot Plans

The preferred location for level instrument is away from the turbulence at the liquid outlet nozzle. Instrument nozzles shall be located in the quiet zone of the vessel ie. on the opposite side of the weir or baffle or near the vapour outlet.

Process nozzles should be located at minimum from the tangent line.

The relief valve should be placed at a point on the top of the drum where the access platform can also provide access to other valves connected to the top of the drum.

The pressure connection should be placed in the vapour space at the top of the drum, so that the face of the pressure gauge is visible from the ground or platform.

The temperature connection is usually close to the bottom outlet, pointing towards the access aisle or platform.

Manholes can be positioned at the top, at the side or at one end of the vessel.

Steam-out connections should be located at the end opposite to the maintenance access with additional vent in the bottom section of the drum as steam has a lower molecular weight.

The vent connection should be located in the top section of the drum at the end opposite the steam out connection.

The drain should be located in the bottom section of the drum.

PLATFORM ARRANGEMENTS

A platform with good guard railings is necessary for access if the manhole is more than 3.5 m above grade.

Platforms on drums are required for access to valves, instruments, blinds and maintenance accesses.

Platforming should be kept to the minimum necessary to provide safe and suitable access to manholes and operating valves.

PIPING ARRANGEMENTS

Piping for process drums should be arranged in conjunction with nozzle locations, platform arrangements and the drums location to related equipment.

Piping should be positioned to facilitate the installation of supports with sufficient flexibility to absorb any excessive stresses during operation.

Relief valves open to atmosphere on low elevated horizontal or vertical drums should be positioned to allow the discharge piping to be routed to a convenient and safe location. Closed system relief valve should be located at a convenient platform adjacent to the drum above the relief valve header.

Normally, the liquid or vapour inlet is at the top and at one end of drum with liquid outlet on the bottom and vapour outlet at the top at opposite end.

Drain and vent lines may be located centrally or at the ends if the drum is horizontal and if desired, the drain valve may be placed at the low point of the outlet piping.

Horizontal drums should always have small slopes towards the drain points.

Level, pressure and temperature instruments are used to control the operation of the drum and should be in a position for optimum operation and maintenance.

Level controllers, switches and gauges are either located individually or grouped from a common bridle or stand pipe. The controller must be operable from grade or a platform, switches, gauges and pressure / temperature connections may be operable from a ladder if no platform is available at the required elevation.

The fixed saddle shall be on the same side of the vessel as the pump suction nozzle. In case of requirement of an emergency isolation valves, these valves shall be located directly on the vessel nozzle. The position of the support shall allow for the installation of the valve and its actuator.

In order to gain access for maintenance and operations purpose, level transmitter (LT) and Level indicator (LT) nozzles shall be arranged on either side of the access ladder with their centres 400mm from the ladder centerline.

All instruments shall be grouped on vessels and drums such that the minimum number of ladders and platforms is required, without any compromise on accessibility.